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48 terms across valves, steam systems, pumps and piping. Use browser search (Ctrl+F) to find a term quickly.

A

Actuator

A device that provides the mechanical force to open or close a valve without manual effort. Industrial actuators are either pneumatic (using compressed air), electric (using a motor gearbox), or hydraulic. Pneumatic actuators are preferred for fast operation and intrinsically safe (non-sparking) applications. Electric actuators are used where instrument air is not available or where precise positional control is needed. Most actuators for quarter-turn valves (ball, butterfly) are rotary; those for gate, globe, and control valves are linear. The actuator must be sized to the valve's operating torque (for rotary) or thrust (for linear) with a safety margin.

Affinity Laws (Pump)

Three relationships that describe how a centrifugal pump's performance changes when its rotational speed changes. If speed changes by a ratio n: (1) flow changes proportionally to n; (2) head changes proportionally to n²; (3) power consumed changes proportionally to n³. The cube relationship for power is why variable frequency drives (VFDs) save so much energy — reducing pump speed by 20% reduces power consumption by nearly 50%.

ANSI

American National Standards Institute. In the context of valves and piping, "ANSI" usually refers to pressure-temperature rating classes for flanges and valves: ANSI Class 150, 300, 600, 900, 1500, and 2500. These ratings specify the maximum allowable working pressure at a given temperature. ANSI Class 150 at ambient temperature allows approximately 19.5 bar(g); Class 300 allows approximately 51 bar(g). Often used interchangeably with ASME B16.5 (the actual standard for flanges and fittings).

API

American Petroleum Institute. API standards are widely used in oil, gas, and petrochemical service. Relevant references include API 6D (pipeline valves), API 600 (bolted bonnet gate valves), API 607 (fire testing for quarter-turn valves), and API 598 (valve testing). When a specification requires API compliance for a valve, it is typically an API 6D or API 600 gate valve, or a valve tested to API 607 for fire safety.

B

Backpressure

The pressure present in the downstream pipe or condensate return system, acting against the outlet of a valve or steam trap. Backpressure is always considered relative to inlet pressure — the difference (differential pressure) is what drives flow through a trap or control valve. In steam trap selection, the backpressure ratio (backpressure absolute ÷ inlet pressure absolute) is critical: thermodynamic disc traps fail to operate reliably if backpressure exceeds 50% of inlet pressure.

Ball Valve

A quarter-turn valve using a spherical closure element (the ball) with a cylindrical bore drilled through it. Open: bore aligns with pipe, minimal pressure drop. Closed: bore perpendicular to pipe, tight shutoff. Available as full bore (bore equals pipe ID) or reduced bore. Soft-seated (PTFE) ball valves achieve Class VI bubble-tight shutoff. Metal-seated types handle high temperature or abrasive service. Standard choice for small-to-medium bore clean service (gas, water, steam, chemicals).

Bellows Seal

A sealing arrangement in a valve where the stem is sealed by a thin-walled metal bellows welded to both the stem and the valve bonnet, providing a hermetically tight seal. Used on valves where any leakage to atmosphere is unacceptable — cryogenic service, toxic or radioactive media, high-purity gas. The bellows replaces the conventional packing gland. Also called a bellows-sealed valve. These valves are more expensive than packing-sealed types and require careful installation to avoid bellows fatigue.

BEP (Best Efficiency Point)

The point on a centrifugal pump's performance curve where hydraulic efficiency is maximum — the pump converts the most input power into useful pumping work and generates the least heat and vibration. Operating far from BEP causes increased vibration, seal wear, bearing wear, and heat generation. Good pump selection practice targets 80-110% of BEP flow for continuous duty. A pump running at 40% of BEP flow is likely to have a short service life.

Blowdown

Deliberate discharge of water or steam from a boiler or pressure vessel to remove dissolved solids, scale-forming minerals, or sludge. There are two types: continuous blowdown (a controlled bleed from the water surface to maintain TDS levels) and intermittent/bottom blowdown (periodic discharge from the mud drum to remove settled sludge). Blowdown represents an energy and water loss from the system. Blowdown heat recovery using a flash vessel and heat exchanger is a common energy-saving measure on medium to large boilers.

