Page 1 of 3 123 LastLast
Results 1 to 13 of 33

Thread: PUMP QUESTION

  1. #1
    Join Date
    Apr 2001
    Posts
    327
    I have a nursing home that has heat pumps in every room and a heat exchanger in the mechanical room tied to the heat pump loop and the cooling tower. The cooling tower pump keeps loosing its prime and the system shuts down. The strainers are clean and the pump will not pull the water from the tower.The tower is not piped properly, the suction comes out of the pump and goes up five feet and outside and down five feet to the tower.We thought the water was flowing back to the tower when the power went out and the pump would lose its prime but I found a check valve in the suction pipe yesterday so it was not flowing back and the power did not go out yet it still went down.I got it to run yesterday by removing the check valve and running the main pump (5 hp B&G) and the 7.5 hp back up at the same time. This got the system up and running but the pump is still cavitating(I can hear it).
    The tower is an open BAC and the sump is full so I am pretty confident that I have a full column to the pump.
    My question is will scaled plates on the heat exchanger cause my pump to cavitate. The water goes from the pump to the exchanger to the tower and back to the pump.The building is three years old, there is chemical in the system. The system has done this in the past maybe three times before this summer,(maintenance guy keeps great records)but this summer it has gone down maybe ten or fifteen times so something is wrong.

    Sorry so long but want to giv e as much info as possible. Thanks for all replies

  2. #2
    Join Date
    Jun 2003
    Location
    in a tree looking in your window
    Posts
    1,175
    Originally posted by cardinal ac
    The tower is not piped properly, the suction comes out of the pump and goes up five feet and outside and down five feet to the tower.
    Sounds like you have an inverted trap there, is there any kind of vent in the horizontal line that goes thru the wall? I dont believe scaled plates will cause your problem, sounds like you have some kind of restriction in the suction piping leaving the tower(birds,racoon,bags,who knows). The inverted trap is going to need to be full of water to get the pump primed, can you get the air out of it.
    If you dont stand behind our troops, please feel free...........to stand in front of them.

  3. #3
    Join Date
    Sep 2002
    Posts
    186
    Sounds like your suction is getting plugged. Check your suction pipe for a rust hole or leaking threads. Did you check your flow rate back to your tower? You might run a temperary hose and footvalve. This would let you know if the suction is a problem. If the discharge is the problem reverse the flow and collect the water so you get rid of the scale. The last time I did this we used a horse tank and some 2" hoses and a gas trashpump. (there pump if they let you)It worked good haven't been back.
    Because chicks dig it.

  4. #4
    Join Date
    Apr 2001
    Posts
    327
    I closed off the pump discharge and bumped the water in the suction side to 30 psi with a hose, to check the piping and the check valve (this is how I found out there was a check valve buried in the insulation at the tower) the check held and the piping held.The only other place is in the tower but nothing is accessible. The screen is clean in the tower.
    There are no vents on the suction line.
    If I can't push enough through the exchanger then I can't be pulling enough through the pump, does that make sense at all or am I grasping at straws.
    The sprayers in the tower do not seem to be spraying very hard but is that an indication of a suction or discharge problem.
    The specs on the heat exchanger say it should have a ten psi drop and my inlet is 23psi and outlet is around five. The temp drop is around 25 degrees. There are no records of what the original pressure drop was or temp rise.

  5. #5
    Join Date
    Sep 2002
    Posts
    186
    Are you sure the ck valve is opening. It can hold pressure but not open all the way, this would cause the problems your having.
    Because chicks dig it.

  6. #6
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,985
    Originally posted by cardinal ac
    I The specs on the heat exchanger say it should have a ten psi drop and my inlet is 23psi and outlet is around five.
    Specs say 10psi and you have 18psi accross the heat exchanger. Sounds like you should look at the heat exchanger.

  7. #7
    Join Date
    Apr 2001
    Posts
    327
    The flapper from the check is laying on the roof so it is not a problem anymore, but I still had a very hard time getting the pump going and it is still cavitating. The heat exchanger needs to be cleaned but I am not sure if it will solve the problem and to take it apart will take at least a day and the residents paying 2-4 grand a month get ugly when the air is not on.

  8. #8
    make sure your piping doesnot have any place where it can trap air. then clean cooling tower, orfices, strainers,and condenser barrell. if that doesn,t work call for service.
    also make sure pump isnt running backwards.

  9. #9
    Join Date
    Oct 2002
    Posts
    970
    I just cleaned out a h/exchanger on a similiar system friday, it was plugged with all kinds of crap. (about 30 plates). You may want to clean it out with that much p/drop and td. Take an apprentice for sure.
    I work with the Chiller Whisperer...

  10. #10
    Join Date
    Sep 2002
    Location
    Deer Trail, CO
    Posts
    216
    The inverted trap on the suction side of the pump will kill you on an open tower. The centrifugal pump cannot lift the water up out of the tower through your inverted trap to the system. Centrifugal pumps have to have a specified amount of static suction pressure to operate. Sounds like if the tower had been piped straight through the wall into the suction side of the pump the system would be better off. Usually we like to see the tower a little higher than the pump to create some pressure on the inlet of the pump but I have seen pumps put only a foot below the tower in a properly designed system.
    Don't let your ego get in the way of a good decision

  11. #11
    Join Date
    Mar 2003
    Location
    Cincinnati
    Posts
    7,977
    Couple of things to look at. You didn't mention if you've looked at the impellor in the pump. And another does this BAC have those perforated tubes on the suction line that act as strainers and assure that you only pull water from the bottom of the basin? I've seen those get plugged up pretty good. Lastly if those spray nozzles are those black plastic ones withe the slot in them pull a few and poke around those headers will get plugged up with scale really bad and reduce flow trhough the tower.

  12. #12
    Join Date
    Jan 2004
    Posts
    1,176
    MDP is right on the money.A pump like yours is not designed to lift water.I was involved in a similar job a few years ago and the engineers solution was to put a check in the suction side of the pump,well the pump did not have enough power to open the check and lift the water.It was a cluster and I was there weekly repriming the pump.I ended up having to change the spring in the check valve to a weaker one so you might want to do the same.it worked alot better after that but it still wasn't perfect.I don't know why engineers don't pull there heads out of there asses when they design condensor loops.The outlet of the tower always needs to be at a higher elevation than the pump or the pump will always cavitate.Also you mention you did pull the check so now you can't shut it down as all that water will flow back into the tower.You really should repipe the loop or reinstall the check with the weakest spring possible and see if that works, another thing you might do is set up the controls ,if there is any, to start both condensor pumps and then shut 1 down.

    [Edited by acjourneyman on 07-18-2004 at 10:16 AM]

  13. #13
    Join Date
    Oct 2003
    Location
    edgewood nm
    Posts
    73

    trap

    I agree with MDP.If your inverted trap is above the water line of tower sump, even by several inches you will have a problem. Unless that pump runs constant even a small pocket of air will stall a centrifugal pump. Especially since you now know that the check valve is out of the loop. Everytime the pump shuts down the level in the suction line will equalize with the level in the sump. A dirty HE will further exzasperate the problem.

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event