Water Hammer Pump Sudden Shut off
Water Hammer Pump Sudden Shut off
(OP)
I would like to know what the effects would be if a pump had to suddenly be stopped due to, for example, a power failure. Is there significant risk of water hammer? What measures are recommended for preventing water hammer?
In my specific situation, I will be running 3 pumps, sucking and discharging into common manifolds, with each pump delivering around 870m3/hr. Suctions 18" and discharges 16".
In my specific situation, I will be running 3 pumps, sucking and discharging into common manifolds, with each pump delivering around 870m3/hr. Suctions 18" and discharges 16".





RE: Water Hammer Pump Sudden Shut off
Reverse flow can be avoided by a check valve, however it may still be a good idea to take a look at what can happen, if the check valve malfunctions or breaks.
There are a number of ways to prevent or mitigate transient surges, a fast acting control valve, a soft starter, a surge "accumulator", a pressure relief valve (although sometimes they might complicate the problem), a recycle valve back to suction that can be slowly closed. Best way is dependent on how you want to operate and the cost of the device.
See this recent thread for another case I just finished discussing.
thread378-222072: Software solutions
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
RE: Water Hammer Pump Sudden Shut off
The theoretical maximum transient pressure can be found by assuming that velocity head (v^2/2/g) is totally converted to pressure head, which you must both add and subtract to the operating pressure at any point within the system to find the total pressure. Add transient pressure to the operating pressure when the wave is outbound and subtract on the return. Actually some energy is dissipated via friction and expansion of pipe and product as the wave travels, so it should be less than the max, but reflections from any points where the velocity changes, at reducers, tees, etc. can affect the original waves at inopportune times, causing two or more transient waves to add together, so the "theoretical maximum" is not including reflected additions when multiple waves are possible.
A computer analysis is really the best way to solve anything other than a very simple system.
And you're absolutely correct in that VFDs might be nice for startups, but do not help at all in power failure situations. Transients have also been known to shatter the disks in check valves ...among other things.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
If I am wrong, please be gentle.
By the way, once a motor has tripped, there is no connection to permit a "back surge". However, the other two pumps will abruptly shift to a different point on the pump curves. I would not be surprised to see a load surge developed by the two running pumps. This may be mistaken for a surge from the tripped pump.
Thanks.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Water Hammer Pump Sudden Shut off
Yes it is correct that the transient is positive on the upstream side and negative on the downstream side of a closed valve. The downstream wave loses amplitude with friction and compression of product ahead of it. When it is reflected, it continues to lose energy on the way back and the amplitude of that one wave is always less when it returns. If the pressure goes below atmospheric pressure, or external water pressure on an offshore pipeline, a pipe of relatively thin wall thickness to diameter ratio might collapse. At least all of that is true for a flat pipeline and a valve that does not have a feedback and control function. If I said or implied something differently, sorry for the confusion.
Now, once you start adding pumps, control valves, relief valves and realistic elevations to the Joukowski equation, it loses its conservativeness. You must consider the action time of the controls in conjunction with the transient pressures wave leaving, arriving or passing by and how much energy you have stored in the pressure expanded pipeline, in the compressed product itself and the potential energy of all product at higher elevations downstream (and sometimes upstream too). If you are pumping to a high elevation, which is what a lot of pumps do, there is a lot of stored energy up there that is more than ready to go back to where it came from. When the pump stops, it can be free to do so. Mr. Murphy says it tends to do it at inconvenient times.
I'm not following what you mean by "connection". Are you saying a discharge shut-in valve, or a check valve closes to "disconnect" the pipeline from the pump?
An example of a case I was referring to would be something like, only one pump running and discharging into a downstream pipeline that has pressured the product and pipeline to its operating point pressure. The pump's motor abruptly stops running and the pump stops turning in a 1/2 second or so. The pipeline is still pressured to operating pressure and that hi pressure product will begin to flow back through the pump's discharge into the suction of the pump, tending to reverse spin the pump, effectively turning it into a turbine. If two additional pumps were running and discharging into the same manifold, they would only make things worse, since the manifold and pipeline would then have a continuing source of product and pressure, instead of a decreasing amount of product and pressure as was the case with only one pump running and tripping.
