The case of the Failing Feedwater Pumps, this weeks case

RayWohlfarth

So this Friday's case is about a large apartment building with a low pressure steam system. The owner said the pumps on the duplex boiler feed unit (Two pumps, one tank) were failing at an alarming rate. He lost five pumps in five years. The boilers were running at 2 psi steam pressure.
Each pump costs about two thousand dollars. The system has a working water treatment system and they check the readings monthly. As usual, I will let you know the answer Friday morning at 6am EST. In the mean time, have fun detectives figuring this out.

EBEBRATT-Ed

Bad traps, leaking buy condensate too hot

JUGHNE

Trying to pump steam.

retiredguy

I think that the problem was that the pumps were trying to pump too much water and needed a down stream cock or flow control valve to add head to the pump to control the amount of flow. This could overload the pump motor causing premature motor failure and also could cause the pump to cavitate which would ruin the pump impeller.

PC7060

Anyone diagnose the failure mode of the pumps?

jumper

PC7060 said:

Anyone diagnose the failure mode of the pumps?

My question as well. Generally motor burns out more often than pump. Two phasing. Generally one hears cavitation from insufficient NSPH.

So my first guess is that pumps are oversized so that they happily destroy themselves. See oversized pumps for diverse situations. Difficult to explain that bigger or more is not always an improvement.

mattmia2

Stem in the tank is usually the cause of premature failure. Not entirely enough information for why. Could be failed steam traps. Since they say it is running at 2psi i'm going to guess it is a vapor system and that pressure is letting far too much steam in to the emitters through the metering devices so it is blowing through in to the returns and in to the tank.

Pumpguy

The word FAILING doesn't identify the problem. First need to know what type of failure has happened?

Shaft seal and motor bearing failures are most common. Next I would say burnt out motors and cavitated impellers.

Of course there's always customer neglect. If leaking shaft seals and worn out motor bearings are neglected, the pump will rattle itself to destruction and go up in smoke.

I one time had a building manager call in to say "The pumps been noisy for 6 weeks. Can you come out and fix it right away?"

Pumpguy

The view shown doesn't identify what brand of pump this is.

Some pumps like this don't have an external copper tube air vent line for the shaft seal cavity. Instead, they have an internal drilled passage to vent the air. This air vent hole needs to be lined up with a matching hole in the volute. If they're not lined up, air in the shaft seal cavity won't vent, allowing the seal to run dry and overheat.

Even with the holes lined up, they can get clogged during the off season. When this happens, the seal cavity can't vent air so the seal runs dry and overheat.

Changing to an external copper tube vent line will solve this problem.

RayWohlfarth

Lots of great thoughts this week. Thanks for that. In this weeks case, the bottom of the tank was filled with mud. The owner never bothered to clean it. It should have been cleaned especially after the first pump failure. The mud was pulled into the pump and when the boiler was off, the mud hardened and became almost like cement. Here is the link to the case. https://youtube.com/watch?v=zCdqTu3ci7g

jumper

Now that is truly idiotic. OTH better to wreck pump than boiler?