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  1. #1
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    Two part ammonia pump question



    What if any, would the detriments be to having a hermetic ammonia pump grossly oversized? My thinking is that one that is too far oversized would cause turbulence in the vessel as the excess ammonia is bypassed through the minimum flow orifice. The pump was oversized with the expectation that additional cooler space would be added soon after the last expansion. However, that has not happened. The pumps had always made noise and the discharge valve has always had to be throttled down to stop the noise (taking into account all other possibilities to eliminate cavitation).

    Secondly, along with the above issue, the bypass and motor recirculation piping goes against manufacturer recommendations. So much so that the pump manual has the warning on page 1 in bold black letters "do not pipe the pump bypass and motor recirculation into a common header". However, both pumps bypass and motor recirculation piping goes into a common header (without check valves) which then dumps into the wet suction return from the evaporators. I found this causes flow through the standby pump and cavitates the pump in service.

    The standby pump is now isolated, but hoping for insight on the rest. Thanks,



  2. #2
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    Re: Two part ammonia pump question

    Interesting case.
    Should be no problem with oversize pump if piping was designed proper for the pump capacity. waste of power tho. return of pump bypass should not cause problems inside vessel if piped proper. Bypass actually supposed to prvent pump from being deadheaded. Pump flow should be controlled at the evaps where there should be hand expansion valves designed for throttling service. Pump discharge valve likely a manual shut off valve and this type valve not good for control per se, probably have to almost close the valve to get any amount of throttling plus run risk of wire drwaing the valve over a period of time. In regards to putting backflow into standy pump would need to see piping to comment but sounds like the two bypass lines are joined before going into common larger return header from evaps.... not a good design. So you valved off the standby pump to solve the problem. Why would you leave the standy pump open to circuit anyway??

  3. #3
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    Re: Two part ammonia pump question

    The standby pump has had the valves open since the day they were installed in 2004. I entered the picture in 2006 and have fought to break a history of "operator hands off mode". Now after proving myself and turning the thought process into refrigeration becoming a profit center rather than a cost center we are moving forward. The pump controlls were set up to automatically switch the pumps weekly. I now do this manually usually on a monthly basis and once the standby is at room temp, check for oil.
    The engineering team outsourced several items in the last expansion and the piping was approved and passed on to the contractor who installed the pumps. My thought process leads me to beleive that the contractor would have caught this, but that's history now. The pipe size was increased during the change though. The lines to each evap are 1" and the metering valves are open 1 turn. The NPSH is 5' but I run the head at about 7' above pump suction centerline.
    Suction pressure is 35-40 psig and there is a 35 pound differential. 125 TH @ 137 GPM. Currently I'm trying to talk the team into a VFD to see if slowing the pump down will illeviate the issue and open the discharge valve fully. However, I have to check with the pump manufacturer beforehand.

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