Pump test 3

[bksprinkler]

One of the drawbacks of working almost entirely in the office is that I miss out on a lot of the field experience. But I recently had the opportunity to witness the startup of a vertical turbine pump. The startup went well, and the pump seems to be operating correctly, but we ran into some trouble trying to do a flow test.

The pump is a 2500 gpm @ 115 psi vertical turbine taking suction from a wet well in the side of a pond. The test mechanism is just an 8" pipe that extends about 20' out the side of the pump house and discharges back into the pond. There are two 8" butterfly valves and a flow meter in the test pipe. With the pump running and the first valve fully open, the second valve was slowly opened. As the valve is opening, the water begins to shoot out of the test pipe, about 20-30 feet over the pond, but the flow is really low. As the vale opens more, the volume of water getting past the valve fills up the pipe and the discharge at the end transitions from a thin stream shooting out the end to a full stream pouring out the end of the pipe; maybe a 5' arch. At this point the flow meter is reading about 600 gpm. As the valve opens more, the stream comes out with more force, but is still solid and continuous.

Here is where the trouble starts. As the flow exceeds 1000 gpm, we start to hear some noises coming from the butterfly. The discharge is still mostly smooth, but it starts to pulsate. As the flow approaches 1200 gpm, the noise from the valve is so bad we have to shut it down. Several more tests, varying the length of the discharge pipe and checking the butterly for damage, and nobody is comfortable being in the pump room opening the butterfly past 1200 gpm. This is where my lack of field experience comes in. In theory, I have a few ideas of what is causing the problem. But, after all the investigating, all I am left with is that there is a problem with the butterfly valve being used to throttle the test.

So, to those with more extensive pump testing experience, is it possible that the butterlfy is causing some turbulence and/or cavitation? My only solution is to replace the butterfly with a gate valve, thinking that the more I can get the gate out of the path of the water, the less turbelence there will be. Any thoughts? Has anyone encountered a similar problem?

 

[bmlxd40]

This is the first time I've heard of an initial pump test using only a flow meter. Usually, a test header, hoses, and Hose Monsters are used, at least in my area. Its also quite common for the pump rep to bring his own set of play pipes, sometimes mounted to a pipe rack on a pickup, or a custom built trailer.

As to the issue with the butterfly valves, I've never personally throttled with the b'fly valves, but I've heard of similar experiences from co-workers, just not quite to the extent that you witnessed. My guess would be the same as yours, cavitation and/or turbulence. It would make me very nervous also, especially since I've personally witnessed the innards of a b'fly valve break off and get lodged in a six inch elbow.


How close are your valves to the flow meter? I know there are minimum distances required from fittings that change the direction of flow, both before, and after, the flow meter (I think six pipe diameters before, and three after, but It's been a while since I installed a flow meter). I cant remember if there is a minimum distance on valves also. Though I don't think it's the source of your problem, I cant help but wonder if the cavitation/turbulence could be increased by the valves being to close to the flow meter, if this was the case in your installation. Either way, cavitation or turbulence can't be great for the accuracy of the flow meter.

 

[bksprinkler]

This is the first test I have seen without a test header. In theory, there isn't much difference, but obviously the theory doesn't help miuch in this situation. The flow meter is more than 5 diameters downstream from the first butterfly, and the second is more than two downstream from the meter, per the manufacturer (it's actually printed on the side of the meter, so you'd have to try real hard to screw that up with this meter).

I can't quite wrap my head around why we would have such a severe cavitation problem with the butterfly. But, I haven't seen a pump test for such a large pump, or one without a header. I'm hoping the gate valve will solve the problem.

 

[FFP1]

I doubt the butterfly valve is the problem (as long as the control valve directly upstream of the fire pump is a gate valve; not a butterfly)

The acceptance test should be completed using a test header (NOT using a flow meter). The test header results whoulc be used to confirm the flow meter is properly calibrated and fully functional.

The cavitation is probably being caused by turbulent water flow due to the water being discharged into the pond. If the water discharge back into the pond is remotely close to the suction pipe for the fire pump, the turbulant water (significant concentration of air bubbles) will draw into the suction side of the fire pump and cause exactly what you are describing.

 

[bmlxd40]

FFP1, I hadn't thought of that, but it makes perfect sense.

 

[pipesnpumps]

Bernoullis principle. As velocity increases in the localized very low pressure zone in the eddy currents on the downstream side of the butterfly valve it flashes into water vapor and immediately collapses back to water downstream.

You can't throttle 115 psi across a butterfly valve! Need a globe valve to do that. I would put it downstream of the flow meter if possible. If not, then well upstream


Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[pipesnpumps]

Re read the thread. Seriously sounds like the valve, air bubbles won't sink down to pump suction. I have heard the exact noise you describe across a BF valve. It sounds like a banshee scream or like rocks are being sandblasted inside the pipe. In my case I added another butterfly valve and took the pressure drop across two valves.

Another thought is could the flowmeter orifice be too small for the pump flow? What is the max flow for the meter?

Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[KDLLC]

Might try rotating the BFV's and flowmeter 90 degrees, 180 degrees or 270 degrees. Since the disk in the BFV is only partially open @ 1200 gpm, it's orientation relative to the upstream fittings might compound the turbulance coming from those fittings. Just a thought.

