Vacuum pump head scratcher

NYLamb · November 2022

Hello all,

I work at a (once high pressure) housing development that was converted to low pressure in the early 2000s. We utilize vacuum pumps to distribute steam throughout the 23 zones on the property (150 buildings) and have a head scratcher going on in one of our zones.

We have 23 of these https://media.statesupply.com/filemanager/4/2/4280.pdf Mepco vacuum pumps distributed throughout the property.

The problem vacuum pump in question is not building vacuum beyond 9". It sounds a little whiney but we've taken the entire pump apart. Impellers are all good, check valves are holding, no obstructions on the pump inlets/orifices, and the pump is taking condensate.

Checking the trends, when our steam valves start to cut back and close off overnight, the pump starts building vacuum again (as much as 21" when the valves completely shut).

The zone contains 6 buildings and when checking how the steam is distributed among them, the steam distributes two ways, with the shorter run ending in the building containing the vacuum pump.

Our initial thought was that the pump was getting bogged down with excess steam attributed by bad traps blowing by. Changing all problem traps led to no change.

We tried adding city water to the pump to cool down the condensate and saw no change.

We've bled air via the petcock over and over assuming it's air-bound. No change.

We pressurized the zone and found a few leaks in the system but nothing substantial. Those were corrected to no avail.

The most baffling part to me is the pump is incapable of building a vacuum even with condensate isolated from the machine, yet builds vacuum overnight when steam is secured (which led us to believe the issue was temperature related).

Thought we were on the right track with the replacement of traps but turned out not to be the case. Wondering if anyone here has any ideas. With the weather getting colder we're looking to get this issue resolved fairly quickly.

Thanks

Jamie Hall · November 2022

You say you have isolated the condensste from the pump, but is the condensate actually isolated from the vapour stream going to the pump? And, if not, what is the temperature of the condensate? The boiling point of water at 9" of vacuum is somewhere around 190 F, and if the condensate is hotter than that and has access to the vapour stream going to the pump, that's all you are going to get.

Steamhead · November 2022

Hopefully @Pumpguy will chime in here.

Pumpguy · November 2022

I would concur with @Jamie Hall. Condensate temperature is a limiting factor when it comes to vacuum pump performance.

Before we move on to other possible causes of poor performance, we really need to know what temperature condensate this pump is seeing. We should also know if the temperature is constant, or if it varies. If it varies, what is the range? And how does the vacuum pump perform in relation to the various temperatures?

I would also be curious to know the exact model number of the MEPCO pump set in question, as well as the EDR load of the system's radiation, how that EDR load was determined, and the system's steam supply pressure.

Valving off the vacuum pump package from the return line and seeing what maximum vacuum can be achieved will tell us the maximum theoretical vacuum that can be achieved, but that tells us nothing about the pump's air removal capacity in terms of CFM. When it comes to vacuum pumps, the CFM is the most important factor to consider.

We can discuss these issues later, but first lets find out what condensate temperature we're dealing with and how it affects the vacuum pump's performance.

EBEBRATT-Ed · November 2022

I would usually suspect traps blowing through but the OP says that has been addressed

NYLamb · November 2022

Pumpguy said:

I would concur with @Jamie Hall. Condensate temperature is a limiting factor when it comes to vacuum pump performance. Before we move on to other possible causes of poor performance, we really need to know what temperature condensate this pump is seeing. We should also know if the temperature is constant, or if it varies. If it varies, what is the range? And how does the vacuum pump perform in relation to the various temperatures? I would also be curious to know the exact model number of the MEPCO pump set in question, as well as the EDR load of the system's radiation, how that EDR load was determined, and the system's steam supply pressure. Valving off the vacuum pump package from the return line and seeing what maximum vacuum can be achieved will tell us the maximum theoretical vacuum that can be achieved, but that tells us nothing about the pump's air removal capacity in terms of CFM. When it comes to vacuum pumps, the CFM is the most important factor to consider. We can discuss these issues later, but first lets find out what condensate temperature we're dealing with and how it affects the vacuum pump's performance.

While in operation, condensate temperatures fall between 140-150F. We've isolated the pump from condensate and even with cold city water were unable build a vacuum beyond 8-9". These pumps are rated for a maximum of 240 F.

The pump will only build a vacuum when the corresponding steam valve for the zone shuts completely. It pulls down to 22" when this occurs. It's the reintroduction of steam in the system that causes the pump to bog down and remain stagnant at 9".

We installed the pump just last year. It was the 23rd we've installed and the last to be installed. We are just trying to eliminate any possibilities on our end before Mepco comes on site to diagnose being that it's still within it's warranty period.

Pumpguy · November 2022

If the condensate temperatures are as originally stated, we're okay there so long as they're always as stated. I have to say that, IME, that is not always the case.

The fact that we get good vacuum performance some time, and poor at others tells me that from a mechanical standpoint there's nothing worn or broken within the pump. We need to be looking for something variable that the pump is seeing.

My experience case in point, when the thermostat calls for heat and the boiler is making steam, we get high temperature condensate that causes the system vacuum to fall off.

When the thermostat is satisfied and the boiler shuts down, the condensate comes back cooler and the vacuum levels return to normal.

I'd like to see a chart showing how the condensate temperature varies over a 24 hour period, and what the vacuum readings are over that same 24 hour period.

NYLamb · December 2022

Pumpguy said:

If the condensate temperatures are as originally stated, we're okay there so long as they're always as stated. I have to say that, IME, that is not always the case. The fact that we get good vacuum performance some time, and poor at others tells me that from a mechanical standpoint there's nothing worn or broken within the pump. We need to be looking for something variable that the pump is seeing. My experience case in point, when the thermostat calls for heat and the boiler is making steam, we get high temperature condensate that causes the system vacuum to fall off. When the thermostat is satisfied and the boiler shuts down, the condensate comes back cooler and the vacuum levels return to normal. I'd like to see a chart showing how the condensate temperature varies over a 24 hour period, and what the vacuum readings are over that same 24 hour period.

Sorry for late reply, the problem was resolved... After trying everything on our end, we reached out to manufacturer.

There is a stainless steel Venturi here (essentially a thick washer) on each of the pumps. The manufacturer was doubtful they were the issue considering how new the pump was but gave us a set anyways. To the naked eye, the set we received was identical to the set we took out but they were in fact the problem.
Most likely sized wrong from the factory by a hair because originals had zero wear.

Under load vacuum now pulling 21" plus.

I thank you for your responses and apologize for my late one!!!

EBEBRATT-Ed · December 2022

glad u found the fix

Vacuum pump head scratcher

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