Storm pump discharge piping banging
Storm pump discharge piping banging
(OP)
We have installed 30 HP storm pumps with about 500 lineal feet of 6" discharge piping. A check valve is installed in the pit near the pump discharge. The pump lifts the water approx 40 feet along the route (inside and outside of the building) with quite a few offsets. The pipe is hung from a trestle and the building steel. The final point of piping discharge has a 10' drop into a pit. A soft start/stop was installed to control the pumps. When the pump stops the pipe bangs violently at some offsets about 160' upstream from the discharge point. This is also near the highpoint of the system.
There is no noise or movement when the pump starts. The check valve slamming after the pump stops could be causing a shock wave but there is no reaction anywhere near the beginning of the run. Maybe the tail end of the system trying to drain the final leg while air is trying to move upward is causing the problem. We are thinking of adding a air relief valve at the highpoint. Any thoughts or experience with this problem?
Thanks in advance.
There is no noise or movement when the pump starts. The check valve slamming after the pump stops could be causing a shock wave but there is no reaction anywhere near the beginning of the run. Maybe the tail end of the system trying to drain the final leg while air is trying to move upward is causing the problem. We are thinking of adding a air relief valve at the highpoint. Any thoughts or experience with this problem?
Thanks in advance.





RE: Storm pump discharge piping banging
I think you are on the right track of the problem.
You observe the banging at pump-stop (soft-stop included) only. Questions for detailed diagnosis (see causes below): One bang at one location or at several locations? One bang or several? If several damped in a sinus-curve lessening way?. Banging near top-point of system away from or near check-valves?
Banging in a system is (almost) always caused by water-hammering. The waterhammering is a sudden rush/stop of the water, resulting in a pressure peak in the fluid, with a changing of pressure direction.
My guess for possible causes (could be several in combination):
a) Pipeline from top-point down will act as a vacuum pump and create an underpressure. When normal air-pressure re-enters this will cause a larger reversing pressure. Pressure will be stopped at the checkvalve (and/or bends).
b)It might, but less likely, be caused by air pockets trapped in the system, and giving back-pressure when the pump stops. (More likely to occur at start)
c) The third possible cause is a wrong type/dimension and/or placement of the checkvalve. In this last case the checkvalve will close suddenly by and after the fluid has started flowing in the reverse direction, and will often be diagnosed by bangs at the valve in a repeating sinus-curve.
d) The checkvalve might be of a type reacting at the lesser fluid at soft-stopo flow by starting gulping and slam shut by reverting fluid stream between the gulps. (Valve is too large to be kept open at the lower flow, but is then more likely to also occur at start up, but not necessarily))
Suggestions for improvement, not giving priority to the single points.
1. Check piping layout and dimensioning, including pump and checkvalves.
2. Change checkvalve to soft-closing type (non-slam nozzle check valves)
3. Checkvalve, soft closing, at outlet because of large length of pipeline.
4. Air outlet/inlet valves at highpoints.
5. Securing/clamping etc of the pipeline to be improved?
RE: Storm pump discharge piping banging
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Storm pump discharge piping banging
as you indicate, if you have a pipe leg draining to the sump then an air valve (possibly a goose neck) at the high point might be useful
RE: Storm pump discharge piping banging
RE: Storm pump discharge piping banging
However, it seems that your pump system is designed poorly. The 6-Inch pipe for this application should have been designed for a velocity of approximately 5 ft/sec.
The 30 Hp pump will probably put out around 900 gpm which is around 10 ft/sec, which makes the design of the piping system more difficult.
You are getting a water hammer when your system shuts down.
Your options are:
1. Throttle the pump back as much as possible.
2. Install a smaller pump.
3. Install larger pipe.
4. Install a vacuum breaker at the top. Note that you can expect some leakage from this device at startup.
RE: Storm pump discharge piping banging
are the hangers free to swing such that the pipe hits the structure? appears you may need some restraint in both the x and y directions at the 10' offset.
does the soft start ramp down properly?
RE: Storm pump discharge piping banging
Thanks to all for the help.
RE: Storm pump discharge piping banging
Above ground piping systems may be designed as restrained or unrestrained. Selection of the design method is dependent on variables such as operating temperature, flow rates, pressures, and piping layout. The high velocity that you have pushes you into the restrained category.
The restrained system is referred to as an anchor and guided design. Anchors are employed to restrain axial movement as well as to provide vertical support. It will help if you install anchors in the middle of the horizontal runs, each time your pipe changes direction.
If you want to add supports, you should work that out with the structural engineer that designed the roof. Many of the roof designs are very economical and do not have an allowance for additional loads.
The least cost alternative is probably to just throttle the pump back.
RE: Storm pump discharge piping banging
acdcal: interesting problem. Seems that the problem is covered (partially) by my suggestion marked a): weight of fluid drawing down, air for pressure-equalizing pressing up, solved by letting air come in on top.
In addition, of course, the ever-present question: why wasn't it equipped according to the rules first time around, with an air inlet/outlet valve at top-point as normally prescribed. (Rethorical question, no answer needed
RE: Storm pump discharge piping banging
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Storm pump discharge piping banging
RE: Storm pump discharge piping banging
RE: Storm pump discharge piping banging
The problem seems to be that the momentum of the fluid when the pump shuts down is causing a vacuum situation to occur. Installation of a vacuum relief valve seems to have cured the problem.
The only question is how the poster obtained the vacuum relief valve so quickly.
RE: Storm pump discharge piping banging
RE: Storm pump discharge piping banging
The soft/start on the pump will eliminate some of the water hammer, but because of the extreme flow rate, will not eliminate all of the water hammer. Surely, the situation would be worse without the soft start.
Here is a description:
As a further example of inertia, Fig. 3.1-a shows a pump discharge pipe. At a very small moment of inertia of pump and motor, the failing pump comes to a sudden standstill, which has the same effect as a suddenly closing gate valve, only this time on the downstream side of the gate valve. If mass inertia causes the fluid flow on the downstream side of the pump to collapse into separate columns, a cavity containing a mixture of water vapour and air coming out of solution will be formed. As the separate liquid columns subsequently move backward and recombine with a hammerlike impact, high pressures develop. The phenomenon is referred as liquid column separation" or macro-cavitation4.
http://www.ksb.com/linkableblob/ksb-pk/80892-19367...
RE: Storm pump discharge piping banging
RE: Storm pump discharge piping banging
Without the surge analysis, I would caution on adding restraints and supports, as making the pipe more rigid can change how the pressure waves propagate and increase the damage.
If macro cavitation is still forming, you will get destruction of the equipment over time even though the sounds have lessened.
The vacuum relief may have completely solved the problem, but without analysis you can never be sure.
Other solutions may be needed such as: a flywheel on the pump, multiple air relief/vacuum relief, surge anticipator valve and/or surge tank(s).
Hydrae
RE: Storm pump discharge piping banging
While I believe flexibility can be very good in some applications or with good lateral support, and I have not seen enough sure details of flow or the pump etc. to know exactly what is going on here (with transients and/or system resonance etc.?), if we are talking about grooved end steel piping on e.g. simple rod hangers and with pretty high flow (with no effective lateral stability provided, as pipes strapped down etc.?), I wonder if this could also be a factor in why system is a little lively?