Engineering context

FluidFlow resolves relieving flow and pressure drop as part of the steady-state network at the relieving condition. It supports calculations to API RP 520 Part 1 and ISO 4126-1. For rupture discs, FluidFlow uses the Resistance-to-Flow Method (Kr).

Engineering workflow (hydraulic)

1. Determine the relief device locations by placing a relief valve on all static equipment stamped per ASME Section VIII, both sides of the heat exchanger, all positive displacement pumps and compressors, and any liquid-filled system that can be blocked in.
2. Identify the relief scenarios by drawing a system boundary around the equipment within block valves and the fire circle. Evaluate the system to determine if it can be overpressured by connected systems or by internal causes. Simultaneous independent initiating events (double jeopardy) are not normally combined as a single relief case unless connected by process, mechanical, or electrical cause. The governing scenario, the one requiring the largest orifice area, sets the design basis for the PSV.
3. Determine the required relief rate by applying a mass/volume balance. Make the relief flow large enough that pressure does not exceed the allowable relieving pressure. All physical properties, such as temperature, density, latent heat, compressibility, etc., are evaluated at the relieving pressure (set pressure + allowable accumulation).
4. Determine the required orifice area by applying the appropriate sizing equation (API RP 520 Part 1 or ISO 4126-1). Select the next standard API orifice size from API 526 (D, E, F, G, H, J, K, L, M, N, P, Q, R, T) or from the valve manufacturer’s catalogue that equals or exceeds the calculated area. Consider the allowable accumulation based on the fire case, single valve (non-fire) case, and multiple valve case.
5. Determine the actual (rated) relief flow based on the selected valve. The selected valve will generally be larger than the calculated area. This rated flow (not the required flow) is the basis for inlet and outlet piping hydraulics in new installations.
6. Size the inlet piping based on the 3% criterion. The frictional pressure loss in the inlet piping must not exceed 3% of the PSV set pressure (gauge). If the 3% criterion cannot be met: increase the inlet line size, reduce the inlet pipe length and fittings, or switch to a pilot-operated PSV with remote sensing.
7. Size the outlet piping and collection system based on the backpressure criterion. The maximum allowable backpressure must meet the criteria per valve type (conventional, balanced bellows, and pilot-operated).
8. Steps 4 through 7 are iterative. Backpressure assumed in Step 4 must be confirmed by the outlet calculation in Step 7. If the confirmed back pressure exceeds the assumed value, re-size the orifice area with the corrected back pressure correction factor (Kb) and repeat until convergence.

How FluidFlow helps

FluidFlow models the relieving condition within the same steady-state network as the rest of the system. With the relief device, its inlet and outlet piping, and the fluid properties in one model, FluidFlow performs the initial PSV orifice sizing (Step 4) and calculates the inlet frictional pressure drop and outlet back-pressure (Steps 6 and 7) — giving the qualified engineer consistent hydraulic inputs to support the final sizing confirmation with the valve manufacturer and the compliance check against ASME BPVC Section I and VIII.

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