When sizing safety valves, many engineers take catalog capacities at face value – but the actual discharge coefficient (Kd) is often lower than the published number.The main safety valve product names of China Safety Valve Network include:,Air Compressor Safety Valve,A69Y Main Safety Valve,(W Series) Welding Safety Valve,Anti-sulfur Spring Type Safety Valve,ANSI Bellows Safety Valve,Air, Gas ValveAmmonia-used Safety Valve,A28X Air Compressor Safety Valve,AD Safety Valve Muffler,Bellows Spring Fall Lift Safety Valve,Built-in Safety Valve,Butterfly Spring Type Safety Valve,Copper Safety Valve,Duplex Safety Valve,Double Lever Safety Valve,Explosion Wave Proof Safety Valve

Catalog Kd is ideal

Catalog Kd values are measured at full lift, clean air, steady-state flow. For full-lift valves, Kd ranges 0.7–0.85 per ASME. But field media are rarely air, and conditions are seldom steady.

Factors that reduce Kd

• Different media: Steam typically has 5–10% lower Kd than air; liquid Kd for conventional valves is only 0.4–0.6.

• Two-phase flow: Requires correction per ISO 4126-10 or API 520 Part I – Kd may drop below 0.5.

• Inlet pressure loss: Exceeding 3% of set pressure prevents full lift, reducing Kd by 10–20%.

• Backpressure: For conventional valves, Kd drops linearly when backpressure exceeds 10% of set pressure.

Quick check

Do not use catalog capacity directly. Estimate actual capacity as: Actual ≈ Catalog × (actual Kd / catalog Kd) × density correction. If below required, go to a larger orifice. For steam and gas, use 0.85× catalog capacity as a conservative rule; for liquids, 0.7×.

Bottom line: Always add margin when sizing safety valves. Catalog Kd is lab data – field conditions degrade it. Check inlet loss, backpressure, and phase – then size with confidence.

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