Specifying a ball valve looks straightforward until a line fails. A valve that is "the right size" on paper can still leak, seize, or corrode inside six months if the seat, material, or end connection is wrong for the duty. This guide lays out the dimensions a sourcing engineer actually checks before raising an RFQ.
1. Start with the Application, Not the Catalogue
Before anything else, write down four numbers: design pressure, design temperature, medium chemistry, and duty cycle. Everything downstream — material, seat, port, actuation — follows from these. A brass valve is fine on city water; it is not fine on demineralised water at 180°C or on sour gas. "Application first" is also what your factory will ask for before committing to a material certificate, so you may as well have the answer ready.
2. Size and Pressure: DN, PN, Class, and WOG
Nominal size (DN) is the pipe-bore reference in millimetres; DN50 ≈ 2-inch NPS. Size the valve to the line, not to the flow you wish you had — over-sizing buys nothing but cost and a throttling problem.
Pressure ratings come in three flavors and they are not interchangeable:
| System | Meaning | Where used |
|---|---|---|
| PN (e.g. PN16, PN40) | Nominal pressure, bar, ~20°C reference | EN / metric markets |
| ASME Class (150, 300) | Temperature-de-rated rating per ASME B16.34 | North America / oil & gas |
| WOG (e.g. 1000 WOG) | Cold, non-shock working pressure in psi | Plumbing / commercial |
A DN50 3-piece SS316 ball valve in the PrimeFlow range is rated PN16 / 1000 WOG / Class 150 — three numbers because three markets ask three different questions (PrimeFlow catalogue, indicative). Read all three against your line, and remember the rating falls with temperature. ASME B16.34 is the source of truth for Class pressure-temperature; ISO 17292 covers metallic ball valves for petroleum, petrochemical and related services.
3. Body, Ball, and Seat Material
Material selection is driven by the medium and temperature, not by what is in stock. Indicative guidance:
| Service | Body & ball | Seat | Note |
|---|---|---|---|
| Domestic water, HVAC | Brass / SS304 | PTFE | NSF/ANSI 61 for drinking water |
| Treated water, low-pressure steam | SS304 | R-PTFE | Better chemical resistance |
| Chemical, marine, coastal | SS316 | R-PTFE | Mo content resists chloride pitting |
| Higher temperature service | SS316 / Carbon steel | R-PTFE or PEEK | Verify seat temp limit first |
| Non-potable, large bore | Cast iron / Carbon steel | PTFE | Butterfly valve often cheaper above DN150 |
On seat material: PTFE is typically rated to about 180°C; R-PTFE (glass- or carbon-filled) pushes that higher and resists wear. Always quote the seat limit, not the body limit — the seat usually fails first. The PrimeFlow SS316 3-piece valve carries a −20 to 180°C envelope, which is typical of a PTFE/R-PTFE seat configuration (indicative).
4. End Connections and Port Type
End connections drive install cost more than valve cost:
- Threaded (NPT / BSPP / BSPT) — DN8 to DN50, fast, cheap, not for high vibration.
- Socket weld — DN8 to DN50, leak-tight, permanent.
- Butt weld — process lines, DN50 up, requires a qualified weld procedure.
- Flanged — DN15 up, ASME B16.5 / EN 1092-2, easy to service and replace.
Port type is where specs quietly lose money. A full port (full bore) valve has a bore equal to the line — use it where pressure drop matters or where the line is pigged. A reduced port valve is cheaper and smaller and is fine for most isolation duties. If you cannot afford the roughly 5–10% pressure drop, or the line is pigged, specify full port explicitly — the factory will default to reduced otherwise.
5. Manual vs. Actuated
Most specs leave actuation as an afterthought. Decide before you order: an actuated valve needs a different ISO 5211 mounting pad, a longer stem, and often a double-block design. Rough guidance:
- Manual lever / gear — isolation, infrequent operation, economical up to about DN200.
- Pneumatic actuator — fast cycling (sub-second), fail-safe with spring return, plant air required.
- Electric actuator — remote control and positioning, slower cycle, no compressed air needed.
Quote the actuator mounting standard (ISO 5211) and the torque at your worst-case differential pressure, not catalogue torque. A factory that cannot give you a torque curve is a factory that has not sized the actuator.
6. Standards, Certifications, and Documentation
The documents that separate a usable valve from a return-shipment:
- ASME B16.34 — pressure-temperature ratings for flanged and welded-end valves.
- ASME B16.10 / EN 558 — face-to-face and end-to-end dimensions (so the valve fits the gap).
- ISO 17292 — metallic ball valves for petroleum, petrochemical and related services.
- API 6D / API 608 — pipeline valves and metal ball-valve specifications for oil & gas.
- MSS SP-110 — ball valves, a common reference in North American specs.
For export, request the certificate set your destination actually audits: ISO 9001 for the quality system, CE / PED for the EU pressure directive, ATEX for explosive atmospheres, NSF/ANSI 61 for drinking water in North America, WRAS for the UK, and API 6D marking for pipeline service. Always ask for EN 10204 3.1 material certificates on body and ball; a 3.2 certificate with a third-party stamp costs more but is the only thing some EPCs will accept.
Frequently Asked Questions
What is the difference between PN and WOG?
PN is a metric nominal pressure in bar at a ~20°C reference, used in EN markets. WOG (Water-Oil-Gas) is a cold, non-shock working pressure in psi, common in plumbing specs. They are not directly convertible: PN is part of a temperature-de-rated family, WOG is a single cold number.
Full port or reduced port — which do I need?
Full port for pigged lines, pressure-drop-sensitive service, and slurry. Reduced port is acceptable for most isolation duties and costs less. If the line is pigged, always full port — no exceptions.
PTFE or R-PTFE seats?
PTFE for clean, low-abrasion service up to about 180°C. R-PTFE (reinforced) for higher temperature, frequent cycling, or slightly abrasive media — it wears slower and holds pressure longer.
Which material for seawater or brine?
SS316 is the baseline; for continuous seawater, specify a super-duplex (e.g. ASTM A890 grade 6A) or a lined valve. Plain SS316 will pit — the molybdenum slows the process, it does not stop it.
What documentation should I request for EU and North American export?
EU: CE/PED conformity, ATEX if explosive atmosphere applies, EN 10204 3.1 MTC. North America: NSF/ANSI 61 for potable water, UL/FM for fire protection, CRN for Canada. Add a 3.2 MTC where the EPC requires third-party witness.
Next step
If you have a line sheet with medium, pressure, and temperature, that is enough to raise an RFQ. Share the duty conditions and the destination market — PrimeFlow will return material, seat, end-connection, and certification options against your spec rather than a generic catalogue page.
Request a quotation → View a 3-piece SS316 ball valve