Axial Thrust

[PTT]

Hi,

Anybody knows how to calculate or approximate the axial thrust acting on a centrifugal pump open impeller?

Regards

 

[MECHROSSI]

Hello:
Axial thrust depends of impeller design, i.e. Do you have an impeller with rear vanes?, the calculations are depending of pressure on impeller back side.

Regards

 

[Artisi]

MECHROSSI - I agree that it depends on impeller design, it is also an function of the pressure on the inlet side of the impeller, but I fail to see why would rear impeller vanes affect axial thrust. Rear impeller vanes are usually there to reduce the pressure seen in and around the stuffing box and to assist in clearing the area when handling any solids etc. The alternative to rear vanes is balance holes thru the impeller from inlet side to the rear of the impeller in an attempt to bring the rear of the impeller to near inlet pressure.

 

[bk19702]

Impeller back vanes have a big impact on axial thrust if the recommended clearance between the vanes and casing cover is not adhered to. If the clearance is excessive, higher than normal thrust loads can occur in the direction of pump suction. I've seen this first hand on some old IR MFL overhungs which have back vanes on the impeller. In two particular failures, the clearance between vanes and casing cover were in excess of 0.080" (OEM recommended clearance was 0.040 - 0.060"). In both failures, the thrust bearings failed with pretty spectacular results.

 

[MartinSr00]

I would treat it as if it is closed. Assume that on the suction side, the diameter of the suction "wear ring" (that's really not there), is where the shroud stops on the ID. Then it's a piston problem. We used to assume that the discharge pressure in the side-wall space is equal to .7 times the impeller differential head pressure.

 

[PTT]

Thanks guys for your fast reply to my request.

On the impeller there're back vanes. Not very high i.e about 7/64" and also six balancing holes of 3/8" dia. The impeller outside dia. is 13", it's an open impeller with six vanes. The height of the vanes is 1-1/8". The speed is 1800 rpm. The impeller eye area is 27.6 sq.in. It's a small acid slurry pump and i think the back vanes are designed on this impeller to ensure clearing around the stuffing box area instead of thrust reducing effect on this one.

By the end of the day, I will be able to obtain the outlet and inlet pressure.

With balancing holes is the axial thrust can be neglected?

Thanks!

 

[Pumpone]

Calculation of Axial Thrusts depend of
Impeller type and details,
Impeller dimensions,
TDH, GE,
Ns,
A good calculation method is made in ANSI/HI 1.3-2000,
You can aprox for open impeller

 

[PTT]

Thanks Pumpone, I will check in the ANSI document to get an approximation of the axial thrust.

Regards