PSV data sheet explained
Every line of the data sheet, who fills it in, and a blank copy in Excel and PDF.
PSV sizing and selection
We size every valve from your process data before we quote it. The relief cases follow API 521 and the required area follows API 520. The orifice letter comes from API 526.
The short answer
Sizing finds the smallest flow area that passes the required capacity within the allowable overpressure. We take the next larger standard orifice, check back pressure and inlet loss, and then select the valve type and materials.
The selection comes first, then the drawings, then the quotation.
Relief cases
API 521 lists the causes of overpressure to check for each piece of equipment. The case with the largest required capacity governs the size of the valve. Your process engineer sets the cases and the capacities. Common cases are:
- Blocked outlet
- External fire
- Control valve failure
- Loss of cooling or reflux
- Heat exchanger tube rupture
- Thermal expansion of blocked-in liquid
- Utility failure, such as power, cooling water, or instrument air
Data sheet lines 13, 21, and 39 carry the case and the capacity. When the fire case needs a different area, we size both and list both.
Required area
API 520 Part I gives the sizing equations for each phase. Each equation takes its inputs from the data sheet and applies correction factors.
| Phase | Main inputs | Correction factors |
|---|---|---|
| Gas or vapor | Capacity, relieving pressure and temperature, molecular weight, Z, and k | Kd, Kb, Kc |
| Steam | Capacity, relieving pressure, and degree of superheat | Kd, Kb, Kc, KN, KSH |
| Liquid | Flow, specific gravity, set pressure, and back pressure | Kd, Kw, Kc, Kv |
| Two-phase | Flow and properties of both phases | Method agreed for the project, such as the omega method in API 520 |
- Kd
- Coefficient of discharge. Preliminary sizing uses the effective values that API 520 gives for each phase. The final check uses the certified value of the selected valve.
- Kb
- Back pressure correction for gas and vapor.
- Kw
- Back pressure correction for liquid on balanced bellows valves.
- Kc
- Combination correction when a rupture disc is installed under the valve. API 520 gives the value to use when the pair has no certified value.
- Kv
- Viscosity correction for liquid.
- KN
- Correction for high-pressure steam, applied above the pressure that API 520 states.
- KSH
- Superheat correction for steam.
The relieving pressure in these equations is absolute: set pressure plus allowable overpressure plus atmospheric pressure.
Allowable overpressure
The pressure rise allowed above MAWP during relief is called accumulation. The applicable code sets its limit. The limit differs for one valve, for several valves that share a duty, and for an external fire case. Your design code and project specification give the values that apply. We size to the allowable overpressure stated on data sheet line 30.
With the set pressure at MAWP, accumulation and overpressure are the same figure.
Orifice selection
We compare the required area with the effective areas in API 526 and take the next letter at or above it. The letter fixes the standard inlet and outlet sizes for the pressure class. See the API 526 orifice table.
The final check uses the actual orifice area and the certified coefficient of discharge of the selected valve. Its rated capacity must be at least the required capacity.
When the required area is larger than the T orifice, two or more valves share the duty. Valves of our other series, outside the API 526 letters, are sized from their own areas.
Back pressure and valve type
Back pressure decides the valve type. Data sheet lines 27 to 29 give it. The back pressure limit of each type comes from the applicable code and the project specification.
| Back pressure | Valve type |
|---|---|
| Constant superimposed back pressure, and low built-up back pressure | Conventional spring-loaded, such as LFO |
| Variable superimposed back pressure, or higher built-up back pressure | Balanced bellows, such as LFB |
| High or varying back pressure | Pilot-operated, such as LFXD |
On a conventional valve with constant superimposed back pressure, the set pressure on the test bench is lowered by that back pressure. This bench value is the cold differential test pressure (CDTP).
Inlet pressure loss
API 520 Part II gives a guide for the non-recoverable pressure loss between the vessel and the valve inlet. A loss above the guide can make a spring-loaded valve open and close rapidly, which is called chatter.
The inlet piping is part of your layout. Data sheet line 17 gives us the line size for a first check. Where the layout cannot meet the guide, tell us, and the selection looks at the inlet piping and the valve type together.
Materials and options
Selection then fixes the series, the materials, and the options. Materials follow the line material, the medium, and the temperatures on data sheet lines 18, 22, 26, and 35. Options such as levers, test gags, and soft seats follow lines 52 to 57.
The limits of each product line are compared on the products page.
What happens after sizing
The order of work is the same for every inquiry:
- We send the selection for each tag: series, orifice letter, inlet and outlet size and rating, and materials.
- Once you confirm the selection, we send the valve drawings.
- The quotation lists each tag with the selected valve, materials, and price.
Projects often change the process data during design. Send each new revision of the data sheet, and we repeat the sizing for the tags that changed.
Send your data for sizing
A data sheet is best. Medium, set pressure, required capacity, back pressure, and relieving temperature are enough to start. The PSV data sheet page explains every line.
Questions engineers ask
What information is needed to size a relief valve?
Fluid and phase, required capacity, set pressure, allowable overpressure, back pressure, and relieving temperature. Gas sizing also needs molecular weight, Z, and k. Liquid sizing needs specific gravity and viscosity.
How is a fire case relief valve sized?
API 521 gives the method for the heat input from an external fire and the vapor flow that follows from it. The applicable code sets the accumulation allowed for a fire case. Your process engineer sets the fire case data, and we size the valve from it.
Why is the selected orifice larger than the calculated area?
API 526 orifices come in fixed sizes, D to T. We take the next letter at or above the calculated area, so the valve has some spare capacity. We do not go a size larger without a reason, because an oversized valve can chatter.
Can you size from the line size and the set pressure alone?
No. The line size says nothing about the flow the valve must pass. Without the required capacity and the medium, the orifice letter is a guess. Send the five starting items and we size the valve properly.
Related pages
API 526 orifice sizes
Effective areas of the orifice letters D to T in square inches and square centimeters.
Pressure relief valve terminology
Overpressure, accumulation, blowdown, and the three kinds of back pressure.