4 replies to this topic

[manizheh]

Dear All,

Would you please advise me about an special formula for back pressure calculations in relation with set pressure.
Also I need to know first of all I shall calculate the back pressure quantity and then specify the valve type or I shall specify valve type at first?

Thanks for your advise

[pleckner]

manizheh:

There is no "special formula" to calculate back pressure on a PSV. It is strictly a matter of hydraulics. You start at the point of known pressure, which is usually some downstream point in your system, and work backwards to the flange on the PSV. For a gas, you can use Isothermal gas equations or adiabatic gas equations. The Isothermal gas equation is easier to use and for the most part perfectly acceptable. If you have CRANE TP410, they give a gas flow example using the adiabatic aproach with an expansion factor to account for the change in density as the pressure decreases. You can work from the PSV to the outlet if you want but doing this way becomes trial-and-error.

As far as which to do first doesn't matter.

Some guides on type of PSV selection:

Balanced Bellows

Use this if the PSV is going to discharge into a common header so as to not only mitigate the effects of superimposed back pressure but the effects of built-up back pressure as well.

Conventional

Use this if this the only PSV in this system or the PSV is going straight to atmosphere and you can stay within allowable variable back pressure limits.

If you need an understanding of the terminology and concept I presented above, then you are too new to this type of analysis. PSV system analysis is critical and must not be done by a novice without tight supervision. I would strongly suggest you let someone else more experienced handle this or make sure that person is standing over your shoulder and teaching you as you go along.

You can ask specific questions on this message board as you go along and we will be very happy to assist. However bear in mind that it took many years for most of us to become knowledgable and comfortable enough to be confident we can do these type of calcualtions and analysis properly.

[wael]

Hi Phil
I noticed that while doing backpressure calculations we are getting high backpressure for the outlet expanders ( the PSV outlet flange is smaller than the line )
The question is do I have to gradually change to avoid the backpressure increase
Thanks

[mishra.anand72@gmail.com]

Could you please specify that PSV is discharging into some common header or to Atm. Also if PSV is discharging into common header, could you please specify normal pressure in common header.

[wael]

Could you please specify that PSV is discharging into some common header or to Atm. Also if PSV is discharging into common header, could you please specify normal pressure in common header.

Hi Mishra

We are quoting the constant (flare ) back pressure as 0.5 psig


Thanks