Walk through any refinery, power plant, or LNG terminal and you will find the same six families doing almost all the work. They differ in how they stop or modulate flow, and that single design decision drives where they fit.

The four-family overview you will find in most introductory posts is a useful starting point, but it under-counts what real projects need. Once you add in check valves for pump protection and plug valves for chemical service, you cover the practical universe.

Most procurement documents do not say "gate valve." They say "API 600 gate valve" or "API 602 forged gate valve." The API number tells the foundry exactly what to build. Memorizing five or six of them will let you read almost any datasheet.

• API 600 / ISO 10434 — Bolted-bonnet gate valves for refinery and chemical service, generally NPS 2 and above, Class 150–2500.
• API 602 / ISO 15761 — Compact forged steel gate, globe, and check valves, typically NPS ½ to 2, Class 800–4500. Common for instrument and small-bore lines.
• API 603 — Cast corrosion-resistant gate valves (CF8, CF8M, CN7M) for low-pressure corrosive service. Many buyers reach for API 600 by default, then realize they needed 603 for a stainless line.
• API 6D / ISO 14313 — Pipeline valves (gate, ball, check) for transmission pipelines. Required for long-distance oil and gas lines, often with pipeline-spec extensions such as NACE and low-emission packing.
• API 594 — Check valves: swing, dual-plate, and axial (nozzle). The first place to look when specifying a non-return valve.
• API 609 — Butterfly valves: lug- and wafer-type, used widely in water and HVAC and increasingly in low-pressure hydrocarbon service.

Once you have picked the family and the API standard, three technical decisions will determine whether the unit price is $400 or $4,000. Lock these down in the RFQ and the quote you receive will be tight.

3.1 Pressure class (ASME B16.34)

Class 150, 300, 600, 900, 1500, and 2500. As a rule of thumb, each step up in class roughly doubles the wall thickness and the body weight, which means more material, more machining, and a higher price. Most refinery and chemical process lines sit at Class 150 or 300. High-pressure headers, steam systems above 100 bar, and transmission pipelines push to 600, 900, or 1500.

3.2 Body material

The body is where corrosion, temperature, and pressure meet. The most common body materials you will see on datasheets are:

• WCB (carbon steel, ASTM A216) — general-purpose for non-corrosive service up to ~425°C.
• WC6 / WC9 / C5 / C12 (chrome-moly cast) — high-temperature steam and refinery.
• CF8 / CF8M / CF3 / CF3M (austenitic stainless) — corrosive and hygienic service.
• LCC / LCB (low-temperature carbon) — down to -46°C.
• Monel, Inconel, duplex, super-duplex — for seawater, sour service, or highly corrosive chemicals.

3.3 End connection

The end connection decides what you need to buy around the valve. Pipe flanges are the default for most process lines because they make maintenance easier. Butt-weld ends are preferred for high-pressure and high-temperature where leak-tightness matters most. Socket-weld and threaded ends are used in small-bore, instrument, and utility lines. Mixing them in one project usually creates headaches at site — pick one philosophy and stick with it.

Beyond the base spec, many projects add a list of supplementary requirements. These are not optional — they change the materials, the testing, and the paperwork, and they always add cost. Build them into the RFQ from day one.

• NACE MR0175 / ISO 15156 — required for sour service (H₂S). Restricts hardness, mandates certain alloys, and excludes some casting repairs.
• API 607 / ISO 10497 (fire-safe) — soft-seated valves for hydrocarbon service where external fire is a credible risk.
• API 624 / ISO 15848 (low fugitive emissions) — graphite vs. low-emission packing, increasingly required in oil & gas and pharmaceutical projects.
• Cryogenic (-46°C and below) — extension bonnets, impact-tested bodies, special greases. Common in LNG, ethylene, and air-separation plants.
• PMI (Positive Material Identification) — required for stainless, duplex, and alloy bodies to avoid mix-ups in the warehouse or at site.

The most common mistake we see in valve RFQs is that the valve is sourced separately from the pipe, the fittings, the flanges, and the gaskets. The result is four or five POs, four or five delivery dates, and a long list of small mismatches to fix at site: flange bores that do not match pipe IDs, gaskets that do not match the flange class, stud bolts that are the wrong length.

