The relief load for a pool fire has nothing to do with how much water is in the vessel. It is a heat balance: the fire delivers a certain Btu/h through the wetted shell, the water boils at the relieving pressure, and the valve has to pass the steam that heat generates. Inventory only sets how long the valve blows, not how big it is.
The spread comes entirely from one input: whether the site has adequate drainage and firefighting. Get that decision documented before anyone orders a valve.
Is this a pressure vessel or a boiler? Read the nameplate first
The code stamp on the vessel decides which rulebook governs, and the two rulebooks give different valves and different sizing bases. Settle this before touching a formula.
| What the nameplate shows | Governing basis | Effect on sizing |
|---|---|---|
| ASME Section I "S" stamp (fired steam generator) | Section I safety valve rules | Section I "V"-stamped valves only, capacity based on boiler heat input, no fire-case accumulation credit |
| No code stamp, atmospheric or low-pressure storage tank | Tank venting rules such as API 2000, not API 521 vessel fire case | Emergency venting for fire exposure is a different equation set; an open vent may already be the relief path |
An 80 psig set pressure on something called a "water storage vessel" deserves a second look. At that set point the contents sit at roughly 336°F saturation when the valve lifts. If the vessel is really an atmospheric tank with a gauge hatch, the fire case is a venting problem, not a PRV problem, and the numbers below do not apply. If it carries a U stamp and an 80 psig MAWP, proceed.
Specify by orifice letter, set pressure, and code stamp, not by the noun.
Three routes to the load, and which one applies to water
The CGA S-1.2 approach, familiar from ammonia and other liquefied-gas storage, is the wrong tool here. Its posted Gi and Gu values are tabulated for specific gases and vessel types under the CGA scope; they are not a general fire-case method for a steam-generating water vessel.
| Method | What it needs | Where it fits |
|---|---|---|
| CGA S-1.2 Gi/Gu tables | Listed fluid, listed container type | Compressed and liquefied gas containers in CGA scope; no coverage for water-to-steam fire case |
| API 521 heat input + API 520 Part I orifice | Wetted area, drainage/firefighting status, insulation conductance, latent heat and relieving pressure | Any Section VIII vessel with liquid inventory exposed to external pool fire |
| PRV vendor sizing program | Same inputs, plus valve model and backpressure | Confirmation and stamped-capacity selection, after you own the inputs |
Use API 521 to compute the load and API 520 Part I to convert it to orifice area. Then run the same inputs through a vendor program as an independent check — it applies the manufacturer's certified coefficient of discharge and will pick the actual model, but it will not question a wetted area you typed in wrong.
How much heat does the fire put in?
API 521 expresses fire heat absorption as a power law on wetted area, in the form Q = C × F × Aws0.82 with Q in Btu/h and Aws in ft². Two coefficients apply: the lower one when prompt firefighting and adequate drainage keep a pool from persisting under the vessel, the higher one when they do not. Confirm both coefficients and the environment factor F against the edition of API 521 your project cites, then work the numbers:
- Confirm the wetted area. API 521 counts only surface wetted by internal liquid and only up to a fire-exposure elevation above grade — commonly 25 ft. A total shell area of 400 ft² is not automatically 400 ft² of wetted area. Check the normal liquid level and the vessel elevation before accepting the figure.
- Set the environment factor F. Bare vessel, F = 1.0. Insulation credit requires a fireproof system that stays in place during the fire and resists dislodgement by a hose stream; the reduced F comes from the insulation conductance table, not from the jacket's normal service rating.
- Compute Aws0.82. For 400 ft², that is 136.
- Compute Q. Adequate drainage and firefighting: 21,000 × 1.0 × 136 ≈ 2.86 × 10⁶ Btu/h. Without them: 34,500 × 1.0 × 136 ≈ 4.70 × 10⁶ Btu/h.
- Get the relieving conditions. Single valve, fire contingency, 21% accumulation: P1 = 1.21 × 80 + 14.7 = 111.5 psia, saturation near 336°F, latent heat approximately 882 Btu/lb from steam tables at that pressure.
- Divide. W = Q / hfg.
| Case | Q (Btu/h) | W (lb/h steam) |
|---|---|---|
| Adequate drainage + firefighting | 2.86 × 10⁶ | 3,240 |
| No credit taken | 4.70 × 10⁶ | 5,330 |
Water's latent heat is high compared with hydrocarbons, so the mass flow looks modest. Do not let that seduce you into a small valve — steam at 111 psia is not dense, and the orifice check is what decides.
