PSV Relieving Pressure: How Do I Set It Below MAWP?

Start Here: The Undersized Result Is an Input Problem

Your fire-case sheet returns a required orifice several letter sizes above the installed valve. Nothing about the tank changed. The only line that moved is relieving pressure.

The numbers on this job: tank MAWP 430 psig, installed PSV set pressure 420 psig541.2 psia. If you carried relieving pressure at 535 psia, you sized the valve at 98.9% of the critical pressure. At that point the latent heat of vaporization has almost collapsed, and the fire-case rate W = Q / Hvap goes to infinity as Hvap goes to zero.

That is not a valve fault. The valve is not undersized in any absolute sense. You fed the sizing equation a relieving pressure sitting on top of the critical point, and the equation did exactly what it should. Fix the relieving pressure before you touch the orifice selection.

Check 1: Draw the Code Ceiling From MAWP, Not From Set Pressure

This is the check that resolves the 522.9 versus 535 psia argument, and it is why API 520 8th ed. Tables 2 through 6 show identical relieving pressures whether set pressure equals MAWP or sits below it.

Overpressure is referenced to set pressure. Accumulation is referenced to MAWP. Those two are the same number only when set pressure equals MAWP. The code limit is on the vessel, so accumulation governs. ASME Section VIII Div 1 gives the familiar ceilings: 10% for a single relief valve, 16% for multiple valves, 21% for fire exposure.

Outcome of this check: anything above 535.0 psia is outside code and you go back. Anything at or below it is legal. That still leaves you a window, not a single answer.

Check 2: Relieving Pressure Is a Design Value You Choose

Relieving pressure has a lower bound and an upper bound, and you pick a number inside them. The lower bound is the set pressure — below set the valve is not lifting and you have no capacity to credit. The upper bound is the maximum allowable accumulated pressure, 1.21 x MAWP for fire.

With set pressure at 350 psig, that window is 350 psig to 520.3 psig. Nothing in the code forces you to design to the top of the window. Applying 21% to set pressure is a convention, not a requirement; designing to a lower accumulated pressure is always permitted. When set pressure sits below MAWP, the reverse also holds — you may design to an accumulation that is a larger percentage of set pressure than 21%, as long as the vessel never exceeds 1.21 x MAWP.

The logic closes on itself and it is conservative. You pick P_r, compute the required rate at P_r, and size the valve to pass that rate at P_r. The installed valve then holds accumulation at or below the pressure you assumed. Do the calculation correctly and the real fire will not push the vessel past your design point.

Check 3: Is There Any Latent Heat Left at That Pressure?

Pull up a pressure-enthalpy diagram for the fluid and keep the picture in your head. The two-phase dome narrows as pressure rises and pinches shut at the critical point. Hvap is the width of that dome. It shrinks steeply above a reduced pressure of roughly 0.9 and ceases to exist at Pc.

Read Hvap from a property table or REFPROP at the saturation temperature corresponding to your chosen relieving pressure — not at operating conditions, and not at the normal boiling point.

• 535.0 psia — 6.2 psi below critical. Vanishing Hvap, exploding required rate, and vapor density, Z and k all degrading at the same time. This is the number that failed your existing valve.
• 522.9 psia — 18.3 psi below critical. Still inside the steep part of the curve. The difference against 535 psia is small, which is exactly what you observed.
• 438.2 psia — 103 psi below critical, 81% of Pc. Off the knee of the curve, with a real latent heat and a workable orifice.

Outcome: if your relieving pressure lands at or above 541.2 psia, jump to Check 5. If it lands within a few percent of critical, go to Check 4 and buy margin by lowering set pressure.

Check 4: How Far Can You Drop the Set Pressure?

Two hard bounds:

• Floor. Maximum operating pressure is about 200 psig, so the minimum defensible set pressure is roughly 220 psig (operating plus 10%). Set any lower and you will chatter and leak in normal service.
• Ceiling. Set pressure cannot exceed MAWP, 430 psig.