I've had cases where a slight overpressure is relieved through a relief valve, which stops product flowing into the pipeline downstream as pump discharge flow is diverted to the relief valve. The pressure near the relief valve begins to decrease and the relief valve closes. Product already downstream of the pipeline then comes to a stop and velocity head is converted to pressure head, increasing the pressure and reopening the relief valve again, just as the pump's discharge control valve is opening back up again. It created a continuous cycle between the pump discharge pressure control valve, the relief valve and the pipeline downstream that was difficult to stop without shutting down the pump and completely restarting the system again from 0.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
I misunderstood one of the responses. I thought that someone was expecting a power surge of some type and was pointing out that the trip that had caused the problem would have disconnected the electrical feed to the pump, making an electrical surge from the tripped pump impossible but still allowing load surges from the running pumps.
Sorry for any confusion I may have caused. My bad.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Water Hammer Pump Sudden Shut off
If there is no control valve that slams shut on power failure, and the pump simply stops pumping, where does the shock wave start? I accept that the flowing liquid stores significant energy as velocity head and that this will be dissipated into the pipe supports as it slows down, but I find it hard to imagine this being catastrophic. Shouldn't pipe support design take this case into account?
If the pump is pumping against a zone of stored pressure (compressed gas or high static head) I guess the liquid would reverse quickly after the pump stopped and the check valve on the pump discharge would slam shut and I can see this being a problem. But what happens in the situation where we have a cooling water circuit and the water is simply circulating back to the tower? Is it dangerous to stop this type of pump suddenly (as opposed to closing a valve suddenly)?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Water Hammer Pump Sudden Shut off
I have not looked at a continuous loop, but it seems obvious that the positive wave continues upstream and the negative wave continues downstream and, if the two can add together at any point, trouble errupts. I'll add that to my list for future study.
To tell you the truth, I haven't thought very much about what happens to the supports, but I suspect it could involve rather large forces.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
A piping system that can't tolerate a power failure to a pump won't last very long.
RE: Water Hammer Pump Sudden Shut off
Lord Kelvin [PLA, 1883-05-03]
Stop messing around and go model the system. If you cannot hire someone who can. Then you will have some "numbers" not just "feelings".
Go to www.pipingdesign.com and look at the papers there on surge and its prevention.
regards
Geoff Stone
Surge Analyst.
RE: Water Hammer Pump Sudden Shut off
Stanier: I agree that numbers are essential. However, there is some theory behind the numbers, and even should you get a whole lot of numbers, you need the understanding in order to determine whether or not those numbers are realistic. Furthermore, we don't all have the access to the engineering tools needed to model this, and we also don't all have the backing of management to support the idea of taking such a "possible" problem (because this is all it is in their eyes) to some specialist who is just going to cost the company a whole lot of money. My working environment is not an engineering company, actually it's a farm. So for me, at least this is a starting point to get an idea of how likely a problem this is in my scenario.Thanks for the link.
And thanks all for the input from others...
RE: Water Hammer Pump Sudden Shut off
RE: Water Hammer Pump Sudden Shut off
RE: Water Hammer Pump Sudden Shut off
Centrifugal pumps both start and stop spinning rapidly enough to cause surge.
Shozza,
If the motor becomes a generator, I can only see the pump and fluid slowing down faster, possibly creating a larger net surge, since the motor would be sapping that energy from the spinning momentum of the pump. Am I incorrect? If I am, why?
TechnoZA,
What's the rated power, head and flow of these pumps, suction and discharge pressure, and diameter and length of the manifold and the downstream pipe?
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
To start with, submersibles will push water out of a river to our delivery pump station, and delivery pumps from here to one of our dams.
Submersibles:
3 operate in parallel. (Figures below per pump)
dP = 200kPa
Q = 868 m3/hr
P = 57.7 kW
Delivery Pump Specs:
3 operate in parallel. (Figures below per pump)
dP = 617kPa
Q = 868 m3/hr
P = 180kW
Pumps delivery through 2 x 400mm and 1 x 500mm pipelines in parallel. Manifold dia 600mm
Total system static head 60m
dH of subs to deliveries 12m
Length from subs to deliveries about 80m
Length from deliveries to dam about 4500m
RE: Water Hammer Pump Sudden Shut off
RPM submersible pumps?
RPM delivery pumps?
How high are the delivery pumps over the submersible pumps?
