check-valve

name	check-valve
description	Check valve design and selection — swing/lift/ball/tilting disk/dual plate types, cracking pressure, Cv flow coefficient, water hammer prevention, API 594/API 6D, backflow prevention.
metadata	{"priority":7,"promptSignals":{"phrases":["check valve","non-return valve","backflow prevention valve","swing check valve","tilting disk check","water hammer check valve"],"minScore":3}}

Check Valve Design and Selection — Complete Skill

Check Valve Types

Swing Check Valve

Operation: hinged disc pivots from seat; opens with forward flow; gravity/reverse flow closes Cracking pressure: low (0.02–0.05 bar); good for low-ΔP applications Flow characteristic: full-bore when fully open; Cv per pipe ID roughly same as gate valve Closure behavior: disc swings back; potential for slam on pump shutdown → water hammer Applications: low-pressure liquid; horizontal and vertical-up installation Limitations: poor performance in vertical down; slam risk; not suitable for pulsating flow

Lift Check Valve (Piston/Ball Check)

Operation: piston, disc, or ball lifted by flow; gravity reseats on flow reversal Cracking pressure: moderate (0.03–0.1 bar) Sealing: tighter than swing (guided closure); better for gas Installation: horizontal only (piston lift needs gravity to close); ball type: any orientation Applications: high-pressure gas/steam; reciprocating compressor discharge Limitations: higher ΔP than swing at same Cv; not suitable for solids in flow

Tilting Disk Check (Dual Plate Wafer)

Operation: two spring-loaded half-discs; close quickly on flow reversal Cracking pressure: spring assists closure; can set cracking pressure Low water hammer: fast closure minimizes reverse flow momentum Wafer design: sandwiched between flanges; compact; low weight Applications: large pipelines; pump discharge; HVAC systems; high-frequency cycling CV: typically higher Cv per size than swing; lower ΔP at same flow

Ball Check Valve

Operation: ball seats against conical seat; flow lifts ball; gravity/spring closes Applications: sewage, wastewater, slurries (no disc edge to catch solids) Orientation: vertical up preferred; works horizontal (side ports for horizontal)

Flow Coefficient Cv

Definition: Q = Cv × √(ΔP/SG) [US: Q in GPM; ΔP in psi; SG = 1 for water] Or (metric): Q = Kv × √(ΔP_bar) [m³/hr; Kv = Cv / 1.156]

Check valve fully-open Cv (approximate by nominal pipe size):

NPS [in]	Swing Cv	Tilting disk Cv
2	75	110
4	400	600
6	900	1400
8	1700	2600
12	5000	7000

Sizing for minimum opening angle: Cv_required = Q / √(ΔP/SG); select valve with Cv > Cv_required; check opening angle at minimum flow

Minimum flow for stability: Valve must open ≥ 60–70° for stable operation (disc must not hover/chatter) If Q_min < Q_for_stable_opening: valve chatters → stem/disc wear → noise → failure

Cracking Pressure

Cracking pressure: minimum differential pressure to open valve from closed position P_crack = F_closure / A_disc [Pa; F_closure = spring force + disc weight; A_disc = disc area]

For vertical upward flow (against gravity): P_crack ≈ ρ × g × h_disc / A_disc [varies by design; typically 0.02–0.15 bar]

For spring-loaded: P_crack = F_spring / A_disc [adjustable; useful for back-pressure setting]

Water Hammer Prevention

Water hammer pressure rise (Joukowsky): ΔP = ρ × a_wave × ΔV [Pa] a_wave = √[K/(ρ × (1 + K × D/(E_pipe × t)))] [wave speed in pipe; K = bulk modulus of fluid; D/t = pipe flexibility] For steel pipe, water: a_wave ≈ 1200–1400 m/s ΔV = velocity change at valve closure

Closure time for no pressure surge: t_close < 2L/a_wave [sec; L = pipe length; a_wave = wave speed; sharp closure = instantaneous ΔP] For t_close >> 2L/a_wave: surge reduced by wave reflections

Check valve water hammer severity: Swing check: long closure time → large reverse velocity at closure → high ΔP Tilting disk / dual plate: fast spring-assisted closure → minimal reverse flow → low ΔP

API 594 (Check Valves): Testing: hydrostatic shell test at 1.5× rating; seat leakage test at rated pressure Swing check: API 594 or API 6D (pipeline service) Tilting disk: API 594

Anti-Slam Features

Weighted disc: provides faster closure via gravity assist; reduces slam vs. unweighted Spring-assisted closure: spring force closes valve before full reverse flow develops Damper/dashpot: hydraulic damper decelerates disc closure → controlled closure → no slam External cushioning piston: hydraulic cylinder connected to disc shaft → adjustable closure rate

Backflow Prevention (Code Requirements)

Backflow prevention for potable water: Reduced pressure (RP) assembly: two independent check valves + relief valve; AWWA C511 Double check valve (DC): two independent check valves; for non-health hazard connections Requirements: ANSI/ASSE 1013 (RP) or 1015 (DC)

API 594 vs. API 6D

Aspect	API 594	API 6D
Application	Wafer/lug/flanged; plant piping	Pipeline; flanged end
Testing	1.5× pressure; seat leakage	Full API 6D testing protocol
Pigging	Not required	Pig-passable bore required

Output

Provide: check valve type (swing/lift/tilting disk/ball), nominal pipe size [in or DN], design Cv, flow rate [m³/hr or GPM] and ΔP at operating [bar], cracking pressure P_crack [bar], water hammer ΔP estimate [bar], closure time [s] vs. 2L/a_wave, anti-slam features (damper/spring), installation orientation requirement, backflow prevention standard (if potable water: ASSE 1013/1015), and applicable standard (API 594, API 6D).