Thrust force calculation and restraint

[KaBone]

I've seen a few refences that calculate the thrust force based on the pipe pressure, pipe area, and angle of deflection.
I get the thrust force from static pressure wanting to blow things apart.

What about the force of the moving water being accelerated in a different direction or complete stop?
Isn't that water hammer?

 

[bimr]

Yes, the force of the moving water being accelerated in a different direction or complete stop is considered to be "water hammer". However, the forces that occur as a result of the water hammer event need to be addressed or engineered out of the piping system.

I have witnessed the fire department closing the hydrants too quickly causing a water hammer event that resulted in broken pipes. The fire department is supposed to be trained so that they don't open and shut valves quickly.

 

[1503-44]

Restraint forces at bends are due to differing lines of action between fluid pressure and at changes of pipe cross-sectional area, such as those that occur at reducers, tees and partially closed valves, as PxA differ, even though there may be no change in direction of flow.
Total pressures is the result of both static and dynamic pressures.
Static forces are the result of static pressure only with no associated movement of fluid.
Dynamic forces and pressures are those due to both change of momentum and change of fluid velocities. Changes of momentum can occur during steady state, via change of direction, and in transient flows, via changes in fluid velocity.
Water hammer is the result of all those forces and pressures that are due to transient flows.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[KaBone]

Big mahalos for the replies - So there is a dynamic thrust force at a bend. The formula for thrust:
F = P * A * 2sin(delf angle)
does not appear to take this into account.

Is the dynamic thrust considered negligible?

 

[bimr]

It depends:

Link

 

[1503-44]

Force is generated by by the flowing mass from its change in direction going round the bend exactly the same as P x A. As your pressure is probably calculated using only friction loss considerations, it does not contain pressure variations at the bend that are the result of the fluid hitting the side wall of the pipe as it goes round the end bend and is redirected towards the outlet. Those extra pressures only occur in the vicinity of direction changes from the momentum of the fluid changing its direction. On a 90° bend from the x axis you have fluid entering the bend moving at some velocity Vx, then being slowed down to a stop in the x direction and accelerated back to its entry velocity, but now in the Y direction. So reaction forces on both x and y axis are generated in addition to the pressures generated by fluid flowing along the pipe in a straight line.

These Momentum generated forces ate not present when there is no flow. Then you only have P_static x A x sin df to account for. When the pipe is flowing at steady state, you have P_operating x A x sin df + steady state momentum forces. And during water hammer, you have water hammer pressures and momentum forces at a higher, lower and maybe even reversed direction to consider.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[1503-44]

I should point out that a pipeline is usually designed for a stated maximum pressure along the entire pipeline, or at least a significant segment length and that design pressure should include the maximum water hammer pressure. So your Pressure x Area calculations will (or should) include the sum of all the pressures that make up the stated maximum. But the momentum forces will usually not be included in that design pressure, as they are a local phenomenon, existing only in the vicinity of bends and tees.

As the maximum pressure is usually only experienced at a limited number of points along a pipeline, the pressure at any given random point will most likely be well under the design pressure and adding the momentum forces is unlikely to exceed pipe design pressure. However it is a good thing to keep that possibility in mind and avoid changes of direction in regions that do operate at pressures equal to, or close to the maximum. Avoid changes of direction near pump discharges and low points of a pipeline when possible and check that the momentum forces do not cause excess stresses or restraint forces when you cannot.





A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[KaBone]

big mahalos for the replies and guidance.