Nash Jennings Vacuum Pump Losing Prime

Chgo399

I’m somewhat perplexed as to why one of the two Nash Jennings vacuum pumps I operate has recently started losing its prime. During a norma start up steam will be present in the system as well as vacuum. The two vacuum pumps will kick on and get up to around 9” and then one of them loses prime and the system drops to 3” since the single pump can’t handle the load itself. We were able to pipe in a valve on the inlet side of the pump to add water to the pump. When we do that the pump picks up and runs fine until it reaches set point (13”). Once it comes back on it will lose prime again. Any thoughts on what may be happening? Bad check? Damaged pump? From what I’ve read elsewhere, it seems like the liquid ring inside the pump seems to not be present for one reason or another. Thanks in advance!

Pumpguy

@Chgo399, Can you be more specific as to what model (Nash called them SIZE NUMBERs) pump you have?

Perhaps you could post pictures of your unit.

I assume you have a duplex unit where 2 vacuum pumps are connected to the same receiver tank, correct?

There is one series of Nash units where the symptoms you describe are somewhat a "nature of the beast" type of thing, although that series is not typically capable of going out to the vacuum levels you describe.

That said, if you have the unit I think you do, there is an adjustment and some tweaking that can be done that might resolve this problem.

In addition, it is possible to add a separate stand alone vacuum pump piped into this unit. We have done this many times and this is a foolproof way to resolve this kind of problem.

Feel free to call me at your convenience 1-888 853 9963 and we can discuss the details. If you are in the Chicago area, and by your handle I think you are, I might be able to visit your site to advise what can be done.

dopey27177

Nash vacuum pumps were used on low vacuum vacuum systems.

The three inches of vacuum are used to to circulate the steam around the system.

If you need to run your system at a higher vacuum the differential switches need to be adjusted to operate both pumps together till the vacuum you desire.

remember pumps use electricity, your fuel savings benefit may not be justified by the added electrical usage.

Typically higher vacuum occurs on startup because the condensate water temperature is low. as the condensate temperature heats up and gets closer to steam temperatures the vacuum drops down.

When condensate temperatures get into the 180 degree range team will form in the eye of the impeller and some cavitation will occur and vacuum will not increase.

If you are running high return temperatures you may have defective steam traps.

If your building has wet returns the first traps to check out are the drip traps at the end of the steam mains and riser drips.

That being said you may need to replace or repair steam traps at the radiators.

I hope the water you are adding is not from a domestic water source, because if it is you may say good by to your boiler.


Jake

Pumpguy

I believe the pump in question is an older style Nash Jennings Vacuum Heating Pump having both the vacuum rotor and the condensate impeller spinning together on a common shaft.

Almost certainly this is in a commercial or institutional building, and not residential.

I have seen cases where this style of Nash Jennings Vacuum Heating Pump will steal water from the vacuum side of the pump to the water side, which then discharges it back to the boiler, and leaves no water to seal the vacuum side.

Later designs changed certain details and did not have this problem.

If I am correct, a simple solution is to add a solenoid valve to the condensate discharge line, controlled by a float switch. This arrangement will insure there is always enough water in the tank to seal the vacuum pump.

@dopey27177, Nash Jennings Vacuum Heating Pumps are frequently used on high vacuum, subatmospheric systems, operating as high as 22" Hg. vacuum on the return lines.

You are very correct about high temperature's negative effect on the performance of vacuum pumps on steam heating systems.