Quick estimate: gas service
Required orifice area, orifice letter and valve size for a conventional valve with no back pressure.
Assumes: API 520 Part I critical gas flow, 10 % overpressure (minimum 3 psi), Kd = 0.9 × 0.975, ideal gas (Z = 1), no back pressure, no rupture disc upstream. Orifice areas and pressure limits are those of the Flammer 66 Series.
What the full safety valve calculator does
- Gas, steam, liquid and two-phase flow, including sub-critical gas flow, steam superheat and the Omega method for flashing mixtures.
- External fire case to API 521: heat input from the wetted area, then the vapour load and the orifice.
- Conventional, balanced-bellows and pilot-operated valves, with the back-pressure factors and the limits of each type checked for you.
- Automatic choice of Flammer series (9, 33 or 66), orifice, inlet and outlet size, flange class and model code.
- Reaction force, cold differential test pressure and seat-tightness test values on the datasheet.
- A live valve drawing with its main dimensions, and a two-page datasheet.
How safety valve sizing works
The valve must pass the worst-case relief flow while the vessel pressure stays within the accumulation the code allows. The relieving pressure is the set pressure plus the allowable overpressure plus atmospheric pressure. For a gas in critical flow, API 520 Part I gives:
Kb corrects for back pressure on a balanced-bellows valve and Kc is 0.9 when a rupture disc is installed upstream. The orifice chosen is the smallest standard letter whose area is at least A and whose pressure rating covers the set pressure. The guide Safety valve orifice letters D to T lists every letter with its area.
Choosing the valve type
| Valve type | Use when | Back pressure |
|---|---|---|
| Conventional | Discharge to atmosphere or to a header with low, constant back pressure | Built-up back pressure no higher than the allowable overpressure |
| Balanced bellows | Variable or higher back pressure, corrosive or fouling discharge systems | Up to about 50 % of set pressure, with a capacity correction |
| Pilot operated | Operating pressure close to set pressure, high back pressure, large sizes | Largely unaffected |
Common questions
Which standards does the safety valve calculator follow?
API 520 Part I for sizing, API 521 for the external fire case, ASME Section VIII Division 1 and, for boiler steam valves, IBR Regulation 293.
What do the orifice letters D to T mean?
They are the standard orifice sizes of API 526 flanged safety valves, from D (the smallest) to T (the largest). Each letter has a fixed flow area and a matching inlet and outlet size. See the guide on orifice letters for the full table.
Why is the capacity multiplied by 0.9?
ASME requires the rated capacity of a safety valve to be 90 % of the capacity measured in certification tests. The tool applies this derating by default, and it is mandatory when ASME VIII or IBR is selected.
Can it size for liquids, steam, two-phase flow and fire?
Yes, in the full tool. The quick estimate on this page covers a gas with no back pressure only.