10 replies to this topic

[M Menessy]

Dear members,
I would like to know if the pressure drop calculations in the PSV inlet line are done based on stagnation or static pressure.
The main concern that in a case where PSV is installed on atmospheric tank, there is a reducer which decreases the size from 10" to 6". When I used hydraulic calculations software, the staic pressure drop was higher than the stagnation pressure drop (which is expected due to velocity increase across the reducer). So, which pressure drop is the real one and which one will be used to check the 3% role.
Many thanks
Menessy

[fallah]

I would like to know if the pressure drop calculations in the PSV inlet line are done based on stagnation or static pressure.
The main concern that in a case where PSV is installed on atmospheric tank, there is a reducer which decreases the size from 10" to 6". When I used hydraulic calculations software, the staic pressure drop was higher than the stagnation pressure drop (which is expected due to velocity increase across the reducer). So, which pressure drop is the real one and which one will be used to check the 3% role.
 

 

Menessy,

 

The low pressure PSV's on the atmospheric storage tanks, mostly weight loaded, have modulating characteristics hence 3% pressure drop rule isn't applicable for inlet lines of them to see if there would be the possibility of chattering...of course reduction in PSV capacity due to high pressure drop along the inlet line should be considered.

Edited by fallah, 20 April 2018 - 12:52 PM.

[M Menessy]

Dea Mr. Fallah,
Thanks for your kind reply and noted. What about the other part of the request which is regarding the stagnation pressure drop vs. the static pressure drop.
Thanks in advance
Menessy

[fallah]

Thanks for your kind reply and noted. What about the other part of the request which is regarding the stagnation pressure drop vs. the static pressure drop.
Thanks in advance
 

 

Menessy,

 

The basis for pressure drop calculation along the PSV inlet line to check about the 3% rule is the set pressure equal to "stagnation pressure" minus "frictional pressure loss", because the valve disc is held open by the stagnation pressure of the relieving gas.

Edited by fallah, 21 April 2018 - 01:02 AM.

[M Menessy]

Dear Mr. Fallah,
Thanks again for your kind reply. If we applied Bernoulli equation to calculate the pressure loss (taking into consideration the limitations), the difference in the static pressure terms is the ACTUAL pressure drop which will occur. Would you please confirm this statement. If there is a reducer in the inlet of the PSV, most hydraulic calculation spreadsheets calculate the pressure drop from K factor without considering that part of pressure energy is converted into kinetic energy. Is it correct.
Best regards
Menessy

[fallah]

Menessy,

 

The total allowable pressure loss is 3% of the set pressure. This value should be included "vessel exit loss", "size change loss" and the "losses due to straight pipe and direction changes". Obviously the part of pressure energy converted to kinetic energy or vice versa which is recoverable shouldn't be included in allowable pressure loss and just those non recoverable energy due to friction or size and direction change should be included.

[M Menessy]

Dear Mr. Fallah,
Thanks a million for your kind replies. Now the situation is clear but I have one open point which I need your help to close. If it is required to perform real and actual pressure drop calculations for PSV inlet line (aside from the 3% role) to know the actual inlet pressure at the PSV inlet flange, shall I consider, in addition to the terms you have specified in the previous reply, the effect of the velocity change across the reducer or not. I know that this is a part of the recoverable pressure drop but I need to confirm.
Thanks in advance
Menessy

[fallah]

Menessy,

 

In fact, there is no way transitioning from higher size to lower size through a pipe reducer without tolerating a little bit friction and drag losses. Then the pressure energy upstream of the reducer is partly converted to the recoverable kinetic energy and partly has to be considered as non recoverable energy losses.

 

The non recoverable energy losses due to passing through the reducer can be estimated by a K factor of around 0.52 for a typical pipe reducer relative to the upstream velocity.

[M Menessy]

Dear Mr. Fallah,
I used Crane to get the K factor according to the diameters ratio then substituted in Bernoulli equation to know the static pressure downstream the reducer by knowing the pressure abd velocity upstream the reducer and the velocity downstream it. The static pressure drop is much higher than the stagnation pressure drop which resulted in lower static pressure than the stagnation one. Which one will be actual inside the pipe and which pressure will be measured using pressure measurements devices.
Best regards
Menessy

[M Menessy]

Dear Mr. Fallah,
I used Crane to get the K factor according to the diameters ratio then substituted in Bernoulli equation to know the static pressure downstream the reducer by knowing the pressure abd velocity upstream the reducer and the velocity downstream it. The static pressure drop is much higher than the stagnation pressure drop which resulted in lower static pressure than the stagnation one. Which one will be actual inside the pipe and which pressure will be measured using pressure measurements devices.
Best regards
Menessy

[Alfreedo]

As I understand it, you are only to consider non-recoverable losses.  In other words, losses due to energy shift from potential to kinetic energy are to be ignored.  I always go stagnant - stagnant pressure.  I reasoned this to myself as being ok because the velocity head (momentum) would be acting directly on the poppet/plug in the PSV thus driving it open.  Think pitot tube stagnant pressure gauge which takes the velocity head (momentum) into account.

 

On the backpressure side, the omnidirectional static pressure should be used I believe because the pressure at the outlet of the momentum does not directly act on the plug from the back end as I understand it.  So I would use the static (gauge) pressure at the face of the PSV flange for backpressure calculations.

Edited by Alfreedo, 21 December 2022 - 05:26 PM.