Butterfly Valve

A quarter-turn valve using a disc mounted on a central shaft as the closure element. When open, the disc rotates to roughly parallel with flow (not completely out of the flow path — a butterfly valve always has some restriction). Three main designs: concentric (resilient rubber seat, PN10-16, utility service), double-offset (composite or soft insert seat, PN25-40, general industrial), triple-offset (metal seat, PN40-100, steam, high-temperature, fire-safe service). Lighter and more compact than ball valves at large diameters; dominates sizes above DN200.

C

Cavitation (Pump)

A destructive phenomenon where local pressure in the pump falls below the vapour pressure of the liquid being pumped, causing vapour bubbles to form and then collapse violently as pressure recovers. The bubble collapse implosions erode impeller surfaces, create severe noise and vibration, and eventually destroy the pump. Prevented by ensuring adequate Net Positive Suction Head Available (NPSHA) at the pump suction. If cavitation is occurring, check whether the suction pipe is too long, too small, or has too many fittings; whether the liquid temperature is too high; or whether the pump is running too far off BEP.

Cavitation (Valve)

Different from pump cavitation. In a control valve, cavitation occurs when the local pressure in the valve trim falls below the liquid's vapour pressure, creating vapour bubbles that collapse on the downstream side of the trim. Causes noise, vibration, and rapid erosion of valve trim. Prevented by special anti-cavitation trim designs (labyrinthine paths that dissipate pressure in stages) or by selecting a valve Cv such that the pressure drop across the valve does not cause vapour formation.

Check Valve

A valve that allows flow in one direction only, closing automatically when flow attempts to reverse. Used to prevent backflow into pumps, compressors, and feed systems. Types include swing check (a disc on a pivot — simple, low pressure drop in open position, not suitable for pulsating flow), dual-plate check (two spring-loaded semicircular plates — compact, fast-closing, suitable for vertical installations), and axial flow/nozzle check (disc moves axially against a spring — low cracking pressure, very fast closing, used on high-velocity gas lines and pump discharge). Duo-Chek (dual-plate) and Noz-Chek (nozzle type) are specific product families in the Crane ChemPharma range.

Condensate

Liquid water formed when steam loses its latent heat to a process or to the surroundings. In a steam system, condensate continuously forms in pipes, heat exchangers, and steam-traced lines. Condensate is valuable: it is hot (typically 80-100°C depending on system pressure and trap type) and it is treated water (it has already been chemically dosed through the boiler feed treatment system). Returning condensate to the boiler feedtank saves fuel, water, and chemical treatment cost. See our guide: [Condensate Recovery: How to Calculate Your Steam Losses and ROI](/blog/condensate-recovery-roi/).

Cv Value

A dimensionless flow coefficient that expresses how much fluid a valve can pass under defined conditions. Technically defined as the flow of water in US gallons per minute at 60°F through a fully open valve with 1 psi pressure drop. A valve with Cv = 100 passes 100 USgpm of water at 1 psi differential pressure. Used to size control valves and check that an isolating valve does not create excessive pressure drop when open. European equivalent is Kv (m³/h at 1 bar pressure drop), where Kv = 0.865 × Cv.

D

Diaphragm Valve

A valve where flow is controlled by a flexible diaphragm (rubber or PTFE) pressed against a weir or compressor in the valve body. The fluid contacts only the diaphragm and body — no stem, packing, or mechanical parts are wetted. The standard choice in pharmaceutical manufacturing (Saunders brand diaphragm valves are the industry benchmark), food and beverage (where crevice-free, cleanable valves are required), and corrosive chemical service. Not suitable for high-pressure or high-temperature service. Available in liner types (rubber-lined for corrosive), natural rubber, EPDM, and PTFE-lined for the most demanding chemical service.

DN (Nominal Diameter)

DN stands for Diamètre Nominal (French) — the nominal pipe bore size in the ISO/metric system. DN15 = 15mm nominal, DN50 = 50mm nominal, DN100 = 100mm nominal, and so on. DN is not the actual bore (a DN100 pipe does not have a 100mm internal bore — the actual bore depends on the pipe schedule). It is a nominal designation to identify pipe and fitting sizes for compatibility. The equivalent in the imperial/ANSI system is NPS (Nominal Pipe Size): NPS 2 is approximately equivalent to DN50. Note that above NPS 4, the NPS number and DN number diverge — NPS 8 is DN200, not DN8.

