Selecting Set Pressures for Multiple PSV Installations

Two pressure safety valves may share the same set pressure when the governing rules allow it; staggering is not automatically mandatory. For a multiple-device, nonfire case, the cited limit places the first device at no more than the maximum allowable working pressure (MAWP), each additional device at no more than 105% of MAWP, and accumulated pressure at no more than 116% of MAWP. Select the actual set pressures from the credible relief-load range and valve stability analysis: use equal settings only when simultaneous operation remains stable, and stagger them when lower-load cases need one valve while the governing case needs both.

Pressure and Capacity Limits

The number that matters is the required relieving rate at each credible pressure-producing scenario, not only the largest rate. The installation described has a governing overfilling load of 187914 kg/hr and uses two identical PSVs in service together. Both are currently set at the protected equipment design pressure, but design pressure and MAWP must be checked as separate entries on the vessel documentation before using either in a set-pressure calculation.

Quantity	Cited nonfire limit	Where to read or confirm it
First-device set pressure	Not above 100% of MAWP	Protected-equipment records, relief-device datasheet, and governing code edition
Additional-device set pressure	Not above 105% of MAWP	Each valve datasheet, certificate, and set-pressure record
Accumulated pressure with multiple devices	Not above 116% of MAWP	Relief calculation for the operating, nonfire contingency
Governing relieving load in this installation	187914 kg/hr from overfilling	Scenario-based relief-load calculation

API 520 Part I, section 5.4.2.2.2, is cited for these multiple-device limits and refers to ASME Section VIII requirements. ASME Section VIII, UG-134, is cited as allowing additional devices to be set higher, but not above 105% of MAWP. The mathematical condition for an additional device is therefore:

Pset,additional <= 1.05 × MAWP

An additional valve set at 1.00 × MAWP satisfies that upper bound. The wording permits a higher setting within the limit; it does not, by itself, require the higher setting. Applicable law, the adopted code edition, owner specifications, and equipment certification still control the final selection.

Symptom-to-Cause Reading

Chatter is rapid opening and closing, not merely two valves lifting near the same pressure. It occurs when flow, pressure, valve dynamics, inlet loss, outlet backpressure, and blowdown interact so that a valve cannot sustain a stable open position. Equal set pressures can increase exposure to that operating region, but equal settings alone do not prove the cause.

Observed behavior	Likely mechanism	Deciding check
Both valves lift during a small-load scenario	Available relief area is large relative to the instantaneous load	Compare each scenario load with the certified capacity of one valve at its applicable relieving conditions
Pressure falls immediately after both valves open, followed by repeated cycling	Combined discharge reduces vessel pressure below the valves' stable open or reseating region	Review the pressure trace with set pressure, overpressure, and blowdown data
Only one valve cycles	Valve-specific inlet loss, backpressure, mechanical condition, or setting difference	Inspect the two flow paths and compare test certificates and maintenance records
Valves operate smoothly only at the largest load	Both valves are adequately loaded at high flow but excessive area is exposed at lower flow	Map all credible loads against one-valve and two-valve capacity
Both valves lift together without instability	Simultaneous operation is dynamically stable for that event	Confirm accumulated pressure and total credited capacity in the relief calculation

A broad range of credible relieving loads strengthens the case for staging. A narrow range clustered near the two-valve requirement may support equal settings, provided hydraulic and mechanical checks show stable operation.

Flow and Thermal Mechanism

This is flow and force balance, not logic. Vessel pressure produces opening force on each valve. Once a valve lifts, discharge changes vessel pressure, inlet pressure loss, and outlet backpressure. Those changes alter the net force holding the disc open. If two equal-set valves begin passing flow together during a scenario that needs less than their combined capacity, the resulting pressure reduction can remove the opening force from one or both valves.

The disc then moves toward the seat. Flow falls, vessel pressure recovers, and the valve opens again. Repetition creates chatter, with impact loading on the seating surfaces and moving parts. Staggering creates a pressure interval in which the first valve carries the lower load alone. The second valve joins only when pressure continues rising because the first valve cannot pass the developing load.

Thermal behavior also belongs in the scenario calculation. Fluid state and temperature affect density, mass flow, flashing or vapor generation, and certified valve capacity. A capacity stated for one set of relieving conditions cannot be transferred to another scenario by comparing mass-flow numbers alone. Read the required properties and correction inputs from the approved relief calculation and valve sizing record.

