Chemical Engineering Vocabulary: 100 Key English Terms
Chemical engineers work with English every day: process flow diagrams, P&IDs, equipment datasheets, simulation reports, safety studies and licensor manuals are almost always written in it. Knowing the core vocabulary of unit operations, reactors, heat and mass transfer, process control and process safety helps you follow design reviews, talk to vendors and international teams, and explain clearly what is happening in the plant.
100 terms
11 topics
51 core terms
200 example sentences
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No account needed. Read the meaning, see where the term is used, then study the examples. Click any word in the lesson for help. You can also select a word and press Alt + Enter.
Process Fundamentals and Balances
unit operation
noun
Meaning
A basic physical step that appears in many different processes, such as distillation, filtration, drying or heat exchange. Any plant can be described as a chain of these steps.
Where you use it
Used at university and in design work to break a process into blocks. Courses and textbooks are often called 'Unit Operations', and engineers say a plant 'consists of' or 'combines' several unit operations.
Examples
The downstream section has three main unit operations: extraction, evaporation and crystallization.
Before you size anything, draw each unit operation as a block and label every stream.
Usage note
A unit operation is a physical change (separation, mixing, heating). A chemical change in a reactor is sometimes called a 'unit process'.
Common phrases
key unit operation · separation unit operation · combine unit operations · unit operations lab
A calculation based on the rule that mass is not created or destroyed: everything that goes into a system must come out, react or build up inside it.
Where you use it
The first calculation in almost every design, troubleshooting or lab report. Engineers do, close, check or set up a mass balance around a unit or the whole plant. 'The balance doesn't close' means the numbers in and out do not match.
Examples
Our mass balance around the dryer doesn't close; we are missing about 4% of the water.
Start with an overall mass balance, then write a component balance for ethanol.
Usage note
'Material balance' means the same thing. Turkish speakers sometimes say 'mass equilibrium', which is wrong here.
Common phrases
overall mass balance · component mass balance · close the mass balance · mass balance around
A calculation that tracks all the energy entering and leaving a system as heat, work and the enthalpy of the streams, so that heating and cooling needs can be found.
Where you use it
Done right after the mass balance in design and in troubleshooting. Engineers use it to size heat exchangers, calculate steam or cooling water demand and check reactor heat removal. Usually written as 'heat and mass balance' or 'H&MB' in project documents.
Examples
The energy balance shows that the reactor needs about 350 kW of cooling at full load.
The feed is colder than we assumed, so please update the energy balance before the vendor sizes the reboiler.
Usage note
In project documents 'heat and mass balance' is a fixed phrase, not 'mass and heat balance'.
Common phrases
heat and mass balance · overall energy balance · perform an energy balance · energy balance on the reactor
A drawing that shows the main equipment of a plant, how the streams connect them and the key operating data such as flows, temperatures and pressures.
Where you use it
Prepared early in a project, before the P&ID. It usually has a stream table at the bottom. Engineers draw, issue, review and mark up a process flow diagram, and everyone calls it the PFD.
Examples
Check the stream table on the process flow diagram for the design temperature of the feed.
The process flow diagram shows only the main equipment; valves and instruments are on the P&IDs.
Usage note
Do not mix up the PFD and the P&ID: the PFD shows the process, the P&ID shows every pipe, valve and instrument.
Common phrases
issue the process flow diagram · stream table · PFD revision · draw a process flow diagram
A way of producing in separate loads: materials are charged into a vessel, processed for a set time and then emptied before the next load starts.
Where you use it
Typical of pharmaceuticals, fine chemicals, paints, food and specialty polymers. Each load is a 'batch' with its own batch number and batch record. Engineers talk about batch time, cycle time and batch-to-batch variation.
Examples
The resin is made in a batch process, and each batch takes about nine hours.
Moving from a batch process to continuous production would cut our labor costs.
Usage note
A 'semi-batch' process adds one reactant gradually during the batch, which helps control heat release.
Common phrases
batch reactor · batch time · batch record · batch-to-batch variation · run a batch
A way of producing in which feed enters and product leaves without stopping, usually for months at a time at nearly constant conditions.
Where you use it
Typical of refineries, petrochemicals, fertilizers and bulk chemicals with high volumes. Engineers compare batch and continuous operation and say a unit 'runs continuously' or 'operates around the clock'.
Examples
At 200,000 tonnes per year, a continuous process is far more economical than batch reactors.
In a continuous process, a small upset can affect hours of production.
Usage note
Stress falls on the second syllable: con-TIN-u-ous.
Common phrases
continuous operation · continuous production · run continuously · switch to a continuous process
The condition in which temperatures, pressures, flows and compositions at every point in a process stay constant over time.
Where you use it
Used in calculations (most balances assume steady state) and in operations: after a change, operators wait for the unit to 'reach' or 'settle at' steady state. The opposite is 'transient' or 'dynamic' conditions, such as start-up.
Examples
Wait until the column reaches steady state before you take samples.
This is a steady-state simulation, so it won't show what happens during start-up.
Usage note
Steady state is not the same as equilibrium: a column at steady state still has heat and material flowing through it.
Common phrases
reach steady state · steady-state operation · steady-state simulation · at steady state
The material that enters a unit or a plant to be processed, such as crude oil to a refinery or the liquid stream entering a column.
Where you use it
Used in every unit. Operators increase, cut, divert or stop the feed; engineers talk about feed composition, feed rate, feed quality and feedstock. Also a verb: 'we feed the column at tray 18'.
Examples
Cut the feed to 70% until the compressor is back online.
The feed contains about 2% water, which hurts the catalyst.
Usage note
'Feedstock' is the raw material for a whole plant; 'feed' can be any stream entering any unit.
A stream that takes unreacted material or a process fluid from a later point in the process and sends it back to an earlier point, so that it is not wasted.
Where you use it
Common when per-pass conversion is low. Engineers must include recycle streams in the mass balance and often add a purge stream so that inerts and impurities do not build up. 'Recycle' is also used alone as a noun and verb.
Examples
Argon builds up in the recycle stream, so we purge a small part of it continuously.
Without the recycle stream, almost 80% of the hydrogen would be lost.
Usage note
In process engineering 'recycle' means sending material back inside the process, not collecting waste for recycling.
A vessel or piece of equipment where a chemical reaction is carried out under controlled temperature, pressure and mixing.
Where you use it
Central to every chemical plant. You charge, heat, cool, pressurize and empty a reactor. Common types are batch reactors, stirred tanks (CSTR), tubular or plug flow reactors and fixed-bed catalytic reactors.
Examples
We charged the reactor with 2 tonnes of monomer and heated it to 80 °C.
The reactor temperature rose too quickly, so the operator cut the feed.
Usage note
Stress is on the second syllable: re-AC-tor.
Common phrases
batch reactor · jacketed reactor · fixed-bed reactor · charge the reactor · reactor temperature
A substance that makes a reaction go faster, or more selectively, without being used up itself. In industry it is often a solid in pellets placed in a reactor bed.
Where you use it
Used in refining, petrochemicals, ammonia, polymers and pharmaceuticals. Catalysts are loaded, activated, poisoned, deactivated, regenerated and replaced. 'Catalyst life' and 'catalyst change-out' are common in planning meetings.
Examples
Sulfur in the feed is poisoning the catalyst, so conversion has dropped by 5%.
The catalyst change-out is planned for the next turnaround.
Usage note
The verb is 'catalyze' (US) or 'catalyse' (UK); the adjective is 'catalytic'.
Common phrases
catalyst bed · catalyst activity · catalyst poisoning · regenerate the catalyst · catalyst change-out
The amount of desired product you actually get, usually shown as a percentage of the amount you could get in theory from the raw material fed.
Where you use it
Used in reaction engineering, production reports and economics. Yield is improved, increased, lost or calculated; plants report 'yield on feed' or 'overall yield'. It is also a verb: 'the reaction yields 92% product'.
Examples
After the new catalyst was installed, the yield went up from 86% to 91%.
Most of the yield loss comes from side reactions at high temperature.
Usage note
Yield, conversion and selectivity are different. Conversion counts how much feed reacted; selectivity says how much of that went to the right product; yield combines the two.
Common phrases
product yield · overall yield · yield loss · improve the yield · theoretical yield
The fraction of a reactant that has reacted, usually given as a percentage of the amount fed to the reactor.
