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Skidmore Rotary Vacuum System not pulling vacuum

Good Afternoon All,

I recently got pulled on a job to perform start-up on a refurbished Skidmore Rotary Vacuum, model B-20 - 10,000 EDR, that one of our salesmen sold to a local buidling owner. This is a 9 story building with roughly 250 rooms. All the heat is CI radiators.

System Background:

This building was built sometime in the early 1900's, i believe around the 20-30's. When the owner bought the building in the 80's the 1962 model Skidmore was not functional. This is a one pipe steam system with a gravity fed return. All steam equipment is in the basement. The natural gas boiler runs around 5 pounds.

Because the vacuum unit has not been running since at least the 80's they were relying on increasing the steam pressure in the winter to heat the upper floors and then letting gravity feed back to a separate boiler feed tank. He got tired of cold calls, brought in a contractor and here we are.

Skidmore has a rebuild program for these units. Once it was re-built and on site, I was given the task of helping the contractor with install and start-up. I tried to do my homework as best as possible. I read "25 steps in the repair of a Vacuum steam system." I have read through chapter 12 of the "lost are of steam heating" and followed all the manufacturers IOM.

Due to the above information, the owner performed a trap audit and then ended up replacing all the bad radiator traps. We also made sure to install the equalizer loop correctly and installed a fast acting solenoid valve on  the boiler feed tank discharge.

The problem:

The unit performs its function beautifully when not on the system. When we shut off the return gate valve, the unit charges to -7.5" w.c. and then shuts off, if/when we start to loose vacuum, it kicks back on. It is properly primed and does feed the boiler feed tank the condensate that is returned. The problem is, we are unable to pull the building system down past 1.5" w.c.


Vacuum was checked at the top floor at one of the radiators. We can feel the vacuum, but still read 1.5" w.c.

All the vacuum breakers that were on the system, have been removed.<span style="font-size:16pt">******** IS THIS OK FOR AN ACTIVE VACUUM SYSTEM?*******</span> I'm reading conflicting statements that say some vacuum systems have or don't have vacuum breakers.

My guess is that there is a leak somewhere in the distro/return system. There is one F&T trap on the end of main drip, about 30 feet away from the vacuum unit. The rest of the traps are thermostatic radiator traps.

I should also state that there are (3) condensate return mains coming back to a common header, feeding the vacuum unit. if i valve off the main building return, i can get the unit to pull almost 3" w.c. once i open the valve back up, i drop to around 1.5"-2" w.c.

I really just want to verify that there is not something I am missing.

Any and all input appreciated.




  • jumper
    jumper Member Posts: 2,255
    one pipe return ?

    Is the main at the top of the building? Is the return dry or wet ?

    When you measure suction on a vacuum system you have to account for the boiler temperature. If it's boiling no pump can reduce pressure.

    Why are there traps on one pipe steam ?
  • mturner
    mturner Member Posts: 5
    @ jumper

    I apologize, this is 2-pipe system. The main is in the basement, supplying risers that feed each floor.

    "Is the return dry or wet"

    - These definitions still confuse me but i

    believe that it is a dry return. (The   condensate return piping is

    always above the receiver tank, pitched toward the tank)

    "When you measure suction on a vacuum system you have to account for the boiler temperature"

    - Can you please elaborate?

      I know that our temperature at the unit is less than 180 deg F at -1.5" w.c.


  • mturner
    mturner Member Posts: 5
    vacuum pressure

    One more thing, i stated in. w.c. this is actually inHg.


  • Dave in QCA
    Dave in QCA Member Posts: 1,785
    edited November 2012
    More information needed

    I am a little confused... what is the blue tank in the foreground?  It lookes like its got a big vent coming off the top.  How does the condensate get back to the boiler?

    Generally speaking... Vacuum systems have no vent and no vacuum breakers.  The only point where venting occurs is the exhaust port of the vacuum pump.  It looks like there are two of them on this system, and they go up overhead and the dump on the floor.  Is that what I'm seeing?

    It sounds like your system is not tight and is allow air back in.  You need to be able to draw a vacuum on the return lines.  When the boiler is not making any steam, and thus the traps are open, the vacuum will carry through out the system, into the steam mains and to the boiler itself.  This will lower the boiler temperature of the water, the temperature of the steam, and also cause for very fast and even circulation of the steam/vapor.

    Please take more picture, edit previous posts to removed information about being a one pipe system, etc. 

    Is there a pump on the condensate outlet of the vacuum pump set?  Or... perhaps a solenoid that the tells the pump when to send condensate to the boiler?... or is it simply gravity?  Again, why the big tank in the foreground?

    Do any drawings exist from when the building and boiler system was built?
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
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