Cascaded Relief Valve Sizing: Designing Failsafe Overpressure Protection on Multi-Stage Skids

The Perils of Multi-Stage Pressure Reduction

When reducing bulk LPG from 1750 kPa to an appliance burner pressure of 3.5 kPa, the pressure drop must be cascaded across multiple regulator stages. However, an insidious failure mode exists: 'wide-open failure'. If the high-pressure first-stage regulator debris or diaphragm ruptures, full tank pressure slams into the intermediate piping. If the subsequent stages and relief systems aren't sized correctly, the low-pressure components will shatter, dumping explosive gas into the facility.

In a cascade system, the safety relief valve (SRV) on the first stage must either be sized to exhaust the *entire* capability of the failed regulator at its maximum inlet pressure, or the subsequent piping must be rated to absorb the shock.

API 520 and Flow Capacity

The first step in calculating SRV capability is identifying the 'worst-case scenario'. For a multi-stage skid, the worst case is usually a blocked discharge. The burner is off, and the primary regulator fails completely open.

• Inlet Conditions: What is the maximum possible pressure from the bulk tank? In summer, a black LPG tank can reach 1750+ kPa.
• Regulator Orifice: Using the wide-open Cv (flow coefficient) of the failed regulator, calculate the maximum kg/hr (Standard Cubic Feet per Hour) of gas that can pass through the orifice at 1750 kPag.
• Relief Valve Capacity: The SRV protecting the intermediate stage must have an exhaust capacity in kg/hr *greater* than the wide-open failure flow of the regulator it protects.

Hydrostatic Relief on Liquid Lines

While vapor pressure relief prevents regulator failure, hydrostatic relief valves (HRVs) prevent pipe explosions. Liquid LPG has an extraordinarily high coefficient of thermal expansion. If liquid is trapped between two closed valves (e.g., between the bulk tank's excess flow valve and the vaporiser's inlet solenoid) and the sun heats the pipe, the liquid will expand. Because liquid is incompressible, massive hydrostatic pressure builds instantly. Without a tiny HRV set at 2400-3100 kPa to burp a few drop of liquid, the solid steel pipe will literally tear itself apart.

Never pipe an SRV discharge directly back down into a contained area. For hazardous indoor installations, SRV exhaust ports must be manifolded (using piping significantly larger than the SRV outlet to prevent backpressure) and piped to a safe, static-free atmospheric vent outdoors.