Where you use it
Used in reactor design, lab reports and plant monitoring. Engineers talk about per-pass conversion and overall conversion, and say conversion is high, low, increasing or limited by equilibrium.
Examples
Per-pass conversion is only 15%, so the unreacted gas is recycled to the reactor.
Higher temperature gives higher conversion but lower selectivity.
Usage note
In chemical engineering 'conversion' never means a unit conversion unless the context makes that clear; say 'unit conversion' for that.
How fast reactants are used up or products are formed, usually given as moles per unit volume per unit time. It depends on temperature, concentration and catalyst.
Where you use it
Used in kinetics, reactor design and safety studies. Engineers measure the reaction rate, write a rate equation (rate law) and find the rate constant. 'Kinetics' is the general word for the study of reaction rates.
Examples
As a rough rule, the reaction rate doubles for every 10 °C rise in temperature.
We need lab data to fit the reaction rate before we can size the reactor.
Usage note
The doubling rule is only a rule of thumb; the real effect depends on the activation energy of the reaction.
Common phrases
reaction rate constant · rate of reaction · increase the reaction rate · rate law
The average time that material spends inside a vessel or reactor, found for a simple case by dividing the volume by the volumetric flow rate.
Where you use it
Used in reactor design, settling tanks, dryers and scale-up. Engineers increase, reduce or calculate residence time and talk about the residence time distribution (RTD). 'Holdup time' is used in a similar way for separators.
Examples
At the higher feed rate, the residence time drops to 20 minutes and conversion falls.
The separator was sized for five minutes of liquid residence time.
Usage note
In reactor textbooks the symbol τ (tau) is often called 'space time'; for constant density it equals the residence time.
Common phrases
average residence time · residence time distribution · increase the residence time · liquid residence time
How much of the reacted feed turns into the desired product rather than into unwanted by-products, usually given as a percentage.
Where you use it
Used in catalyst testing, reactor optimization and economics. Engineers say selectivity is high, drops with temperature, or improves with a new catalyst. There is often a trade-off between conversion and selectivity.
Examples
The new catalyst gives 96% selectivity to ethylene oxide.
If we push conversion higher, selectivity drops and we make more CO2.
Usage note
Use 'selectivity to' or 'selectivity towards' the product: 'selectivity to propylene'.
Common phrases
high selectivity · selectivity to · product selectivity · improve selectivity
Short for continuous stirred-tank reactor: a tank with a mixer where feed flows in and product flows out all the time, and the contents are assumed to be perfectly mixed.
Where you use it
One of the two ideal reactor models taught in every reaction engineering course, next to the plug flow reactor. Engineers say 'model it as a CSTR' or 'two CSTRs in series'. Said letter by letter: C-S-T-R.
Examples
In a CSTR, the outlet composition is the same as the composition inside the tank.
Using two CSTRs in series gets us closer to plug-flow performance.
Usage note
Some books call it a 'backmix reactor' or 'mixed flow reactor (MFR)'.
Common phrases
CSTRs in series · ideal CSTR · model as a CSTR · CSTR volume
An ideal tube reactor in which the fluid moves forward like a series of thin slices with no mixing along the length, so the composition changes from inlet to outlet.
Where you use it
Used as a design model for tubular reactors, packed catalyst beds and long pipes. Abbreviated PFR. Engineers compare PFR and CSTR volumes and say a reactor 'behaves close to plug flow'.
Examples
For the same conversion, a plug flow reactor usually needs less volume than a CSTR.
We modeled the cracking furnace coil as a plug flow reactor.
Usage note
'Plug' here means a moving slice of fluid, not a stopper or an electrical plug.
Describes a reaction or process that releases heat to its surroundings, such as combustion, neutralization or most polymerizations.
Where you use it
Used in reactor design and process safety. Strongly exothermic reactions need good cooling, controlled dosing and protection against runaway. Engineers say a reaction is 'highly', 'mildly' or 'strongly' exothermic and talk about the heat of reaction.
Examples
The nitration is highly exothermic, so the acid is dosed slowly with full cooling.
Because the reaction is exothermic, the outlet of the catalyst bed is 40 °C hotter than the inlet.
Usage note
The noun 'exotherm' means the temperature rise you see when an exothermic reaction starts: 'we saw a 15 °C exotherm'.
Describes a reaction or process that takes in heat from its surroundings, so heat has to be supplied to keep it going, as in steam reforming or cracking.
Where you use it
Used in reactor and furnace design. Endothermic reactions are often run in fired heaters or with interstage reheating, and the temperature drops as the reaction proceeds if heat is not added.
Examples
Steam reforming is strongly endothermic, so the tubes sit inside a large furnace.
In the endothermic dehydrogenation, the gas is reheated between the reactor beds.
Usage note
Remember: exo = out (heat goes out), endo = in (heat goes in).
The minimum energy that reacting molecules need for a reaction to happen. A high value means the rate is very sensitive to temperature; a catalyst lowers it.
Where you use it
Used in kinetics and safety studies. Engineers find it from lab data with the Arrhenius equation, k = A·exp(−Ea/RT), and give it in kJ/mol. It explains why a small temperature rise can speed up a reaction a lot.
Examples
The activation energy is about 90 kJ/mol, so around 80 °C a 10 °C rise more than doubles the rate.
The new catalyst lowers the activation energy, so we can run the reactor 30 °C cooler.
Usage note
Written Ea or Eₐ. The 'apparent' activation energy measured in a plant can be lower than the true value when mass transfer limits the rate.
Common phrases
apparent activation energy · Arrhenius equation · lower the activation energy · activation energy barrier
The slowest step in a series of steps, such as reaction, diffusion or heat removal, which controls how fast the whole process can go.
Where you use it
Used in kinetics, catalysis and troubleshooting. Engineers ask whether a process is limited by kinetics, mass transfer or heat transfer, because only speeding up the rate-limiting step will help.
Examples
Gas-liquid mass transfer is the rate-limiting step, so a faster agitator will help more than extra catalyst.
In the plant, heat removal, not chemistry, became the rate-limiting step.
Usage note
'Rate-determining step' means the same thing and is more common in chemistry textbooks.
A substance made in a process alongside the main product. It may be sold, used elsewhere in the plant, or treated as waste.
Where you use it
Used in reaction engineering, economics and environmental reports. Engineers try to reduce unwanted by-products, separate them from the product and find a value for them. Also spelled 'byproduct'.
Examples
Hydrochloric acid is a by-product of the chlorination step and is sold to a nearby plant.
At higher temperatures we form more heavy by-products that foul the reboiler.
Usage note
A 'side reaction' is the reaction that forms the by-product; 'co-product' is used when the second product is also wanted.
A piece of equipment that moves heat from one fluid to another without letting them mix, for example from hot oil to cold water through the walls of tubes or plates.
Where you use it
Found in every plant. Engineers size, rate, clean and bypass heat exchangers. The main types are shell-and-tube, plate and air-cooled exchangers. People often shorten the word to 'exchanger'.
Examples
The feed is preheated in a shell-and-tube heat exchanger using the hot reactor effluent.
We need to open the heat exchanger and clean the tubes during the shutdown.
Usage note
Turkish 'eşanjör' comes from French 'échangeur'; in English say 'heat exchanger' or 'exchanger'.
A number that shows how easily heat passes from one fluid to another through a wall, measured in W/(m²·K). The higher it is, the smaller the exchanger can be.
Where you use it
Used in exchanger design and performance monitoring. The overall value is called U or 'U-value' and appears in Q = U·A·ΔT. Engineers estimate, calculate and track it; a falling heat transfer coefficient usually means fouling.
Examples
The overall heat transfer coefficient has fallen by 30% since the last cleaning.
Water on the tube side gives a much higher heat transfer coefficient than oil.
Usage note
Do not confuse it with thermal conductivity (k), which is a property of a material, not of a whole exchanger.
Common phrases
overall heat transfer coefficient · film heat transfer coefficient · clean heat transfer coefficient · estimate the heat transfer coefficient
A heat exchanger that cools a vapor until it turns back into liquid, for example at the top of a distillation column or at the exhaust of a steam turbine.
Where you use it
Used in distillation, refrigeration and power plants. Engineers talk about the overhead condenser, total and partial condensers, and condenser duty. A 'condenser limitation' often caps column capacity on hot summer days.
