Hydraulic Loads at Pump Casing due to Improper Flexible Coupling Installation 4

[IBechir]

Greetings

I came across the following paragraph in a pumping handbook (vibration and noise. (1998). In pumping station design (2nd ed.). butterworth-Heinemann. ) regarding hydraulic loads imposed on pump casings due to improper installation of flexible joints at pumps nozzles for pipes equal to or larger than 6". It was only described in the paragraph shown below so if someone could shed some light on this issue I will be much obliged. My goal is to find out how this applies to an HVAC pump installation with 350mm Dia suction pipes for each of two duty pumps in parallel. Each pump is 108 lps @ 450kPa, 1450rpm and 98 kw motor.

Pump Support
The suction and discharge piping end flanges and the
opposing pump nozzles must be properly supported.
Avoid the use of unrestrained pressure-bearing
"expansion" or "flexible" joints at pump nozzles for
pipes equal to or larger than 150 mm (6 in.) in diameter
unless the contained pressure times the nozzle
cross-sectional area is within the manufacturer's nozzle
load limits, and the pump and piping natural frequencies
are well removed from the range of vane
pass frequencies. Although such joints relieve any
piping thermal expansion or Bourdon-tube effects
from "loading" the equipment nozzles, they do not
allow the piping to absorb the cross-sectional nozzle
hydraulic load (i.e., the pressure times the open area).
This nozzle hydraulic load can produce a large thrust
perpendicular to the nozzle opening and severely load
the pump casing. Failure to account for such loads has
caused serious operating alignment problems, casing rubs, and system damage in many installations.​

Thank You

 

[JJPellin]

An unrestrained expansion joint will tend to produce an axial thrust within the pipe. The pressure in the pipe it working to extend the length of the expansion joint. Pressure times area equals force. These forces will be transferred to the equipment that the pipe is attached to. Large expansion joints tend to have axial retention rods designed to limit this axial expansion and control these forces. We commonly use these on blowers and condensing steam turbines. I have attached a sketch of one of our joints.



Johnny Pellin

 

[bimr]

How does this apply?

One can not expect the pump manufacturer to design the pump to support unknown piping loadings. The piping should be supported and anchored independently of the pump.

The hydrostatic force equals area time pressure. For a 350mm dia pipe at 40 psi, that equates to 6,000 lbs load.

 

[LittleInch]

I penned a reply earlier which seems to have got lost in transmission, but agree. I got 43kN force.

What the author of the original piece is saying is that this pressure equates to a force on the nozzle. With rigid piping this force is met by an equal reaction from the piping which is usually anchored or restrained in some way. with a flexible bellows or other joint which is designed not to transmit load from piping to nozzle, it also works the other way and doesn't support the nozzle. Pressure acts in all direction and you need to resist it in all directions.

The pumps are designed to accept certain nozzle loads - look them up and see if you have a problem or not. If you do then support the flange better or use rigid pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IBechir]

Thank you all. I understand better now.

JJPellin: " The pressure in the pipe is working to extend the length of the expansion joint. "

Isn't that the 'bourdon-tube effect' that the author referred to?

 

[JJPellin]

I am not familiar with the term "bourdon-tube effect" in the context used above. But, I would say, no. The force trying to extend the pipe is just pressure acting over an area. In context, the reference to bourdon-tube seems to be saying that the expansion joints reduced stress which might be created from a pipe which is not straight trying to straighten out when pressure is introduced. I picture the bourdon tube in a standard pressure gauge.

Look at the attached image from the post linked below. This pump has small expansion joints of the type that might be used in your application. They have axial rods to restrict the axial expansion due to pressure. This limits the loads imposed on the pump from this effect.

http://www.eng-tips.com/viewthread.cfm?qid=383136

Johnny Pellin

 

[IBechir]

Thanks man.

 

[bellowsmfg]

IBechir,

If you are still there, the effect that is been covered here is called "Pressure Thrust" will is equal to the effective area of the expansion joint (Which is usually bigger that the OD of the pipe) X the pressure in the pipe.

Expansion joints are designed to allow movement in particular directions. Tie-rods prevent axial movement or can limit them. If they do, then they are transferring the pressure thrust created by the bellows section of the joint. If there are not tie-rods the pressure thrust will act on both your pump nozzle and the nearest anchor and or supports. If you don't have something restraining the other end you're going to be in for a surprise. And not a good one.

Note that pumps designed per API-610 are designed with the allowable nozzle loads in the tables provided by the code. And in turn the pump foundation, base plate etc, should be able to deal with the loads as well (To my civil engineering friend above). That is a code requirement.

Expansion joints should be selected to solve pipe stress issues that cannot be solved by other means. But when they are selected you need a full understanding of what they are, what each ones purpose is (universal, pressure balance, in-line pressure balance, tied, gimbal, single bellow). This is because expansion joint selection must go hand in hand with the pipe support design for the system. They must be designed to work together.

To help you understand single bellows here is an exercise for you. Let's assume you installed a bellow on the suction nozzle and discharge nozzle of your pump. Assume the effective bellows areas are around 7.5% greater that the pipe OD. As the suction and discharge pressure will differ greatly, calculate each pressure thrust. Then using the nozzle dimensions and orientation of the pump calculate you out of balance moment and forces acting on the pump. This will give you an understanding of why your paragraph above recommends not to have unrestrained pressure bearing expansion joints. The recommended expansion joints will normally have restraints on them.

Conor Walshe

http://www.bellowsmfg.com

Bellows Manufacturing and Research, Inc.

https://bellowsmfg.com