Amps/kilowatts follow GPM?

Discussion in 'Pumps and Tanks Well Forum & Blog' started by chas2010, Aug 21, 2010.

  1. chas2010

    chas2010 New Member

    Messages:
    3
    Location:
    Novato CA
    I read here that amps follow gpm. I need some clarification. If I close the valve that controls the water out of my well I would expect the the well pump is pumping against an increased head pressure therefore the pump would have to work harder resulting in a higher amp draw, but in fact the amps drop to about a third. Why? If the vavle is closed wouldn't the head pressure climb to a point that the pump shuts down? If it is still running, and it is because there are amps being drawn, where is the water going? Can someone smarter than me explain this whole thing? Thanks all. Chas
  2. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Au contraire; closing the valve reduces the outflow, thus the WORK the motor is doing. Same with a vacuum cleaner, that high whine you hear when the hose is plugged is the motor revving up because of REDUCED work load.

    I never got a third reduction in my bench tests on pumps, but valveman will give you his much longer experience in restricting the flow of submersible pumps. Some more, some very little. But the rule is reduced flow means reduced load. Now, if you reduce to zero or near, you will overheat the pump "deadheading" and burn it up or shut off the high temp cut off. The water isnt going anywhere its just revolving in the stages and making heat from friction.

    If you want maximum EFFICIENCY, you carefully design-choose your pump- to work in its sweet spot [BEP- best efficiency point] most of the time. No so easy and requiring a lot of understanding of pumps and tanks and patterns of use, that may change with the owners and seasons.

    This is regarding CENTRIFUGAL pumps, and other rules apply to the hundreds of other types made

    Maximum current draw in a centrifugal pump occurs in high flow no head situations. Low flow, high head causes the opposite.
    Last edited: Aug 21, 2010
  3. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas
  4. chas2010

    chas2010 New Member

    Messages:
    3
    Location:
    Novato CA
    Ballvalve - thanks for taking the time to repsond. I am still confused about where the water pressure being created, but I will read your explanation several times until it sinks in. Thanks again. I my follow up with another question or two. Chas
  5. chas2010

    chas2010 New Member

    Messages:
    3
    Location:
    Novato CA
    valveman - thanks for your article. I am still confused but I will read your article several times until it sinks in. Thanks again. I my follow up with another question or two for you as well as ballvalve. I really appreciate the help. Chas
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,267
    Location:
    Cave Creek, Arizona
    Water has weight, and it takes energy to push it up a pipe. If the valve is closed the pump is not lifting any water, or weight, so the energy comsumption goes down. It takes very little energy to "circulate" the water in the pump. The more water the pump "moves" the greater the power consumption. A deadheaded pump will overheat, the water will change to steam, and that will cause cavitation, which will in turn damage or destroy the impellers.
  7. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas
    True. And water weighs about 8.3 pounds per gallon, so it takes more horsepower to lift 10 GPM, which is 83 pounds per minute, than to lift 1 GPM, which weighs 8.3 pounds per minute. Deadheading means 0 flow rate, which will destroy a pump. But it takes very little flow to keep the pump from overheating. With most pumps, as little as 1 GPM of flow is all that is needed to keep the pump nice and cool. It is hard to understand because it is counter intuitive. You would be surprised at the number of pump engineers, installers, and manufacturers that do not understand this property. I use this as my yardstick to measure the knowledge of people in the pump industry. If they understand this phenomenon, then they probably understand pumps fairly well. If they don't understand this phenomenon, then you need to find another "pump expert".
  8. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    What I don't understand about the CSV is why you do not get some cavitation at those low flows of 1GPM? Or do you, and todays submersibles with small diameter polymer impellers do not erode?

    I would guess with a stationary metal impeller centrifugal with large diameter impellers, the minimum GPM must increase quite a bit. [?]
  9. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas
    Cavitation happens when the NPSHA is less than the NPSHR. The NPSHR is actually reduced as the flow is reduced. Many times just reducing the discharge flow rate of a centrifugal will eliminate cavitation. Ie; gravel noise from the pump, close discharge valve, gravel noise goes away. Reducing the discharge flow rate, no matter which kind of valve you use, will reduce NPSHR and not cause cavitation.

    At 1 GPM, what you get is more recirculation than cavitation. Recirculation can cause cavitation like wear but, usually on different areas of the impeller. The fact that the CSV always allows 1 GPM, never 0 GPM, keeps enough cool water passing through the pump that the temperature rises very little. As long as the water stays cool, it doesn’t flash when re-circulating, which could cause wear.

    Impellers made with materials of high tensile strength are more resistant to wear from cavitation. Soft brass might wear away when Stainless Steel and plastics, which are normal materials for impellers, work very well. Some impellers are made with an acetal plastic that is even self lubricating.

    Actually it is the increase in the number of impellers that requires more flow for cooling. A single impeller in an end suction type pump doesn’t touch anything. The only heat produced is from the friction between the water and the tip of the impeller blades. Cool water has great lubricating factors. So again, as long as you have 1 GPM of cool water passing through, there is great lubrication and very little friction or heat.

    With pumps that have multiple impellers, there is normally a rubber bushing touching the shaft between each impeller. The more impellers, the more bushings, and the more heat produced. So a pump with 20 impellers, needs more cooling flow than a pump with a single larger impeller.

    A pump with multiple stages also builds more pressure than a pump with fewer impellers. The 1 GPM through a CSV is just an average. It will change from .4 to 1.8 GPM, depending on the differential pressure. So a pump with many impellers will get 1.8 GPM for cooling, while a low head pump will only have .4 GPM for cooling. This way the CSV keeps the minimum flow as low as possible, while automatically adjusting to match the pumps cooling requirements.
  10. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    So then is it correct to say a deadheaded centrifugal pump overheats from simple friction rather than cavitation? HJ mentions steam and then cavitation, but I do not believe you can have cavitation "in" steam.

    As you know there are machines designed to create hot water or steam via controlled cavitation, but said machines have rather large flows of water thru them.
Similar Threads: Amps/kilowatts follow
Forum Title Date
Pumps and Tanks Well Forum & Blog Follow up question on well pump breaker tripping Oct 28, 2005

Share This Page