Examples
On hot days the air-cooled condenser can't keep up and the column pressure rises.
Part of the liquid from the condenser goes back to the column as reflux.
Usage note
In electronics 'condenser' is an old word for capacitor; in a plant it always means a heat exchanger.
A heat exchanger at the bottom of a distillation column that boils part of the bottom liquid to produce the vapor that rises up the column.
Where you use it
Used in every distillation column. It is usually heated with steam or hot oil. Common types are kettle and thermosiphon reboilers. Operators raise reboiler duty to send more vapor up the column.
Examples
Increase the steam to the reboiler slowly or you'll flood the column.
The thermosiphon reboiler needs no pump because circulation is driven by density difference.
The build-up of unwanted deposits such as scale, coke, polymer or biological growth on equipment surfaces, which reduces heat transfer and increases pressure drop.
Where you use it
Used in exchanger design, operations and maintenance planning. Designers add a fouling factor (fouling resistance) to the calculation; operators watch for signs of fouling and schedule cleaning. The verb is 'foul': 'the tubes are fouled'.
Examples
Fouling on the cooling water side has cut the exchanger duty by a quarter.
We added a fouling factor of 0.0004 m²·K/W for the crude side.
Usage note
Not related to 'foul' in sports. In a plant it always means dirty deposits on surfaces.
The amount of heat that a heat exchanger, reboiler, condenser or furnace must transfer per unit time, usually given in kW or MW.
Where you use it
Used on datasheets and in design and performance checks. Engineers calculate the heat duty from the energy balance and use it to size equipment. People often just say 'duty': 'the condenser duty is 4 MW'.
Examples
The heat duty of the feed preheater is 1.8 MW at design rates.
Fouling has reduced the exchanger's heat duty by about 20%.
Usage note
In this sense 'duty' has nothing to do with tax or a person's obligation.
An outer shell around a reactor or vessel through which steam, hot oil or cooling water flows to heat or cool the contents.
Where you use it
Typical of batch reactors in pharmaceuticals and fine chemicals. Engineers talk about jacket temperature, jacket inlet and outlet, and half-pipe or dimpled jackets. A 'jacketed reactor' or 'jacketed vessel' has one.
Examples
Switch the jacket from steam to cooling water as soon as the reaction starts.
The jacket can't remove heat fast enough in the 10 m³ reactor, so we added an internal coil.
Usage note
In plant English a jacket is part of the equipment, not clothing.
A way of separating a liquid mixture by boiling it and condensing the vapor, using the fact that the components boil at different temperatures.
Where you use it
The most common separation method in refineries and chemical plants. It takes place in a distillation column with a reboiler at the bottom and a condenser at the top. Engineers talk about the overhead or distillate, the bottoms, the feed tray and the number of stages.
Examples
The solvent is recovered by distillation and sent back to the reactor.
The distillation column can't reach 99.5% purity at this reflux ratio.
Usage note
The column itself is a 'distillation column' or 'tower'; 'distiller' usually means a company that makes spirits.
A separation method in which one or more components of a gas dissolve into a liquid, for example CO2 being removed from gas by an amine solution.
Where you use it
Used in gas treating, emissions control and acid production. It is carried out in an absorber or scrubber column; the loaded solvent is then sent to a stripper or regenerator. Engineers talk about absorption efficiency and solvent circulation rate.
Examples
Hydrogen sulfide is removed by absorption in an amine contactor.
Absorption improves at lower temperature, so the lean amine is cooled first.
Usage note
Absorption (into the bulk of a liquid) is different from adsorption (onto the surface of a solid such as activated carbon). Turkish speakers often mix the spellings.
Common phrases
gas absorption · absorption column · absorption efficiency · physical absorption · chemical absorption
A separation method that uses a solvent to pull a component out of a liquid or solid mixture, based on how much better it dissolves in the solvent.
Where you use it
Used in pharmaceuticals, aromatics recovery, metal refining and food processing. In liquid-liquid extraction the two phases are the raffinate and the extract. Engineers choose the solvent, the solvent-to-feed ratio and the number of stages.
Examples
The product is recovered from the aqueous layer by extraction with ethyl acetate.
Three extraction stages give more than 98% recovery.
Usage note
In mining, oil and gas, 'extraction' can also mean getting raw material out of the ground, so check the context.
Separating solid particles from a liquid or a gas by passing the mixture through a porous medium, such as a cloth, mesh or cartridge, that holds the solids back.
Where you use it
Used across the industry, from catalyst recovery to water treatment. The solids that build up form a filter cake and the clear liquid is the filtrate. Common equipment includes filter presses, Nutsche filters, bag and cartridge filters.
Examples
Filtration is slow because the crystals are too fine and blind the cloth.
After filtration, the cake is washed with cold water and sent to the dryer.
Usage note
The verb is 'filter', not 'filtrate'. 'Filtrate' is a noun: the liquid that passes through.
Forming solid crystals from a solution, usually by cooling it or evaporating the solvent, so that a pure solid product can be separated.
Where you use it
Common in pharmaceuticals, fertilizers, sugar and salt production. Engineers control supersaturation, cooling rate and seeding to get the right crystal size. The equipment is a crystallizer; the liquid left behind is the mother liquor.
Examples
A slower cooling rate during crystallization gives bigger crystals that filter more easily.
We add seed crystals at 45 °C to start crystallization in a controlled way.
Usage note
British spelling: crystallisation. Both are correct.
Removing water or another liquid from a solid, usually by heating it with hot gas or under vacuum, until it reaches the required moisture content.
Where you use it
Often the last step before packing a solid product. Common equipment includes spray dryers, fluid-bed dryers, rotary dryers and vacuum dryers. Engineers talk about moisture content, loss on drying (LOD) and drying time.
Examples
Drying takes twelve hours because the cake still holds 30% solvent.
The spray dryer outlet temperature controls the final moisture after drying.
Usage note
The equipment is a 'dryer' (also spelled 'drier', which is more often an adjective: 'drier air').
In distillation, the amount of condensed liquid sent back to the top of the column divided by the amount taken off as product (L/D).
Where you use it
One of the main handles on a distillation column. Raising the reflux ratio improves purity but costs more energy in the reboiler and condenser. Designers often choose about 1.2 to 1.5 times the minimum reflux ratio.
Examples
We raised the reflux ratio from 2.5 to 3 to get the top product back on spec.
Running at a high reflux ratio is wasting steam, since the product is already well over spec.
Usage note
'Total reflux' means all the condensed liquid goes back to the column and no product is taken, often used during start-up.
Common phrases
minimum reflux ratio · increase the reflux ratio · total reflux · reflux drum · reflux pump
A horizontal plate inside a distillation or absorption column where rising vapor bubbles through the liquid flowing across it, so the two phases exchange components.
Where you use it
Used when talking about column internals, design and troubleshooting. Common types are sieve, valve and bubble-cap trays. Engineers count trays from the top or bottom, talk about tray efficiency and flooding, and specify the feed tray. The alternative is packing.
Examples
The feed enters on tray 18, counting from the top.
Gamma scanning showed that two trays had collapsed during the last upset.
Usage note
'Plate' is also used for tray, and 'theoretical stage' or 'theoretical plate' is an ideal tray used in calculations.
The movement of a component from one place or phase to another because of a difference in concentration, for example from a gas into a liquid or from a liquid into a solid.
Where you use it
The theory behind distillation, absorption, extraction, drying and many reactor problems. Engineers talk about mass transfer rate, mass transfer coefficients and whether a process is 'mass-transfer limited'. Usually taught together with heat transfer and fluid flow as transport phenomena.
Examples
Better gas dispersion improved mass transfer and the hydrogenation finished in half the time.
The reaction is fast, so the overall rate is limited by mass transfer, not kinetics.
Usage note
Turkish university courses often say 'kütle aktarımı'; in industry 'kütle transferi' is more common.
Common phrases
mass transfer coefficient · mass transfer rate · mass-transfer limited · interphase mass transfer
The movement of molecules from an area of high concentration to an area of low concentration caused by their random motion, without any bulk flow.
Where you use it
Used in mass transfer theory, catalyst design and membranes. Engineers describe it with Fick's law and a diffusion coefficient (diffusivity). Inside porous catalyst pellets, slow pore diffusion can limit the reaction rate.
