PSV CDTP calculation based on operating temperature or relieving temperature

[nasim123]

Dears
I need your advice pls

As per API RP 520,(page 48) the temperature correction factor (multiplier) is typically required when the relieving temperature exceeds 250°F. However in Dresser PSVs(Consolidated PSVs)they are referring the operating temperature not relieving temperature. In Crosby PSVs, they are referring relieving temperature.

Which one will follow?

 

[The Obturator]

The best answer is "Whatever the spring temperature is at moment of relief" !. Granted you cannot always determine that. API-520 (July 2014) paragraph 4.2.3 states "Inlet Temperature" as opposed to Operating Temperature. However, it does state that other factors, including the fluid can influence it. CDTP does tend to be a often argued point. There is good reason for either operating or relieving temperature based on the heat conducted to the spring. It is not an exact science at all whatever the case. Each manufacturer will state a different correction factor and quite likely on what it is based. Generally it is the relief temperature taken (and as the inlet temperature). Note that it does not apply to open bonnet SRV's or those applications which state a "fire" temperature. Note that you are maybe referring to an older API-520.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[nasim123]

Dear Avalveman, While calculating the CDTP, we are referring the operating temperature. If the operating temperature is more than 120 Degree F, then we need to multiply temperature correction factor with set pressure, then only we will get the CDTP. Except Crosby design pzvs, we are referring whether the operating temperature is exceeds the limit or not. We are not referring the relieving temperature.

What is the reason behind this rule?

 

[The Obturator]

nasim123. Where are you getting the 120F figure from ? As previously, the method differs between manufacturers and their design. Manufacturers aCCDTP is based on their design, experience, testing and reports from the field. CDTP temperature correction continues to be argued. You really need to look at it case by case. What if an operating pressure is say 10C and the relief temp say 300C ? do you set the SRV higher with a correction factor of say 2% (CDTP = Set P x 1.02) or leave it for 10C operating temp ? How critical is opening point ?, has valve been set within tolerance ? You must apply good engineering practice with manufacturers advice to ascertain best CDTP setting. Per my earlier opening statement, base CDTP on the spring temperature at the moment of relief and against the manufacturers temperature correction multiplier. That's the closest optimum CDPT you can get.

Per ISO, only the term Safety Valve is used regardless of application or design.