Surge forces in sewer force main

[PELS]

We just designed a sewer force main connecting three lift stations. The first station has an 8" PVC FM that discharges to a gravity manhole and into the second lift station. The length is 3180 feet. The second station has a 14" PVC FM that extends 9087 feet to the third lift station.

The force mains and lift stations are currently under construction. Out of the blue in the midst of construction, the sewer agency asked us to provide evidence that "surge forces" will not be a problem in the force mains. This is the first time we have had an agency request this data.

Has anyone else had to deal with "surge forces" and what kind of data must we produce to satisfy the agency. I understand that surge forces are due to sudden changes in pressure due to starting and/or stopping pumps or power failures. Once determined, are there such things as "surge relief valves" to address the forces?

 

[RWF7437]

Yes, there are such things as surge relief valves. Some are hydraulic and some are simply spring loaded check valves which do not depend on electrical power and may, therefor, be preferable. One maker of such valves in the US is Claton Valve. There are others.

Why does it take so long for regualtors to ask such questions,.......... one often wonders.

 

[RWF7437]

Also see the related discussion thread under Water Treatment and Distribution in this forum. Much good information there.

Good Luck

 

[dicksewerrat]

With an open end on the pipe there will be much less surge pressure. If they didn't spell out the concern and or the control of surge pressures in the spec. you are talking about an extra. what kind of pressures are the systems supposed to operate at now?

 

[KRSServices]

I suspect the regulator has experienced lift stations with large vertical lifts and inadequate surge relief, particularly at the check valves. If the motors do not have soft start/stop over time the check valve will take a ounding to the point of failure. In other cases, such as HDPE pipe, the momentary pressuring of the pipe causes the pipe walls to flex slightly. Over time, these to have been known to fail. Maybe that is one suggestion as to why they are asking the questions.

KRS Services

http://www.krs-services.com

 

[ewald]

You need to do a surge analysis using a computer program. Water hammer can be a big problem in PVC pipelines because PVC do not handle cyclic pressures very well. You will need to use a higher pressure class pipe to overcome that problem. Relief measures can be , soft stop pumps, air valves (anti slam air valves are available) or surge air vessels. You will need to analize this to know what relief neasure will work. The profile Surge 5 is a surge program for one.
Ewald.

 

[cvg]

I would disagree that checking a force main design for surge is an extra. A 9,000 feet long force main is quite long and one could expect that water hammer may be an issue. This is something that should have been considered in the original design. I wouldn't expect the owner to make the determination that surge would not be a problem (that's why they hire an Engineer). And if they did, I would want to verify that for myself and specifically exclude it from the scope of work on the basis that it is assumed unnecessary.

 

[KRSServices]

I agree with cvg.

You should be able to quickly approach the design engineer and communicate the concerns by the agency. THe engineer, will then be more than happy to provide you with the answer because the analysis, albeit maybe cursory in nature, is one component of complete design. From this analysis, the engineer should have been able to determine whether soft starts/stops are/were required, the pipe diameter, the pipe wall thickness, check valve requirements (and locations along the forcemain and thrusting requirements. If your engineer had not done this, perhaps a professional reminder is in order because you may have more questions now than just the agency query.

KRS Services

http://www.krs-services.com

 

[stanier]

The design pressure of the material should include for surge and fastigue. If you have selected the material before establishing the surge pressure then you have been negligent in you engineering. This has exposed you to possible claims of damage should there be a failure of the pipeline.

The damage may be physical in nature to persons or the infrasatructure or worrst still may be an environmental spill. I would check your insurance company and check if you have environmental insurance. There is a good chance that you dont because it is expensive or impossible to obtain.

You are putting your company at risk by not engaging a professional engineer skilled in waterhammer analysis to engineer this aspect of the pipeline.

Now to the material of choice. PVC can be brittle below 5 degrees C and thus may well not be the material of choice. There have been numerous failures of the material over the years attributable to surge pressures. This has been particularly so where there have been surface defects from manufacturing or installation. The surface defects on the inside of the pipe are more detrimental as they initiate crack propogation when the inner surface is under tensile stress. ie when the maximum pressure is applied.


Anothe rproblem with thermoplastic materials is ther celerity is reduced due to the lower modulus of the material. Hence the speed of a transient wave is reduced. The time taken for the wave to travel from one end of the pipeline is longer than legacy materials. Hence the time to close a valve to ensure that Joukowsky pressure waves dont oicccur is much longer than for steel or DI.

Also if the main has column separation occuring the pressures generated can be far higher than theose predicted by Joukowsky.


The science of waterhammer analysis is a complex one and borders on the black arts. The computer p[rograms available do the maths for the analysis but dont do the design for you. For that you need a wealth of experience hopefully from other peoples mistakes.


Search for Hytan on google and look at that website. The professor who wrote that software allows his lecture notes to be read. Then go to

http://www.aft.com

and look at Impulse. One of the best transient analysis packages.

Sharing knowledge is a way to immortality

 

[PELS]

Thank you everyone for your thoughtful replies. It is nice to have this sense of community in the engineering profession. May you all have a prosperous year!

PELS

 

[SPLIT]

A waterhammer develops not only anpressure increase but impulsive loads.Thse loads are aplied in the reducers, elbows. etc.In an elbow can be evaluated as delta p per cross piping section /cos 45°(delta p is the waterhammer pressure increase )
For more details you can see ASCE Recomended practices for buried pipes.