Increasing the speed of centrifugal pump

[Chiefkocak]

I have a Worthington centrifugal pump running at 1200 RPM moving sea water. I have a electric motor that will mount up perfectly that turns at 1800 RPM. I want more flow, without changing the pump what will changing the speed do? The Worthington manuals pump curves are set up for a specific RPM, so they are no help.

 

[pmover]

of more importance is the increased differential head across the pump at the higher rpm. another factor is the npsha. higher rpm pumps will likely require increased npsha.

for example, at same inlet conditions, the discharge pressure will be higher at the higher rpm. is the piping system rated for the higher pressures? relief valve settings/capacities?

before increasing speed, i'd have the mfg evaluate the pump's performance at the higher rpms for the design/actual process conditions.

good luck!
-pmover

 

[25362]

Is the motor capable to handle a 1.5[sup]3[/sup] times greater HP demand ?

 

[timbones]

You can use the pump similarity rules to scale the pump curve to give you a rough estimate.

Q is proportional to N*D^3
H is proportional to N^2*D^2
P is proportional to rho*N^3*D^5

So for your pump, Q (flow) will be 1.5 times original Q. H (differential head) will be 1.5^2=2.25 times original H. P (power) will be 1.5^3=3.375 times original power.

 

[TenPenny]

As stated above, the hp at 1800 rpm will be about (1800/1200)^3, or about 3.5 times the hp at the lower speed.

The question is, can the pump handle this power? Also, can the pump run at the higher speed?

What is the model and size of the pump?

 

[Artisi]

Although the earlier info. is important to you it doesn't tell you if the pump can operate at this speed.My suggestion is to contact the manufacurer with pump model, impeller diameter, coupling detail, motor detail and the information you want. Who else is better to advise you than the manufacturer.

 

[Chiefkocak]

Thank you all for the rapid response. I'm a new member here. A Chief Engineer on a US flag steam powered merchant ship.
All the info you have provide points that my 1800 RPm motor will handle the load. Original in service motor is 15 HP the 1800 is 25 HP. These Worthington LRV type pumps are built like a tank to take the neglect that only sailors can dish out.

 

[Artisi]

Sorry to say that 25hHP @1800rpm is insufficient: let assume that the power required at 1200rpm was 12hp, if you now run the pump at 1800rpm the power could be in the order of:-

(1800/1200)^3 X 12 = 40HP (minimum)

However, this will depend largely on the head imposed on the pump by the pipe work configuration and the static head - you also need to review the NPSHr versus the NPSHa --- this is NOT a function of them being built like a tank but a matter of physics and wishful thinking will not overcome an NPSH problem.

You need a proper review of this installation by someone who understands pumps and the ramifications of increasing the speed by 50%.

 

[BigInch]

The only good news is that, if you can get the motor and the NPSHr.. I'm pretty certain that the pump will run well at 1800 rpm. As you say... built like a tank. BUT, there are still many reasons why you might not really want to do that. You will have 225% more pressure and 50% more flow, so pressure rating of piping, valves, fittings and any attached equipment may be exceeded. Velocities in piping, valves and equipment might get relatively high, which could cause water hammer and erosion of valve seats, etc. If its potentially dirty sea water, tripple that problem's magnitude. Pump maintenance requirements will also be increased. Wiring and power distribution to the new motor will probably also be at risk. Check those ratings too. You might even exceed and anchor bolt loads at the motor base plate, as start up torque will be considerably higher. I'd recommend that you don't do it at home alone.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

I have no doubt that this particulat pump can run at 1800rpm - as a matter of fact it may have been designed for this speed in the first place or even 3000rpm (50Hz) or 3600rpm (60Hz).
My previous comment stands - contact the manufacturer.

 

[vpl]

I would like to reiterate BigInch's point. Even if the pump can handle it, can the rest of the system?

Higher flow can result in flow-induced vibration in systems not designed for it. This has caused cracks in piping, espectially small instrument lines, and occasionally valves and even large components (such as a steam dryer). Higher velocity (than originally designed) will also wear out heat exchanger tubes and throttle valves. And, of course, there is flow accelerated corrosion, especially downstream of elbows.

Increased flow is not always a good thing.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[vspek]

You have to ask a different question. What can I get out of the Pump, if I replace 15 Hp 1200 RPM motor to 25 Hp 1800 RPM.
You have to remember that, impeller can be modified to keep power consumption under 25 Hp.
From 1996 I was modifying impellers for Kocak to fix Goulds pumps operating away from BEP and breaking the shafts.
The same can be done in your case of replacing motor.
The similar problem happens, when companies are buying European ships and converting from 50 HZ to 60 HZ.

Best Regards.

 

[Artisi]

Just restating my earlier comment;
"You need a proper review of this installation by someone who understands pumps and the ramifications of increasing the speed by 50%."
This means a complete study of the pipe work and anticipated flow rate, pump performance and power limitations for the speed / flow / impeller diameter etc.

 

[djack77494]

Chief,
I think you've gotten plenty of replies that should lead you to the correct conclusion that you should not willy-nilly replace the motor with a higher speed motor. Despite its build and reputation, the pump and/or other components of the system may not do well at the revised conditions that will occur. Decide what you want in terms of new flowrate and head and determine the NPSH you have available. Armed with that information you should be able to predict using the pump affinity laws how the existing pump will perform at the new speed. Now with help from the pump vendor, determine if you should trim the impeller to achieve the desired performance.
Doug