Multistage or split-casing

[eyemount]

I have a problem determining the best pump to use. I have a clear water tank (88m head/180m3/hr). This is said as low flow/high head. Anybody knows which pump is best to use? And the reasons and prooves are?

 

[vanstoja]

For best hydraulic efficiency you need to select a driver running speed and calculate specific speeds to go along with design flowrate which enables use of the so-called Worthington curves(in RPM,GPM,Ft.units) to estimate hydraulic efficiency. If my conversions are correct I get
N ns HydEff
3600 1554 0.79
1800 777 0.72
4634 2000 0.80 (Maximum efficiency for Q=792.6GPM)

 

[Artisi]

To answer your question - if it was my project, I think that for 180m3/hr at 88THD I would be looking at a small multi-stage pump. However,whether to use multi-stage or split case will be your decision based on an engineering approach and is dependent on what is available, price, overall efficiency, application, space available, back-up from supplier etc etc.
There is no hard and fast rules as to what is the best unit.
Get bids for both types if you have any doubt and make a close comparision of each.

Naresuan University
Phitsanulok
Thailand

 

[Scipio]

First off, all pumps are split case, I'm assuming you're looking at a multistage pump, and deciding between a radially split multistage pump (casing closure is radial to the shaft), and an axially split multistage pump (casing closure lies parallel to the axis of the shaft).

Generally, axially split casings have lower pressure and temperature limits than radially split casings. Depending on manufacturer and design standard, axial splits can be used to up about 14,000 kPa and 200°C, beyond that you should look at radially split. Beyond that, like Artisi said, there's a lot of other factors that can influence the decision. Surrounding space, for instance, if you have to replace the rotor on an axial split pump, the top cover comes off, and the rotor is lifted up out of the top of the pump - here you need to make sure you have plenty of overhead space available. Most radial split pumps have a 'pullout space' at the non-drive end of the pump that you have to pull the rotor assembly out of horizontally to replace - here you need to make sure there's lots of open floorspace at the non-drive end of the pump.

The pump suction conditions will be a factor as well. If you have plenty of NPSHA, go with a 2 pole motor. At 3600 rpm, you shouldn't even need a multistage pump. A pump with a 10000 suction specific speed should require about 2.7 meters NPSHR at 3600 RPM. If you have it, I'd go with a single stage end-suction pump unless you have money to burn.

If NPSH is low, you may have to run at lower speeds. Lower speeds mean lower specific speeds and lower efficiencies that over the long run can justify going to multiple stage designs.

Also, if NPSH is low, you may want to go to a double suction impeller. In that case, even if you go with a single stage pump, you may still find yourself with a axial split case design - I'm not sure why, but every single stage double suction pump I've seen is axially split.