1. Body material by service
The valve body is the pressure-containing part. Code compliance (ASME B16.34, EN 12516, API 6D) dictates the candidate material list, but plant economics narrow it sharply.
- WCB carbon steel — clean utility water, hydrocarbons up to ~425°C, most steam service. Cheap, weldable, well-understood.
- WC6 / WC9 low-alloy — high-temperature steam, refinery service above 450°C, hydrogen-bearing streams.
- CF8M / CF3M (316 / 316L) — most chemical service, food and beverage, mildly corrosive media.
- Duplex / super-duplex (CD3MN, CE3MN) — seawater, chloride- bearing streams, hydrocarbon with H2S.
- Hastelloy C-276 / Alloy 22 — strongly oxidising acids, mixed-acid service, hypochlorite.
The industrial valve reference lists common ASTM body grades with code references and typical service envelopes.
2. Trim selection
Trim — the disc, seat ring, stem, and any seating surface — sees the highest velocity flow and most of the chemical attack. Most plants over-specify the body and under-specify the trim, then watch the disc erode out at six months.
Two rules that hold up across services:
- Trim hardness ≥ pipe spec. A 316L disc in a 316L pipe wears at the same rate as the pipe. A stellite-faced disc in 316L pipe outlives the pipe.
- Two-phase service kills soft-seated trim. If the line sees flashing, cavitation, or particulate, spec hardened (or coated) metal trim and a multi-stage cage.
For control valves specifically, see the valve positioner reference for how trim style affects positioner tuning.
3. Seat and seal materials
For on-off ball and butterfly valves, the seat material defines the leakage class and the temperature window.
- PTFE / RTFE — universal first choice up to ~200°C, Class VI bubble-tight. Cold flow under sustained pressure is the failure mode.
- PEEK — to ~260°C, better creep resistance than PTFE, good chemical resistance, higher price.
- Graphite-reinforced PTFE — higher temperature with retained sealing, common in hydrocarbon service.
- Metal-to-metal (stellite / hardfaced) — high-temperature steam, abrasive slurries, fire-safe service. Leakage class is typically Class IV to V (not bubble-tight).
For gate-valve-specific seat geometry, see the gate valve reference.
4. Stem packing
Stem packing is the single most-replaced wear part in process-plant valves. The two dominant systems:
- PTFE chevron / V-ring — clean service, low fugitive emissions, easy to retrofit. Limited above 200°C.
- Flexible graphite — high temperature, fire-safe, good for steam. Higher stem friction; usually needs a positioner with extra torque.
Low-emissions packing certified to API 622 or ISO 15848 cuts fugitive hydrocarbon losses sharply but typically increases stem friction by 20-40%. Size actuators accordingly — undersized actuators on low-emissions packing will stall on the stickiest part of the stroke and slowly destroy the positioner.
For high-cycle service (more than 50 cycles per day) plan to replace the packing on a fixed maintenance interval rather than on leak detection. The first leak typically means the packing has already shed graphite particles into the bonnet, and a clean repack requires a full bonnet disassembly.
Live-loaded packing
For services where the stem temperature cycles significantly (steam, hot oil), spring-loaded "live-loaded" packing follows the gland thermally and avoids the classic problem of packing that's tight cold and leaks hot. The retrofit is cheap if you spec it at purchase; field retrofitting is labour-intensive because most live-load kits require a different bonnet profile.
5. Special services
Sour service (NACE MR0175 / ISO 15156)
Any line with measurable H2S in a wet stream falls under NACE MR0175. The spec restricts material hardness, weld procedure, and post-weld heat treatment. A standard 316L disc may pass the chemistry check but fail the hardness limit.
Cryogenic
Below -50°C carbon steel becomes brittle; specify austenitic stainless (CF8M) or aluminum bronze body and extended-bonnet construction so the packing runs at ambient temperature.
Fire-safe
Per API 607 (quarter-turn) and API 6FA (gate/check), a fire-safe valve must seal within stated leakage limits during and after a 30-minute fire exposure. This drives metal-seated construction and graphite secondary packing.
6. Procurement traps
Three common overspending patterns to avoid:
- Specifying a full Hastelloy valve where only the trim sees the corrosive phase. A WCB body with C-276 trim and PTFE seat is often half the cost and lasts as long.
- Buying duplex stainless because the pipe is duplex, when the duplex is specified for chloride stress-corrosion that the valve internals will never see at the actual service temperature.
- Demanding low-emissions packing on every valve when the regulatory driver only applies to ~15% of the plant valve population.
Lead time as a hidden material decision
Body forgings in standard ASTM grades (A105, WCB, CF8M) are typically held in stock by major foundries and ship in 6-10 weeks. Specialty alloys (Hastelloy, Alloy 22, Tantalum-clad, super-duplex) frequently quote 20-32 week lead times because the foundry has to schedule a heat for your order. If your project schedule can't absorb that, the engineering team needs to know before the material decision is locked in.
For pipeline and refinery service the API 6D and API 600 vendor pre-approval lists shape what you can actually source on time. Adding an unlisted vendor can mean a 12-week pre-qualification overhead.
Data-sheet hygiene
The single most expensive line on a valve data sheet is the one nobody fills in. Examples that cost money downstream:
- "Service" left as "process fluid" without composition, pH, or impurities — forces the vendor to spec defensively, usually upgrading material.
- "Insulation thickness" left blank — vendor picks a low-cost shield that doesn't fit your final lagging design.
- "Cycle frequency" omitted — vendor assumes infrequent operation; for high-cycle service the actuator and packing both have to be different.
For control-valve and isolation-valve cross-references, the industrial valve reference includes a worked checklist of the 18 fields that should never be empty on a valve purchase requisition.
Frequently asked questions
Is 316L stainless always enough for chemical service?
No. 316L handles a wide range of organic and mildly corrosive media, but fails fast in low-pH (<4) or high-chloride service, especially at elevated temperature. Pull the relevant corrosion chart for your specific media-temperature combination.
Can I downgrade the body material if the trim is upgraded?
Frequently — yes. A WCB body with hardfaced or Hastelloy trim and PTFE seat works for many chemical services where a full-alloy body would be overspecified. Validate against your specific service envelope and code requirements.
What's the difference between Class IV and Class VI leakage?
Class IV allows ~0.01% of full open flow as leakage past a closed seat. Class VI is essentially bubble-tight (0.0001% range). Soft-seated valves achieve Class VI easily; metal-seated rarely better than Class IV-V.
Do I need NACE MR0175 if H2S is only present during upsets?
If the design basis includes upset conditions with H2S above the threshold (0.05 psi partial pressure for sour gas), then yes — NACE applies. Document the assumption in the line list so future inspections can verify.
When does PTFE seat material become a liability instead of an asset?
PTFE seats lose integrity above roughly 200°C continuous service and creep under sustained high pressure. They also degrade under steam-and-vacuum cycling. For high-temperature steam, hot oil, or cryogenic service, switch to PEEK, RPTFE with filler, or metal-to-metal seats validated for the actual duty cycle.