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Pump head increased... How??

Pump head increased... How??

Pump head increased... How??

Currently due to the increase of pressure drop through the pipes, my pump can no longer support the pump head. The pump is already purchased; may I know what should I do? I was thinking to change the motor to a larger HP one, then I can at least support about 15ft of head. But do you know how much approximately will it cost to change a 7.5hp to a 10hp motor in a end suction centrifugal pump?

RE: Pump head increased... How??

You can probably increase the impeller size in the pump.  Ask the manufacturer for pump curves for larger impellers, and that will tell you flow at TDH.  Impellers are usually cheaper than a new motor.


RE: Pump head increased... How??

You need to decrease the pressure drop through your piping or produce more head from your pump.  Reducing the dP likely means increasing your discharging piping size (pressure drop is proportional to the ratio of the diameters to the 5th power).  Is the installed impeller in the pump the maximum?  A larger impeller will give you more head, that might drive you to the next sized motor, starter, etc.

You can also look whether you can reduce the flow rate but I doubt that's a feasible solution.

Other options.  Can you lower the delivery pressure or increase the supply pressure?  If you have a control valve, does resizing the control valve to a larger trim get you enough additional pressure drop (my experience would say no but I'll throw them out as an option to consider).

A 10 hP motor isn't very expensive.  In terms of cost, replacing the impeller is likely the least expensive option as pointed out by Roach.  Upsizing the pipe, if it’s relatively short (and isn’t installed ) might be another option.  If the piping is relatively long, replacing the entire pump will the least expensive solution.

RE: Pump head increased... How??

Generally when a pump is ordered it is powered thru it's curve meaning that for the impellor size no matter what the pressure or flow (since they are related) the pump will not overload.  Increasing the hp without changing the impellor will not do anything.  If you change the impellor more than likely you will exceed the hp power curve but depending where you are on the curve you maybe within the hp rating (generally 1.15 service factor on the motor).  If you have a relief valve it can protect the motor and system.  In small pump system like this it may be cheaper to just buy a new pump, it is much easier than ordering a impeller or changing out a motor (motor and pump alignment are very important).  I usually don't see impellor changeouts unless the pump is special or 25 hp and above.

RE: Pump head increased... How??

You can underfile the edges of the impeller vanes.  This will get you up to 3% more head.

RE: Pump head increased... How??


Can you please explain the procedure for underfiling?

RE: Pump head increased... How??

Underfiling is beveling the suction side trailing edges of the impeller blades (concave blade side with backward curved blades. This changes the discharge angle slightly, increasing the head. It should be a controlled process applied to every blade and requires shaving or filing away the metal. Overfiling, done to the pressure side of the blade trailing edges (convex side) has the opposite effect and reduces head.

RE: Pump head increased... How??

for a given pump you could increase rpm and the suite the new motor.
Check for the nominal current at name plate,
read with a amprobe wath curren are you runig yet , maybe
you will not need to change motor.


RE: Pump head increased... How??

Also buying a cheap inverter and ramping up your Hz is an option

RE: Pump head increased... How??

Increasing motor size will do absolutly nothing,assuming you have a maximum diameter impeller.

RE: Pump head increased... How??

Underfiling (also called backfiling) will only give you short term head increases and is therefore only a short term fix.  Pump companies will sometimes do that to the new impeller to help it pass it's performance test especially when using HI "A" criteria.  Underfiling makes the vanes very sharp - almost like a knife edge.  But like everything in life, nothing is free.  As the pump operates, you will notice that the head will drop back to what it "should" be as the vanes become more dull.

You could put a 100hp motor on the pump and still get no more out of it if you don't change the impeller.  A 10hp motor shouldn't cost you but a couple hundred dollars or so - depending on the specs.

So, what's cheaper?  Changing the pipe to a larger size or purchasing a totally new pump?  Given that it's a 7.5hp pump, I would venture a guess to say that buying a new pump will be cheaper.  Like other people suggestes, at the least, you should consider a larger impeller if possible.  You would have to pay for a new impeller, a new motor, and probably a few gaskets.  Don't forget the labor to take the pump apart, remove the old impeller, put the new impeller one, and put the pump back together.  You're probably looking at around $800 to do all of that.

Tim S.

RE: Pump head increased... How??

To tstead
Underfiling is not a temporary fix for any impeller to which it is applied. The removed metal is gone forever and the associated flow discharge angle change that raises pump head will last as long as the impeller. Underfiling may be a "temporary fix" to the first of many impellers of the same design to be manufactured because it involves more work and should , for the sake of manufacturing costs, be replaced by an alternate solution. If the impeller diameter is oversized , as is usually done to assure head requirments are met. then the best alternate solution to underfiling is to increase the final maching outside diameter to get the required head without underfiling.

RE: Pump head increased... How??

