6 replies to this topic

[Olidin]

I have this perculiar case with a very small relief valve: http://www.kunkleval...asuvmodel30.asp

 

Often, these valves are on air filters, passing very small amount of relief air in whatever scenario that are applicable. 

 

When we consider "piping" for these valves, there are no inlet or outlet piping. However, when we do consider "exit loss" at the outlet of the PSV, we have a high outlet pressure drop (about 40-50%) without any outlet piping at all. 

 

What is your opinion on including an exit loss for the outlet pressure drop when there is no piping?

Edited by Olidin, 11 October 2013 - 03:20 PM.

[Bill B]

Not completely sure of your set up, it appears that you have a RV screwed directly into vessel, or attached by flange to a nozzle. Since there is no piping, you will have no back pressure. Per ASME/API you do not apply the Crane exit head loss.

The inlet is another matter. Since you are attached to the vessel, you will have at least a velocity head due to the inlet (.25 - .5 - .75) depending on whether the inlet is rounded at the tank surface, flat or penetrates. To that you may need to add a short piece of pipe?

Hope this helps.

[Olidin]

I see, I think when I say exit loss, I mean exiting from the relief device outlet. 
 
Exiting from the vessel to the relief device is "inlet loss" in my head. 

 

... Per ASME/API you do not apply the Crane exit head loss...

 

Interesting, do you know which API or ASME specify that? I was looking through both to find such statement and couldn't find them. In fact, API 521 7.2.2 specify that 

 

"The developed back pressure of this system should include all pressure losses, such as exit losses, friction losses and kinetic energy loss"

 

Of course, API failed to define "exit loss" for a system without any piping at all. 

[Bill B]

Regarding the exit head, I am not sure where to find that specifically in ASME. You may want to check CCPS "Guidelines for Pressure Relief", Chapter 3 which has a discussion on the subject. Bottom line, convention is to not add the Crane exit loss (e.g. K = 1) for RV bp calcs.

[ryn376]

The exit loss term is only required when exiting into a confined space. 

 

According to "Chemical Engineering Fluid Mechanics" by Ron Darby (author of the 3K Method) pg 213:

 

A note is in order relative to the exit loss coefficient, which is listed in Table 7-5 as equal to 1.0. Actually, if the fluid exits the pipe into unconfined space, the loss coefficient is zero, because the velocity of a fluid exiting the pipe (in a free jet) is the same as that of the fluid inside the pipe (and the kinetic energy change is also zero). However, when the fluid exits into a confined space the kinetic energy is dissipated as friction in the mixing process as the velocity goes to zero, so the loss coefficient is 1.0. In this case the change in the kinetic energy and the friction loss at the exit cancel out.

[fallah]

 

Often, these valves are on air filters, passing very small amount of relief air in whatever scenario that are applicable. 

 

When we consider "piping" for these valves, there are no inlet or outlet piping. However, when we do consider "exit loss" at the outlet of the PSV, we have a high outlet pressure drop (about 40-50%) without any outlet piping at all. 

 

What is your opinion on including an exit loss for the outlet pressure drop when there is no piping?

 

 Olidin,

 

The main point is: in such systems the flow through the PSV is mostly choked and the critical flow pressure (Pcf) will exist at the exit plane of the PSV flow nozzle while the back pressure is the pressure at the PSV outlet flange. Then most choked flow pressure normally very higher than atm pressure, especially in the PSVs with lower outlet flange size, might be transferred to PSV outlet flange as the point of back pressure indication. This causes the back pressure would be very high even with no outlet piping. Obviously, after outlet flange the pressure would be dissipated as noise and...

[S.AHMAD]

1. The maximum allowable pressure drop for inlet piping is 3% and the outlrt piping is 10%.

2. If you don't have inlet and outlet piping, then obviously the installation meets the pressure drop requirements.