1. Why "an industrial valve" is never just one product

If you put a refinery, an ethylene plant, an LNG terminal, and a hydrocracker side by side, the equipment that looks most interchangeable — a flanged steel valve bolted into a line — is in fact seven different product families governed by seven different API standards. Specifying "a gate valve" without naming the standard is the most common reason a delivered batch ends up on a rejection pile.

For a procurement engineer or EPC piping lead, the question is not "which valve is best." It is "which combination of body material, pressure class, end connection, seat design, and standard is fit for this specific line, this fluid, and this temperature window." The reference is API's Recommended Practice 602-style guidance for the hydrocarbon processing industry, which maps each valve function to a small set of standards and forces you to compare them on the same axes.

This guide walks through those seven families, the standards that govern them, and the procurement logic that lets you pull the whole package — pipe, industrial valves, pipe fittings, pipe flanges, gaskets, and stud bolts — from one mill-direct supplier rather than five different trading desks.

2. The four functions a valve performs on a process line

Before any standard is named, the function has to be locked. API guidance groups process-line valves into four jobs:

• Shut-off (isolation) — fully open or fully closed, nothing in between. The valve family here is the gate, ball, plug, and butterfly.
• Backflow prevention — closes on its own when flow reverses. The dedicated family is the check valve, including the stop-check hybrid.
• Throttling (flow control) — sits at a partially open position for long periods. The dedicated family is the globe valve.
• Overpressure protection — handled by safety relief valves, which sit outside the API 600-series standards and are not in this guide's scope.

3. Gate valves — API 600, 602, 603, and where each one fits

Gate valves are the workhorse isolation valve on every refinery and petrochemical header. The wedge slides in and out of the flow path; when fully open, the bore is unobstructed and pressure ｄｒｏｐ is minimal. The confusion starts because three different API standards govern what looks like the same product.

API 600 — the heavy-duty refinery gate valve

API 600 covers gate valves from NPS 1 and up, pressure classes 150 through 2500, with flanged or butt-weld ends. It was written for refinery service and specifies a robust thick-wall design rated for temperatures up to 538°C (1000°F). Body and bonnet wall thicknesses are heavier than ASME B16.34 standard class, giving extra corrosion allowance. The wedge is usually a solid or flexible wedge, with the flexible design tolerating body distortion from pipeline stress or thermal expansion without sticking in the closed position.

API 602 — the small-bore forged gate valve

API 602 covers gate valves up to NPS 4, pressure classes 150 through 1500 including the Class 800 intermediate rating. In a process unit you will see API 602 valves at NPS ½ to NPS 2, Class 800, with threaded or socket-weld ends. Standard port is reduced, with full port as an option. Bodies are usually forged — common materials include A105 carbon steel, LF2 for low-temperature service, F11/F22 for chromium-moly service, F51/F55 duplex grades, and F304/F316/F321 stainless for corrosive service. Where threaded or socket-weld ends are not acceptable (such as compressor lube-oil headers), flanged and butt-weld ends are also available.

API 603 — the corrosive-service gate valve

API 603 was developed to give a lower-cost alternative to API 600 for corrosive, lower-pressure service. It uses stainless steel or nickel alloy bodies, with wall thicknesses matching ASME B16.34 — lighter than API 600 but adequate for the intended service. Pressure classes are 150, 300, and 600, sizes NPS ½ and up, flanged or butt-weld ends. Hard-facing on the seat surfaces helps prevent galling between stainless wedge faces and stainless body seats.

Pressure-seal bonnet gate valves for hydrogen service

Above Class 600, especially in hydrocracker and hydrotreater loops, gate valves with a pressure-seal bonnet design are commonly specified. The bonnet seal becomes tighter as line pressure rises. MSS SP-144 covers this design. Use is generally limited to non-corrosive service to protect the seal ring, and reassembly after maintenance should be done with manufacturer support. Spare seal rings are usually OEM-only.

4. Ball, plug, and butterfly valves — quarter-turn isolation

Where you need fast operation and frequent cycling, gate valves give way to quarter-turn designs. The trade-off is body geometry: a ball or butterfly in the flow path is not as full-bore as an open gate.

