Well Pump Control & Surge Protection 3

[ChristyB]

I am working on making improvements to an existing water system and have a few questions. Background: 66 homes spread over 6 miles; distribution pipes are 4", 2", and 1.5"; Currently the well's 10 hp submersible pump runs continuously at about 90 gpm, causing the reservoir (located 2 miles away) to continuously overflow.

My preferred pump control option is to use a radio SCADA system to turn the pump on and off based on the reservoir water level. Since I will be turning the pump on and off, I am concerned about pressure surges, but don't want to make this system more complicated than it needs to be.
> Is a pump control valve needed? It seems like they require a lot of additional valves (surge anticipator valve, check valves, and isolation valves) plus a way to discharge water
> Is a check valve sufficient? (there is aleady a check valve installed)
> Would a pressure reducing valve work?
> Would a cycle stop valve work?

I am also not sure about needing to install a soft start for the pump since it would now be turning on and off. Are there any advantages to intalling a soft start? Are they just an added expense that isn't really necesary? Would having a softstart reduce the potential for pressure surges?

Sorry for the long list of questions. I would appreciate any insight you can give.

Thanks.

 

[bimr]

Further information is needed before one can make a reply.

Is there an elevation difference?

When you say reservoir, are you talking about a water supply tank?

And are you pumping directly from the well to the tank?

Or are you pumping into the water distribution system?

Well depth?

 

[ccor]

Only one way to find out, do the calcs. Bimr is right though, can’t tell from you post what happens between the pump to the pond?

The 4" line at 90 gpm results in a velocity of 2.3 ft/s so surge will likely not be a problem unless you have a large static lift. If all 90 gpm is going through the other lines (9.2 ft/s & 16.3 ft/s) it's probably significant.

Joukowski's (I’ve seen it spelled three different ways) equation is probably adequate here if the system is not looped. You will need to know the existing pipe material and wall thickness.

IMO soft starts are usually a waste of money for pumps of 20Hp or less.

 

[ChristyB]

Here is some more information:

The elevation difference between the pump and the reservoir (really an 18,000 gal concrete tank) is about 300 ft.

The well pumps through the distribution system to the tank.

The line between the well and tank is 4".

Other lines in the distribution system are 4" and smaller.

The system is not looped.

The well is 120' deep and the static water level is 86'.

I am not sure about the pipe wall thickness. They are a mix of new and old PVC lines.

Where do I find Joukowski's equation?

Thanks

 

[ChristyB]

Based on ccor's comments, I looked up in previous threads about the Joukowski's equation and found that the pressure increase, DH = c (wave speed) * Dv (change in pipe flow velocity) / g

In my Pumping Station Design Book, I found that the wave speed for PVC pipes is typically 1000 - 2000 ft/s, to be conservative, I used the maximum. Since the maximum change in velocity is about 2.4 ft/s, I get the pressure increase to be 149 ft, or 64.6 psi. My starting pressure is about 130 psi, so if I understand things correctly, the pressure surge would be 194.6 psi (give or take a bit).

Would this pressure surge only occur near the pump, or would it occure throughout the system?

Christy

 

[bimr]

You definitely should have a pump control valve if you will be starting and stopping the pump. The pump control valve should include a slow closing check valve. Contact Cla-valve for recommended sizing.

The pump control valve will eliminate most of the pressure surges when starting the pump. The slow closing check valve will decrease the back surge should a power failure occur.

If you are discharging into the top of the reservoir, your suggestion of a level control system along with SCADA on the reservoir will work.

Turning the pump on/off is probably the least expensive method to control the system. If you are pumping into the bottom of the reservior, you can also use an inexpensive pressure switch to control the pump operation (instead of level control).

You may want to consider the use of a variable speed drive for the well pump. The benefit of the variable speed drive is that surge pressures from the starting/stopping of the pump will be eliminated.

 

[BRIS]

The pipe network can obviously accommodate the surge pressure because it is existing and it has not blown up yet with switching on and off of the pump!.

You are going to with on and off much more frequently. The problem with uPVC pipe is it suffers fatigue and needs to be down rated for surge pressure. The amount of down rating depends on the frequency of the surge. Unlike ductile iron pipe surge pressures in PVC pipe have to be accommodated within the pressure rating of the pipe.

A 10hp pump is not a big pump and adding a soft start/stop should not be too costly– I would be tempted to add a soft start /soft stop controller.

Controlling the pump on prssure may not be easy if the rising main is also feeding into distribution.

 

[bimr]

Bris,

ChristyB has posted that the pump operation is being changed from a situation where the pump is running constantly to possibly an on/off scenario.

It should be obvious that the surge pressures are going to be more of a problem with the new operating scenario.

The much higher short term pressure rating of PVC pipe allows one to take advantage of plastic pipe's inherent property of being able to handle short term pressure surges. Non-plastic pressure pipes display insignificant difference betweent short term and long term design strength.

Having said that, it is prudent to design an installation to minimize surge pressures.

 

[BRIS]

I relise the change but presumably the pump has been turned on and off over its life to date and nothing as blown up. The frequency of surge pressure increase with the new scanario and PVC pipe is susceptable to fatigue failure so there may be a long term problem. Typical de-rating factors are: (UK Water research Council).

Fatigue re-rating factors

Hourly Total factor
frequency cycles
50 years
0.2 73,000 0.7
1.0 438,000 1.3
2.0 876,000 1.5
5.0 2,190,000 2.0
10. 4,380,000 2.5
50. 22,000,000 4.0
two starts per hour requires a derating of 1.5.

For a pipe velocity of 0.7m/sec and a wave speed of about 360 m/sec the surge pressure by Joukowasky equation will be about 2.5 bar. If the pipe class is 12 bar and there are 2 starts per hour then, unless I was to make a more rigorous calculation, I would design for an allowable long term steady state pressure of 6.3 bar. (12-2.5)/1.5

 

[Bjegovic]

first of all, transient analysis shall be done independently of the pump operating mode in the design stage, considering worst case scenario (stop can occur due to power failure, for example - don't expect that VFD will help you there).

For this system, you shall go with start/stop mode to save water and energy, or design the VFD to keep constant level, or... consider also functioning of water disinfection system or other plant components.

there are also many ways to relieve water hammer.

VFD, Soft starters, PR and isolating valves are not of much help, theoretically speaking.

Some pressure relief valves, soft closing check valves, vacuum breaker valves may help. Routing of pipeline is crucial in water hammer calcs (vacuum can be more dangerous that overpressure for PVC pipes), many other factors shall be considered.

However, a 0.5 m3 hydrobox i.e. surge tank (serially made, shall not be expensive) will clear any doubts you may have regarding transients, given that the system already works.

 

[stanier]

see

http://www.pipingdesign.com.

There are papers there on Avoiding Surge in a Piping Systems. Also one on the Risks associated with surge, commercial and technical!

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[QQstuff]

a pump control valve is a main valve with different pilotary installed, so you only need one valve for pressure reducing and such and such. Pump control valve can open slowly and close quickly, it can do more than a check valve which only close quickly.