Pump mechanical seal starts leaking with water temperature drop
Pump mechanical seal starts leaking with water temperature drop
(OP)
The pump is a horizonally mounted, closed impeeler, single stage, end suction dual volute, top centerline discharge centrifugal pump, 1800 RPM, 12" suction and discharge.
The service is a chilled water pump that circulates cold water from an AC plant. It initially runs with no mechanical seal leakage, but once the chiller is turned on and the water drops below about 50°F, the seal starts to leak. When the chiller is turned off and the water rises back to ambient (~70°F), the leakage remains. Once the pump is left idle over night, it starts up with no leakage the next morning until the chillers are turned on and the water drops below 50°, then the leakage returns and repeats the pattern. Cycling the pump off/on after the water warms back up doesn't stop the leakage...only sitting overnight does. We’re puzzled as to why the leakage remains when the chiller is turned off and the water warms up above 50°--if it’s thermal pipe strain, it seems this should be relieved once the pipe warms back up to ambient. Perhaps there’s some hysteresis in the pipe strain that relieves itself over night due to structural vibrations or other longer term effect.
I suspect there is likely some static strain preexisting as a result of bad piping alignment, which is then exacerbated by the relatively small temperature change when the chillers are turned on, enough to distort the casing, causing shaft strain and the seal leakage.
At this point, it is impractical to break the piping connections to verify piping alignment. Any other ideas for confirming if static and/or thermal piping strain could be causing the leakage, or any thoughts on what else the problem could be? Both the seal and pump manufacturers are stumped.
The service is a chilled water pump that circulates cold water from an AC plant. It initially runs with no mechanical seal leakage, but once the chiller is turned on and the water drops below about 50°F, the seal starts to leak. When the chiller is turned off and the water rises back to ambient (~70°F), the leakage remains. Once the pump is left idle over night, it starts up with no leakage the next morning until the chillers are turned on and the water drops below 50°, then the leakage returns and repeats the pattern. Cycling the pump off/on after the water warms back up doesn't stop the leakage...only sitting overnight does. We’re puzzled as to why the leakage remains when the chiller is turned off and the water warms up above 50°--if it’s thermal pipe strain, it seems this should be relieved once the pipe warms back up to ambient. Perhaps there’s some hysteresis in the pipe strain that relieves itself over night due to structural vibrations or other longer term effect.
I suspect there is likely some static strain preexisting as a result of bad piping alignment, which is then exacerbated by the relatively small temperature change when the chillers are turned on, enough to distort the casing, causing shaft strain and the seal leakage.
At this point, it is impractical to break the piping connections to verify piping alignment. Any other ideas for confirming if static and/or thermal piping strain could be causing the leakage, or any thoughts on what else the problem could be? Both the seal and pump manufacturers are stumped.





RE: Pump mechanical seal starts leaking with water temperature drop
If you have Taco/Armstrong pumps you can expect leaks.
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
Johnny Pellin
RE: Pump mechanical seal starts leaking with water temperature drop
On this pump, there's zero leakage when the pump is first started for the day, then it first begins to drip when the water drops below about 50°F, then increases to a steady stream with further cooling to about 40°. When the chillers are turned off and the water warms up above 50°, the steady stream leakage continues, even with cycling the pump on and off. Then the next morning after the pump has been off over night, the seal is miraculously leak tight when the pump is started until the water drops below about 50°. This cycle has been repeatable for several days.
So the main questions are:
1) Why does the seal start to leak only at lower water temperatures (but within the seal's design range)?
2) If it leaks because of a temperature effect, why doesn't it stop leaking when the water temperture rises again?
3) What can explain why it seals again after sitting idle overnight?
RE: Pump mechanical seal starts leaking with water temperature drop
Initial thought, you've basically added a lot of details and possible variables to the situation that "it runs for a little while before there is visible leakage."
Start the pump an hour early and leave the chiller off, see if it takes the same amount of time to see external leakage, or if it is really somehow dependent upon the chillers operating.
Just to mention it, is it possible that the leakage is actually condensation?
RE: Pump mechanical seal starts leaking with water temperature drop
It is beginning to sound as thought the chilled water is actually not the cause, but something else so perhaps look at what heats up reagardless of the pump temperature that then cools down overnight.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
LittleInch: The pump is close-coupled...the impeller is mounted directly on the motor shaft extention, with only a short portion of the shaft exposed to the cold water, and as mentioned above to 1gibson, the pump was run for extended periods previously without the chiller, during which it did not leak. So it has seen many cycles of the motor heating and cooling off without any leakage until pumping cold water. I can't get away from the piping system having some hysteresis in the thermal load it is putting on the casing that takes overnight to relieve itself somehow (gradual relaxation due to external structural vibrations maybe?), but nobody else thinks this is the case.
ianhoag: That's a good thought that I'll look into more. I can't imagine that any differential expansion/contraction over such a relatively small temperature variation can be an issue, but i'll try to vet that out to be sure.
Thanks for all the thoughts so far!
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
RE: Pump mechanical seal starts leaking with water temperature drop
http://www.johncrane.com/~/media/J/Johncrane_com/F...
■ Temperature:
Up to 121°C/250°F
■ Pressure:
Vacuum to 13.7 bar g/200 psig
■ Speed:
3600 rpm up to 62mm/2.500"
1800 rpm for seal sizes between 62mm/2.500"
and 135mm/5.250"
900 rpm for seal sizes greater than 135mm/5.250"
that was going to be my next suggestion. taking temp readings on the seal. laser gun will work.
RE: Pump mechanical seal starts leaking with water temperature drop
NegativeZero: It is interesting that the Crane spec sheet doesn't identify a low temperature limit. Both Crane and the pump manufacturer have been consulted on this problem, and both are at a loss to explain how a seal that begins leaking when the water temperature drops, and does not stop leaking when the water temperature is raised again, but miraculously seals again the next day after not running over night.
I found this article yesterday that gave me an idea to explain how the seal "fixes" itself over night: http://en.wikipedia.org/wiki/O-ring
Skip down to the section about the Challenger disaster, that discusses Richard Feynman's demonstration where he drops an o-ring into ice water and then shows its loss of pliability. The key part that struck me is: "The material of the failed O-ring was FKM which was specified by the shuttle motor contractor, Morton-Thiokol. FKM is not a good material for cold temperature applications. When an O-ring is cooled below its Tg (glass transition temperature), it loses its elasticity and becomes brittle. More importantly, when an O-ring is cooled near, but not beyond, its Tg, the cold O-ring, once compressed, will take longer than normal to return to its original shape." Per the seal parts list, 2 o-rings and a gasket are fluorocarbon material (FKM). Does it sound plausible that the chilled water is cooling the fluorocarbon o-rings and/or gasket material to where it loses enough elasticity to begin leaking, and then “will take longer than normal to return to its original shape”, thus not resealing itself for several hours (overnight) later? Although it can't really "return to its original shape" as it is contrained within the seal parts, the sealing force my gradually restore itself over an extended period.
Maybe I'm clutching at straws here, but there has to be a physical explanation for something that is happening in such a repeatable fashion. It has stumped the seal and pump manufacturers as well as other engineers here, and the powers that be here have discounted my theory that thermal pipe strain distorting the pump casing that for some reason takes overnight to relieve itself explains the seal works again the next day, until the chiller is turned on again. Their best plan is to replace a $4000 seal when there is an available repair window to see if that fixes it.
RE: Pump mechanical seal starts leaking with water temperature drop
http://www.johncrane.com/~/media/J/Johncrane_com/F...