Improper service for existing pumps?

[jari001]

Hi everyone,

I have a sump pit (see attachment for details) that drains via a lead lag pumping system. The fluid being pumped out is 90^oC WFI (water for injection/very pure water). I am taking the vapor pressure of the water at this temperature to be 526 mmHg. The pit is arranged such that the pumps are bolted to the top of the sump pit, have legs that extend into the pit and have 2" clearance from the floor of the pit. The maximum distance from the surface of the water in the pit to the center line of the pump is ~70 inches (70 inches length of the legs is by best approximate to center line of the pump given the drawings).

Pump details:
GPM = 25
Head (ft) = 110
RPM = 3500
Discharge line is 2"
HP = 3

One of the maintenance techs has noticed that this pit is pumping out slower than normal. My initial diagnosis is that the pumps were
not designed for this fluid temperature, so the NPSHr is too high (Can I get a stupid check on this thought?). The first complicating factor is that the stream of water into the sump generates a lot of steam as it flows into the and waits to be pumped out. Could the steam alone be forming a vapor lock situation that impedes the flow?

If further system details are needed, please ask and bear with me as I hunt for them. I will be on site tomorrow so I will have a better chance to get information.

Thanks,
~J

 

[Artisi]

If the units ran before successfully, then it's a fair bet it's an installation problem not a hydraulic problem - certainly sufficient info has been given to sort it out.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[jari001]

I was able to confirm that the installed pumps are in fact submerged pumps, Ventriflo is the supplier. I am waiting for them to get back to me with the pump curve for the model I have. Strap on flow meter is available and I plan to correlate it to the transducer level readings.

Since we are in shutdown now, I have set out a plan to do the flow testing that's been suggested. Unfortunately, the hot water loop gets deactivated for line maintenance, so I can do a room temp water run soon but a hot water run won't happen until mid/late January. I have also requested the techs disassemble the pumps and check valves for visual inspection as part of the flow testing experiment.

 

[LittleInch]

Good feedback. Let us know how it goes.

room temp test will sort out pump damage or passing NRV so could find / eliminate some aspects of what might be going on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[QualityTime]

[ol 1]
[li]This pump is very small and is rated at 25 gpm. A strap on flow meter is not very accurate and the accuracy is dependant on a lot of things. I assume this sump is not that big?[/li]
[li]Would it be easier to uncouple the discharge pipe and use a stop watch, bucket, throttle the pump discharge and use a pressure gauge?[/li]
[li]I assume you know how to do a pump test to recreate the pump curve?[/li]
[/ol]


I am going to go out on a limb and take a wild guess....LOL. The pump suction is clogged. If it is clogged you run the risk of cavitating. Lets all have a gentlemen's bet on the possible cause! Things are getting a little dull. This should liven it up

 

[JohnGP]

The pit dimensions are known so flow rate can be determined by the drop in level over time. Manufacturer's pump curve will likely be for water at room temperature anyway, so can be directly compared with your test results on cold water. Inspecting pumps and piping components will be invaluable as well.

 

[LittleInch]

A point to look at is whether the pump is actually matched to the duty. You say head is 110ft. Is that measured? Will be interstices to see if you're actually flowing faster than your 25 gpm.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jari001]

Hi everyone,

My contract at this site is ending at the end of this week because the main project I was working on is now done(New Year surprise for me!). I briefed the engineering team on what information I had and recommended the following actions be taken:

1) Test both pumps independently and in tandem with water at both temperature ranges
2) Dissasmble and visually inspect the pumps and check valves while shutdown is still underway
3) Review change control documents to find the design intent documents
4) Recalculate sizing parameters to verify original calculations

I also told the group that my opinions was either new pumps or a heat exchanger would be needed. The less desirable solution was to be okay that both pumps run for a long time to keep the hot water level down.

Thanks for everyone's time, too bad I couldn't come back with a definitive close to this thread...

 

[QualityTime]

A

http://wwwww....So

we are left hanging....

 

[Artisi]

I wouldn't worry too much about a definite close - it's fairly normal here and we have learnt to live with it - masochist's comes to mind.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)