effect on pump operates at differnt viscosity 2

[upm]

Gents,
If the centrifugal pump operates at different fluid , hence viscosity is different, does the head remains the same assuming no change to the system dynamics or resistance , thus the seystem curve remains the same too??? i know for a lighter fluid the discharge pressure reduces at the pump flange (static energy) however the head should remain the same??? i am really confused pls assisset
upm

 

[imok2]

Here is what is going to change when you pump viscous fluids with a centrifugal pump:

1.The brake horsepower requirement will increase.
2.You will notice a reduction in the head the pump will produce.
3.Some reduction in capacity will occur with moderate and high viscosities.
4.The pump's efficiency will decrease.

High viscosity fluids are better handled with positive displacement pumps that are affected differently than centrifugal pumps by a change in fluid viscosity:

At a constant speed, changes in viscosity will have very little affect on capacity.
The total head will probably increase with viscosity because of higher system resistance.
The brake horsepower (kilowatts) will increase with capacity.
The efficiency probably will not be affected because of less leakage through the internal pump clearances. In some cases the efficiency will incease.

 

[upm]

sorry it was my mistake as i wanted to say when the density is changed

 

[Artisi]

If you increase the density (specific gravity) of the pumped liquor and assuming it behaves like water, the flow and developed head will remain the same but the discharge pressure will increase as will the input power required.It is therefore important that you check if you have power reserve at the duty point and sufficient power across the pump curve for start up and any upsets to the system.

International College
Naresuan University
Phitsanulok
Thailand

 

[upm]

Does this mean the work done by the pump remains the same and only the static energy (input power) increases??? In that case during pump evaluation, how do we calculate the head,as normally we use the design diff pressure and specific gravity to calcualte the head. as the discharge pressure will increase thus DP reduces , the head will reduce as well. if we did not know that the s.g has changed we might conclude the pump performance is dropping?
any one can assist in field pump evaluation in such case assuming the speed/system resistance remains constant
upm

 

[PUMPDESIGNER]

upm,

Head is always the same regardless of the specific gravity.
I therefore avoid pressure in the calculations because my puny brain gets confused and then I add that to the puny brains in my fellow workers and we have the 3 stooges going on, and I am Moe by nature so I end up hitting people all the time and going to jail.

Sooo,

I make the conversion to pressuure as needed to deal with people in the field. I work in head all the way, make the simple conversion to pressure to answer specific questions, discuss field problems, etc.


PUMPDESIGNER

 

[quark]

Pumpdesigner is right by hook and crook

(huh I am not a coward, just stopping PD going to jail one more time)

Centrifugal pumps, irrespective of the pumped fluid, develop same head and power drawn by a pump mainly depends upon the mass flow rate of fluid moved through it(and head is generally used in the power equation rather than pressure). Power increases with increase in density and this is taken care in the power equation by the term specific weight.

Work done by the pump increases and subsequently you should increase the power input also. If you don't have sufficient capacity prime mover, it may overload and your system will be destroyed.

Regards,

 

[TD2K]

All good points. If you go back to basics, remembering that the head from a centrifugal pump is constant for a given flow (for low visc fluids), the effect of a changing SG is easy to see

BHp = H*Q*SG/(3960*eff)

H - head, ft
Q - flow, gpm
SG - specific gravity
eff - pump efficiency

For a constant flow, a centrifugal pump always puts out a constant head. Therefore, as the SG increases, the BHp requirement is directionally proportional to the change in SG.

Although the discharge pressure will change for a changing SG, the head doesn't for a constant flow.

Viscosity, as previously discussed, lowers the head and flow from a pump and increases the BHp requirement. Lots of pump handbooks will have the method for adjusting the pump curve and BHp requirements for a high visc fluid.

 

[upm]

Thanx guys for the help and it is very clear now . I need to simplify it to our operators to understand
upm

 

[PUMPDESIGNER]

quark,
You are a funny guy, and correct of course about the power relationship to flow.


PUMPDESIGNER