3 replies to this topic

[M Menessy]

Dear Members,

I would like to check the following statement which is extracted from draft copy of API RP520 Part 2, Sixth Edition:

 

“Kinetic energy losses, including the pressure loss as a result of the flow impinging on the valve disc, are considered recoverable and do not need to be included in the pressure drop calculations. However, even when kinetic energy losses are non-recoverable, they are small in comparison with other losses and therefore can be neglected”

 

My question is how the kinetic energy will be recovered in the PSV inlet line (when kinetic energy changes across a reducer for example) and when it cannot be recovered. Is it related to the pressure build-up when PSV starts to open. Also, in the above statement, non-recoverable kinetic energy can be neglected so, can we neglect it in actual calculations especially when PSV is installed on atmospheric tank.

 

Your kind help is highly appreciated.

 

Menessy

[fallah]

Menessy,

 

Kinetic energy change across, say a reducer, is comprised of two parts; one part is recoverable due to converting to static pressure energy and the another one is non recoverable due to turbulent effects. Non recoverable pressure loss due to friction cannot be neglected and is the source of pressure build up.

 

You can neglect non recoverable kinetic energy in the pressure drop calculation even when PSV is installed on atmospheric storage tank.

[M Menessy]

Dear Mr. Fallah,

Thanks a million for your kind reply. When the velocity downstream the reducer is larger than the velocity upstream the reducer (which is logic), the static pressure downstream the reducer will lower than upstream it due to friction and converting part of pressure energy into kinetic energy. Can I consider that the actual pressure drop across the reducer is the difference in static pressure upstream and downstream it.

 

Let me explain the case in better words. In one of the projects, one PSV suffers from chattering due to excessive pressure drop in the inlet line so, it was proposed to keep the PSV flange size as it is and increase the size of the inlet line (by one or two more sizes) then reducer is used to reach the size of the PSV flange. I doubt that the reducer will cause more pressure drop than the original case which was really the result after performing hydraulic calculations. This large pressure drop is due to the conversion of pressure energy into kinetic energy. So, will this be the actual case or shall I not consider the pressure drop due to conversion into kinetic energy.

 

Best regards,

Mohamed

[fallah]

Mohamed,

 

Non recoverable pressure drop across the reducer is due to conversion a part of kinetic energy to some wasted energies such as turbulancy, noise,...

 

Then by increasing the size of inlet line you, in fact, have reduced the non recoverable pressure loss due to friction and non recoverable pressure loss due to passing through the reducer is very small comparing to pressure gain by increasing the size of inlet line. Hence the balance would be positive in benefit of reducing the total non recoverable pressure loss...