Valve Selection for
Process Service —
Types, Standards and Criteria

The valve is the most numerous single component in any process piping system and one of the most frequently misspecified. A valve that is the wrong type for its service will either fail to perform its intended function or fail prematurely — and in pressure systems, valve failure rarely announces itself conveniently. Correct valve selection requires matching the valve's design and materials to three things: the intended function (isolation, regulation, or non-return), the process conditions (fluid, pressure, temperature, and any hazardous properties), and the operational requirements (frequency of operation, required shutoff class, and maintenance access).

This article covers the principal valve types used in industrial process piping — gate, globe, ball, butterfly, plug, needle, diaphragm, and check — their design characteristics, the standards that govern them, and the selection criteria that determine which type is appropriate for a given service.

The Three Functions — Isolation, Regulation, Non-Return

Before selecting a valve type, the required function must be defined precisely. The three fundamental functions are distinct and not interchangeable:

• Isolation (block valves) — fully open or fully closed, providing positive shutoff when closed and minimum pressure drop when open. Gate valves, ball valves and butterfly valves in their isolation variants are designed for this service. Operating an isolation valve in the partially open position causes erosion of the valve internals and is not an acceptable practice.
• Regulation (control valves) — modulating the flow rate by operating at intermediate positions between fully open and closed. Globe valves, needle valves and specially rated ball and butterfly valves designed for throttling service. A valve designed only for isolation that is used for regulation will fail rapidly.
• Non-return (check valves) — allowing flow in one direction only, closing automatically when reverse flow occurs. Swing check, lift check, dual-plate check, and axial check valves.

Gate Valves

The gate valve uses a flat or wedge-shaped disc (the gate) that rises and falls vertically to open and close the flow path. When fully open, the gate is retracted completely into the bonnet and the bore is clear — pressure drop is minimal. Gate valves are not suitable for throttling: a partially open gate valve creates severe turbulence on the downstream face of the gate, causing vibration and erosion. Gate valves are also slow to operate, typically requiring multiple handwheel turns to full travel, making them unsuitable for applications requiring rapid operation.

Gate valves are the default isolation valve for general process piping in carbon steel, low-alloy steel and stainless steel. API 600 (steel gate valves with flanged ends, bolted bonnets) is the principal standard for full-bore gate valves in oil, gas and petrochemical service. API 602 (compact gate valves with flanged, threaded, socket-weld or butt-weld ends) covers smaller bore valves to 4". ASME B16.34 defines pressure-temperature ratings for all valves manufactured to US standards. For European procurement, EN 13709 applies to steel gate valves in general-purpose and petroleum industry service.

Globe Valves

The globe valve uses a disc or plug that moves perpendicular to the seat, throttling flow through an S-shaped or Z-shaped internal flow path. This geometry makes globe valves inherently better for regulation than gate valves — the relationship between stem travel and flow rate (the valve's inherent characteristic) is well-controlled and predictable. The Z-pattern flow path also produces higher pressure drop when open than an equivalent gate or ball valve, making globe valves less suitable where minimising pressure drop is critical.

Globe valves are specified for manual throttling, for frequent operation (their design tolerates repeated opening and closing better than gate valves), and for high-pressure or high-temperature service where the bolted bonnet and rising stem design offers reliable sealing. They are the standard manual regulation valve for steam service, and are common in utilities (cooling water balance valves, chemical dosing) where precise flow control is needed manually.

Ball Valves

The ball valve uses a spherical rotating plug with a through-bore. Quarter-turn operation gives fast open and close, and when fully open the bore diameter matches the pipe bore — minimal restriction. When closed, the ball presents a solid sealing surface against upstream and downstream seats simultaneously, giving excellent shutoff. Ball valves dominate modern process piping installations as isolation valves because of their compact form, low operating torque, full bore option, low maintenance requirements, and availability in a very wide range of sizes, materials and pressure ratings.

API 608 (metal ball valves in flanged, threaded, butt-weld or socket-weld ends) covers standard ball valves for process piping. API 6D (pipeline valves) applies to larger bore ball valves in pipeline service. Floating ball designs (ball retained by seat rings) are standard to around Class 600 and DN200; trunnion-mounted designs (ball supported on trunnions) are required for higher pressure or larger bore where operating torque of a floating ball design would become excessive. Soft-seated ball valves (PTFE, PEEK, or elastomer seats) provide excellent shutoff (ANSI class VI zero-leakage) but have temperature limits; metal-seated designs tolerate higher temperatures and cryogenic conditions but have higher minimum shutoff leakage class.

Butterfly Valves

The butterfly valve uses a disc rotating about a central axis — quarter-turn operation like a ball valve but with a much more compact, lower-weight body that is particularly advantageous in large diameters. When fully open, the disc remains in the flow path and causes moderate pressure drop (higher than ball or gate). Butterfly valves are specified where weight and face-to-face length are primary considerations and moderate pressure drop is acceptable.