Double Block and Bleed

A valve configuration that provides two independent seals with a bleed port between them. Used in applications where positive isolation is critical — before entering a confined space, before hot work on a pressurised line, in sampling connections. A double block-and-bleed (DBB) arrangement confirms isolation by opening the bleed valve between the two blocks: if pressure appears at the bleed, one of the block valves is leaking. Available as separate valves or as integral DBB valves in a single body.

Dryness Fraction

Also called steam quality. A measure of the proportion of true dry steam (vapour) versus entrained moisture (liquid droplets) in a steam flow. Dryness fraction of 1.0 means 100% dry steam. Dryness fraction of 0.95 means 95% steam and 5% liquid water by mass. Most industrial processes require steam with a dryness fraction above 0.95 — wet steam carries liquid droplets that blanket heat transfer surfaces, cause water hammer, and erode steam trap orifices. Steam separators and correct steam main sizing maintain high dryness fraction to process equipment.

F

Face-to-Face Dimension

The dimension between the inlet and outlet flange faces of a valve, measured along the pipe axis. Standardised in ISO 5752 (metric) and ASME B16.10 (imperial). A valve supplied to the correct face-to-face standard can replace another valve of the same size and pressure class without modifying the pipe flanges or pipe support positions. When ordering a replacement valve, confirming face-to-face compliance with the applicable standard avoids on-site fitting problems.

Flash Steam

Steam generated when high-pressure condensate is released to a lower pressure. When condensate at high pressure passes through a steam trap or pressure-reducing device, the sudden pressure drop causes a portion of the liquid to evaporate. This "flash" steam is a usable energy source — it carries the same heat content per kg as live steam at the lower pressure. Flash steam is most efficiently used via a flash vessel: flash steam goes to a low-pressure header or low-pressure use, and the remaining liquid condensate drains to the return system.

Fugitive Emission

Unintended leakage of process fluid from valve stem seals (packing), gland seals, or body-bonnet joints to the atmosphere. Fugitive emission control is important in the hydrocarbon and chemical industry — volatile organic compounds (VOCs) escaping from valve glands contribute to air pollution. Low emission (LE) valves are designed and tested to proven stem seal specifications (ISO 15848, API 622) that limit fugitive emissions below defined thresholds. Specified by environmental and regulatory requirements in petrochemical and pharmaceutical plants.

G

Gate Valve

A linear-motion valve using a wedge-shaped or parallel-slide gate as the closure element. The gate is raised fully out of the flow path when open (minimal pressure drop, full bore) and lowered into the seat when closed. Gate valves are not suitable for throttling — a partially open gate valve vibrates, cavitates, and erodes rapidly. Used only for isolating service (fully open or fully closed). Widely used in older plants on water services, steam mains (larger sizes), and pipeline isolation. Being progressively replaced by ball valves in smaller sizes and high-performance butterfly valves in larger sizes for new installations.

Globe Valve

A linear-motion valve with an internal baffle that forces fluid to change direction as it passes through. The closure element (plug) lifts away from a circular seat to open. Unlike gate valves, globe valves can throttle — the plug-to-seat geometry allows controlled flow restriction. Used in steam service (globe valves with metal seats handle high temperature better than soft-seated ball valves), in applications requiring frequent operation, and in control valve bodies. Higher pressure drop than ball or gate valves in the open position, so not used where minimising pressure drop is critical.

H

Head (Pump)

The energy imparted to a fluid by a pump, expressed in metres of liquid column height rather than pressure units. Head = pressure (Pa) ÷ (density × gravity). Expressing pump performance in head allows the same pump curve to apply regardless of the fluid density — a pump delivering 20 metres of head delivers 20 metres of head for water, oil, or any other liquid (though power consumed varies with density). The system head is the total head the pump must overcome: static head (height difference between suction and discharge) plus friction losses in the pipe, fittings, and equipment.

