8 replies to this topic

[jitendraprocess]

Dear all,

Please guide regarding inlet line size for PSV

Inlet line size should selected such that pressure drop should be less than 3% of set pressure ?
Also we can calculate relief rate for fire case scnerario but how to calculate for block discharge case?
For calculating line size, at which pressure i.e. set pressure or relieving pressure, we have to take properties?

Please guide me.
Awaiting reply.

[jrtailor09]

Jitendra process,

You are Right Pressure drop at inlet line shall not exceed 3% of set pressure.
Block discharge means maximum flow at inlet of line when outlet valve is closed.
You have to consider all properties at relieving condition.

Regards,

[Robert Montoya]

Here are the answer:

For calculate the drop pressure use the equation 3-19 Crane:

Qh=1.0312*Y*D ^2/Sg*(SQRT(Delta P*density/Kt)



The drop pressure should be less 3% of set pressure and take into account losses of pipe and fittings. The mínimum length shall be 3 ft.
For block discharge case; consider the max. Flow through the FV/PV/LV whatever is your case.
Additional information in: http://www.cheresour...page__hl__crane

[jitendraprocess]

While calculating PSV sizing, at relieving pressure calculate relieving temp. So we have to take latent heat at that temp. but ProII & chemcad both are giving different latent heat ? This latent heat is of saturated liquid at relieving pressure * temp. condition. Is it right?
please guide me

Thanks

Edited by jitendraprocess, 18 July 2012 - 06:02 AM.

[Robert Montoya]

The relief condition shall be 21% of the set pressure for case fire, but if you case is block discharge case is 10% of set pressure.
Is very important if you evaluated fire case, although your design case is block discharge, the relief conditions of the heat latent shall be at 21% of the set pressure.

[jitendraprocess]

The relief condition shall be 21% of the set pressure for case fire, but if you case is block discharge case is 10% of set pressure.
Is very important if you evaluated fire case, although your design case is block discharge, the relief conditions of the heat latent shall be at 21% of the set pressure.

Dear sir,

I have calculated orifice size which is too small i.e. 0.003 sq. inch , so i selected 1D2 having area 0.11 sq. inch which is too much excess.
Is there any smaller size like 0.5D1 or 3/4D1 Please guide me.
Also i have considered constant back pressure which is 10% less than set pressure because i dont know flare header pressure.

[Robert Montoya]

Ok this valve size is very small, once that you are sure that apply in that pipeline.
Regarding the size, in fact there is another size smaller depends of the vendor, for example FARRIS has a valve more small. but the problem is the procurement process which practically will be for the few vendors at time of public the bidding in case your procurement process is so. If not so you can enter the web page and select FARRIS depending on the flow and the valve set point of relief.

[fallah]

Is there any smaller size like 0.5D1 or 3/4D1 Please guide me.

Hi,

Farris series 2700 relief valves with "C" orifice is an example.

Fallah

Edited by fallah, 23 July 2012 - 02:41 AM.

[pravin164]

Dear OP,

Dont you think you are mixing different things in one post, and the orginal thread regarding the properties at relieving conitions for blocked discharge overpressure scenario has been ruled out.

You should always consider the relieing pressure as the set pressure + overpressure, and for any overpressure scenario the rlieving properties should be taken at this conditions only (Offcourse overpressure is diferent for different contingency). Also please note that PSV inlet line sizing is done using the relieving conditions properties and tail pipe sizing is at back pressure conditions (built-up+ superimposed)

Pravin