Examples
Smaller catalyst pellets reduce the diffusion path and improve activity.
Diffusion in liquids is about ten thousand times slower than in gases.
Usage note
The verb is 'diffuse' (/dɪˈfjuːz/); the adjective 'diffuse' (/dɪˈfjuːs/) means spread out and is pronounced with an s sound.
Common phrases
diffusion coefficient · molecular diffusion · pore diffusion · Fick's law of diffusion
The pressure of the vapor above a liquid when the two are in balance at a given temperature. It rises with temperature, and the liquid boils when it equals the surrounding pressure.
Where you use it
Used in distillation design, storage, pump NPSH checks and safety. Volatile liquids have a high vapor pressure. Engineers estimate it with the Antoine equation or read it from data tables and simulators.
Examples
Butane has a high vapor pressure, so it is stored in pressurized spheres.
At 60 °C the vapor pressure of the solvent is too high for this pump's suction.
Usage note
US spelling is 'vapor', UK spelling is 'vapour'. Keep one spelling in a document.
The temperature at which a liquid's vapor pressure equals the surrounding pressure, so that it boils. It goes down under vacuum and up at higher pressure.
Where you use it
Used when choosing separation methods, solvents and operating pressures. Engineers compare the boiling points of components and talk about 'close-boiling' mixtures that are hard to separate. The normal boiling point is measured at 1 atm.
Examples
We run the column under vacuum to lower the boiling point and protect the product.
The two isomers have boiling points only 3 °C apart.
Usage note
For mixtures, engineers use 'bubble point' and 'dew point' instead of a single boiling point.
Common phrases
normal boiling point · boiling point elevation · close-boiling · lower the boiling point
A measure of the heat content of a substance, used to calculate how much energy is needed to heat, cool, vaporize or react it at constant pressure. Usually given in kJ/kg or kJ/mol.
Where you use it
Used in every energy balance and in steam tables. Engineers talk about stream enthalpy, enthalpy change (ΔH) and enthalpy of reaction. Only changes in enthalpy matter, so values are given relative to a reference state.
Examples
Take the steam enthalpy from the steam tables at 10 bar.
The enthalpy of reaction is −95 kJ/mol, so we have to remove a lot of heat.
Usage note
A negative enthalpy of reaction (ΔH < 0) means the reaction releases heat.
Common phrases
enthalpy change · enthalpy of reaction · enthalpy of vaporization · specific enthalpy
The state in which a liquid and its vapor exist together with no net change, each with a fixed composition. It is the basis for designing distillation and flash separations.
Where you use it
Abbreviated VLE. Engineers fit VLE data, choose a thermodynamic model (NRTL, UNIQUAC, Peng-Robinson) in the simulator and plot x-y diagrams. 'Bad VLE data' is a classic reason for a column not meeting spec.
Examples
Check the vapor-liquid equilibrium data before trusting the column simulation.
Ethanol and water form an azeotrope, which you can see clearly on the vapor-liquid equilibrium plot.
Usage note
Equilibrium is pronounced /ˌiːkwɪˈlɪbriəm/, with stress on 'LIB'.
Common phrases
VLE data · VLE model · x-y diagram · binary vapor-liquid equilibrium
The equation PV = nRT, which links the pressure, volume, temperature and amount of a gas. It works well at low pressure and high temperature.
Where you use it
Used for quick estimates of gas density, volume and flow. For high pressures or non-ideal gases engineers add a compressibility factor (Z) or use an equation of state such as Peng-Robinson. Temperature must always be in kelvin.
Examples
Using the ideal gas law, one kilomole of gas at 0 °C and 1 atm takes up about 22.4 m³.
At 150 bar the ideal gas law is not good enough; use the simulator's equation of state.
Usage note
Use absolute pressure and temperature in kelvin; using gauge pressure or °C is a very common exam and design mistake.
Common phrases
ideal gas law · ideal gas behavior · compressibility factor · equation of state
The amount of heat needed to raise the temperature of a material by one degree. Per kilogram it is called specific heat capacity; water's is about 4.18 kJ/(kg·K).
Where you use it
Used in every sensible-heat calculation: Q = m·cp·ΔT. Engineers take values from data books or simulators and use cp for constant pressure. It is needed to size exchangers and estimate how fast a vessel heats up or cools down.
Examples
Oil has about half the heat capacity of water, so you need twice the mass flow for the same duty.
Check the heat capacity of the mixture, not just the pure solvent.
Usage note
'Specific heat' and 'specific heat capacity' are used interchangeably; written cp (or Cp).
Common phrases
specific heat capacity · heat capacity at constant pressure · mean heat capacity · cp value
The heat taken in or given out when a substance changes phase, for example from liquid to vapor, without any change in temperature.
Where you use it
Used in condenser, reboiler, evaporator and dryer calculations. Engineers talk about latent heat of vaporization and of condensation; condensing steam gives up its latent heat, which is why steam is such a good heating medium.
Examples
Water's latent heat of vaporization at 100 °C is about 2,260 kJ/kg, more than five times the heat needed to warm it from 0 to 100 °C.
Most of the dryer's energy goes into supplying latent heat to evaporate the water.
Usage note
'Sensible heat' changes the temperature; 'latent heat' changes the phase. 'Heat of vaporization' is the same as latent heat of vaporization.
Common phrases
latent heat of vaporization · latent heat of condensation · sensible and latent heat · heat of vaporization
The temperature at which a gas or vapor mixture, when cooled at constant pressure, starts to form the first drops of liquid.
Where you use it
Used in gas processing, drying, compressed air and distillation. Engineers specify water dew point and hydrocarbon dew point for natural gas and instrument air. The matching term for the first bubble of vapor when heating a liquid is the bubble point.
Examples
Instrument air is usually dried to a dew point of −40 °C so that no water condenses in the lines.
Keep the flue gas above its acid dew point to avoid corrosion in the stack.
Usage note
Pronounced /duː/ in American English, like 'do', and /djuː/ in British English.
Common phrases
water dew point · hydrocarbon dew point · acid dew point · bubble point and dew point
The amount of fluid that passes a point in a given time, given either as mass (kg/h) or as volume (m³/h).
Where you use it
One of the most common words in control rooms, datasheets and balances. Operators increase, reduce or stabilize a flow rate; engineers specify mass flow rate or volumetric flow rate. In speech people often just say 'flow': 'the feed flow is low'.
Examples
Increase the cooling water flow rate to 40 m³/h and watch the outlet temperature.
The design mass flow rate of the feed is 12,500 kg/h.
Usage note
Turkish 'debi' has no English cognate; do not write 'debit', which means a bank withdrawal.
Common phrases
mass flow rate · volumetric flow rate · design flow rate · increase the flow rate · flow rate measurement
A machine that moves a liquid through pipes by adding pressure to it. Most plant pumps are centrifugal; others are positive displacement types such as gear or diaphragm pumps.
Where you use it
Used every day in operations and design. Operators start, stop, prime and switch pumps; engineers size them using a pump curve and specify head, flow and NPSH. Critical services often have a spare: 'pump A running, pump B on standby'.
Examples
Switch to the standby pump; pump A is vibrating badly.
The pump can't deliver the design flow against the new discharge pressure.
Usage note
The vowel is a short /ʌ/, close to a Turkish 'a': /pʌmp/. Do not read the 'u' as a Turkish 'u'.
A device in a pipe that starts, stops or adjusts the flow of a fluid, either by hand or automatically.
Where you use it
One of the first words new engineers hear on site. Valves are opened, closed, throttled, isolated, locked and leak-tested. Common types are gate, globe, ball, butterfly and check valves; 'block valve' and 'isolation valve' are used for shut-off duty.
Examples
Close the inlet valve and drain the filter before you open it.
A check valve on the discharge stops backflow when the pump trips.
Usage note
Pronounce the final 'v': /vælv/. 'Valf' with an 'f' sound is a Turkish habit.
Common phrases
isolation valve · check valve · open a valve · close a valve · valve position
The loss of pressure as a fluid flows through a pipe, valve, filter, bed or piece of equipment, caused mainly by friction.
Where you use it
Used in line sizing, pump selection and troubleshooting. Engineers calculate, measure and allow for pressure drop; a rising pressure drop across a filter, catalyst bed or exchanger is a classic sign of plugging or fouling. Often written ΔP and said 'delta P'.
