When To use a Surge Valve

[glapfk11]

Does anyone know when to use a surge relief valve?

I.e. what conditions call for one--> To clarify I am working on a pump station design retrofit. I'm trying to figure out whether or not I need to add a surge relief valve to the pump discharge header. It was suggested that I may need one on the discharge header b/c there has been a "significant increase" in the static head. What does an increase in static head have to do with wether or not a surge relief valve is needed?

 

[stanier]

Relief Valves generally do not act quickly enough to reduce a surge. The non guided type called Neyptic valves are designed for surge. You may also need a air valve and I suggest you go to

http://www.ventomat.com

and read there what type and the technology involved in air valve selection.

As for the surge valve you need to undertake a surge analysis for the system.

Go to

http://www.pipingdesigners.com

and look there at the paper on Risks of Surge in Pipeline Systems. My contact details are on the paper. I undertake surge analysis for companies all over the globe so may be able to help. If not there are many companies in the USA who provide such a service. ww.aft.com can do it for you.

 

[bimr]

The static head does not have much to do with the potential for surge except that the maximum water pressure is a combination of the surge pressure and the static pressure.

A surge relief valve is used to minimize the effects of water hammer. The potential for a water hammer is unique to the application and is a function of the number of pumps, pipe velocity, discharge piping layout, type of valves and valve closing times, variable speed drives, power failure, elevation, etc.

You need to do an analysis of the pump station and distribution system to see if you will have a surge. For example, suppose the power fails, what happens? Is the outgoing water going to reverse direction and slam the pump discharge check valves closed creating a water hammer?

Do you have a long pipeline with high velocity? If the pump stops quickly, what will happen?

You are somewhat fortunate that you are modeling an existing pump station. You can start your analysis by discussing the potential for water hammer with the current water department people. What happens with the current system? Is water hammer presently a problem?

It is good practice to select equipment to minimize water hammer. Select valves that will close slowly rather than slam. Use lower pipe velcities. Plan for emergencies such as power failure. Use electrical equipment that has soft starts.

Contact GA Industries for further information. They will do a simple surge analysis for you for free.

http://www.gaindustries.com/

 

[glapfk11]

Stainer- I could not find the Paper you spoke of on the piping designer's website...do you know excatly where I find it on the site?

Bimr- as far as I know there haven't been any problems with water hammers at the plant's pumping station. Perhaps the surge valve was suggested to me because there was concern about the added water pressure due to the increase in static head....

The max pipe velocity will be 8ft/sec. The new pumps (3+1 spare) will be on Vfd's and there is a backup generator in case of a power failure. So perhaps the risk is low??? I've contacted GA industries as you suggested so we will see what they come back with.

Thanks for all of your help thus far...if you can think of any other resources that may help me to gain a better understanding of the conditions that can lead to a water hammer please let me know.

 

[bimr]

You should be aware that the water hammer can happen very quickly such as in the time that it takes for a valve to close. The water hammer may occur before the emergency generator has time to start.

The VFD's will help you to minimize the problem.

You also have not mentioned the capacity of the lift station or the destination of the outgoing water. However, I suspect that if you use slow closing check valves, you should not create a problelm. I think the recommended check valves for this application would be the weighted hinged type. Good luck.

 

[glapfk11]

Thanks Bimr...the existing check valves are of the weighted hinge type. It is my understanding that this type of valve would help alleviate some of the system pressure by allowing some the flow to drain back into the wet well through the pump as the valve slowly closes. This seems to be in line with your recommendation.

as an FYI we are using dry pit non-clog centrifugal pumps to draw water from a wetwell and to pump it over to a new, elevated headworks (~800 ft of discharge piping). Total pumping capacity is 8.5 mgd. how Does the cacity of the pump station come into play?

 

[semo]

I'll agree with Bimr that the VFD's will help.

I had one instance years ago in a water system where the pumps (vertical turbine) were increased in capacity without upsizing the distribution lines; therefore, the dynamic head also increased which placed a large system pressure on the nearby distribution system. At low points in the system, water heater valves were blowing.

An existing pressure relief valve was removed during the upgrade with the idea that solid state starters would allow us to ramp up the flow at a slower rate and control the surge. This worked fine for the first 80% or so of the ramp; however, the solid state starters did not allow the linear ramping that VFD's will and the last 20% caused a surge that traveled throughout part of the system.

Once the pressure relief valve was added and adjusted, the surge was controlled. It did not eliminate the surge, it just reduced the maximum pressure of the surge to an acceptable level.

Note that this works on pump startup. On loss of power, the flow may decrease quickly causing a negative surge in the immediate area. The pressure relief valve won't help until the surge rebounds and it will only help then if the rebound pressure is as high as the valve setpoint.

If this is a sewer system, will the surge do any harm to the system? You really need a hydraulic analysis of the pump and discharge piping. Another option might be to examine upsizing the discharge piping and thereby reducing the dynamic pressure.

 

[bimr]

Water hammer refers to fluctuations caused by a sudden increase or decrease in flow velocity. With more mass (capacity), you have larger forces to contend with.

http://www.nesc.wvu.edu/ndwc/pdf/OT/TB/TB_Wi03_Water_Hammer.pdf

 

[rconner]

glapfk11,
I believe the paper stanier was probably referring to is at

http://www.pipingdesign.com/articles/piping_surges.pdf.

 

[stanier]

glapfk11,

Paper is here.

http://www.pipingdesign.com/articles/piping_surges.pdf

 

[stanier]

There are two sorts of damage from surge. The obvoius one is pipeline failure. The more onerous and costly is fatigue damage that results in component wear over the life of the asset. Many studies have concluded that the initial cost of an asset is only 12% of the whole of life costs. It doesnt need much increase in maintenance for costs and resources to mount up over the years. Because its incremental it isnt identified by the "beanies" so easily!

That is where VFD drives are a potential for savings cost. Not only can you run your plant more efficiently to cater for diurnal demand but they protect when there is a surge for frequent stopping and starting. They are of NO use when there is a power failure and so the surge analysis is still required for this event.

Building a plant and then testing it to see if there is surge is 18th Century mentality. Much the same as increasing pipe wall thickness and flange rating to cater for a high surge and ignoring the faitgue damage.

We have the tools to analyse surge why not use them? You wouldnt NOT use FEA to analyse the stress in a pressure vessel nozzle or a relief valve sizing program to determine the relief valve in a process plant. So why avoid a surge analysis. Just because its hard and you dont understand it? Thats a fools paradise.

Get help, learn to use and understand surges.

I am a surge analyst and I offer no apologies for pushing my barrow. I am astounded though that professional engineers ignore the requirements of standards that need surge pressure to be taeken account of in their design. Just think what the coroner might say if that was your decision to not do it?