Effect of higher speed and higher hp motor 2

[extremechanical]

Hi, I have a pump which is beneath a tank. It's actual motor is .63 hp, 1200 rpm
Maintenance wants to couple it with 1.5 hp motor with 1800 rpm

I want to know what is effect when a pump is coupled to higher rpm and higher hp motor than vendor designed values

Will it increase velocity at suction and then what is the effect on npsh. What if the pump is not flooded .

Please explain if suction speed or npsh changes also come into effect .

 

[TD2K]

If you run the pump at 1800 rpm rather than 1200 rpm the flow is going to increase. How much depends on the new pump curve and the resistance curve of the system it is pumping into. If it's on a flow control loop then the flow will be constant unless you chose to set it but then I would have to wonder why they are installing a larger and higher speed motor. Suction speed and NPSH will both be affected but you need the vendor's curves to quantify how much. You need to look at your system to decide if you have sufficient NPSH for the new pump.

 

[LittleInch]

What effect?

See affinity laws. Assuming the pump is centrifugal and the impellor diameter remains the same then the impact could be

flow COULD increase by 1.5 (prop to rot speed), Head incrases by 2.2.5 (prop to rot speed ^2), power required COULD increase by 3.75 (prop to rot speed ^3)

Suction veleocity depends on whether flow is controlled to a set value or not - Depend son your system which we can't see as you've not provided any details, draiwngs or sketches....

NPSHR will increase regardless of flow increase. How much is difficult to judge, but the vendor might have curves for a different speed

If a centrifugal is not flooded it has difficulty pumping unless it is a pump able to self prime.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

You need to define what you are doing - or what maintenance intend doing and why. Increasing the speed to 1800rpm has the potential to overload the proposed 1.5hp motor.
This may not be the only problem - to clarify the suitability of the pump you need to have a pump curve for 1200rpm (and preferably a curve for 1800rpm) together with all relative operation data included and a system curve or at least know the actual duty / performance (flow and head)of the pump under current conditions.
If you can supply pump flow and total head for the current operation and supply the actual static head on the system it is possible to fairly accurately calculate the total head for the new speed.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

That small of motor and pump are most likely designed for a 5 year lifetime like most appliances.

If you speed it up, it will pump at a faster rate for some time and then fail. At any fixed speed, the NPSH required by a centrifugal pump will increase with increase in flow from rated flow. At substantially increased flow from design flow the increase in NPSH (R) is very rapid.

 

[extremechanical]

Thanks a lot for all replying

 

[mohammedwais]

Hi extremechanical ,

accd to your post I have to inform you this increasing for the Hp will not make any effect on the pump or on your system but increasing for the RPM could provide you a bad consequences. How ?

Actually,the main effects will appear on following :
1) Pump's component (mechanical seal, impeller, etc)
2) System application ( HVAC ,Fire fighting , etc)
3) Energy saving

Lets go the result of increasing the RPM :

1) Increasing to discharge ( GPM & Head are increased)
2) Increasing for the consumption power
3) Increasing For NSPHr
4) Increasing for the control panel capacity ( breaker, contactor, etc)
5) Increasing for the total system cost
6) Inceasing for the Running cost
7) Unsuitability for the FF systems + HVAC systems + HEATING system

 

[BigInch]

Moh'd. It's a very tiny pump.

 

[Softedge]

extremechanical,

my suggestion is that you follow the recommendations by littleinch. the other posts have various errors in the relationship between the pump and system. basically the pump will add energy based on its performance curve balanced with the system requirements, the intersection of the pump and system curves. the expected change in the system operation should be based on the supplier pump curve for the new speed and if not available the affinity laws as noted by littleinch.

arthur

 

[BigInch]

The 1.5 HP will be 2.38 X currently rated power. Motors can usually reach 1.5 X rated torque during startup. Under those circumstances that little 0.63 HP pump could see torque that is 3.57 X normal. That is probably well above the safety factor of its shaft design.

 

[Artisi]

BigInch, I would say that the shaft sizes on these small pumps are usually governed by mech-seal and bearing sizes available rather than calculated power considerations. But it's all academic now as the OP seems to have either decided for or against the change or opted out of the conversation.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[BigInch]

Yes, most likely true.

My mistake. Hopefully this thread will die peacefully now.

 

[Artisi]

Die out Like many others, unfortunately there are others that should die out very early in their life.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)