momentum in elbows

Dear engineers
as you know the momentum force in elbows resulted from bellow formula
F =ro*Q(V2-V1)
How should we consider the momentum force in our calculation?
does caesar consider it?
regards
jozm

Caesar II does not automatically consider this force unless you turn on the Bourdon pressure effect under Environment->Special Execution Parameters while in Input. This is pipe strain due to pressure, not related to momentum. Usually, when we talk about momentum we are considering unbalanced forces on pipe legs for a short duration such as you have with waterhammer or slug flow, not static analysis where these forces are balanced.

In steady state flow cases, the force required to decelerate the flow in the X-direction in an elbow turning from flow along the X-axis to one of the other axes is balanced off by the force that was used to accelerate the flow at the other end of the run in the X-direction. So the only thing affected is the axial stress along the run parallel to the X-axis, and the Bourdon effect stresses in the elbow. Usually these are not worth considering.

We worry about this effect for transients because the passing of a slug or shock wave affects the elbows within the system non-simumtaneoussly. The forces within the elbows are no longer balanced, and therefore can cause deflection of the pipe, sometimes by significant amounts.

Thanks for your useful information mr brown and craigb, but as you know the definition of bourdon effect is as follow :This effect considers the elongation of pipes under pressure tensile stresses as a loading condition.
but you have mentioned; This is pipe strain due to pressure, related to momentum.
what is the relation between bourdon effect and momentum force(mentioned formula)?
regards
jozm

The Bourdon effect in CII cannot consider Momentum since the flow velocity, V, is not known. Ths is a pressure effect. Pressurized pipe stretches, but for Steel pipe the stretch is very small: Delta= FL/(AE). You cannot "see" it. (F= P*inside area, A above is cross sectional area).

Of course the Bourdon effect is from the Bourdon pressure gage (and Mr. Bourdon) where a curved tube tries to "straighten out" when pressurized and moves a dial indicator. An elbow is a curved tube.

For steady state flow in pipes there is a momentum force at each elbow and they will all cancel out if you do a free body diagram of the closed system. But again the pipe is stretching ever so slightly (E is BIG) due one pressure and two momentum. For most liquid systems, V is 3 to 15 ft/s and P*A is usually much higher than rho*v^2.

Watch out in "unrestrained" HDPE piping systems. This stretch you can witness. E is not so big.

Enough already.

Typo in my post, should have read NOT related to momentum... have edited it to read correctly.

Use Bourdon "2" in C2 when modeling "bent pipe" as your bend elements, but use Bourdon "1" for forged elbows.

That bourdon tube effect that straightens bends works by turning the ovalized cross section back to circular.

Yes, Bourdon effect and momentum are different but they kinda' work the same way.

I have seen a thin-walled stainless line get pretty banged up. It is an empty line that quick gets flooded (acid dump?). The momentum loads did the damage.