High-Performance Butterfly Valve

A butterfly valve designed beyond the concentric resilient-seated configuration — typically double-offset or triple-offset. Double-offset: stem is offset from both the pipe centreline and the disc sealing surface, causing the disc to cam away from the seat as it opens, eliminating rubbing wear. Triple-offset: three geometric offsets ensure the disc makes purely rotational, non-rubbing contact with the seat only in the final degrees of closure, allowing metal-to-metal seating. High-performance butterfly valves achieve Class V or VI shutoff, handle pressures to PN100 and temperatures to 600°C+, and are used in applications where a concentric butterfly valve would fail prematurely (steam, high-pressure gas, fire-safe service).

I

IBR (Indian Boiler Regulations)

The Indian Boiler Regulations 1950, framed under the Indian Boilers Act 1923. IBR specifies design, construction, inspection, testing, and operational requirements for steam boilers and related pressure equipment in India. IBR applies to boilers above 1 kg/cm² pressure and 0.5 m² heating surface, and to steam pipes with bore above 79mm at pressures above 3.5 kg/cm². Equipment used on IBR-classified lines must meet IBR certification requirements. Administration is through state Boiler Inspection departments. See our guide: [IBR Compliance for Steam Systems in India](/blog/ibr-compliance/).

Impeller

The rotating element inside a centrifugal pump that imparts energy to the fluid. As the impeller rotates, centrifugal force accelerates fluid outward from the eye (centre) to the periphery, converting shaft energy into velocity, which the volute converts to pressure. Impeller types include open (no shroud, suitable for solids-bearing fluids), semi-open, and closed (fully shrouded, highest efficiency for clean liquids). Impeller diameter affects head and flow — reducing impeller diameter (trimming) reduces head and flow and is a common adjustment to match a standard pump to a specific duty.

L

Latent Heat

The energy absorbed or released when a substance changes phase at constant temperature. In steam engineering, the latent heat of vaporisation is the energy required to convert liquid water at boiling point into steam at the same temperature and pressure. At atmospheric pressure, latent heat of steam is approximately 2,257 kJ/kg. This is the energy that steam carries and delivers to a process when it condenses — it is far larger than the sensible heat content of hot water at the same temperature, which is why steam is such an efficient heat transfer medium. As steam pressure (and therefore temperature) increases, latent heat decreases.

Lug Type (Butterfly Valve)

A butterfly valve mounting style where the valve body has threaded lugs (inserts) through which the flange bolts pass and are secured — the bolts do not pass through to the opposite flange. This allows either side of the flanged piping to be unbolted and removed independently, making lug-type butterfly valves suitable for dead-end service (where one side of the line can be blanked off, isolated, or removed separately). Compare with wafer type, which relies on both flanges remaining bolted to hold the valve in place.

N

NPSH (Net Positive Suction Head)

The difference between the absolute pressure at the pump suction and the vapour pressure of the liquid at the pumping temperature, expressed in metres of liquid. NPSH Available (NPSHA) is a system property — calculated from the suction side conditions (source pressure, static head, pipe friction losses). NPSH Required (NPSHR) is a pump property — specified by the manufacturer and varies with flow rate. To avoid cavitation, NPSHA must always exceed NPSHR by a margin (typically NPSHA ≥ NPSHR + 0.5 to 1.0 metre). NPSH is most critical for hot water, volatile hydrocarbons, and high-flow pumps.

P

P&ID (Piping and Instrumentation Diagram)

A schematic diagram that shows all the piping, equipment, instruments, and control systems in a process plant or section. P&IDs use standardised symbols (ISA 5.1 for instruments, ISO 10628 for equipment) and show pipe sizes, line numbers, valve types, instrument tags, and control loops. The P&ID is the primary reference document for valve selection, instrumentation procurement, and process hazard analysis. A line number on a P&ID typically encodes the fluid, pipe size, material class, and insulation requirement.

PN Rating

Pression Nominale — the pressure designation used in the European/ISO pipe and valve standard system. PN16 means the fitting is rated to 16 bar at 20°C (ambient). PN25 = 25 bar at 20°C, PN40 = 40 bar at 20°C. The actual allowable pressure decreases as temperature increases, following pressure-temperature tables in the relevant standard (EN 1092 for flanges, EN 12516 for valves). Roughly corresponds to ANSI pressure classes: PN16 ≈ ANSI Class 150, PN40 ≈ ANSI Class 300, PN100 ≈ ANSI Class 600.

