Need opinion on large steam system

Mike_Sheppard

Some background:

There’s a building I do work at in Baltimore. Two years ago they had their boilers replaced. It is a large building. I can’t remember the amount of floors off the top of my head, but let’s say 15. They had old firebox boilers. The system used vacuum pumps and was operating at 7-10 psi because they also use steam heat exchangers for hot water. Boilers maintained 10 psi for the heat exchangers and they open and close a motorized valve for the building.

Well obviously the system never worked well. They tried to master trap the building because they didn’t want to look for bad radiator traps. They decided it was time to replace the system. It was a multimillion dollar job. They replaced the boilers. Removed the vacuum pumps. Installed a standard feedwater unit. Still master trapped the building. Didn’t fix any radiator traps. Still using motorized valve for building and running building at 10 psi.

I want to propose to them some form of solution to restore the system to what it should be.

I know they need the higher pressure for the heat exchangers for hot water, and that requires constant pressure available, hence the need for the motorized valve.

What do you guys think of leaving the motorized valve, putting a pressure reducing valve before it to limit a maximum of 2psi in the heating system, and putting a new vacuum pump system in? Obviously they would need to go through the building and replace traps because if not it would destroy their vacuum pumps (like it kept doing to the old ones).

Has anyone here had experience with this kind of situation? Has anyone needed to use a pressure reducing valve in a heating system? Looking for any advice.

EBEBRATT-Ed

Sounds like a good plan to me. I only see 1 potential issue (other than getting the $$$$$ for the upgrade)

I would consult with Sarco or Spence or Watson McDaniel someone who makes steam pressure reducing valves to see if this can be done. Because you are only reducing the pressure a few # sizing of this valve is critical. They may want 2 or 3 smaller valves in parallel for control purposes.

Once had a job in Hartford Ct where than ran 30# for some reason and reduced it to low pressure 5 psi.

Mike_Sheppard

@EBEBRATT-Ed_9 good thinking. I will look in to that. We have a complex here in DC that heats from a central plant and sends high pressure steam out to multiple buildings where the pressure is stepped down. But that’s a much larger pressure differential than this.

RedHook

Hi Mike,
Is the valve line sized? You could size the valve to take a larger pressure drop than the existing valve resulting in a lower system pressure. Do you know the EDR of what the valve is servicing?

What controls the valve? I assume it is floating or modulating, not on/off.

Steamhead

@Mike_Sheppard , which building is this?

I wonder if this was originally a Dunham Vari-Vac system?

RayWohlfarth

Mike If this were mine I would suggest steam trap inserts for all the traps and be sure you had vacuum breakers on either side of the zone valves. I would look to install a domestic water heater and turn the pressure down to 2 psi

JUGHNE

I agree with Ray.
That boiler runs 10 PSI year around just for DHW?
Small hospital here had that situation. Boiler room in July was about same temp as water delivered.
Installed 2 HTP SS DHW for less than the replacement of the 40 year old tank and steam HX would cost. Plus higher efficiency for DHW production and cooler boiler room/building.

ttekushan_3

Either way, getting the steam pressure down to about 2 PSI for space heating is what you want.

And yes, you want the vacuum return restored. @Pumpguy can help you out. The whole return system was designed and sized for it!

Agree the steam trap cage units should all be changed at the radiators or coils.

Finally, lose the master trap!

So I concur with your thinking, the question being how to handle DHW. One thing about steam is how outrageously fast hot water can be heated and the resulting very short recovery time. The application will be your guide. You may very well be served by separating that function.

One More Thing: this being an imperfect world, I suggest mounting an aquastat at the vacuum pump inlet that can interrupt the motor starter for the vacuum pump(s) if high temperatures are encountered. For example, a liquid ring vacuum pump begins to lose effectiveness as temps rise above 140°F or so. I set it around 160°F so pump damage can't occur at all. It's easy enough to accomplish with a setup which has a separate vacuum pump motor. At this juncture you have that choice to specify such an arrangement.

While this does interrupt the vacuum, it does so when the system is fully saturated and air is already purged. Vacuum is always available when a call for heat is initiated and when the system cools down so as to return condensate after the call for heat has ended. Most of the time it won't interrupt the vac pump at all.

Long and short of it is, I like a vacuum pump to last a LOT longer than the 1st failed steam trap!

Pumpguy

Thanks @ttekushan for the recommendation.

In addition to what has been recommended above, I would recommend splitting the returns; high temperature condensate from the heat exchangers flow into a separate, possibly high temperature 2' NPSH type, condensate pump, and cooler low temperature condensate from direct radiation into the vacuum pump.

Both units would discharge their condensate direct to the boiler feed tank.

Another idea would be to use a PRV on the direct radiation steam supply, and reduce this steam down to 1.5 or 2 PSI.

This arrangement would probably negate the need for a high temperature cut off switch for the vacuum pumps, allow the vacuum pumps to operate more efficiently, and add to their life.

If the returns are to remain with all returning to the vacuum pump, I would not recommend the use of any vacuum breakers. The recommend arrangement is to use equalizer lines as shown in the attached file.

ratio

What about inlet orifices on the space heating system, sized for the 10# inlet pressure plus vacuum? Then, it could be modulated somewhat by reducing the vacuum in the system; and you'd be able to keep the 10# system because everyone knows that higher pressure works better.

EBEBRATT-Ed

Need to know more about the hot water load. Sounds like big shell and tube converters or is it just DHW?

Mike_Sheppard

This is what they use for hot water. One of them.

Mike_Sheppard

In the old system the heat exchanger did not run through the vacuum pump.