Choosing & Specifying a Valve

Q: What should I know about Choosing & Specifying a Valve?

Isolation < DN150 → ball or gate. Isolation ≥ DN150 → butterfly. Throttling/control → globe (or characterized ball). Backflow → check (spring/nozzle on pump discharge). Slurry → knife-gate/full-bore ball. Flammable/ESD → fire-safe ball (API 607).

Q: Which standards or codes apply?

high ΔP with flashing/cavitation, sonic gas velocity, cryogenic/LNG, erosive slurry, lethal/toxic service, or rapid cycling. These need anti-cavitation/multistage trim, special materials and detailed analysis — not a catalogue valve.

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Quick pick (TL;DR)

Isolation < DN150 → ball or gate.
Isolation ≥ DN150 → butterfly.
Throttling/control → globe (or characterized ball).
Backflow → check (spring/nozzle on pump discharge).
Slurry → knife-gate/full-bore ball.
Flammable/ESD → fire-safe ball (API 607).

Step 1 — Gather the input data

Pull these from the line list, P&ID and process datasheet first:

Size & schedule; design + operating P & T.
Fluid, phase, ρ, viscosity, % solids, flow.
Corrosive species → material/trim.
Function.
Shutoff class, ΔP, end connection.
Fire-safe, low-emission, NACE, cryogenic.

Step 2 — Decision logic (with numbers)

Throttling → globe/characterized ball; never gate/ball.
Isolation & <DN150 → ball or gate.
Isolation & ≥DN150 → butterfly.
Flammable/ESD → fire-safe ball (API 607).
Slurry → knife-gate/full-bore ball.
Pump discharge → spring/nozzle check.
Then set class (P–T), seat (temp), operator (size).

Decision flowchart

What is the function?

Throttling / controlGlobe / characterized ball

Non-returnCheck — spring / nozzle

Isolation → size?< DN150 → Ball / Gate≥ DN150 → Butterfly

Modifiers — flammable → fire-safe ball; slurry → knife-gate; >200°C → metal seat; sour → NACE.

Quick selector (try it)

Function

Size

Fluid

Temperature

Step 3 — Service selection table

Service	Size	Valve & class	Body / trim / seat
Cooling water	DN150+	Butterfly wafer, 150 RF	DI / CF8M disc / EPDM; gear ≥ DN200
Steam	≤ DN80	Gate OS&Y, 300 RF	A105 / 13Cr trim, bolted bonnet
LPG / gas	All	Ball fire-safe, 300 RF	WCB / 316 / RPTFE · API 607
Instrument air	DN15–25	Ball full-bore, 800	A105 / 316 / RPTFE
Flow control	Per Cv	Globe control + positioner	Characterized / anti-cav trim
Slurry	All	Knife-gate / full-bore ball	Hardened / replaceable seat
Pump discharge	All	Dual-plate / nozzle check	Spring, non-slam
Firewater	DN100+	Gate/butterfly, 150/300 · UL/FM	DI / CF8M / EPDM · position indicator
Flare / relief	All	Full-bore ball/gate · API 607	WCB / 316 full-bore (no pockets)
Chemical dosing	DN15–50	Diaphragm / lined ball, 150	PFA/PTFE lined
Tank drain	DN50+	Knife-gate / full-bore ball	Hardened / CF8M
Cryogenic / LNG	All	Extended-bonnet ball/globe, 150–300	316L · BS 6364 cryo-tested

Reference values you'll reuse

Parameter	Typical limit
Liquid velocity	~1–2 / ~2–4 m/s
Gas velocity	~15–25 m/s
Soft-seat limit	~ −20 to +200 °C
Metal seat	High temp / abrasive
Lever up to	~ DN150–200
Gear from	~ DN200+

Step 4 — Worked calculations

Formulas used below:

Velocity v = Q / A , A = (π/4) · d² Control valve coefficient Cv = Q · √(SG / ΔP) (Q in US gpm, ΔP in psi) Actuator torque T_act ≈ k · ΔP · d³ (rises with the cube of size)

Quick unit conversions

From	To	Multiply by
bar	psi	14.5
m³/h	US gpm	4.40
mm	inch	0.0394
m³/h → m³/s	—	÷ 3600
°C → K	—	+ 273.15

Example A — 8″ cooling water

Given Service = cooling water (clean), isolation Size = DN200 (ID ≈ 0.20 m) , Q = 200 m³/h Design = 10 barg / 45 °C Step 1 — Area A = (π/4) × 0.20² A = 0.0314 m² Step 2 — Velocity v = (200 / 3600) / 0.0314 v = 1.77 m/s (< 3 m/s ✓ OK) Step 3 — Type & class DN200 ≥ DN150 → butterfly 10 barg @ 45 °C → Class 150 ample water @ 45 °C → EPDM seat ; DN200 → gear Result Butterfly wafer, Cl150, DI body, CF8M disc, EPDM seat, gear

