Calculate cold differential test pressure (CDTP) for a relief valve from its set pressure, back pressure and temperature correction, or solve in reverse.
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Cold Differential Test Pressure (CDTP) Formula
For an applicable conventional spring-loaded valve procedure, subtract constant superimposed back pressure before applying the manufacturer temperature multiplier K:
- CDTP is the ambient bench test pressure.
- SP is service set pressure; SBP is constant superimposed back pressure present before opening.
- K is the applicable dimensionless bench-pressure multiplier: 1 for no correction, 1.01 for a 1% increase.
Select the required result and enter the displayed gauge pressures and manufacturer multiplier. Pressure units convert while preserving the entered quantity; choose the desired result unit separately. Optional tolerance gives an arithmetic test band only. No particular standard tolerance is assumed.
A manufacturer-confirmed balanced-bellows procedure may use CDTP = SP * K without a generic back-pressure subtraction. Pilot-operated valves require their exact model and vent-arrangement procedure, so this calculator does not produce a generic pilot result. Built-up and variable back pressure are outside this bench-adjustment model. Confirm the valve manual and test procedure before use.
Pressure Unit Conversion Factors Used
These factors show how each pressure unit is converted to psi before the formula is applied.
| Unit | Multiply by to convert to psi | Example |
|---|---|---|
| psi | 1 | 100 psi = 100 psi |
| bar | 14.5037738 | 10 bar = 145.0377 psi |
| kPa | 0.1450377377 | 500 kPa = 72.5189 psi |
| atm | 14.6959488 | 2 atm = 29.3919 psi |
| mmHg | 0.0193367747 | 760 mmHg = 14.6959 psi |
Back Pressure Terms in the CDTP Calculation
| Term | Meaning | Role in the formula |
|---|---|---|
| Set pressure, SP | The pressure at which the valve is intended to open under service conditions. | Starting pressure before back pressure adjustment. |
| Total back pressure, BP | The total pressure on the outlet side of the valve. | Context only; not an input to this calculator. Use the verified constant pressure present before opening. |
| Built-up back pressure, BUP | Back pressure generated after the valve opens due to flow through the discharge system. | Excluded from the bench subtraction; it develops after opening. |
| Superimposed back pressure, SBP | Back pressure present before the valve opens. | For the applicable conventional procedure, subtract before multiplying by K. |
Example Calculations
Example 1: Calculate CDTP
For an applicable conventional valve, use service set pressure 150 psi, verified constant superimposed back pressure 20 psi and manufacturer multiplier K = 1 (no temperature correction).
The cold differential test pressure is 130 psi.
Example 2: Calculate Set Pressure
For an applicable conventional valve, use CDTP 180 psi, verified constant superimposed back pressure 25 psi and manufacturer multiplier K = 1.
The set pressure is 205 psi.
FAQ
What does CDTP mean?
CDTP means cold differential test pressure. It is the pressure used when testing a pressure relief valve on a test stand at ambient temperature, adjusted for the effect of superimposed back pressure. For the applicable conventional procedure, CDTP = (set pressure minus constant superimposed back pressure) times the manufacturer temperature multiplier.
Why is built-up back pressure subtracted from total back pressure?
Total back pressure includes superimposed and flow-generated built-up components. The calculator requires the verified constant superimposed component directly; it does not infer it from operating discharge measurements. Built-up back pressure is not a bench subtraction.
Can CDTP be higher than set pressure?
Yes. A manufacturer temperature multiplier greater than 1 can make CDTP exceed service set pressure. This calculator does not accept negative superimposed back pressure. The applicable equation and multiplier depend on the valve procedure.
