Unbalanced Forces Due to Surge Relief across a DanFlo SPX surge skid.

[Tom Connor]

Unbalanced force determination around a critical surge relief system has illustrated what I've considered to be unreasonable. I'm interested to discuss this with other SR Engineers that may have faced this issue. The currently designed system sketch is attached. The classical calculations (shown) illustrate an unbalanced force across the relief skid of 60 kips which cannot be mitigated. I feel something is missing in the calculation or I've over complicated the issue. The static load when pressure reaches 180 psi is 44 kips in the pipe segment immediately upstream of the surge valve; the downstream pipe segment has a static pressure of approximately 10 psig; 20" pipe is 0.375 wt.

Tom Connor, PE
Operational Fluid Mechanics

 

[1503-44]

Why do you think it is unreasonable. As you have already surmized, force = pressure x area. Π * 10^2 * 180 = 56.5K as the static load, more or less. Keeping velocity changes SLOW, static load roughly converts to an equal amount of thrust unbalance (during steady state flow). Accounting for high dynamics (solely from a structural/stress engineer's perspective), you might want to think about applying a "dynamic factor" of up to around 2 x your static load to estimate max dynamic loading, maybe less as the concurrent static load should drop somewhat as the 180 psig should tend to reduce towards outlet pressure as the valve opens.

I general I would want to avoid so many 90degree bends around a surge skid. Use some 45deg elbows there, or better yet slowly curve the pipe up out of the ground in as straight a manner as possible from ... whereever you have located the anchor block. Anchor block? With surge skids I tend to prefer an anchor block, a bit unusual for me, rather than depending entirely on what might be a questionable location of a "virtual anchor", which could actually be far away from the skid. And after the surge skid, keep the surge flows as straightly aligned as possible there too. If you go back underground, again avoid the 90deg bends there too.

 

[LittleInch]

Even on a simple calc where does 60 kips comes from?

a 16" valve with 180 dpi ( there will be some pressure on the other side?) gives you 35 kips.

Also that elbow US will give you thrust in the opposite direction? ( might be balanced by the other elbow to be fair but could you make the pipe bigger downstream to reduce velocities and thrust??

you can run a transient piping analysis to give you true data and you will clearly have some forces to deal with but its a big valve and a lot of flow so will need to be pretty well restrained.

what's the problem here?

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