How high is the reservoir over the delivery pumps?
Do you have check valves between the submersible pumps and the delivery pumps?
And between the delivery pumps and the manifold?
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
Submersibles to Delivery Pumps has dH of 12m
Delivery pumps to downstream reservoir dH of 48m
Certainly check valves btw. delivery pumps and manifold. Not 100% sure if there are check valves between subs and deliveries. We're busy upgrading, so could always add.
Thank you!
RE: Water Hammer Pump Sudden Shut off
There will be an equilibrium flow and dynamic head with three pumps running.
There will be an lower equilibrium flow and dynamic head with two pumps running.
When a pump is stopped the system will coast down from one state to the other.
Think shifting your car out of gear and coasting rather than hitting a wall.
The rate at which the system "coasts down" will depend on the inertia of the fluid in the piping, the dynamic and static heads, and on the differential head across the pump.
Think coasting on the level versus coasting up hill.
I would expect the flow to continue through the unpowered pump due to inertia, and the flow rate to decay with no serious pressure transients.
HOWEVER, the flow through the stopped pump will drop to zero and then reverse. At this point the check valve should close.
But, if the speed at which the flow decays and reverses is short in relation to the closing speed of the check valve, so that there is significant back flow through the check valve before it can physically close, the check valve will slam shut generating pressure transients that may damage the check valve and/or other components.
In the event of a power failure, all six pumps will stop. I would expect the flow rate to drop to zero and then reverse. Any pressure transients would depend on the closing time of the various check valves relative to the time for the flow to stop and reverse.
Hello BigInch. If this post has any validity, great.
If this just causes confusion then please RF it.
Thanks.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Water Hammer Pump Sudden Shut off
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
Model the system with pumps at different speeds. You will soon see that the pump does not add to the system after a very small deterioration in speed.
If it was possible to guess engineering data there would not be the 15 or so transient analysis software packages out there that enable you to model such systems. All the graphical techniques would not have been developed to estimate the size of a pressure transients. Books would not have been written and in addition all the inquests undertaken for those killed in surge incidents would have been a waste of time.
I hope you have plenty of insurance and do not spending time in jail.
Down load a demo from AFT and look at a system response to what you are talking about.
The reason you pay for specialists to engineer things is the same as why you engage a lawyer, tax adviser, accountant, plumber, electrician etc etc. You cannot do the necessary work and need help. Judges frown on you not spending the money to engineer things.
Do you guess the wall thickness of a pressure vessel or pipe, adequacy of a foundation, the earthquake resistance of a building? If you do you do not belong on this forum.
RE: Water Hammer Pump Sudden Shut off
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
Stanier:
All I ask is the opportunity to gain understanding and knowledge. I'm not attempting to be an expert, or to make decisions on the matter. The stimulating discussions and ideas are useful to me. At least I get the opportunity to gain some understanding from those with much more experience than me. Your point about the seriousness of the matter and the need for professional advice is taken. Thank you.
RE: Water Hammer Pump Sudden Shut off
c = speed of the wave 600 -1100 m/sec (typically). L = pipe length
If the stop is instantaneous the pressure change (m head) Dh is DV x C/g (DV = change in velocity 2 m/sec, g = 9.81). Dh = 122 to 244 m head (12 to 24 bar). (Joukowsky equation)
Your pipe length is 4500m. The pipe period will be around 10 seconds (time of travel). Submersibles will stop almost instantaneously, the bosste pumps will stop in less than 10 seconds.
Pressure of +/- 12 to 25 bar are likely to be a problem !!.
Take Stanier's advice and run a model.
Brian
RE: Water Hammer Pump Sudden Shut off
I modeled several pump stops both lift and mains and according to my results, its not happening. I don't believe the JoukowskI equation applies since there is no actual valve closure anywhere and I show water continues to be pumped, as the pump quickly slows down, so hydraulically instantaneous or not, pump stops don't get a big pressure build. I agree, as I have noted, my results should not be considered definitive and obviously TechnoZA needs to confirm that, but that's what I got.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Water Hammer Pump Sudden Shut off
(in fact the Joukowsky equation applies to each decrement/increment in velocity caused by pump stop/start)
A by pass around the booster station may be a simple and economic solution here, but I agree with Stanier you cannot risk not doing a study.