Examples
The pressure drop across the filter has doubled since Monday; it's probably blocked.
A 4-inch line would give too much pressure drop, so we went to 6 inches.
Usage note
'Pressure loss' and 'differential pressure (DP)' are also used; on instruments you will see 'PDI' for a differential pressure indicator.
Common phrases
high pressure drop · pressure drop across · allowable pressure drop · differential pressure · calculate the pressure drop
A fluid's resistance to flow: honey has a high value, water a low one. Measured in pascal-seconds or, more often in practice, centipoise (cP).
Where you use it
Used in pump selection, pipe sizing, mixing and quality control. Viscosity falls as liquids get hotter, which is why heavy oils and resins are kept warm. Water at 20 °C has a viscosity of about 1 cP.
Examples
The resin's viscosity is too high at room temperature, so the line is steam-traced.
As viscosity rises, the centrifugal pump loses head and efficiency.
Usage note
Dynamic viscosity (cP, Pa·s) and kinematic viscosity (cSt, m²/s) are different; always check which one a datasheet gives.
A dimensionless number (Re = ρvD/μ) that compares inertial forces with viscous forces and tells you whether flow is laminar or turbulent.
Where you use it
Used in pipe flow, heat transfer and mixing calculations. In pipes, flow is usually laminar below about 2,100–2,300 and fully turbulent above about 4,000. Engineers calculate Re first, then pick the right friction factor or heat transfer correlation.
Examples
With this oil the Reynolds number is only 900, so the flow is laminar.
Keep the Reynolds number above 10,000 in the tubes for good heat transfer.
Usage note
Always 'Reynolds number', with the s and a capital R; it is named after Osborne Reynolds.
Common phrases
calculate the Reynolds number · high Reynolds number · pipe Reynolds number · impeller Reynolds number
A machine that raises the pressure of a gas by reducing its volume. The main types are centrifugal, reciprocating and screw compressors.
Where you use it
Used for process gas, refrigeration, hydrogen recycle and instrument air. Engineers talk about suction and discharge pressure, compression ratio, surge and anti-surge control. A compressor trip often shuts down a whole unit.
Examples
The recycle gas compressor tripped on high vibration and the reactor had to be depressurized.
Gas leaves the compressor much hotter, so there is a cooler after each stage.
Usage note
Pumps move liquids; compressors and blowers move gases. Do not call a gas machine a 'pump' in English.
The forming and violent collapse of vapor bubbles in a liquid when local pressure falls below its vapor pressure, usually at a pump inlet or across a valve.
Where you use it
A common pump and valve problem. Operators recognize it by a noise like gravel in the pump, vibration and falling discharge pressure. It damages impellers and valve trims. The fix is usually more NPSH: higher suction level, lower temperature or less suction pressure drop.
Examples
The pump sounds like it's full of gravel; that's cavitation from the low tank level.
Cavitation has eaten away the edges of the impeller vanes.
Short for net positive suction head: the margin between the pressure at the pump inlet and the liquid's vapor pressure, expressed as meters of liquid.
Where you use it
Checked in every pump selection. NPSHa (available) comes from the system design; NPSHr (required) comes from the pump vendor's curve. NPSHa must be higher than NPSHr with a safety margin, or the pump will cavitate.
Examples
With the tank at minimum level, NPSH available is only 2.1 m against 2.5 m required.
Raising the suction vessel by two meters solved the NPSH problem.
Usage note
Said letter by letter: N-P-S-H. Always make clear whether you mean NPSH available (NPSHa) or NPSH required (NPSHr).
Smooth flow in which fluid moves in parallel layers with little mixing between them. In pipes it happens at low Reynolds numbers, typically below about 2,100.
Where you use it
Common with viscous liquids, small channels and slow flows. Engineers note that laminar flow gives poor heat transfer and mixing, and use different pressure drop equations for laminar and turbulent flow.
Examples
The polymer solution is in laminar flow, so the heat transfer in the cooler is poor.
In laminar flow the pressure drop rises in direct proportion to velocity.
Usage note
The opposite is 'turbulent flow' (türbülanslı akış); between them is the 'transition region'.
Common phrases
laminar flow regime · fully developed laminar flow · laminar and turbulent flow · transition region
Short for piping and instrumentation diagram: a detailed drawing of every pipe, valve, instrument, control loop and safety device in a unit.
Where you use it
The key document for design, construction, HAZOP, operation and maintenance. Engineers review, mark up, redline and 'walk down' P&IDs (check them against the real plant). Said as 'P and I D' or 'P and ID'.
Examples
The bypass valve is missing from the P&ID, so please add it in the next revision.
We'll go through the P&ID node by node in the HAZOP tomorrow.
Usage note
Some companies write 'piping and instrument diagram'; both expansions are used.
Common phrases
P&ID review · mark up the P&ID · as-built P&ID · P&ID walkdown
The target value that a controller tries to hold, such as a temperature of 85 °C or a tank level of 60%.
Where you use it
Used constantly in control rooms. Operators raise, lower, change or ramp a setpoint; engineers check how far the process value is from the setpoint. Also written 'set point' or abbreviated SP, next to PV (process variable) and OP (output).
Examples
Lower the reactor temperature setpoint to 78 °C until the cooling water problem is fixed.
The level keeps swinging around the setpoint, so the controller probably needs retuning.
Usage note
Turkish operators often say 'set etmek' or 'set değeri'; in English say 'change the setpoint', not 'set the set'.
Common phrases
change the setpoint · temperature setpoint · setpoint change · track the setpoint · remote setpoint
An automatic valve that opens and closes in small steps on a signal from a controller, so that flow, pressure, level or temperature stays at the setpoint.
Where you use it
The most common final control element. Engineers size control valves using Cv, choose fail-open or fail-closed action, and check percentage opening. Operators say a valve is '30% open', 'wide open' or 'sticking'.
Examples
The control valve is 95% open, so we have almost no room to increase the flow.
The steam control valve fails closed if instrument air is lost.
Usage note
'Fail-closed' (FC) and 'fail-open' (FO) describe where the valve goes when power or air fails; they are a key safety choice.
Common phrases
control valve opening · fail-closed control valve · size a control valve · sticking control valve
A control method that measures the process variable, compares it with the setpoint and adjusts the output, usually a valve, to correct the difference.
Where you use it
The basic idea behind almost every control loop in a plant. Engineers compare feedback control with feedforward control, which acts on a measured disturbance before it affects the process. Also called closed-loop control.
Examples
Feedback control alone reacts too late to feed changes, so we added a feedforward signal.
The tank level is held by simple feedback control on the outlet valve.
Usage note
In everyday English 'feedback' also means comments on your work; in control it means the measured signal sent back to the controller.
Common phrases
feedback control loop · feedback and feedforward control · closed-loop control · negative feedback
The most common type of process controller. It sets its output from three terms: proportional (the present error), integral (past error) and derivative (how fast the error is changing).
Where you use it
Found in every DCS. Engineers 'tune' a PID controller by adjusting gain, integral time and derivative time. Many loops use only P and I terms; derivative is common in temperature control. Controllers run in manual, auto or cascade mode.
Examples
The PID controller on the reactor temperature is too aggressive and keeps overshooting.
Put the PID controller in manual while we check the transmitter.
Usage note
Say the letters one by one: P-I-D. Many real loops are PI controllers, with the derivative term switched off.
Common phrases
tune a PID controller · PID tuning · controller gain · put the controller in manual · cascade control
A field instrument that measures a value such as pressure, temperature, level or flow and sends it as an electrical signal, often 4–20 mA, to the control system.
Where you use it
Used by instrument technicians, operators and control engineers. Transmitters are calibrated, zeroed, ranged, replaced and checked when a reading looks wrong. On P&IDs they appear with tags such as PT (pressure), TT (temperature), LT (level) and FT (flow).
Examples
The level transmitter is reading 0%, but the sight glass shows the tank half full.
Ask the instrument tech to calibrate the pressure transmitter on the discharge line.
Usage note
Instrument tags follow the ISA letter code: the first letter is what is measured (P, T, L, F) and T at the end means transmitter.
An instrument that measures the flow rate of a liquid or gas in a pipe. Common types are orifice plate, magnetic, Coriolis, vortex and ultrasonic meters.