 

[LCREP]

If you have fire hydrants down stream from the pump, flow the hydrants using a few hose monsters, and see what the pump can do. You will need at least 3-4 outlets flowing to get 3,750 gpm. Take the discharge pressure at the pump. At least you will know if the pump is operating properly. I hate flow meters around the pump. But since this is discharging water back to the pond I do not have a problem with it, and NFPA 25 says it is OK too. Have you called the flow meter mfg to see what they say the problem?

****************************************
Fire Sprinklers Save Firefighters’ Lives Too!

 

[FFP1]

The arrangement would be better with a gate valve directly upstream of the flow meter; therefore, you could eliminate the possibility of turbulent water flow in the vicinity of the flow meter. I am old school and I almost always use a test header if installed.........butterfly valves work just fine (upstream of the flow meter) as long as the upstream valve is fully open and >5 feet upstream of the flow meter. Use the downstream valve to control water flow vaolume (as the original post indicates).

I know for a fact that water arranged to discharge back into an open reservoir can and will be drawn into the suction side of the fire pump (resulting in pump cavitation)........been there done that; resolved the problem after I determined the source of the problem.

 

[bksprinkler]

Thanks for all the responses. They have been very helpful.

I am a little skeptical that the discharge it getting air into the pump suction. It is at least 30 ft away, and even with the pump running at 1200 gpm, it should take quite a while to pull water from the pond into the wet well. Also, we can't hear any cavitation noise from the pump, but that could be because the butterfly is so loud. But the discharge does go back to the pond near the opening to the wet well, so it is possible. Definately something we need to look at.

I am also a little surprised that it would be a problem with the pressure drop across the butterfly. I have done a test with a 295 psi pump discharging to a tank with an 8" butterfly to throttle, and didn't have any trouble. But, that was a 750 gpm pump, so we topped out at less than 1200 gpm.

We do have hydrants downstream of the pump, so if things don't work out with the meter, we can always run a test with the hydrants. But that isn't nearly as easy, and will create a much bigger mess.

 

[b1ueshift]

I've seen similar flow meter arrangements work fine - I don't think there is a fundamental problem with the idea of using a butterfly valve to throttle that pressure and flow.

The pulsing of the flow could be caused by air leaking into the pump. Is there a pressure gauge on the pump inlet? If so, you can see if you have a net vacuum. If there is a pinhole leak anywhere in the suction pipe, it can let a large amount of air entering into the pump.

An air leak in the suction side will cause the pump chambers to fill up with air, which reduces the pumps output flow allowing the air to dissipate then it fills up with air again giving you the pulsing flow.

Incidently, I consider pockets of air in a pump to be very dangerous. I was involved in one project where a high powered pump ripped apart a flexible bellows coupling during a full flow test. I assume it was caused by the shock of a pocket of air passing though it.

 

[bksprinkler]

After replacing the butterfly with a gate valve, the test ran just fine. It may have been a defective valve, but I think more likely it was just a combination of the pressure and high volume of water that caused the problem.

 

[b1ueshift]

Interesting - thanks for the update. I wouldn't have guessed that would fix it.

 

[chicopee]

Butterfly and globe valves should not be used to throddle.

 

[chicopee]

Oops, I meant ball valves and not globe valves.

 

[FIRESPRINK]

I went through most of this thread, but will add that the flow test meters I have used provide specific dimensions to meet for distances from fittings and from the meter. I have installed over 30 flow test meters, we have used them for startups and annual inspections since 1994. They must be installed properly. The butterfly valves are acceptable for this and any flow test type, the Victaulic #735 (10.11 datasheet) venturi flow meter literature states the supply side valve must be 5 pipe IDs from the meter and the discharge valve 2 pipe IDs and throttled. Fully open the supply valve and throttle through the discharge va;ve and IMHO the meter is the BEST way to test. I have been in this industry since 1978, when I started we drove the truck onto the playpipe rack, and then try to balance the flows (I know balance is not required try telling that to some in the industry) through each playpipe tip for up to 8 pipes. With a meter not always but often we have some chattering. Test takes 1/4 to 1/2 the time of same pump hose & playpipe test. Observation-I was supervising a full flow annual pump test for a high rise building where a fire hose had so much friction from the water pulsing through the test hose valve's disc that it BURNED a 2" ID hole in the hose. That was when I began looking for options.

 

[b1ueshift]

I'm inclined to agree that there doesn't seem to be a problem with using a butterfly valve per se. I think either the valve was defective (loose spindle perhaps) or there was some other problem that was fixed at the same time and the contractor didn't draw attention to it.

The manufacturers of flow meters tend to recommend butterfly valves for throttling. I'll go with that for my projects.

 

[inspectortx2]

chicopee

have you ever tried to throttle 3750 gpm with an 8" ball valve good luck. I test pumps on a daily basis and i have put 3750 gpm through a butterfly valve with a header never a flow meter. use an 8" os&y valve and if you still have a problem check you suction screen for holes and think about putting a defuser on your discharge pipe.