Hi fellow stressers!!

Why there is no horizontal reaction force in the PSV for relief load.
It's only shown in the vertical downwards at the outlet of pipe. (Refer to B31.1 Non Mandatory Appendix II figures)

But I have seen many stress engineers apply a thrust force horizontally on the valve centerline itself.

Can this be captured in transient simulation of compressible fluid flow with high mach numbers.

Warm Regards,

There actually are horizontal relief forces, albeit transiently. There is a horizontal reaction force in the PSV in the transient case. However, both API and ASME (at least, for these diagrams) make sweeping assumptions they don't share.
a) pipe does not change fluid flow properties - i.e. infinitely short.
b) pipe only redirects flow.
c) relief conditions at the relief orifice are perfectly expanded outwards to discharge piping conditions - i.e. worst case, with previous assumptions considered.

However, for the case of open discharge relief systems like so, with very short runs of pipe (or even just the length of the bend), horizontal transient loads are very, very, very low.

My experience is that if a given length of pipe is shorter than the speed of sound divided by 1000 Hz, transient effects rarely have impact on SCH 40 piping.

Thanks Michael for that point of view.

Make sense to me, since the open relief system is very short pipe, so wave reflections is unlikely and forces will balance immediately and unbalance load at short pipe is almost negligible.

Maybe need to pay more attention on AIV of PSV due to large pressure difference (e.g., Sound Power Level exceeding 155dB) and might excite high frequency modes of pipe especially for large D/t.

Warm Regards,

Michael, in fact you don't need those assumptions to explain API and ASME diagrams. Flow is for them an isentropic evolution from stagnation ("reservoir status") to a choked status and in flow path may be one or several points where flow is isentropic choked. Flow changes compressible fluid flow properties, but this fact is not relevant when you calculate isentropic choked status (simply said you can do it without knowing such details). An AFT Arrow simulation can give you details, if is of interest.

What they consider particular in such compressible flow is the point where is evident there is a free jet and a choked status in flow (and it is proved to be a reaction force due to the free jet). So for them free jet means reaction force and isentropic choked status calculation is the tool to evaluate the reaction force. "Isentropic" is just the assumption that makes the calculation possible.

@Borzki- why they don't consider the same magnitude of force at relief valve exit flange, even at orifice there is a choked (isentropic or not!) point in flow? Because is not proven to be free jet there- this is my rough explanation. Can be a free jet there just in case PRV/PSV has no piping downstream (and some Vendors give you the reaction force as horizontal at PRV/PSV exit flange, saying they don't know details about your piping system, so they assume to be nothing there!).

Thanks Mariog for that insight.

AFT have developed new software (AFT Xstream) for transient compressible flow which can calculate time history of unbalanced forces.

Warm Regards,

A good article with a nice title: "Gas flow calculation; Don't choke"

https://www.aft.com/technical-papers/gas-flow-calculations-don-t-choke

Borzki, a particular hard problem here is also the geometry of the flow (transient or steady-state); there is a transition from a radial flow in orifice to one that can be considered one-dimensional. For an accurate flow calculation (transient or steady-state), the PRV/PSV orifice doesn't actually act as a restriction orifice in one-dimensional flow.

Thanks Mariog for that additional information.

It's really not a straightforward physics when dealing with compressible flows especially the relief system. The boundary of PSV will be dealt with 3D CFD in this case.

Warm Regards,

You can estimate changes of flow within the pipe as the Fanno flow, if you're concerned about the flowrate increasing or decreasing due to pressure losses.

Because Fanno assumes isothermalism, for gases below the inversion point, you can expect the changes in flow to occur "later" in the pipe due to temperature changes. I.E. loss of pressure results in loss of temperature results in loss of specific volume results in less pressure loss.

This is what I use for open discharge for "long" pipe.

Thanks Michael for sharing your approach.

I will try to decipher it more closely once I am not so busy.

A bit busy this coming holidays for my family vacation (travelling home). Majority of this vacation will be spent just at home due to current condition.

Warm Regards,