Thread: Pump Questions

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    Default Pump Questions

    Hi all. Have a few more questions I've been pondering.

    1. On a centrifugal pump, I understand that at 150 psi the pump is at 100% capacity, 200 psi is 70% capacity, and 250 psi is 50%. I am just wondering what is actually happening inside the pump that is causing the reduction in flow? Is this just the dynamics of the impeller?

    2. A pump textbook I have states that pumps up to 2000gpm are rated for 10 feet of lift. But pumps over 2000 gpm are rated to only 6 feet of lift. Why does a larger pump have less lift?

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    The falling capacities with increasing pressure come from the pump not being operated in it's designed range. If an engineer designed a pump to move "x" amount of water at "y" pressure, then any variation of that flow amount means the pump will operate less efficiently, thus they're flow rates fall with increasing pressures.

    Impeller designs can vary greatly. Some designs lend better to high pressure than high flow. Some designs can do both, but they require massive amounts of input power to do so. Some pumps require more input power to operate in the 50%/250 range, than they require to operate in the 100% range, but again, this varies with the size of the impeller.

    Design variations include, but are not limited to:
    • diameter of impeller eye
    • double suction versus end suction impellers
    • diameter of the impeller's vanes
    • number and angle of impeller's vanes
    • width of the impeller's vanes

    A "rating" on a pump doesn't tell the whole story. For example, every Q-Max model pump that leaves Hale's facility is capable of pumping 2,250 GPM @ 150 psi net. It's up to the truck builder to install it properly, match it with an adequate power train, and equip it with the right quantity and size of valves to allow the desired amount of water to enter and leave the pump.

    My dept purchases that model, but only rates them at 1,000 GPM. You would think these pumps would flow about 1,250 GPM @ 250 psi net, but my experience is that they'll flow at least 1,750 GPM @ 250 psi net, and was when the pump cavitated. Had I introduced another intake line, it would have flowed even more. This kind of performance is not uncommon in modern fire apparatus.

    About the lift issue. I don't know how old the text have is, but present NFPA standards allow any pump, up to 1,500 GPM, to be tested from a lift not exceeding 10'. You can test it at 3' if you like, but nothing greater than 10'. 1,750's should not exceed 8', and 2,000's and up should not exceed 6'.
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    Centrifugal pumps don't really like pressure AND volumes at the same time. Mainly due to the way the pump operates and the way the scroll dumps the water. The higher pressure in the lines creates large flow restrictions and this basically stacks the water in the pump. When that happens you get a reduction in GPM flow. It also causes a LOT of pump heat due to the friction of the water in the pump.

    Larger impeller in the pump takes more water to keep it fed. With the loss in the lines and the loss of pressure on the larger intake size due to surface area you lose lift height.
    The centrifugal pump doesn't actually have a lot of power to pull water because it isn't a positive displacement pump. It relies on atmospheric pressure on the water in the pond/creek to push the water up the suction. The larger diameter suction and the weight of the water in the suction both work against the pumps limited vacuum.

    Of course you can mitigate both problems to some degree.
    Feed the pump with 100PSI water and you increase the volume quite a lot at higher pressures because the pump isn't working against itself and you eliminate the lift problem.

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    Quote Originally Posted by txgp17 View Post
    About the lift issue. I don't know how old the text have is, but present NFPA standards allow any pump, up to 1,500 GPM, to be tested from a lift not exceeding 10'. You can test it at 3' if you like, but nothing greater than 10'. 1,750's should not exceed 8', and 2,000's and up should not exceed 6'.
    I have never heard of reduceing the ammount of lift after 1500 GPM pump. I find this very interesting and would very much like to read the information from where ever you got it from. As all the books and standards I have read indicate that a pump must be tested at 10' of lift through 20' of suction.

    On a side note our main attack pumper has a 1750 GPM Waterous pump that has been rated at the standard of 10' of lift through 20' of suction. And I know for a fact because I have done it I can draft up to 15' with no problems meeting at least 1500 GPM. And I can do 10' lift and flow over 2200GPM through 2 suction lines at over 20' each.

    I guess an easy way to look at it is basicly a standard is a minimum and it is not uncommon to exceed the standard.

