HVACHawaii
Mechanical
Could a short pipe length on the suction of an end suction pump cause it to cavitate? Would a suction diffuser help this condition if so?
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Pump Cavitation 2
- Thread starter HVACHawaii
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I respectfully disagree. There are cases where a short suction line can cause problems. A pump can cavitate from inadequate suction head, as previously noted. But, it can also cavitate from suction recirculation or discharge recirculation. Throwing turbulence into the suction of an end suction pump can change the flow rate at which it will experience suction recirculation cavitation. The preferred arrangement is to have 10 straight pipe diameters before the first reducer, valve or elbow. But, this is hard to justify in most applications. I would still fight to try and get 5 straight pipe diameters if possible. It is very common to put a reducer right at the suction flange. And common practice calls for an eccentric reducer, flat on top to avoid a vapor pocket trap. But, if the first elbow upstream turns the pipe in the vertical direction, it will be self-venting even if the reducer is concentric. I prefer a concentric reducer in this circumstance. I expect a number of people might step up to disagree with me. Smooth, stable, flow with as little turbulence as possible is desirable for good operation with no cavitation.
All I have to do is look around my plant for pumps that consistently have high vibration and poor reliability. A lot of them are piped up on the suction with an eccentric reducer directly to an elbow turning the line to the vertical with a block valve in the vertical leg. This takes up the least space and uses the least fittings. But it does not provide the ideal flow to the pump suction. For some pumps it might not make any difference. But, if you have a pump with a high suction specific speed, or if you have to run the pump well below best efficiency point flow, you may have problems that would not exist with a longer straight run of pipe on the suction.
Johnny Pellin
All I have to do is look around my plant for pumps that consistently have high vibration and poor reliability. A lot of them are piped up on the suction with an eccentric reducer directly to an elbow turning the line to the vertical with a block valve in the vertical leg. This takes up the least space and uses the least fittings. But it does not provide the ideal flow to the pump suction. For some pumps it might not make any difference. But, if you have a pump with a high suction specific speed, or if you have to run the pump well below best efficiency point flow, you may have problems that would not exist with a longer straight run of pipe on the suction.
Johnny Pellin
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- #5
HVACHawaii
Mechanical
Thanks Johnny,
This is what I was thinking as well. Do you have any comment whether a suction diffuser could help alleviate this condition?
John
This is what I was thinking as well. Do you have any comment whether a suction diffuser could help alleviate this condition?
John
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- #6
HVACHawaii
Mechanical
A few more details. 6" pipe on the inlet with basket stainer right at the suction flange of the pump, then about 1 foot of pipe with an elbow up. There are no reducers. The pump sounds if rocks are being pumped through it. I know this is not the case. I have calc'ed the NPSHA and NPSHR, and there is about 8' more NPSH avaialble than required, not taking into account the turbulance efffect from the short "straight" pipe length on the suction. Any thoughts are appreciated.
JJ, You're not disagreeing with anyone in the general sense. JRLake said, "short run or short straight run". You're adding that consideration of poor configuration, which could be problematic with either a long or short suction line.
HVACH... bad configuration. Sorry, no. A very very bad configuration. ... Or .. what do you think about looking again for those rocks?
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"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)
HVACH... bad configuration. Sorry, no. A very very bad configuration. ... Or .. what do you think about looking again for those rocks?
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"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)
- Thread starter
- #8
HVACHawaii
Mechanical
Thanks for the very productive response "BigInch". I understand this is not a optimal configuration. I am trying to remedy a problem with the least cost impact to the contractor/client. I am not the designer just trying to fix what was done.
So, back to my quesition. Does anyone think a suction diffuser will help the situation or does anyone have any "constructive" comments that would provide a minimum cost fix?
So, back to my quesition. Does anyone think a suction diffuser will help the situation or does anyone have any "constructive" comments that would provide a minimum cost fix?
Can you provide a sketch showing relative positions of pump, water level etc, and cannot understand the following statement -
" 6" pipe on the inlet with basket stainer right at the suction flange of the pump, then about 1 foot of pipe with
How can you have a basket strainer at the pump suction flange then 1ft of pipe ---- do you mean basket strainer on the inlet pipe then 1ft of pipe to the pump.