Where you use it
Used for control, mass balances and custody transfer (buying and selling by quantity). Engineers choose the type based on fluid, accuracy and cost, and check straight-pipe requirements. Coriolis meters measure mass flow directly.
Examples
The magnetic flowmeter won't work on this solvent because it doesn't conduct electricity.
The two flowmeters disagree by 6%, so one of them needs to be checked.
Usage note
Also written 'flow meter' (two words). In speech people often just say 'the meter'.
A set of parts that work together to hold one process variable at its setpoint: a sensor and transmitter, a controller and a final element such as a control valve.
Where you use it
Used by control engineers, operators and instrument technicians. Loops are tuned, checked, put in auto or manual and closed or opened. On P&IDs each loop has a tag number, for example TIC-101 for a temperature indicating controller.
Examples
Half of the control loops on this unit are running in manual, which is a bad sign.
Do a loop check on the new pressure control loop before start-up.
Usage note
'Open loop' means the controller is not acting on the process (for example in manual); 'closed loop' means it is in automatic control.
Common phrases
control loop tuning · loop check · closed loop · open loop · cascade loop
Short for distributed control system: the computer-based system that runs the control loops of a plant and gives operators their screens, alarms and trends in the control room.
Where you use it
Used every day in control rooms and projects. Operators watch the DCS, acknowledge alarms and change setpoints on it; engineers configure it and pull trends from it. Safety functions usually run on a separate SIS.
Examples
Pull the DCS trends for the last 48 hours so we can see when the pressure started rising.
The new interlock will run on the SIS, not on the DCS.
Usage note
Smaller plants often use a PLC with a SCADA system instead of a DCS.
The amount of material that a unit or plant processes in a given time, for example tonnes of feed per day.
Where you use it
Used by production, planning and management. Throughput is increased, maximized, limited or reduced; a bottleneck limits the throughput of the whole plant. Often compared with design capacity in percent.
Examples
The column flooding limits our throughput to about 92% of design.
Management wants a 10% throughput increase without new equipment.
Usage note
Capacity is what the plant can do; throughput is what it actually processes.
A closed container designed to hold gases or liquids at a pressure well above or below atmospheric, such as a reactor, separator, drum or column.
Where you use it
Pressure vessels are designed, built and inspected to codes such as ASME Section VIII or the EU Pressure Equipment Directive. Engineers specify design pressure, design temperature and material, and vessels are hydrotested before use.
Examples
The pressure vessel is rated for 16 barg at 250 °C.
Every pressure vessel on site must have a valid inspection certificate.
Usage note
In plant English 'drum' often means a horizontal vessel, such as a reflux drum or knock-out drum, not an oil barrel.
A large container, usually at or near atmospheric pressure, used to hold raw materials, intermediates or products before they are used or shipped.
Where you use it
Found in tank farms at every plant and terminal. Engineers talk about fixed-roof and floating-roof tanks, tank level, working capacity, bunds (dikes) around tanks and nitrogen blanketing. Atmospheric tanks are often designed to API 650.
Examples
The storage tank is at 85% level, so we need a truck by tomorrow morning.
Volatile solvents are kept in a floating-roof storage tank to reduce emissions.
Usage note
A 'bund' (UK) or 'dike' (US) is the wall or basin around a tank that holds a spill.
Common phrases
tank farm · floating-roof tank · tank level · nitrogen blanketing · fill a tank
A motor-driven shaft with one or more impellers that mixes the contents of a tank or reactor to keep solids suspended, spread heat and help reactions.
Where you use it
Used in reactors, blending tanks and crystallizers. Engineers choose the impeller type, speed in rpm and power per volume. Operators check that the agitator is running before dosing, because poor mixing is a common cause of runaway reactions.
Examples
Start the agitator before you add the catalyst slurry.
At the larger scale, the agitator speed had to drop to keep the tip speed the same.
Usage note
'Mixer' is also used, especially for in-line or static mixers; 'stirrer' is more common in the lab.
Common phrases
agitator speed · agitator motor · agitator failure · impeller · start the agitator
The stage after construction when equipment and systems are checked, flushed, tested and run with water or inert materials to prove they work before real start-up.
Where you use it
Used in project and start-up teams. Typical activities are loop checks, flushing, leak tests, pump runs and water runs. Systems are 'handed over' from construction to commissioning and then to operations.
Examples
Commissioning of the cooling water system starts next week, after the flushing is done.
During commissioning we found three transmitters with reversed wiring.
Usage note
In everyday English 'commission' can also mean a sales fee or a committee, so context matters.
The process of bringing a unit or plant from a shut-down state to normal operating conditions, following a written procedure step by step.
Where you use it
Used in operations, safety and project planning. Start-ups are planned, delayed, completed and reviewed, and many incidents happen during start-up and shutdown. As a verb it is written as two words: 'we start up the unit'.
Examples
The start-up was delayed because the furnace burners would not light.
Follow the start-up procedure exactly and log every step.
Usage note
Noun: 'start-up' or 'startup'. Verb: 'start up'. A 'startup' is also a new company, so in a plant context the hyphenated form is clearer.
Common phrases
start-up procedure · plant start-up · cold start-up · after start-up · start up the unit
Stopping a unit or plant, either in a planned and controlled way or suddenly because of a fault or safety system action.
Where you use it
Used daily in operations. Engineers distinguish planned or normal shutdown from emergency shutdown (ESD). Shutdowns are planned, extended and costed. As a verb it is two words: 'shut down the compressor'.
Examples
An emergency shutdown was triggered when the gas detectors went into alarm.
Each day of unplanned shutdown costs us around 150,000 euros.
Usage note
Noun 'shutdown' is one word; the verb 'shut down' is two words.
Common phrases
emergency shutdown · planned shutdown · shutdown procedure · unplanned shutdown · shut down the unit
A large planned shutdown of a plant, usually every few years, for inspection, maintenance, catalyst change, cleaning and installing new equipment.
Where you use it
A major event in refineries and petrochemical plants that takes months of planning. People talk about the turnaround scope, schedule, budget and critical path. In the UK it is often called a 'shutdown' or 'outage'.
Examples
Replacing the column internals is in the scope of the 2027 turnaround.
The turnaround finished two days early and the unit was back on spec within a week.
Usage note
In business English 'turnaround' also means rescuing a failing company, so make the context clear.
A flat ring of soft or semi-hard material placed between two flanges or surfaces and squeezed by bolts to stop leaks.
Where you use it
Used in maintenance, piping and vessel work. Gaskets are replaced, specified by material and pressure class, and must be compatible with the process fluid. Common types are spiral wound, PTFE and compressed fiber gaskets.
Examples
The flange is leaking because someone reused an old gasket.
Use a spiral-wound gasket on the hot oil lines, not a rubber one.
Usage note
A 'seal' usually means a seal on a moving part, such as a pump shaft (mechanical seal); a gasket seals two fixed surfaces.
Common phrases
spiral-wound gasket · replace the gasket · gasket leak · gasket material
Describes a product or stream that does not meet its required specification, for example too much water, the wrong color or too low a purity.
Where you use it
Used in operations, quality control and planning. Off-spec product is sent to a slop tank, reprocessed, blended or sold at a lower price. The opposite is 'on-spec' or 'in spec'. Short for 'off-specification'.
Examples
The product went off-spec for six hours after the reboiler upset.
Send the off-spec material to tank 12 for reprocessing.
Usage note
As an adjective before a noun it has a hyphen ('off-spec product'); after a verb people also write 'off spec'.
Common phrases
off-spec product · go off-spec · off-spec material · back on spec
The supporting services a process plant needs to run, such as steam, cooling water, electricity, instrument air, nitrogen, fuel gas and demineralized water.
Where you use it
Used in design, operations and cost estimates. Engineers calculate utility consumption, size utility systems and include utility costs in operating cost. A utility failure, such as loss of cooling water or power, is a key scenario in safety studies.
Examples
Utilities make up about 30% of the operating cost, mostly steam and power.
Before start-up, confirm that all utilities are available: steam, nitrogen and instrument air.
Usage note
In everyday English 'utilities' are household services such as electricity, gas and water bills.
Common phrases
utility consumption · utility failure · utilities area · utility systems · instrument air
Anything that could cause harm to people, the plant or the environment, such as a flammable gas, a toxic liquid, high pressure or a hot surface.