Plug Valve

A rotary valve using a cylindrical or tapered plug with a port through it. Quarter-turn operation (like a ball valve). Eccentric plug valves (REVO brand) have a plug that cams away from the seat as it opens, reducing seat wear — used in slurry, fibrous media, and services where soft seat ball valves would be abraded. Lined plug valves (XOMOX brand) have a PTFE or rubber-lined body and plug — used for corrosive chemical service where metal-to-fluid contact must be avoided. Plug valves are also used in multiport configurations (3-way or 4-way) for flow diversion or switching.

PRV (Pressure Reducing Valve)

A self-actuating valve that reduces a higher upstream steam (or gas) pressure to a lower, controlled downstream pressure. The downstream pressure is set by a spring or pilot valve, and the PRV modulates its position to maintain that set pressure regardless of variations in upstream pressure or downstream flow. Types include direct-acting (simple spring-loaded, adequate for light duty and low flow) and pilot-operated (using a small sensing pilot valve to control the main valve — better accuracy and stability over wider flow ranges). A separator and strainer upstream of the PRV, and a correctly specified steam trap on the separator, are essential for reliable PRV operation.

PTFE Seat

Polytetrafluoroethylene (PTFE, also known as Teflon) used as the seat material in ball valves, butterfly valves, and check valves. PTFE is chemically resistant to most industrial fluids (exceptions include strong fluorinating agents and some alkali metals), provides bubble-tight shutoff at low seating force, and has a low coefficient of friction (important for reducing valve operating torque). Temperature range is typically -50°C to +200°C for standard PTFE, up to 260°C for reinforced or filled PTFE grades. PTFE seats are not suitable for services with abrasive solids (which cut the seat surface) or very high temperatures.

Q

Quarter Turn

Describes a valve that requires only 90° rotation of the stem from fully open to fully closed. Ball valves, butterfly valves, plug valves, and eccentric disc valves are all quarter-turn valves. Quarter-turn valves are fast to operate and easy to automate with pneumatic or electric actuators. The position (open or closed) is visually apparent from the handle or actuator position indicator — a handle parallel to the pipe axis indicates open, perpendicular indicates closed.

S

Schedule (Pipe)

A designation for pipe wall thickness. Pipe schedule (e.g., Schedule 40, Schedule 80, Schedule 160) defines the wall thickness for a given nominal pipe size. Higher schedule numbers mean thicker walls and higher pressure ratings. Schedule 40 is the standard for most water and steam service at moderate pressures. Schedule 80 is used for higher pressures or where extra corrosion allowance is needed. For high-pressure steam above 25 bar, Schedule 80 or Schedule 160 may be specified. The actual wall thickness depends on both the nominal pipe size and the schedule — a Schedule 40 DN50 pipe has a different wall thickness from a Schedule 40 DN100 pipe.

Seal (Pump)

The device that prevents fluid from escaping along the pump shaft where it exits the pump casing. Two main types: mechanical seal (a pair of precisely lapped rotating and stationary faces held together by springs, with a thin film of fluid lubricating the faces — very low leakage, the standard for most industrial pumps) and packing seal (rings of braided graphite, PTFE, or similar material compressed around the shaft in a stuffing box — tolerates some leakage, simpler, suited to slurries and abrasive media). Mechanical seal failure is one of the most common pump maintenance events — the faces are sensitive to dry running, contamination, and thermal shock.

Specific Speed

A dimensionless parameter that characterises a pump impeller's design geometry and relates its flow, head, and rotational speed. Low specific speed pumps (high head, low flow) have radial flow impellers. High specific speed pumps (low head, high flow) have axial flow (propeller) impellers. Mixed flow impellers are in between. Specific speed is used in pump selection and in diagnosing whether a pump is operating in a regime where it is prone to instability or cavitation.

Socket Weld End

A pipe connection where the valve or fitting has a socket (recess) into which the pipe end is inserted and then seal-welded around the outside. Suitable for small bore piping (typically DN50 and below) in higher-pressure or higher-temperature service where threaded connections are not appropriate. Socket weld valves are common on instrument connections, chemical injection lines, and IBR-classified small-bore steam connections. The pipe must not be bottomed in the socket — a 1.5mm gap must be left to allow for weld contraction.

