Adding a Second Relief Valve to a Fire-Case-Limited Separator

Set pressures (multiple-device rule):

• One device must be set at or below MAWP: the existing valve at 300 psig satisfies this.
• Additional devices may be set no higher than 105% of MAWP = 315 psig. So the second valve may be set anywhere at or below 315 psig; setting it at 300 psig (identical to the first) is also acceptable and simplifies sparing.

Maximum pressure in the fire case: with jurisdiction acceptance of the fire allowance, 121% of MAWP = 363 psig. That is the pressure at which the capacity must be evaluated and the pressure the vessel is permitted to experience in that scenario only.

Important distinction: the 315 psig set point on the second valve is above the vessel MAWP. That is allowed only because it is a supplementary device under the multiple-device rule. It does not raise the MAWP, it does not license 315 psig in any non-fire scenario, and the 116%/121% accumulations do not apply to ordinary process upsets, where 110% is the ceiling for a single device. Do not let the second valve's set pressure be mistaken for a new MAWP.

Not established by the numbers given - and the naive comparison (6,000 + 6,000 = 12,000 > 9,000) is the wrong method.

Capacity must be evaluated at the actual relieving pressure each device sees, not at its nameplate rating point.

• In the fire case the vessel relieves at 363 psig (121% of 300).
• Valve 1, set at 300 psig, is at 21% overpressure at that pressure.
• Valve 2, set at 315 psig, is at about 16% overpressure (363/315 ≈ 1.15).
• Both are above their certified 10%-overpressure rating, so both flow more than 6,000 lb/h - but capacity rises much less than linearly with overpressure, so neither is anywhere near 12,000 lb/h.

Required work: obtain the manufacturer's certified capacity curves and read each valve's capacity at 363 psig (i.e., at its own overpressure percentage). Sum those, compare to 9,000 lb/h with an appropriate margin, and document the calculation. If the sum does not cover the fire load, options are: a larger second valve, a pilot-operated or balanced-bellows valve with higher certified capacity, a rupture disk upstream of a valve, or reducing the fire-case load (drainage, insulation, fireproofing, isolation).

Also verify the discharge system and the inlet line losses at the combined flow - a two-valve installation doubles the flow into a header that may have been sized for one.

Correct answer structure: set pressures are compliant; capacity adequacy is unresolved pending certified data at 363 psig. Never certify a relief installation from nameplate ratings alone.

The test result itself is a pass, barely. ASME set pressure tolerance for pressures above 70 psig is ±3%: 300 ± 9 psig = 291 to 309 psig. A pop at 291 psig sits exactly on the lower limit. It qualifies, but it is not a comfortable result, and 'returned to service without further action' is the weak part of the record.

What should have happened / should happen now:

• Trend it. One reading at the limit is a data point; two in a row is a developing problem (seat damage, spring relaxation, galling, process fouling on the disc).
• Inspect the seat and disc on the next shop visit regardless of the pop result, and check for simmering damage from a valve that opens early.
• Confirm the valve actually saw the fire case last time and that the 6,000 lb/h rating is still the right basis.

Interval: PRD inspection and testing intervals are set per API 576 based on service history, with API 510 setting the outer bound; a common ceiling is 5 years for general service, extensible only with documented service experience or a risk-based basis accepted by the inspector and jurisdiction. Given (a) a pop at the tolerance limit, (b) an increased fire-case duty, and (c) no evidence the valve has been opened since, shorten rather than extend - 2 to 3 years, and no longer than 5 - and re-baseline after a clean bench test with a mid-tolerance result. Record the basis for the interval, not just the date.