API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
(OP)
Hi all,
With a colleague I'm having a discussion on how to interpretarte the API 520 Part II recommendation of not having more than 3% pressure loss at the inlet piping to the PSV. I'm already aware of the following: “The calculation should contain only NON-RECOVERABLE losses. For gases, you DO NOT include acceleration losses as these are recoverable losses. Frictional losses in piping (including entrance and exit losses), fittings and valves, equipment and rupture disks if there was any are the only losses you need to consider. This means you should only be using the incompressible flow equation (i.e. Darcy) in calculating the piping losses.” Which is clearly explained in threat http://www.eng-tips.com/viewthread.cfm?qid=50179
Due to this 3% loss recommendation, you often have to "oversize" the piping to a larger diameter than the actual inlet connection diameter of the safety valve. In our case, we are having a 3" inlet pipeline to the safety valve, but the safety valves inlet connection is 1 1/2" (enough capacity for the relief case we have). Medium is natural gas (compressible fluid), set pressure 10barg. At the very end of the inlet piping we need a concentric reducer 3" x 1 1/2" to be able to install the valve.
In our flow calculations, the pressure drop of the inlet pipe is now mainly caused by the concentric reducer and just a little bit by the 3" pipe (which is a pretty long pipe in this case due to the vessel's double wall design). The concentric reducer is already causing approx. 3% of the pressure drop according our flow calculation.
In these pressure loss calculations, do you normally include the final reducing element to fit the safety valve as part of the inlet piping? Or can this be considered as "part of the safety valve"? I have to add to this that this case concerns a conventional safety valve, not pilot operated or balanced.
Thanks in advance for the feedback!
With a colleague I'm having a discussion on how to interpretarte the API 520 Part II recommendation of not having more than 3% pressure loss at the inlet piping to the PSV. I'm already aware of the following: “The calculation should contain only NON-RECOVERABLE losses. For gases, you DO NOT include acceleration losses as these are recoverable losses. Frictional losses in piping (including entrance and exit losses), fittings and valves, equipment and rupture disks if there was any are the only losses you need to consider. This means you should only be using the incompressible flow equation (i.e. Darcy) in calculating the piping losses.” Which is clearly explained in threat http://www.eng-tips.com/viewthread.cfm?qid=50179
Due to this 3% loss recommendation, you often have to "oversize" the piping to a larger diameter than the actual inlet connection diameter of the safety valve. In our case, we are having a 3" inlet pipeline to the safety valve, but the safety valves inlet connection is 1 1/2" (enough capacity for the relief case we have). Medium is natural gas (compressible fluid), set pressure 10barg. At the very end of the inlet piping we need a concentric reducer 3" x 1 1/2" to be able to install the valve.
In our flow calculations, the pressure drop of the inlet pipe is now mainly caused by the concentric reducer and just a little bit by the 3" pipe (which is a pretty long pipe in this case due to the vessel's double wall design). The concentric reducer is already causing approx. 3% of the pressure drop according our flow calculation.
In these pressure loss calculations, do you normally include the final reducing element to fit the safety valve as part of the inlet piping? Or can this be considered as "part of the safety valve"? I have to add to this that this case concerns a conventional safety valve, not pilot operated or balanced.
Thanks in advance for the feedback!





RE: API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
As for the 3% rule, you may find the following article useful: http://www.eng-tips.com/viewthread.cfm?qid=383741
Dejan IVANOVIC
Process Engineer, MSChE
RE: API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
RE: API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
Is this a general discussion you are having with your colleague, or is it for a specific application? If it's about a specific project or installation, the scope of this discussion could be narrowed quite a lot with some details (PFD, P&ID, specs of the PSV, etc.).
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
RE: API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
Is there any official interpretation or articles discussing using this topic? I don't understand from a practical standpoint how a decrease in static pressure due to a velocity change wouldn't effect a relief valve and potentially lead to chatter?
RE: API 3% Inlet Loss for PSVs: concentric reducer directly at inlet safety valve included in loss calc?
from what I understood reading other threads, this is mainly due to the force the gas or liquid flow is applying due to its kinetic momentum. Try to picture it as a water canon shooting at you: the water has left the hose already (so static pressure equals ambient pressure), but still you fill a lot of force due to the momentum of the fluid. The disc you're lifting in a safety valve only receives static force due to pressure difference when it is still closed just about to open. But when it's fully open, there is a lot of mass flow crashing against the surface of the lifted disc.