Relieving temperature PSV

The relieving temperature for a pressure safety valve (PSV) is the temperature of the fluid at the time the valve is expected to open due to an overpressure event.

Pierre

Assuming you want the temperature at the outlet of PSV, where the pressure is 1.37 bar(a). The process is adiabatic(isenthalpic). Using the steam table software in Excel, the required temperature at the exit of PSV is 144.5 deg C.

relieving T&P&composition are referenced to PSV upstream conditions when a PSV's seat is in an open position

API 520-1-2020
3.1.49 relieving conditions
The inlet pressure and temperature on a pressure-relief device during an overpressure condition. The relieving pressure is equal to the valve set pressure (or rupture disk burst pressure) plus the overpressure. ...

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goutam_freelance said:

Assuming you want the temperature at the outlet of PSV, where the pressure is 1.37 bar(a). The process is adiabatic(isenthalpic). Using the steam table software in Excel, the required temperature at the exit of PSV is 144.5 deg C.

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I don’t think the OP’s fluid is steam. I assume relieving conditions = 7 bar_a and 287 K (14 C).

OP, does the 287 K correspond to 7 bar_a?

Dulli2000 said:

Upstream PSV we have saturated conditions s

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@Latexman , I thought he is using steam.

In adiabatic processes we use T1/p1=Tn/pn (as also API521 says).

The above equation is based on a totally gas filled constant volume vessel (no liquid) operating at a given pressure and temperature with a fixed mass and composition. In this case when heat is added from external fire the pressure and temperature increase follows the ideal gas equation P2/P1=T2/T1. This is not an adiabatic process as heat is added to the gas in the vessel. Note that this equation only applies to vessels without any liquid inventory - only gas.

I calculated a backpressure of 1,37 bara at PSV discharge and have relieving pressure of 7bara (121% set-pressure). Upstream PSV we have saturated conditions so that I get the corresponding temperature from the saturation curve which is 287K.

For a partially liquid filled vessel with vapor space, the relieving temperature WILL correspond to the saturation vapor pressure of the liquid at relieving pressure P1. With heat input to the vessel the liquid will vaporize to gas at a temperature corresponding to the pressure in the vapor space. As vaporization occurs the pressure keeps rising in the vapor space, and so does the saturation temperature, until relieving pressure is reach at which time the temperature of vaporization equals the saturation vapor pressure of the liquid phase at P1.

The calculated relieving temperature seems way off in my eyes. Do I use the right variables here for the relieving temperature?

As discussed there are two different cased above - one with a totally gas filled vessel and one with partially liquid filled vessel with vapor space.

Am I right when we talk about relieving temperature is on the discharge side of the PSV at relieving pressure equal to backpressure?

No - relieving pressure refers to pressure on inlet of relief valve.

Thank you all for very helpful replies.
Right, it is not steam, I am sizing for a vessel filled with liquid NH3 with vapour cushion. It is a very unlikely scenario to get the vessel emptied completely of liquid so that only gas is left. It is more likely that the vessel is filled to 95% for a while. Is there a definition where partially filled stops and completly filled begins?

@Snickster:
1) During evaporation under fire case liquid goes to gas phase. In case of low liquid level in vessel in fire-case, is it still the same scenario if relieving pressure is not reached before all liquid has been evaporated? I am just wondering if this could happen.

2) Also or scenario of partially filled with liquid, is it correct then that the process is still isenthalpic?
For downstream line sizing I need pressure and temperature at PSV outlet in order to get the right density. As written before, I calculated the backpressure at PSV outlet at 1,37bara which is in my eyes equal to PSV outlet pressure. Expansion leads to temperature changes per Joule Thomson effect. But do you still assume saturation? This would simply lead to -27C at PSV outlet.

@Dulli2000

Thank you for letting the cat out at last!

1. If your PSV set pressure is above the saturation vapor pressure at the particular temperature inside the vessel, it is possible that relieving pressure will not be reached just when all the liquid has vaporized. Pressure will increase subsequently on continued heating till the set pressure is reached.

2.

Dulli2000 said:

scenario of partially filled with liquid, is it correct then that the process is still isenthalpic?

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If the expansion happens after the PSV, i.e., without heat input, the process is isenthalpic. It is difficult calculate the combined effect of Joule-Thompson effect along with the pressure drop etc. So use of a database like Peace software is
helpful.

https://www.peacesoftware.de/einigewerte/calc_nh3.php7
Using the above and assuming 7 bara inlet (saturated) and 1.37 bar a outlet the outlet temp is -6 deg C.