Does thrust force appear at the flange of a pump?
Does thrust force appear at the flange of a pump?
(OP)
OK dumb question. When installing/specifying/modeling expansion joints in a piping system, one has to carefully handle and account for the pressure thrust force developed in the line due to the presence of the bellows.
So here is the question. For a line without a bellows, say a pump suction or discharge line, doesn't this force also appear at the flange, and therefore doesn't one have to check this force against the manufacturer's allowable for the pump flange?
So here is the question. For a line without a bellows, say a pump suction or discharge line, doesn't this force also appear at the flange, and therefore doesn't one have to check this force against the manufacturer's allowable for the pump flange?
Thanks!
Pete





RE: Does thrust force appear at the flange of a pump?
In the absence of bellows the longitudinal thrust is taken by the pipe: of course it is present at pump flanges, but is also present during a hydrotest of the pump with covers at the flanges. Hence the manufacturer will take care of it in certifying the pressure rating and that load should not be added again to check the allowable flange loads.
prex
http://www.xcalcs.com
Online tools for structural design
RE: Does thrust force appear at the flange of a pump?
And when designing the foundation, the resultant force in that direction resisted by the foundation will be (P*A) + (net longitudinal force applied by piping due to dead, thermal, seismic, etc.) - yes? Thanks! Pete
Thanks!
Pete
RE: Does thrust force appear at the flange of a pump?
Yes, my opinion is that the allowable loads at the flanges are only coming from external loads (weight, wind, EQ, ...), pressure thrust need not be included, and
No, the force resisted by the foundation will not include P*A, as the longitudinal pressure thrust is resisted by pipe and equipment walls in a circuit with no open discharge and no bellows.
prex
http://www.xcalcs.com
Online tools for structural design
RE: Does thrust force appear at the flange of a pump?
Thanks!
Pete
RE: Does thrust force appear at the flange of a pump?
RE: Does thrust force appear at the flange of a pump?
When there is bellow in the system then the thrust force acting on the elbow or tee is not transmitted to the nozzle due to flexible bellow element. Therefore there will be net force on the nozzle equal to the p * effective bellow area
RE: Does thrust force appear at the flange of a pump?
The only exception is with pressure retaining devices that cannot transmit the longitudinal load: bellows (longitudinal, not angular ones) are the primary example, sliding joints may be another one, no more come to my mind at the moment.
prex
http://www.xcalcs.com
Online tools for structural design
RE: Does thrust force appear at the flange of a pump?
Without a bellows there is no thrust forces on the pump flange. The thrust forces from the internal pressure are restrained by the pipe wall and flange bolts. The pipe wall and the bolts take the forces in tension and cancel each other.
When using a bellows with no bolts to restrain expansion of the bellows due to the internal forces a force of P x the internal area of the pipe cross section are put on the pipe and the pump flange.
Say a bellows with no restraining bolts is subjected to an internal pressure of 150PSIG. Assume the internal diameter of the bellows is 6". The force exerted by the internal pressure of 150PSIG on the pipe area (28.27sqin) is 150 x 28.27 = 4241.16pounds. The pipe must be restrained to resist these forces. The pump flange will be pushed on with 4241.16pounds. The pump probably is not designed to resist these forces. Bolts can be used across the bellows to contain these forces. Bolts across the bellows would resist this force like flange bolts do.
A better way is the put the bellows in a vertical run near the pump. Put a support at the bottom and at the top of the vertical run to resist the pressure forces or use bolts across the bellows. The expansion of the pipe will be taken up latterally (offseting the centerlins on each side of the bellows).
Another thing about expansion joints. If an expansion joint is used to absorb expansion movement axially the forces due to internal pressure and the spring constant of the expansion joint have to be overcome to move the joint. This results in very large forces. It is better to locate the expansion joint so the expansion forces will be absorbed laterally (offseting the pipe centerlines). Doing it this way means that only the latteral spring constant of the expansion joint must be overcome to move the joint. This force is much smaller than the pressure thrust forces.
Example: Using the pipe mentioned above if the expansion joint is used to absorb expansion axially and assuming the spring constant of the joint is 100lb/inch. then the force required to move the joint 1" is 4241.16pounds pressure force plus 100lb/in x 1" = 4341.16 punds. If the same joint is used to absorb the expansion latterally and assuming the latteral spring constant is 200lb/inch, then the force to move the joint 1" would be 200lb/inch x 1" = 200 pounds.
Obviously the forces being much smaller using the expansion joint latterally would result in much smaller forces being exerted on any supports consequently the supports would be smaller. Remember if a joint is used to absorb expansion latterally it should be restained with bolts across it to restain the axial pressure thrust or the pipe must be restrained to resist this force.
Hope this helps.