Surge forces in sewer force main
Surge forces in sewer force main
(OP)
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?
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?





RE: Surge forces in sewer force main
Why does it take so long for regualtors to ask such questions,.......... one often wonders.
RE: Surge forces in sewer force main
Good Luck
RE: Surge forces in sewer force main
RE: Surge forces in sewer force main
KRS Services
www.krs-services.com
RE: Surge forces in sewer force main
Ewald.
RE: Surge forces in sewer force main
RE: Surge forces in sewer force main
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
www.krs-services.com
RE: Surge forces in sewer force main
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 www.aft.com and look at Impulse. One of the best transient analysis packages.
Sharing knowledge is a way to immortality
RE: Surge forces in sewer force main
PELS
RE: Surge forces in sewer force main
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.