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    Quote Originally Posted by Doorbreaker View Post
    Larger impeller in the pump takes more water to keep it fed. With the loss in the lines and the loss of pressure on the larger intake size due to surface area you lose lift height.
    The centrifugal pump doesn't actually have a lot of power to pull water because it isn't a positive displacement pump. It relies on atmospheric pressure on the water in the pond/creek to push the water up the suction. The larger diameter suction and the weight of the water in the suction both work against the pumps limited vacuum.
    I don't want to knit pick, but all pumps rely on atmospheric pressure to get lift. The only way to lift water beyond the 33ft limit at sea level is to somehow increase the atmospheric pressure at your water source or put the pump closer to the source so it can push the water higher instead of "sucking" it up.

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    Quote Originally Posted by txgp17 View Post
    For example, every Q-Max model pump that leaves Hale's facility is capable of pumping 2,250 GPM @ 150 psi net.
    Pretty much true. The Hale rep at the Texas A&M school's Pump Maintenance course a few years back told us that all pumps in the same model line (be it Q-flo, Q-Max, etc.) share the same impeller and volute. The only difference was the gearing in the pump transmission.

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    fordrules: The question about why different pressures and volumes has to do with the motor torque and horsepower curve. Think of impeller width as controlling the required torque of the engine and thus the volume of the pump. More volume = more required torque. The surface ft. per minute at the rim of the impeller determines pressure so that the transfer case ratio sets where in the rpm curve the pump will produce the desired pressure. Finally pressure times volume times a constant equals required horse power. Originally gasoline engines were used for power and the torque curves for them are relatively narrow as well as very narrow horsepower curved situated at much higher rpm. Thus it was necessary to have a two stage pump to allow the P.O. to match engine power curve to the type of discharge desired. High pressure at 1/2 volume = series operation. High volume at 150 psi = parallel operation. With modern high torque diesel engines it is much easier to achieve both pressure and volume from a single stage pump. With a front mount pump on a gasoline engine, the P.O. can quickly reach a point where the motor is developing all the volume it can deliver, and the throttle is screwed all the way out, but the pump can't reach the desired pressure. Not enough horsepower to wind up the pump to reach the desired pressure. Take a 1500 gpm pump at 150 psi. 1500 gpm times 8.33 lbs / gal = 12,495 lbs per minute. 150 psi is a head pressure of 330 feet so the product is 4,123,350 ft lbs / min. Then divide this by 33,000 ft. lbs./ min (1 HP) to convert to water horsepower, or 124.95 HP but there is an efficiency loss through the transfer case and in the pump. Do the same calculation for the 200 psi and 250 psi points on the builders plate. The 250 psi point will require a little less horse power, but at a lot higher rpm.
    Last edited by KuhShise; 03-03-2010 at 08:32 PM. Reason: correct calculations

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    Thanks for all the replies. The pump size vs lift i found in an article on hale's website "priming midship fire pumps" page 7

    http://www.haleproducts.com/_Downloa...%20Answers.pdf

    "Pumps 2000
    gpm and over are rated at a 6 ft. lift."

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    Quote Originally Posted by MColley View Post
    I have never heard of reduceing the ammount of lift after 1500 GPM pump. I find this very interesting and would very much like to read the information from where ever you got it from.
    NFPA 1901, 2009 Edition. Page 49, Paragraph 16.2.3.4.2 and Table 16.2.4.1(a).

    Those limits are placed on the testing of pump. As I stated earlier, many pumps can exceed that in real world examples. But for the purpose of pump testing, we're not supposed to exceed those heights.
    Last edited by txgp17; 03-06-2010 at 01:47 AM.
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    Quote Originally Posted by txgp17 View Post
    NFPA 1901, 2009 Edition. Page 49, Paragraph 16.2.3.4.2 and Table 16.2.4.1(a).

    Those limits are placed on the testing of pump. As I stated earlier, many pumps can exceed that in real world examples. But for the purpose of pump testing, we're not supposed to exceed those heights.
    Thanks for the referance I checked that out this morning. I found that very interesting to read. I find it quite interesting actually becuase I know our 1750 was tested at 10'. Now in saying that out 1750 might actually be a 1500 as I am not sure wheather it is rated in imperial gallons or u.s gallons or what it was tested in. Unfortuanatly I'm not sure if it is just our mechanical division or the Canadian manufactures that do it but they pumps and tanks are bought in one unit of messaurement and rated in the other. And off the top of my head I can't remember which way it goes. It just goes to show though how confuesing ratings can be.

    And of course it probably doesn't need to be said again but standards are the minimum so it is quite common to exceed the ratings.

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