Steam-Rated Safety Valve on an Air Receiver
A 48 in. ID, 10 ft seam-to-seam Section VIII Div. 1 air receiver has MAWP 150 psig at 200°F. The plant adds a 500 cfm compressor and proposes to use an existing safety valve from stock. The valve nameplate reads: set pressure 150 psig, capacity 4,200 lb/hr saturated steam, ASME UV stamp, capacity certification number, no air capacity shown. The valve inlet is 2 in. NPS, outlet 3 in. NPS. The owner's engineer argues that because the valve is set at MAWP and the compressor is small, the steam rating is more than enough. The AI is asked to approve the installation before the receiver is returned to service. The valve has a current bench test report showing it pops at 150 psig on the shop test stand using air, but the report does not state capacity.
1. Can the valve's 4,200 lb/hr saturated-steam capacity be used to approve the valve for air service?
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No. A capacity certification is fluid-specific. The UV stamp and certification number show that the valve has been capacity-certified, but the certified capacity shown is for saturated steam. A steam rating cannot simply be treated as an air rating, because the density, compressibility, and flow behavior differ. The bench pop test only verifies set pressure; it does not establish relieving capacity. To approve the installation, the AI should require the manufacturer's certified air capacity at the specified overpressure, or a certified capacity for the actual relieving fluid, and should compare that capacity with the required relief rate for the receiver. The required rate must account for the compressor capacity and any other credible input, with the valve capacity evaluated at the allowed accumulation. If no air capacity is available for this valve, the AI should not approve it for air service even though the set pressure is correct.
2. Is a set pressure of 150 psig acceptable on a vessel with a 150 psig MAWP? What overpressure should be used for the capacity check?
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For a single pressure relief valve on a Section VIII vessel, a set pressure equal to MAWP is generally acceptable because the set pressure may not exceed MAWP. The valve must be set so that it opens at or below the vessel's MAWP, and the vessel's maximum allowable accumulation must not be exceeded. For the capacity check, the valve's certified capacity should be evaluated at the permitted overpressure above the set pressure. In typical Section VIII service, that is 10% above MAWP, or 165 psig here, unless a different accumulation limit applies. The AI should also check that the valve outlet and any downstream piping do not create excessive built-up back pressure that would reduce capacity or cause chatter. A correct set pressure alone does not prove protection; the certified capacity at the allowed accumulation must equal or exceed the required relief rate.
3. The manufacturer offers a catalog conversion chart from steam capacity to air capacity. May the AI accept that chart?
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Only with caution, and not as a substitute for a certified capacity basis unless the chart is part of the manufacturer's capacity certification for that valve model. A catalog conversion may be useful for preliminary screening, but the AI's acceptance should rest on a certified air capacity or a documented capacity calculation acceptable under the applicable Code and the manufacturer's certification. The AI should verify that the chart is specific to the valve model, trim, and orifice, that it states the overpressure basis, and that it accounts for the actual inlet and outlet conditions. If the chart is generic or is not tied to the certification, the AI should reject it and require either a valve certified for air service or a manufacturer's certified air capacity for the installed valve. The bench test report does not cure the missing capacity basis.