Where is the water level (inlet side) in relation to the pump centre line - above or below.
" 6" pipe on the inlet with basket stainer right at the suction flange of the pump, then about 1 foot of pipe with
How can you have a basket strainer at the pump suction flange then 1ft of pipe ---- do you mean basket strainer on the inlet pipe then 1ft of pipe to the pump.
Where is the water level (inlet side) in relation to the pump centre line - above or below.
If you read between the lines, my point is that the configuration is the problem. IMO you're efforts are better spent rectifing that than adding a difuser. But if you insist, have at it.
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"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)
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"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)
I have no experience with a suction diffuser. I assume you are referring to what I would call straightening vanes. My suggestions would be different. First, I would remove the strainer if you don't need it. Putting the strainer right up against the suction flange is generally a bad idea. And personally, I would rather not have a strainer unless I have a history of scrap metal coming from a fractionator. If there are smaller particles in the stream that could pass through the pump, I prefer to let them pass. A plugged up strainer can cause a failure as well as it can prevent one. If you still have problems, you could try some sort of straightening vanes. But with the limited information available, I don't think this is the most likely problem.
Verify that the pump is running at an acceptable operating point relative to BEP. Just because the pump manufacturer says the pump can run down to 10% of BEP doesn't mean you won't experience any suction recirculation cavitation running there. The closer you can get to BEP, the better. If you can, run a test by changing the flow closer to BEP to see if this eliminates the cavitation.
Johnny Pellin
Verify that the pump is running at an acceptable operating point relative to BEP. Just because the pump manufacturer says the pump can run down to 10% of BEP doesn't mean you won't experience any suction recirculation cavitation running there. The closer you can get to BEP, the better. If you can, run a test by changing the flow closer to BEP to see if this eliminates the cavitation.
Johnny Pellin
- Thread starter
- #13
HVACHawaii
Mechanical
Picture speaks a thousand words. Please remenber this is not my configuration.
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- #15
HVACHawaii
Mechanical
Not really because we need to maintain a walking path on that side.
I ... rest my case.
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"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)
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"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)
Install the flexible coupling and the butterfly valve in the vertical line, move the strainer to the 90 bend with some straight pipe between it and the pump inlet flange, alternately reposition the strainer somewhere else in the system.
Certainly streets ahead of what you currently have.
Certainly streets ahead of what you currently have.
Another idea, before you might want to the try the above, is to ensure that the strainer is properly vented. I can't see the suction side of the pipe, if there is a loop or not. I presume the suction pipe of the pump is properly vented.
You can put the strainer on the suction side as suggest by Artisi, but you should install a DP switch to give an alarm. If not you will end-up damaging the pump when the strainers is filled with rubbish.
You can put the strainer on the suction side as suggest by Artisi, but you should install a DP switch to give an alarm. If not you will end-up damaging the pump when the strainers is filled with rubbish.
If you are looking for a quick and cheap fix, I offer two choices. First, as I already noted, remove the strainer. It is pressure drop. I results in turbulence into the impeller. And, it has a high point trap that will hold a vapor pocket. Make up a spool piece that bolts up in the same space and swap it out. The second option has to do with the high point vapor trap. Remove the top cover on the strainer and add a high point vent to the center of the cover. Vent it completely liquid full and try again.
Overall, this configuration does not look that bad. I don’t like the butterfly valve much. We very rarely ever use them since they can present more flow disruption and turbulence than a gate valve. If you are removing the strainer anyway, I would shorten the new spool piece to make room for a gate valve in place of the butterfly. For the price of one short spool piece and a valve, you can easily determine if these are the problems.
Johnny Pellin
Overall, this configuration does not look that bad. I don’t like the butterfly valve much. We very rarely ever use them since they can present more flow disruption and turbulence than a gate valve. If you are removing the strainer anyway, I would shorten the new spool piece to make room for a gate valve in place of the butterfly. For the price of one short spool piece and a valve, you can easily determine if these are the problems.
Johnny Pellin
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