Where you use it
A key word in process safety. Teams identify, assess, control and eliminate hazards in studies such as HAZID and HAZOP. Safety data sheets list chemical hazards, and site signs warn of fire, toxic or electrical hazards.
Examples
The main hazard in this area is hydrogen sulfide, so always carry a gas detector.
We should eliminate the hazard by using a less toxic solvent, not just add more PPE.
Usage note
Hazard and risk are not the same. The hazard is the source of harm; risk combines how likely the harm is and how bad it would be.
Common phrases
identify hazards · fire hazard · major hazard · hazard identification · hazardous area
An automatic protective function that stops or prevents an action when a dangerous condition is detected, for example closing the feed valve on high reactor pressure.
Where you use it
Used in process safety, control system design and operations. Interlocks are designed, tested, activated, reset and, only with formal approval, bypassed. They are often part of a safety instrumented system (SIS).
Examples
The high-temperature interlock shut off the steam and stopped the feed pump.
Never bypass an interlock without a signed permit from the shift supervisor.
Usage note
Also used as a verb: 'the pump is interlocked with the tank level'.
Common phrases
safety interlock · bypass an interlock · interlock test · reset the interlock
A spring-loaded valve that opens by itself when the pressure in a vessel or pipe goes above a set limit, releasing fluid to protect the equipment from bursting.
Where you use it
The last line of defense against overpressure. Engineers size relief valves for scenarios such as fire, blocked outlet or cooling failure, and they discharge to the flare, a scrubber or the atmosphere. Also called PSV (pressure safety valve) or PRV.
Examples
The relief valve on the reactor is set at 10 barg and discharges to the flare header.
The relief valve lifted twice last night, so we need to find the cause of the overpressure.
Usage note
When a relief valve opens, engineers say it 'lifted' or 'popped'. A 'rupture disc' (patlama diski) is a non-reclosing alternative.
Common phrases
pressure relief valve · set pressure · relief valve sizing · the relief valve lifted · relief scenario
Short for hazard and operability study: a structured team review of a P&ID, section by section, using guide words to find deviations that could cause harm or operating problems.
Where you use it
Run in design projects and before major changes. A facilitator leads the team through 'nodes' and guide words such as no, more, less and reverse, applied to flow, pressure or temperature. The team records causes, consequences, safeguards and actions.
Examples
The HAZOP team found that reverse flow from the reactor could reach the feed tank.
All HAZOP actions must be closed out before start-up.
Usage note
Pronounced as one word, 'HAZ-op', not letter by letter. A HAZID is a broader, earlier hazard identification study.
Common phrases
HAZOP study · HAZOP node · HAZOP action · guide word · HAZOP facilitator
A standard document from the supplier that describes a chemical's hazards, safe handling, storage, first aid, firefighting and disposal, in 16 numbered sections.
Where you use it
Required for every hazardous chemical on site. Engineers and safety staff check the safety data sheet before handling a new chemical, choosing PPE or writing a procedure. Abbreviated SDS; the older name MSDS (material safety data sheet) is still heard.
Examples
Check section 8 of the safety data sheet for the right gloves and respirator.
We can't accept the new solvent until the supplier sends an up-to-date safety data sheet.
Usage note
Under the GHS system the sections have a fixed order: hazards are in section 2, PPE in section 8, physical properties in section 9.
Common phrases
SDS section · up-to-date safety data sheet · check the safety data sheet · MSDS
The lowest temperature at which a liquid gives off enough vapor to catch fire briefly when a flame or spark is brought near it.
Where you use it
Used to classify flammable liquids, design storage and choose electrical equipment for hazardous areas. A liquid handled above its flash point is a serious fire risk. Values are found in section 9 of the safety data sheet.
Examples
Acetone has a flash point well below room temperature, so treat every spill as a fire risk.
The tank is heated to 70 °C, which is above the flash point of the solvent.
Usage note
Do not mix it up with the autoignition temperature, at which vapor ignites with no flame or spark at all.
Common phrases
low flash point · above the flash point · closed-cup flash point · flash point test
An exothermic reaction that gets out of control: heat is produced faster than it can be removed, so temperature, reaction rate and pressure rise faster and faster.
Where you use it
A major topic in process safety, especially for batch reactors. Engineers study it with reaction calorimetry, design emergency cooling, quench or venting, and size relief systems for the runaway case. Typical triggers are cooling failure, agitator failure and over-charging.
Examples
If the agitator stops during dosing, unreacted material builds up and can cause a runaway reaction.
Calorimetry tests showed that a runaway reaction would reach 210 °C within ten minutes.
Usage note
'Runaway' is used as a noun on its own too: 'the reactor went into runaway'.
A safety procedure that isolates equipment from all energy sources and locks and labels the isolation points, so nobody can start it while people are working on it.
Where you use it
Required before maintenance on pumps, compressors, agitators and vessels. Each worker puts a personal lock on the isolation point. Abbreviated LOTO; people say 'apply LOTO', 'the pump is locked out' and 'verify zero energy'.
Examples
Apply lockout/tagout on the agitator motor before anyone opens the manway.
Our lockout/tagout procedure also covers steam, nitrogen and process lines, not just electricity.
Usage note
In the UK the same idea is often called 'isolation' or 'lock-off'.
Common phrases
apply LOTO · lockout point · personal lock · verify isolation · LOTO procedure
An automatic shutdown of a machine or unit by its protection system when a value goes past a safe limit, such as high pressure, low level or high vibration.
Where you use it
Used constantly in operations. Equipment 'trips', 'trips on' a signal or 'is tripped'. Engineers investigate spurious trips (false trips), set trip points and check that each trip is tested. Also used as a verb.
Examples
The furnace tripped on low fuel gas pressure at 3 a.m.
We had three spurious trips last month because of a faulty level switch.
Usage note
In everyday English a trip is a journey; in a plant it almost always means an automatic shutdown.
Common phrases
trip point · spurious trip · high-pressure trip · trip on · trip test
The lowest concentration of a flammable gas or vapor in air that can burn or explode if ignited. Below it, the mixture is too lean to burn.
Where you use it
Abbreviated LEL (or LFL, lower flammable limit). Gas detectors read in % LEL and alarms are often set at around 10–20% LEL. Methane's LEL is about 5% by volume in air and its upper explosive limit (UEL) about 15%.
Examples
The gas detector near the pump showed 25% of the lower explosive limit, so we evacuated the area.
Nitrogen keeps the tank vapor space below the lower explosive limit of the solvent.
Usage note
'% LEL' is not the gas concentration in air: 20% LEL of methane is only about 1% methane by volume.
Common phrases
% LEL · upper explosive limit · flammable range · LEL alarm · below the LEL
To clear a vessel or pipe of air, flammable gas or toxic material by flushing it with an inert gas such as nitrogen, or with steam, before opening it or starting it up.
Where you use it
Used in start-up, shutdown and maintenance procedures. Operators purge with nitrogen until oxygen is below a set limit before introducing hydrocarbons, and purge hydrocarbons out before handing equipment to maintenance. Also a noun: 'nitrogen purge', 'purge stream'.
Examples
Purge the reactor with nitrogen until oxygen is below 2%, then start the hydrogen.
The line was purged and gas-tested before the welders started work.
Usage note
Do not confuse 'purge' with 'drain' (remove liquid by gravity) or 'vent' (release gas to atmosphere or flare).
Common phrases
nitrogen purge · purge with nitrogen · purge the line · purge stream · steam purge
To take a process that works in the lab or a pilot plant and design it to run at a much larger, commercial size.
Where you use it
Used in R&D, process development and project meetings. You scale up a reaction, a process or a product 'from lab to plant'. The noun is 'scale-up', with a hyphen: 'scale-up problems', 'scale-up factor'. The opposite is 'scale down'.
Examples
Mixing and heat removal are the biggest risks when we scale up this reaction.
The process was scaled up from a 5-liter flask to a 10 m³ reactor.
Usage note
Verb: 'scale up' (two words). Noun and adjective: 'scale-up' (hyphen).
Common phrases
scale up a process · scale up from lab to plant · scale-up study · scale-up factor
A small version of a planned production plant, between laboratory and full scale, used to test the process, collect design data and make sample product.
Where you use it
Used in process development before big investments. Teams run campaigns in the pilot plant, test raw materials, measure heat release and produce material for customer trials. Results go into the design basis for the full-scale plant.