Set-Pressure Decision Path

1. Confirm the pressure basis. Read MAWP from the protected-equipment documentation. Check whether the current design-pressure value used for both valves is numerically equal to MAWP; never substitute one label for the other without that check.
2. List every credible scenario. Record the required relieving rate, relieving phase, temperature, pressure, and whether the scenario is fire or nonfire. The cited 116% accumulation and 105% additional-device setting limits apply to the stated multiple-device, operating nonfire case.
3. Establish the one-valve envelope. Determine which scenarios one PSV can handle using certified capacity at the applicable relieving conditions. Include the actual inlet and discharge configuration in the hydraulic evaluation.
4. Establish the two-valve envelope. Confirm that both valves together pass the governing requirement of 187914 kg/hr for the overfilling case. Account for any common piping interaction and backpressure rather than adding nameplate capacities blindly.
5. Select the first set pressure. Keep it at or below MAWP. Choose a setting compatible with normal operating pressure and the valve's certified operating range using the approved datasheet.
6. Select the additional setting. Keep it at or below 1.05 × MAWP. Place it above the first setting when lower-load scenarios can be handled by one valve and staged opening reduces unstable excess capacity.
7. Recalculate accumulation. Demonstrate that the protected pressure remains within the applicable limit when the governing scenario activates the credited devices. For the cited nonfire multiple-device case, the stated ceiling is 1.16 × MAWP.
8. Document the basis. Record which scenarios use one valve, which require both, the selected settings, certified capacities, hydraulic assumptions, and governing code edition.

If the full credible-load list contains cases far below 187914 kg/hr, staging normally provides a cleaner capacity match. If every credible event requires nearly both valves' capacity, equal settings may be workable, subject to the stability and accumulation checks.

Staggered and Equal Settings

Arrangement	Use condition	Main engineering concern
Equal set pressures	Both devices are intended to respond together and remain stable across the credible load range	Small events may expose more relief area than needed; manufacturing and field tolerances can also make actual opening sequential
Staggered set pressures	One valve covers smaller events while both cover the governing event	The second setting and resulting accumulation must remain within the governing limits
Different valve capacities	Load distribution is intentionally matched with unequal areas or certified capacities	Each scenario must be assigned to the correct available capacity and configuration

Staggering addresses load matching; it is not a standalone cure for chatter. Excessive inlet pressure loss, rapidly changing backpressure, unsuitable blowdown, mechanical damage, or a poor installation geometry can destabilize a correctly staggered pair. Conversely, two equal-set valves can operate without chatter when flow demand and system dynamics keep them stably open.

Verification After Selection

1. Check each proposed set pressure against MAWP and the permitted additional-device ceiling.
2. Run every credible scenario through the sizing model with the valves available at that scenario's pressure. A valve set above the first valve cannot be credited as fully open before its activation conditions are reached.
3. Confirm the governing nonfire case remains at or below the cited 116% of MAWP accumulated-pressure limit.
4. Verify one-valve capacity for each scenario assigned to the first stage and two-valve capacity for the 187914 kg/hr overfilling requirement.
5. Review inlet pressure loss, built-up and superimposed backpressure, discharge-header interaction, and manufacturer blowdown data.
6. Compare installed nameplates, seals, certificates, and maintenance records with the approved settings before returning the valves to service.

A pressure trend from a controlled test or actual event should show the first valve lifting, stable pressure response while it carries the assigned load, and second-valve activation only when pressure continues toward its setting. Repeated pressure oscillation, audible hammering, or rapid disc motion means the dynamic problem remains unresolved.

Recurring Design Pitfalls

The first pitfall is treating maximum permitted set pressure as a required set pressure. The phrase “shall not exceed 105%” defines a ceiling. It allows an additional device at 100% when no separate requirement mandates staging.

The second is selecting settings from the governing load alone. A pair sized for 187914 kg/hr may spend other credible events far below that load. Those lower cases decide whether opening both valves introduces an unstable capacity mismatch.

The third is using design pressure as MAWP without checking the equipment record. The calculation must use the pressure basis named by the governing requirement. Another pitfall is mixing fire and nonfire limits; the cited values specifically describe multiple devices sized for operating, nonfire contingencies.

The fourth is assuming identical valves divide flow equally. Small differences in set pressure, inlet losses, outlet losses, spring condition, and installation geometry can make one valve open earlier or carry more flow.

Frequently Asked Questions