Steam Hammer

A mechanical shock in a steam system caused by a slug of condensate carried at high velocity by steam flow suddenly decelerating — when it hits an elbow, valve, or tee. The impact can be violent enough to rupture pipe supports, deform bends, and damage valves. Caused by inadequate condensate drainage (blocked traps, flooded separators, insufficient drip legs) that allows condensate to accumulate in the steam flow. Distinguished from water hammer in liquid systems, though the effect is similar. Correct steam trap sizing and placement at drip legs, correct separator installation, and adequate warm-up rates (for startup of cold steam mains) prevent steam hammer.

Steam Trap

An automatic valve that removes condensate and non-condensable gases from a steam system without allowing live steam to pass through. Essential in every steam application — without traps, condensate would accumulate in heat exchangers and pipes, reducing heat transfer and causing water hammer. Three main operating principles: mechanical (ball float or inverted bucket — responds to liquid level), thermostatic (bimetallic or balanced pressure — responds to temperature difference), and thermodynamic (disc type — responds to velocity and pressure of flash steam). See our selection guide: [How to Select the Right Steam Trap for Your Application](/blog/steam-trap-selection/).

Superheat

Steam that has been heated above its saturation temperature at a given pressure. At 10 bar(g), the saturation temperature of steam is approximately 184°C. Superheated steam at 10 bar(g) and 250°C is superheated by 66°C (the degree of superheat). Superheated steam carries more energy per kg than saturated steam at the same pressure. It is used in turbines (where saturation would cause condensation and blade erosion) and in some process applications where dry steam at a precise temperature is required. Superheated steam cannot be used directly with most steam traps designed for saturated steam service — thermodynamic disc traps handle superheat, but ball float traps do not perform well.

V

VFD (Variable Frequency Drive)

An electronic drive that controls the speed of an AC motor by varying the frequency and voltage of the electrical supply. Used on pumps, fans, and compressors where the flow demand varies — by slowing the motor, the pump delivers less flow at reduced head, and power consumption drops with the cube of speed ratio (affinity laws). A pump running at 80% speed uses only 51% of the power it would use at full speed. VFDs are one of the highest-payback energy efficiency investments for variable-load pumping and fan systems.

W

Wafer Type (Butterfly Valve)

A butterfly valve body that is clamped between two pipe flanges using through-bolts that pass from one flange, through clearance holes in the valve body, and into the opposite flange. The valve has no flange of its own and takes up minimal face-to-face space. Wafer type is the most economical and compact butterfly valve mounting. Its limitation: both flanges must remain bolted to hold the valve — removing either side of the piping while the valve is in place means the valve can fall out. Not suitable for dead-end service. See also lug type.

Water Hammer

A pressure transient (shock wave) in a liquid-filled pipe system caused by a sudden change in flow velocity — typically when a valve closes too quickly, a pump starts or stops abruptly, or a water column decelerates suddenly. The pressure wave can be multiple times the normal system pressure. In poorly designed systems it can rupture pipes, damage flanges, and split valve bodies. Prevented by slow-closing valves, properly specified check valves on pump discharge (to prevent sudden reverse flow on pump trip), air vessels, or surge suppression vessels. Common in municipal water distribution, boiler feedwater, and condensate return systems.

WTA (Knife Gate Valve)

A valve type (WTA is a Crane ChemPharma brand) designed for slurries, pulp, fibrous media, and mining applications. The gate is a thin stainless steel plate (the "knife") that cuts through the medium as it closes, rather than attempting to seal against it. The knife gate valve is not designed for bubble-tight shutoff against clean liquids or gases — it is a bulk shutoff valve for heavy media that would jam or destroy a conventional gate valve or ball valve. Common in paper and pulp mills, mining tailings, wastewater, and heavy slurry chemical plants. /products/steam-traps/ — Steam Traps — products for the terms defined here /products/valves/ — Valves — products for the terms defined here /products/condensate-recovery/ — Condensate Recovery Equipment /blog/steam-trap-selection/ — Steam Trap Selection Guide /blog/condensate-recovery-roi/ — Condensate Recovery ROI /blog/ball-valve-vs-butterfly-valve/ — Ball Valve vs Butterfly Valve Guide /faq — Frequently Asked Questions /resources — Technical Resources and Downloads /contact/ — Contact PCE for technical clarifications

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