Ball valves — API 608

API 608 covers flanged and butt-weld ball valves in Class 150 to 600, plus socket-weld and threaded ball valves in Class 150 to 800. Port options are full, single-reduced, and double-reduced. Two body designs are common: floating ball, where the ball is held in place by the upstream and downstream seats and line pressure provides the downstream seal (typical for smaller sizes and lower classes); and trunnion-mounted, where the ball is fixed on top and bottom stems and seat springs plus line pressure provide sealing (typical for larger sizes and higher classes).

Seat material is the second big decision. Soft seats (PTFE, RPTFE) give bubble-tight shut-off but limit pressure-temperature ratings. Metal seats, usually with hard-facing on the sealing surfaces, handle higher temperatures and are required for fire-safe service. API 607 is the fire test referenced when "fire-safe" or "fire-tested" is specified. For metal-seated ball valves above NPS 3, gear operators or actuators are usually required because of the operating torque.

Plug valves — API 599

API 599 covers plug valves in NPS ½ to 24 for flanged and butt-weld ends, and NPS ½ to 2 for threaded and socket-weld ends. Pressure-temperature ratings follow ASME B16.34 for steel and nickel alloy bodies, and ASME B16.42 for ductile iron. Lubricated plug valves and designs with non-metallic components (seals, sleeves, liners) carry restricted pressure-temperature ratings that must be marked on the nameplate; lubricant and packing are typically rated for -29°C to +107°C (-20°F to +225°F). Non-lubricated designs use metal seats or hydrocarbon-resistant plastic or elastomer sleeve liners. Plug valves are not recommended for throttling.

Butterfly valves — API 609

API 609 splits butterfly valves into two categories. Type A is the concentric, soft-seated utility valve typically rated to ASME Class 125 or 150; the seat seal depends on elastic deformation of the liner at closure, and the valve is position-seated rather than torque-seated. Type B is the high-performance design with an offset disc and seat; this includes single-eccentric (double-flanged or lugged up to NPS 48, wafer up to NPS 24), double-eccentric, and triple-eccentric. Triple-eccentric designs can carry a metal seat and are specified for high-temperature, high-pressure, and tight-shut-off service. Disc protruding past the body face in the open position is normal; lined pipe ends can interfere with disc rotation if not accounted for.

5. Check valves — API 594 and API 602 anti-surge logic

Check valves close automatically on flow reversal, with no external intervention. The most common use is at pump discharge to prevent reverse flow through a stopped pump from a parallel running pump. The selection logic is more nuanced than for isolation valves, because the valve has to close smoothly on a moving fluid column without slamming.

API 594 covers two designs. Type A is the wafer, lugged, or double-flanged single-disc or dual-disc check, in cast iron (ASME B16.1 Class 125 and 250), ductile iron (ASME B16.42 Class 150 and 300), and ASME B16.34 materials from Class 150 to 2500. Body wall thicknesses for cast iron, ductile iron, and B16.34 Group 1 materials match API 600 gate valve thicknesses. Type B is the bolted-bonnet swing check, flanged or butt-weld, in ASME B16.34 materials up to NPS 24; in the fully open position the disc is almost completely out of the flow path, minimizing pressure ｄｒｏｐ and turbulence.

API 602 check valves are small-bore (up to NPS 4), in piston, ball, and swing designs, with wall thicknesses sized for Class 1500. Piston check designs — including the stop-check variant — can trap fluid above the disc and delay opening; pressure balancing from above the disc to the downstream side is sometimes needed to allow unobstructed opening.

In reciprocating compressor or pulsating-flow service, dual-disc, tilting-disc, and axial-flow (nozzle) check valves tend to outperform swing checks because the travel is shorter and the disc mass is lower. Anti-slam or slow-closing accessories can be added to large valves; CFD or surge analysis is sometimes used to predict water-hammer pressure and decide whether the accessory is needed.

6. Globe and stop-check valves — when you actually need throttling

Globe valves are the standard throttling valve, and they double as manual bypass valves around control valves during commissioning and maintenance. The disc moves axially toward and away from the body seat, giving uniform clearance and predictable flow around the disc edge. The seat lasts longer than a gate valve seat under throttling service because the disc does not slide across the seat face.