API 609 (butterfly valves, double-flanged, lug and wafer types) covers process butterfly valves. Three designs exist in practice: concentric (disc axis on centreline — simplest, lower pressure rating), double-eccentric (disc offset from bore centreline — better sealing, higher rated), and triple-eccentric (disc axis also offset from seat plane — suitable for metal-to-metal seating, high temperature and high pressure). Triple-eccentric butterfly valves with metal seats are used in high-pressure steam and hydrocarbon service where a ball valve of equivalent bore would be prohibitively heavy and expensive.

Plug Valves

The plug valve uses a tapered or cylindrical plug rotating a quarter-turn in a matching body. Lubricated plug valves inject grease under pressure between plug and body to maintain seal and reduce operating torque — they are specified for slurry, dirty or viscous fluids where soft-seated valves would be damaged by abrasive particles. Non-lubricated variants with PTFE-sleeved or coated plugs are used for clean fluids in chemical service. Plug valves provide full bore when open and positive isolation, but are less common in modern process piping than ball valves except in specific slurry and sour-service applications.

Needle Valves

The needle valve uses a tapered needle-point plug against a close-matching seat, providing very fine control of small flow rates. They are used almost exclusively for instrument impulse line isolation, flow metering bypass trim, and instrument calibration connections — not in main process lines. Pressure ratings to Class 2500 or higher are available in small bore (6mm to ½" NPS); they are not available in large bore. Material options include 316 stainless, duplex and Inconel for corrosive and sour service.

Diaphragm Valves

The diaphragm valve uses a flexible diaphragm pressed against a weir or through the bore to close off flow. There is no stem packing — the diaphragm is the seal, eliminating any stem leakage path to atmosphere. Diaphragm valves are the standard isolation and regulation valve for hygienic (pharmaceutical, food, bioprocessing) service where crevice-free, cleanable valve internals are required. They are also used for slurry and corrosive fluid handling where a standard valve would be rapidly degraded. EHEDG-certified and ASME BPE-compliant diaphragm valves are available in Tri-Clamp end connections for hygienic applications.

Check Valves

Check valves prevent reverse flow automatically. The principal types:

• Swing check — a hinged disc that swings open under forward flow and closes by gravity or reverse flow. Simple, low cost, suitable for horizontal or vertical upward flow. Slam-close behaviour in rapidly fluctuating flow systems.
• Lift check — disc rises on a guided stem under forward pressure, drops to close. Suitable for high-pressure, high-velocity applications. Must be installed in horizontal lines with stem vertical.
• Dual-plate (wafer check) — two spring-loaded half-discs that open against spring pressure and close rapidly when forward flow drops. Low water hammer on closure, compact wafer body. Standard in large-bore systems and pump discharge lines.
• Axial (nozzle check) — inline disc with spring return, minimum pressure drop, rapid closure — specified for compressor discharge and high-pressure pump systems where slam-close would cause damaging pressure surges.

Valve Materials

Pressure-Temperature Ratings and Valve Sizing

All valves in ASME service are rated according to ASME B16.34, which defines pressure-temperature ratings for each material group at 25°C increments from cryogenic to the material's maximum service temperature. The pressure rating at the design temperature — not the maximum cold working pressure stamped on the body — is the governing design parameter. A Class 300 carbon steel valve rated at 51.1 bar at 38°C is only rated at 38.1 bar at 260°C. Failure to use the rated pressure at actual operating temperature is one of the most common valve specification errors.

Valve sizing for control applications requires calculating the required Cv (flow coefficient) from the design flow rate and pressure drop, selecting the next standard valve size above the calculated Cv, and verifying that the selected valve operates between 20% and 80% open at design conditions to maintain controllability. A control valve that must operate near fully-open or near fully-closed for normal service will not provide adequate control range when process conditions vary.

Valve Datasheets and Documentation

Each process valve in a pressure system should be documented on a valve datasheet covering: service (fluid, pressure, temperature, specific gravity or density at operating conditions), required function (isolation, control, non-return), end connections (flanged class, butt-weld schedule, socket-weld, threaded), body material, trim material, seat type and leakage class (ANSI/FCI 70-2), actuator requirement (handwheel, gear operator, electric, pneumatic), and fail-safe position where actuated. The valve datasheet is a procurement and inspection document, not simply an ordering tool — it defines the acceptable envelope for substitution and is referenced on the P&ID.

Summary

Gate valves remain the default on-off isolation valve for general carbon steel process piping. Ball valves have largely displaced gate valves in stainless steel, cryogenic and smaller bore applications due to their compact form and positive shutoff. Globe valves are correct for throttling and frequent operation. Butterfly valves solve the weight and space problem at large bore. Diaphragm valves own the hygienic market. Check valve selection depends on closure speed requirements and the consequences of water hammer. Every valve specification should state body material, trim material, pressure class, end connection type, and leakage class — and the rated pressure at operating temperature should always be verified against ASME B16.34, not assumed from the nominal class.