What caused the pressure drop, a larger pipe will give you less friction loss, thus increasing pump head.  Either the pump was sized wrong, it is out of ajustment (semi-open impeller) or it is worn out at the seal ring (enclosed) Backfiling will increase eff. However it is probably allready backfiled and there is noway to say it is going to increase a fixed amount, every impeller has different backfile results.  Contact your pump supplier and they can recommend the best remidy for your problem.  Let them know your suction pressure, current pump design, pipe size and lenght,elbows,and required discharge pressure.  This wont cost a dime and you will know exactly what you will have to do.

RE: Pump head increased... How??

More head...hmmm.
You didn't say what you were pumping (abrasive, water, etc.), so I'll take a shot at it.
Put a new suction liner in it.  New liners will decrease the distance between the impeller and liner.  That will help with the head AND the efficiency.

Next would be an increase of impeller trim....or a new trimmed impeller.

As I mentioned, there's not much info to go on, so my suggestions is a general fix.  More info will get better results.


RE: Pump head increased... How??

It seems to be a very very useful Forum. I got lot of doubts in fluid engineering.
I understand that pressure drop is directly proportional to the ratio of diameters to the fifth power.
Can you explain it by giving any examples.


RE: Pump head increased... How??

The easiest way of showing it Peninsula is to do some hydraulic line loss calculations and confirm it for yourself.

Essentially, let's say you have a 4" sch 40 line and you are considering whether to replace it with a 6" sch 40 line.  The ratio of the IDs to the 5th power is (4.026/6.065)^5 = 0.129.  So, the pressure drop through a 6" line is about 13% of that of a 4" line.  It's not an exact number as other factors come into the pressure drop but it's pretty good for an estimate.  This is true for incompressible fluids for virtually any dP (until you start flashing).  For gases, it's valid for pressure drops that don't exceed about 10% of the inlet pressure.

RE: Pump head increased... How??

No one has yet suggested that you might want to model your piping system and see what the pressure drop shold be theoretically.  You may 1)have pipe that has built up deposits on the ID of the pipe thus decreasing its effective diameter for flow, or 2) someone has perhaps modified the pipe system, or 3) someone may have installed a resticting orifice plate for pressure drop or flow measurement, or 4) perhaps there is a plugged screen, or a broken valve stem or etc etc.

Point is make sure there is not a greater problem facing you before you decide to change the pump; because a larger impeller can also create a problem if not matched to the system its in.

The more you learn, the less you are certain of.

RE: Pump head increased... How??

Interesting thread. I had heard of trimming the impeller to decrease performance in an oversized pump, but never the opposite. You learn something new every day.\

Here is one thought that is kind of a longshot.

Regarding changing the motor... there is some limited capability for gaining pump performance if you can reduce the slip.

Some older and smaller motors may have 2% slip or more.  Take a look at your nameplate RPM to get an idea of how much you can gain by buying a motor which operates closer to syncronous speed.

Newer motors designed for high efficiency typically have much lower rotor resistance which results in much lower nameplate slip.  

If you can increase your speed by 1% that is of course 1% flow and 2% head increase.

As was mentioned before, changing motors on this small a machine might be more trouble than it's worth (compared to cost of new pump/motor combination).

RE: Pump head increased... How??

Thank you very much for your reply.

can you explain what will be the calculation pipe contraction.Iam preparing my machine schematic digram .thatwill help you to advise me.

RE: Pump head increased... How??


Your actual operation is not known.

The VFD idea could fix your problem, but if your motor is close to 100% load you won't increase the speed with a VFD.  Remember the HP required increases at the cube of the speed change for centrifugal loads while the head only increases at the square.

One thing I would consider is the possibility of increasing the intake pressure if possible.  Pressure is additive.


RE: Pump head increased... How??

I have briefly looked at the responses thus far, so apologize if I am repeating something previously stated.  Buying a new motor for a centrifugal pump will probably not incraewse the discharge head 15 ft.  Your motor unless it is severly undersized should be run not too far from the nominal operating speed.  Check the amps to make sure your under the full load amps (FLA) at you operating point.

Under filing will get you about a 3% head increase if it hasn't already been done at the factory.

The only true way to increase your discharge head is to increade the pump speed or increase the impeller diameter. Buy purchasing a VFD you can greatly increase the control of your flow and head output often while reducing your energy consumption (costs).  

When purchasing a new pump it is always a wise idea to purchase a pump with an impeller that is 80% of the full diameter impeller.  This gives you room to grow or overcome sizing mistakes made during selection.

You may wish to make an inspection or your current impeller to make sure it is not eroded or damaged.

Greg Case
MechTronix Engineering
Turbomachinery Consultants

RE: Pump head increased... How??

To increase the head of the pump you have increase the diameter of impeller not the motor.

Usually to increase the discharge reduce the diameter and to increase the head increase the diameter of the impeller. Keep in mind this is the thumb rule

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