Example B — 2″ steam

Given Service = saturated steam, isolation Size = DN50 (2″) , Design = 16 barg / 204 °C Step 1 — Seat 204 °C > 200 °C soft-seat limit → metal seat (13Cr trim) Step 2 — Type small bore + on/off + hot → gate (OS&Y) Step 3 — Class 16 barg @ 204 °C: Class 150 derates too far → select Class 300 Result Gate OS&Y, Cl300 RF, A105 body, 13Cr trim, handwheel

Example C — 12″ gas line

Given Fuel gas, DN300 (ID ≈ 0.30 m) Actual flow Q = 4500 m³/h at operating P Step 1 — Area A = (π/4) × 0.30² = 0.0707 m² Step 2 — Velocity v = (4500/3600) / 0.0707 v = 17.7 m/s (< 25 m/s ✓ OK) Decision DN300 isolation, gas → fire-safe ball or triple-offset butterfly tight shutoff → ISO 5208 Rate A, API 607

Example D — control valve Cv

Given Liquid control, Q = 100 gpm, SG = 1.0 Allowable ΔP across valve = 25 psi Step 1 — Required Cv Cv = Q × √(SG / ΔP) Cv = 100 × √(1.0 / 25) Cv = 100 × 0.20 = 20 Step 2 — Select rated Cv size so normal flow ≈ 80% travel rated Cv ≈ 20 / 0.8 = 25 Check verify against choked flow / cavitation (F_L) per IEC 60534

Step 5 — Acceptance / test criteria

Test (API 598)	Pressure	Allowable leakage
Shell	1.5 × rating	Zero
Seat — soft	1.1 × rating	Zero (bubble-tight)
Seat — metal	1.1 × rating	Small, scales with size

Failure modes & symptoms

Symptom	Likely cause
Passing when shut	Throttling damage / debris / worn seat.
Noise & vibration	Cavitation / disc flutter.
Stem leak	Worn / wrong packing.
Hard to operate	Under-sized operator / galling.

Installation & orientation

Respect the flow-direction arrow (check/globe).
Leave clearance for actuator, handwheel, disc swing.
Swing check: horizontal / vertical-up only.
Keep ≥ 5 pipe-diameters from pump/elbow turbulence.
Support heavy valves independently.

Maintenance & spares

Exercise critical valves; check stem packing.
Spares: gaskets, packing, one soft seat per type.
2-year: seat & actuator repair kits.
Rank criticality (A/B/C) for spares/PM focus.

Frequently asked questions

What should I know about Choosing & Specifying a Valve?

Isolation < DN150 → ball or gate. Isolation ≥ DN150 → butterfly. Throttling/control → globe (or characterized ball). Backflow → check (spring/nozzle on pump discharge). Slurry → knife-gate/full-bore ball. Flammable/ESD → fire-safe ball (API 607).

Which standards or codes apply?

high ΔP with flashing/cavitation, sonic gas velocity, cryogenic/LNG, erosive slurry, lethal/toxic service, or rapid cycling. These need anti-cavitation/multistage trim, special materials and detailed analysis — not a catalogue valve.

Severe service — escalate when: High-ΔP flashing, sonic gas, cryogenic, erosive slurry, lethal service or rapid cycling → specialist trim & analysis.

Face-to-face & weight (for layout)

Use ASME B16.10 face-to-face + vendor weight for layout and supports.

Sample datasheet (filled in)

Datasheet field	Value
Tag number	CWS-BV-201
Service	Cooling water supply — isolation
Valve type	Butterfly, concentric, wafer
Size / rating	DN200 (8") / Class 150
End connection	Wafer, B16.5 Class 150 RF flanges
Body / disc / stem	DI A536 / CF8M / 316 SS
Seat	EPDM (bubble-tight, bi-directional)
Design P / T	10 barg / 45 °C
Operator	Gear, handwheel, position indicator
Shutoff / test	Bubble-tight / API 598
Standard	API 609 Category A

Step 6 — Requisition checklist

Type, size, class, end connection.
Body/trim/seat + packing material.
Operator + fail action.
Test class, fire-safe, NACE.
Valve standard + 3.1 certificates.

Cost & lead-time note — Cost: butterfly < ball < gate < globe < control valve. Alloy/actuated = long lead time.

Field pitfalls

Gate cracked open to throttle → wire-drawn seat.
EPDM on hot oil → swells/fails.
Lever on DN250 → needs gear.
No fire-safe on ESD valve → rejected.

Standards — ASME B16.34; API 600/602/609/6D/594; API 598; API 607; ISO 5208/15848.

Glossary

RF / RTJ / FFRaised Face / Ring-Type Joint / Flat Face flange faces.

OS&YRising-stem gate; stem shows open/shut.

WCB / A105Carbon-steel cast / forged body materials.

CF8M / 13CrCast 316 SS / 13% Cr trim.

RPTFE / EPDMSoft seat materials.

FC / FOFail-Closed / Fail-Open on signal loss.

CvFlow coefficient (gpm at 1 psi).

ESDEmergency Shut-Down valve.