A set pressure of 350 psig sits well inside those bounds, leaves 150 psi of margin above maximum operating, and drops relieving pressure to 438.2 psia at full 21% accumulation. That is the move that lets a smaller valve — possibly the existing one — do the job, because the latent heat you divide by is now an order of magnitude larger.

Symptom Versus Cause

Check 5: If You Cannot Stay Subcritical, Stop Sizing for Fire

Above the critical pressure there is no phase change and no latent heat to absorb fire heat input. A PSV on a supercritical fluid in a pool fire provides no real protection regardless of orifice size — you are venting a dense single-phase fluid while the wall temperature keeps climbing. Install the device for code compliance if the vessel requires one, but do not book the fire case as covered by it.

When the fire case cannot be handled by a relief valve, the options are a depressurization system, a non-reclosing device such as a rupture disk, or thermal relief plus mitigation credits — fireproofing, drainage away from the vessel, water spray. API 521 is where you take those credits.

Ask the prior question too. If there is no combustible inventory and no credible pool fire duration, the honest deliverable is a documented non-credible scenario, not an engineered fix for an event that cannot happen. Run the calcs the client asked for, but say so in the report.

Procedure: Re-Rate the Existing Valve to 350 psig

1. Confirm maximum operating pressure. Verify from trend data that the tank never exceeds 315 psig — 90% of the new set pressure. If it does, 350 psig is not available and you go back to Check 4.
2. Fix the design relieving pressure at 438.2 psia (350 psig x 1.21 + 14.7). Record the basis on the datasheet.
3. Read the saturation temperature at 438.2 psia and take Hvap, vapor Z, k and molecular weight at that condition.
4. Compute fire heat input Q from the API 521 wetted-area equation. Decide and document whether you claim adequate drainage and firefighting, and the environment factor F — insulation credit changes the answer more than anything else in this step.
5. Compute required vapor rate W = Q / Hvap, then required orifice area with the API 520 Part I critical-flow vapor equation at the relieving conditions from step 3.
6. Check the two-phase case. A near-full refrigerant tank swells as it heats and can go liquid-full before the PSV relieves; vapor-only sizing then understates the area. Screen this with the DIERS methodology in API 520 Part I.
7. Check inlet pressure drop. Non-recoverable inlet loss at rated flow must stay within 3% of set pressure — that is 10.5 psi at 350 psig versus 12.6 psi at 420 psig. Lowering set makes this tighter, and existing inlet piping that passed before can fail now.
8. Check built-up backpressure. The 10% conventional-valve limit is now 35 psi, not 42 psi. If the tailpipe exceeds it, move to balanced bellows or pilot-operated trim.
9. Order the spring. Dropping 420 to 350 psig is a 16.7% reduction, outside the certified range of a typical spring. Send the valve to a VR-stamp shop for new spring and trim, re-set, capacity re-certification and a new nameplate.
10. Update the PSV datasheet, relief system documentation and P&ID under MOC. State the chosen relieving pressure and why it is below 1.21 x MAWP, so the next engineer does not "correct" it back to 535 psia.

Verify

• Bench pop test at the shop: set point within ASME tolerance of +/-3% for set pressures above 70 psig. Witness it or get the certified test report.
• Confirm the nameplate certified capacity, at the overpressure you designed to, exceeds the required W from step 5. Certified capacity already carries the 0.9 derating — do not apply it twice.
• Confirm the new nameplate set pressure is at or below MAWP and at or above your operating floor.
• Re-check that the sized valve holds accumulation at or below 438.2 psia, and that this stays under the code ceiling of 535.0 psia. Both conditions must be stated in the calculation package.
• After startup, trend tank pressure through a full seasonal cycle and confirm it stays below 315 psig. Simmer at the new lower set is the failure mode to watch for.

When to Escalate

Escalate to the valve manufacturer when the required spring change falls outside the published spring range for the body and trim, or when the certified capacity at your chosen overpressure is not published for that orifice designation — do not interpolate capacity tables yourself. Bring in the Authorized Inspector and the jurisdiction before you change a nameplate set pressure on a code-stamped vessel, and get the two-phase screening reviewed by a relief systems specialist if the tank can go liquid-full during the fire.

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