Centrifugal Pump Impeller Size

Significantly decreased, as the pump would be less resilient against slip; more of an agitator than a pump.

Yes and no. Thank you. Come again.

Ok on a more serious note. I must first say that we do not do homework problems here. This sounds very close to a homework problem. So I will not try to give you an exhaustive explanation for this and other reasons that will become apparent.

There are more aspects to a centrifugal impeller pump than just the impellar size. There's the motor spinning the impellar, the manifold housing surrounding the impellar, the impeller blade pitch, the viscosity of the fluid being pumped and the inlet and exhaust piping. All of these factors affect the discharge pressure one gets. Compounding this problem there's also the possibility that one can run a pump into the non-linear region of cavitation. So can the discharge pressure increase by just changing the impeller size to a smaller diameter, yes it can. Can the discharge pressure decrease by just changing the impeller size to a smaller impeller, yes it can.

Also - it's a single ticket; one cannot come back without replacing the impeller.

Also - the characteristics of a modified impeller cannot be predicted accurately without carrying out practical testing; 1in off the impeller might be too little or it might be too much.

So:

• Consider adding a variable speed, variable voltage drive unit ["VVSD" or "VSD" in colloquial parlance] instead of modifying the impeller.
• For smaller pumps, adding a restriction such as a hand-operated valve downstream of it may be the most cost-effective solution to "too much flow".

"Too much pressure" is either taken care of already by using pressure relief devices, or the system has taken care of it itself, by bursting.

Your pressure, flow and amp draw will all go down.

Exactly how much is anybodies guess.

The pump manufacturer may have a pump curve with a 10" impeller shown, ask them.

hi sajid its burki here, check this link

http://www.maintenanceworld.com/Articles/engresource/centrifugalpumps.pdf

its very good document about centrifugal pumps.

in this read Affinity Law (on 27th page), it gives u relation b/t Head and Impeller size.

best of luck

If you reduce your impeller dia without reducing the housing or volute size the discharge pressure may drop right off to zero.

Is your bulldozer stuck because of a broken pump impeller and you only have a smaller spare that just happens to fit or what?

discharge pressure will reduce.You may even calculate if u have all the required data.

According to the pump laws Q varies wth D,H,the head varies with D^2 and Power varies with D cube

As you can see in a pump typical performance curve, if you consider a constant flow, when you reduce the impeller diameter, your pressure also is reduced, so the power demand will be reduced too.

I see that people are not grasping my point that there can easily be a scenario where an increase in pump pressure happens when one changes only the impeller diameter. (You happen to provide an excellent point to launch my explanation. I hope you don't mind CAR3. )

If the power demand with the larger impeller exceeded the power capability of the motor then depending on the motor torque curves the rotational speed can be slowed but not stalled to produce a much lower head pressure. But reducing the mechanical power demand with a smaller impeller can now allow the same motor to work at anticipated rotational speed and a net higher head pressure occurs.

Not sure I can buy into that. Maybe on very rare occasions.

If FLA+SF are exceeded, which would happen in your scenario, you'd more than likely trip the thermal overload or CB's.

Fine, go ahead and add the condition of tripping the circuit breakers and/or thermal overloads with a full load ampere condition and service factor of an exceeded induction motor. A non-running motor attached to the larger impeller produces no pressure at all. Where a smaller impeller that doesn't overload the protection circuitry produces now considerably more pressure than nothing.

You should notice though in the link Figure 1 curve D for a high slip induction motor produces exactly the torque curve that need not induce a trip condition like my earlier scenario suggests. You should also notice that this motor torque curve is often chosen in oil well pump jacks.

I almost forgot, what in the world does combining Florida with San Francisco have to do with pumps. The only way these two come together in my mind involves a pie incident.

Never was much of an Anita fan, either.

Maybe I'm just a victum of a limited sphere of experience with various types of motors.

Is it in accordance with pump laws?.

I think thats a reach.

If the motor and pump were sized correctly in the first place there should be no change on the DP, and any increase in speed would be taken up in a volute Wash / agitation. Your amperages go up since the motor is doing more work to move less water. Let alone the thumping and cavitation in the volute would drive me crazy.