Globe valves are covered by API 602 up to NPS 4 and by API 623 for NPS 2 to NPS 24. Standard sizes typically max out at NPS 12 for Class 150, NPS 10 for Class 300, and NPS 8 for Class 600, with larger sizes and higher classes available on request. The disc should be at least 20% open during throttling; below that opening, flow-induced vibration can damage the disc and stem. If the bypass duty demands less than 20% opening, an engineered control valve on the bypass is a more reliable solution.

Stop-check valves combine globe and check function: the piston disc can be forced onto the seat mechanically like a globe, and the same disc closes automatically on flow reversal like a check. Available in T-pattern, Y-pattern, and angle pattern, stop-check valves are specified where the line needs both manual isolation capability and automatic backflow prevention.

7. The seven selection factors that decide between two otherwise identical valves

Once function and standard are fixed, the following factors decide between two valves that look the same on paper. They map directly to what an inspector checks during an RFQ review.

• Pressure rating and size — match ASME B16.34 standard class for the chosen body material; confirm the seat rating is not lower than the body rating.
• Fluid service — hydrogen, sour service (NACE MR0175), low-temperature (impact testing per ASME B16.34), oxygen-clean, or sea water each impose a different trim and body spec.
• Structural material — body, bonnet, stem, seat, and disc material; hard-facing selection for high-temperature or erosive service.
• Seat and seal selection — soft vs metal, PTFE vs graphite, fire-safe per API 607, low-emission per ISO 15848.
• Acceptable leakage rate — Class IV, V, VI, or "bubble-tight"; soft seats usually required for Class VI.
• Special features — body cavity relief, purge connections, drain plugs, locking device, extension bonnet for cold service, NACE compliance.
• Flow capacity (Cv) and pressure loss — calculate required Cv at the operating duty and compare against the published Cv for the chosen size; do not oversize.
• Operating frequency — quarter-turn valves cycle faster than multi-turn; for more than a few cycles per day, ball or butterfly is more durable than gate.

8. Bundled sourcing: why pipe + valve + flange + gasket from one mill shortens your schedule

For a typical project RFQ, the valve is only one of five items on the same isometric drawing. The industrial valves are joined to the line by butt-weld or socket-weld fittings, bolted between two pipe flanges (often steel flanges per ASME B16.5 or B16.47) with a gasket and a set of stud bolts and nuts. Each of those five items has its own MTC, its own delivery date, and its own rejection rate.

Sourcing them from five different suppliers adds three structural problems. First, the five MTCs do not always match on heat number, lot, or even material grade, which can hold up the receiving inspector. Second, partial deliveries on a long line force the site team to choose between storing half a system or installing mismatched components. Third, when a flange and a valve arrive with different facing finishes (RF vs FF), or a stud bolt does not match the flange class, the fix is at site, not in the warehouse.

A mill-direct supplier that produces the carbon steel pipe, the butt weld fittings, the steel flanges, the gaskets, the stud bolts, and the industrial valves on the same production schedule can deliver a matched package with one set of mill test certificates, one delivery window, and one point of contact for any non-conformance. For a refinery turnaround with a hard start date, that consolidation is often worth more than any individual price concession.

9. Six pre-PO checks that catch 90% of field problems

Before signing the purchase order, run these six checks against the supplier's datasheet and quality plan. Items that fail here will not be fixable in the field without a return-to-vendor cost.

• Standard and edition match the datasheet. "API 600 gate valve" is not enough; the edition year and supplementary requirements (NACE, fire-safe) must be explicit.
• Body and trim material match the MTC, the nameplate, and the RFQ. Cross-check the MTC heat number, the chemical analysis, and the mechanical properties against the material spec.
• Seat leakage class is named. Class IV, V, or VI, and the test medium (water, air, nitrogen) per API 598 or ISO 5208.
• End connection, facing, and dimension match the mating flange or pipe. ASME B16.5 RF facing on the valve and FF on the flange is a one-way trip to the rework bench.
• Pressure-temperature rating covers the entire operating envelope, including upset cases. Not just the design pressure and design temperature; include pump trip, block valve closure, and fire case.
• Documentation pack is complete and on the PO. MTC per EN 10204 3.1, visual and dimensional report, hydrostatic test report, seat leakage test report, fire-safe test report if specified, NACE certificate if specified.