Examples
The pilot plant runs showed that the filter cake is much wetter than in the lab.
We'll make 200 kg in the pilot plant for the customer's qualification trials.
Usage note
'Pilot' here means trial or test, as in 'pilot project', not an airplane pilot.
Common phrases
pilot plant run · pilot plant campaign · pilot plant data · pilot-scale
Increasing a plant's capacity by finding and removing the equipment limits that stop it from producing more, instead of building a completely new unit.
Where you use it
Common in mature plants. Engineers run test runs, identify the bottleneck (a pump, column, exchanger or compressor), study options and estimate cost. The verb is 'debottleneck': 'we debottlenecked the unit by 15%'.
Examples
The debottlenecking study showed that the overhead condenser is the first limit.
A low-cost debottlenecking project could add 8% capacity with just a new pump impeller.
Usage note
A 'bottleneck' (darboğaz) is the single limit; 'debottlenecking' is the work of removing it.
Common phrases
debottlenecking study · debottlenecking project · identify the bottleneck · capacity increase
The agreed set of data and assumptions that a plant design is based on, such as capacity, feed and product specifications, site conditions and design margins.
Where you use it
One of the first documents in a project, often called the Basis of Design (BoD). Engineers agree, freeze, revise and refer back to the design basis; changes to it later in a project are costly.
Examples
The design basis assumes a summer cooling water temperature of 32 °C.
Adding a second feedstock now would change the design basis for the whole unit.
Usage note
Turkish speakers sometimes say 'design base'; the correct word is 'basis' (plural 'bases').
Common phrases
basis of design · freeze the design basis · design basis document · design case
Using software to build a computer model of a process, with its equipment, streams and thermodynamics, in order to calculate balances, size equipment and test changes.
Where you use it
Used in design, debottlenecking, optimization and training. Common tools include Aspen Plus, Aspen HYSYS, CHEMCAD and the free DWSIM. Engineers build, run, converge and validate a simulation against plant data. Steady-state and dynamic simulations are both common.
Examples
The process simulation predicts 99.2% purity, but the plant only reaches 98.5%.
Before buying a new exchanger, let's test the idea in the process simulation.
Usage note
'The model won't converge' is a very common complaint: the solver cannot find a consistent answer.
Common phrases
steady-state simulation · dynamic simulation · simulation model · the simulation won't converge · validate the model
The smallest stage of process development, using laboratory equipment of a few milliliters to a few liters to test chemistry and collect first data.
Where you use it
Used in R&D and scale-up planning, usually in the order bench scale, pilot scale, then commercial or full scale. Often used as an adjective with a hyphen: 'bench-scale tests', 'a bench-scale reactor'.
Examples
Bench-scale tests showed good selectivity, but we still need pilot data on heat removal.
At bench scale the filtration took minutes; in the plant it takes six hours.
Usage note
'Lab scale' means much the same; 'bench' refers to the laboratory workbench.
Common phrases
bench-scale tests · bench-scale reactor · from bench scale to pilot scale · lab scale
The slow damage of a metal by chemical or electrochemical reaction with its surroundings, such as rust on carbon steel or attack by acids and chlorides.
Where you use it
Used in material selection, inspection and maintenance. Engineers measure corrosion rate in mm per year, add corrosion allowance and choose resistant materials. Types include general, pitting, crevice and stress corrosion cracking. The verb is 'corrode'.
Examples
Inspection found severe corrosion under the insulation on the steam line.
Chlorides in the cooling water are the main cause of corrosion in this exchanger.
Usage note
'Rust' is only corrosion of iron and steel; 'corrosion' covers all metals.
A family of steels containing at least about 10.5% chromium, which forms a thin protective oxide layer and gives much better corrosion resistance than carbon steel.
Where you use it
The most common upgrade from carbon steel in chemical, food and pharmaceutical plants. Engineers specify grades such as 304, 316 or 316L and duplex. Chlorides can cause pitting and stress corrosion cracking in common grades.
Examples
We chose 316L stainless steel because the molybdenum gives better resistance to chlorides.
Stainless steel is not immune to corrosion; warm chloride solutions can still crack it.
Usage note
Engineers often say 'stainless' or 'SS' alone: 'the line is SS316'. 'Inox' is used in some European countries but is rare in English.
The material that a piece of equipment, pipe or part is made from, chosen for strength, temperature, corrosion resistance and cost.
Where you use it
Abbreviated MOC and listed on every datasheet and P&ID line list. Engineers select, specify and upgrade the material of construction, and check compatibility with the process fluid. Typical options include carbon steel, stainless steels, nickel alloys, glass-lined steel and plastics.
Examples
What's the material of construction for the new scrubber? The gas contains wet HCl.
The datasheet lists Hastelloy C-276 as the material of construction for wetted parts.
Usage note
The plural is 'materials of construction', not 'material of constructions'.
Common phrases
MOC selection · upgrade the material of construction · materials of construction · wetted parts
A form of local corrosion that creates small, deep holes in a metal surface while the area around them looks almost undamaged.
Where you use it
Typical of stainless steels in chloride service and of stagnant water. It is dangerous because a few deep pits can cause a leak even when overall wall loss is small. Inspectors report pit depth, and engineers compare grades using a pitting resistance number (PREN).
Examples
The exchanger tubes leaked after only two years because of pitting under deposits.
Duplex steel has much better pitting resistance than 304 in seawater.
Usage note
In engine and bearing work 'pitting' can also mean surface fatigue damage, so check the context.
Extra wall thickness added to a vessel or pipe on top of what strength calculations require, so that it can lose metal to corrosion over its life and still be safe.
Where you use it
Specified on datasheets and line classes. Typical values for carbon steel are 1.5 mm or 3 mm; stainless steel often has little or none. Inspectors compare measured thickness with the original corrosion allowance to estimate remaining life.
Examples
The vessel was designed with a 3 mm corrosion allowance for a 20-year life.
After ten years we've already used up most of the corrosion allowance on the bottom head.
Usage note
Abbreviated CA on datasheets: 'CA = 3 mm'.
Common phrases
corrosion allowance of · use up the corrosion allowance · remaining corrosion allowance · design thickness
Ten questions from this glossary. Pick the right meaning for each term and see your score.
How to learn chemical engineering vocabulary
Read real texts from the field: equipment datasheets, operating manuals, HAZOP worksheets, simulation reports and the notes on P&IDs show the vocabulary in its natural context.
Learn each term with the words that go with it: not just 'reactor' but 'charge the reactor', 'reactor temperature' and 'jacketed reactor'.
Practice reading units and numbers out loud the way engineers do: 'twelve cubic meters per hour', 'three point five bar gauge', 'two hundred kilowatts of heat duty'. English uses a decimal point, not a comma.
Keep a list of pairs that Turkish speakers often mix up, such as absorption and adsorption, yield and conversion, or hazard and risk.
Describe a simple process in a few English sentences, from feed to product, the way you would in a shift handover or a short technical email.
Frequently asked questions
How can I learn English for chemical engineering?
Read real texts from the field and notice how the words are used: textbooks on unit operations, equipment datasheets, operating procedures, HAZOP reports and simulation outputs. Learn terms together with their usual partners (open a valve, raise the setpoint, high pressure drop) and use them right away in your own reports, emails and meetings.
Which chemical engineering terms should I learn first?
Start with the words you hear in every meeting and see on every drawing: mass balance, flow rate, reactor, catalyst, heat exchanger, distillation, pump, valve, pressure drop, setpoint, yield and conversion. Then move on to separation processes, process control, plant operations and process safety. This glossary follows the same order, from the most essential terms to more specialist ones.
Should I say 'process' or 'proses' when I speak English?
In English it is always 'process', pronounced /ˈprɑːsɛs/ in American English and /ˈprəʊsɛs/ in British English. Turkish engineers say 'proses' in Turkish, so be careful not to carry that spelling or stress into English emails and presentations.
Do chemical plants use bar or psi, and Celsius or Fahrenheit?
Most of the world uses SI-based units in plants: bar or kPa for pressure, degrees Celsius, cubic meters per hour and kilograms per hour. Documents from the United States and many licensors still use psi, degrees Fahrenheit, gallons per minute and pounds per hour. Always check the unit and whether a pressure is gauge (barg, psig) or absolute (bara, psia).