We actually had this situation on a heat exchanger for a chiller. "Podunk Inc" came out and threw on what ever impeller they had in their truck. We got the call, thumping hammering, should have been a 5.75 impeller they installed a 5.0. The motor amperages were up and it was hammering since the space between the impeller and volute would not allow the pump to create the proper DP to move the fluid through the condenser. The Motor was working harder to move less water. Once we installed the right impeller the motor amperages dropped, the condenser Delta T increased and the cavitation and thumping were gone.

So one may think that the smaller impeller gives you more speed, but in fact it is the system head that reduces flow and kills the motor.

"what ever impeller they had in their truck"

Width is a big mass-flow factor. Mass flow is hp.

Tangential velocity is pressure.

Clearances count.

Of note is; if you machine down an internally tapered impeller you may reduce the pressure capability but increase the hp against a lesser working head because you have increased the mass flow.

I think that its a reach that somebody would want to change the impeller size if fluid, motor and pump size were sized correctly in the first place. Do you normally change a critical, embedded part of a system because things are working exactly as expected? I'm only proposing that without knowing ANYTHING but that the impeller diameter was reduced by almost 10% that a higher net pressure may happen. Do I need to provide a link for the definition of the word "may". I'll admit that the only way I can conceive a smaller impeller could possibly produce more pressure is if things are failing with the larger impeller. If the motor and larger impeller pump weren't failing why would one want to change anything.

My other reason for ranting on about this is that we so often blind ourselves by expecting every engineering problem as always working linearly. In all engineering disciplines things only work linearly in a certain range of many parameters. We've been given only one parameter and have no idea what other parameters if any were changed, too.

I understand and would agree with the comments when it comes to the engineering. However this is what happened in a practical application on a job. It was explained to us by the on site engineer that the impeller had been struck by debris and the vanes damaged. The contractor that was called prior to us (trunk slammer) used a back woods solution to get by, mind you most back woods solutions work great, this one did not. We took it apart and found the problem and then installed the proper impeller. The system is still running good today and the debris was found in the chiller barrel when we broke it down for a rodding.

For a given RPM, head developed varies in direct proportion to the impeller dia squared, i.e by reducing the dia from 11 inches to 10 inches, the pressure will reduce to 100/121=.8264 times the original head. so it will reduce by nearly, 17.36%.

Irshad

Not 17.37%, then?

But why at all you want to reduce impeller dia.

After all the replies, some are really well given, what do you think, and what you have understood please share your preparedness for home work.

Dear Mr.SajidHussain,

The DISCHARGE PRESSURE WILL COME DOWN as the SQUARE OF THE DIA. At reduced dia, the impeller velocity will come down-for the same speed in RPM and hence the pressure will drop.

In your case you have referred the dia from 11 in to 10 in means, the pressure will be DROPPING TO 10^2/11^2 i.e 100/121=0.8264 of the original pressure at 11 in.

DHAYANANDHAN.S

I think this is a test question and OP is a student. If OP had any idea of pumps, this general question would be intuitive.

Sajid Hussain, why do you ask the question?

OK,

Let me try again. The impeller would be teflon coated to give it resistance to acids, alkalines and other agressive chemicals.

Well.....General question becomes big question when one had no contact with the field since a long time. Anyway, I got the right answer on this forum and thanks to everyone for their time.

Did you pass the test?

Right, will OP give us credit for it?

Nope!

We just gave him a fish.

We gave him a fish but he did bait the hook himself.

I anxiously await your next visit.

I completely agree with PWSlack. The discharge pressure will be decreased. Check your electrical connections all the way to the main panel for loose or bad connections. Get a cheap ammeter and check your motor amperage. If its higher than the name plate on the motor you need to find the reason.

Dear Mr. Sajid Hussain,

Pl. refer Pumps BASIC CHARACTERS which will show

1. Pump CAPACITY varies as PROPORTIONAL to SPEED.

2. Pump HEAD varies as PROPORTIONAL to SQUARE of SPEED.

3. Pump POWER varies as PROPORTIONAL TO CUBE of SPEED.

You have referred Impeller Dia is reduced from 11 Inches to 10 Inches.

Hence Impeller tip velocity will be 10/11 = 0.909 of Original Velocity at 11 inches, speed being the same. Hence HEAD developed with 10 Inch Impeller will be (0.909) x (0.909) = 0.8624 times the Velocity at 11 Inches Impeller, i.e 17.35% Less than original Head with 11 Inch Impeller.

Accordingly Power consumed will be 0.751 times of originally consumed power - 24.89% reduction, and capacity will fall by 9.1 %.

DHAYANANDHAN.S