A mill-direct supplier who produces the carbon steel pipe and stainless steel pipe to ASTM A106 / A312, the pipe fittings to ASME B16.9, the pipe flanges to ASME B16.5, the gasket stud bolt nut sets, and the API 600 / 602 / 6D valves in the same facility can ship one consolidated package. One MTC trail, one heat-number system, one set of certificates.

A clean RFQ should include at least the following ten items. The more complete the RFQ, the closer the quote will be to the final number.

• Type and function (gate, globe, ball, butterfly, check, plug)
• Standard reference (API 600, 602, 603, 6D, 594, 609, ASME B16.34)
• Size: NPS and pressure class (Class 150 / 300 / 600 / 900 / 1500 / 2500)
• Body and trim material (WCB + 13Cr, CF8M + 316, etc.)
• End connection: flanged (RF / RTJ), butt-weld, socket-weld, threaded, or lug/wafer
• Operator: handwheel, gear, electric, pneumatic, hydraulic
• Service fluid and operating temperature / pressure
• Special qualifications (NACE, fire-safe, low-emission, cryogenic, PMI)
• Testing and certification (hydrotest, NDT, EN 10204 3.1 or 3.2, PMI report)
• Quantity, delivery terms, and destination port

Send this list together with the matching line items for pipe, fittings, flanges, and gaskets. A mill-direct quote with the whole piping package will almost always beat the equivalent pieced-together quote once you add freight, inspection, and admin time.

API 600 and API 6D both require hydrostatic and pneumatic testing on every valve, but the project specification can extend this significantly. The practical minimum for project-grade orders includes:

• Mill Test Certificate to EN 10204 3.1 (or 3.2 for critical service), with full chemistry and mechanical results
• Hydrostatic test report (shell + seat, per API 598 or ISO 5208)
• Low-pressure pneumatic seat test for soft-seated valves
• PMI report on every body and bonnet
• NDT report: radiographic (RT) or ultrasonic (UT) for cast bodies, magnetic particle (MT) for welds, dye penetrant (PT) for stainless surfaces
• Fire-safe and low-emission certificates, if specified
• Impact test report at design temperature, when service is below -29°C

A mid-sized refinery recently asked us to quote a steam-header upgrade with the following line items: NPS 6 to NPS 12, Class 300, WCB bodies with 13Cr trim, RF flanged ends, with three of the 30 valves specified for NACE sour service. Here is how the package was structured:

• 8× API 600 gate valves (WCB + 13Cr, Class 300, RF) for main line isolation
• 6× API 600 globe valves (WCB + 13Cr, Class 300, RF) for start-up vent and warm-up bypass
• 4× API 6D ball valves (WCB + 13Cr, Class 300, RF) for quick-isolation duty
• 6× API 594 dual-plate check valves (WCB + 13Cr, Class 300, RF) for pump- and compressor-discharge protection
• 6× API 602 forged gate / globe / check (A105 + 13Cr, Class 800, SW) for the small-bore instrument and drain lines

The same RFQ also called for the matching pipe (ASTM A106 Grade B, NPS 6–12, Sch 40), the butt weld fittings (B16.9 WPB, Sch 40), the ASME B16.5 steel flanges in Class 150 / 300, the spiral-wound gaskets, and the stud bolt / nut sets. All of it left the mill in one consolidated shipment under one MTC trail — which the refinery EPCI used directly in their inspection records.

For buyers who need industrial valves in project volumes — and who want the matching pipe, fittings, flanges, and gaskets in the same shipment — EZ Steel Industrial has supplied API 600, API 602, API 603, API 6D, and API 594 valves for over 30 years. Our manufacturing base in Changsha covers the full pressure-class range from Class 150 to Class 2500, with body materials from WCB and LCC to CF8M, duplex, Monel, and Inconel.

For the supporting piping package, our carbon and alloy steel pipe range covers ASTM A106 / A53 / A333 / A335 / API 5L, our stainless range covers ASTM A312 / A249 / A213 / A269, and our steel flanges cover ASME B16.5 / B16.47. Pipe fittings are produced to ASME B16.9 / B16.11, and matched gasket stud bolt nut sets are stocked for fast-track dispatch.