From Btu/h to orifice area
API 520 Part I sizes steam relief with A = W / (51.5 × Kd × P1 × Kb × Kc × KN × KSH), A in in², W in lb/h, P1 in psia. For this service: Kd = 0.975 (preliminary, vapor, no rupture disc), KSH = 1.0 because the steam is saturated, KN = 1.0 well below the Napier correction threshold, Kc = 1.0 with no disc, Kb = 1.0 for a conventional valve discharging with negligible built-up backpressure.
| Case | Required area (in²) | API 526 orifice |
|---|---|---|
| 3,240 lb/h | 0.58 | H (0.785 in²) |
| 5,330 lb/h | 0.95 | J (1.287 in²) |
The step from H to J is one full drainage argument. If nobody can produce a drainage layout and a fire-water plan for the area under the vessel, size for the higher coefficient and stop arguing.
What moves the answer by a factor of two?
| Input | Where it lives | Effect if wrong |
|---|---|---|
| Wetted vs. total area | Vessel GA drawing, normal level, grade elevation | Direct on Q at the 0.82 power; the single most common oversize/undersize source |
| Drainage / firefighting credit | Plot plan, fire study | 64% swing in Q, one orifice letter |
| Insulation factor F | API 521 conductance table, insulation spec | Credit taken on insulation that burns off or blows off in a hose stream invalidates the whole calculation |
| Liquid-full, blocked-in vessel | P&ID isolation valves | Thermal expansion relief is a separate, earlier contingency; a small thermal valve does not cover the fire case |
| Inlet line pressure drop | Isolation valve, nozzle, inlet piping | Above 3% of set pressure at rated flow the valve chatters and loses capacity regardless of orifice |
| Built-up backpressure | Tailpipe and header | Above 10% of set on a conventional valve, capacity collapses; move to a balanced bellows and apply Kb |
Two more traps. Do not take credit for the water inventory as a heat sink — the fire case is a steady-state heat balance, not a transient. And a vessel with a vapor space relieves vapor once boiling starts; the two-phase methods in API 520 apply to reactive or foamy systems, not clean water in a storage vessel.
How do you prove the valve you bought covers it?
- Pull the manufacturer's certified capacity for the selected model at 80 psig set, saturated steam, and compare it to the required 3,240 or 5,330 lb/h. Certified capacity, not catalog nominal.
- Confirm the valve carries the correct code stamp for the vessel — ASME Section VIII "UV" for a U-stamped vessel, Section I "V" for a fired boiler.
- Recompute inlet non-recoverable loss at the valve's rated flow, not the required flow, and confirm it is under 3% of set pressure.
- Recompute built-up backpressure in the discharge line at rated flow; under 10% of set for a conventional valve, otherwise switch to a bellows design and re-run the sizing with the correct Kb.
- Check the discharge reaction force and the tailpipe support, then confirm the discharge is routed to a safe location for a 336°F steam plume.
- File the sizing sheet with the wetted area figure, the F factor, and the drainage decision written on it, and stamp the required capacity onto the relief device datasheet so the next revalidation starts from the same inputs.
Can I use CGA S-1.2 Gi and Gu values for a water vessel fire case?
No. The CGA tables cover listed gases and container types within the CGA scope. For a water vessel boiling to steam under external fire, use the API 521 wetted-area heat input equation and convert to mass flow with the latent heat at relieving pressure.
Does the amount of water in the vessel change the required relief capacity?
No. Fire relief is a steady-state heat balance — heat in through the wetted shell divided by latent heat of vaporization. Inventory only affects how long the valve discharges before the vessel runs dry, and no capacity credit is given for the heat sink.
Can I take insulation credit to reduce the fire heat load?
Yes, through the API 521 environment factor F, but only for a fireproof system that stays in place during the fire and resists dislodgement by a fire-hose stream. Take F from the insulation conductance table, and re-verify that the banding and jacketing meet those conditions.
Does an 80 psig set pressure mean 80 psig relieving pressure for sizing?
No. For the fire contingency ASME Section VIII permits 21% accumulation on a single valve, so the relieving pressure used in the API 520 steam equation is 1.21 × 80 + 14.7 = 111.5 psia, with saturation near 336°F.
Can I let a PRV vendor's sizing program do the whole job?
Use it to select the model and apply the certified discharge coefficient, but own the inputs yourself. The program will accept a wrong wetted area, a wrong drainage credit, or an unearned insulation factor without complaint, and those three inputs set the orifice letter.