Axial Flow Pump Query 1

[ahg196]

I am having some problems with large axial flow pumps, generating a lot of torque and power and the gearboxes are running very hot. One possible option worth trying is to reduce the speed of the pumps a fraction, and hence the torque and power decrease by factors fo 2 and 3 respectively. This may improve the running conditions of the pumps to an acceptable level. My query is, whn I decrease the pump speed, in order to keep the pump operating at the best efficiency point, will I need to adjust the pitch of the blades? How does the efficiency curve change as the speed is changed?

Thanks in advance

Andrew

 

[electricpete]

1 - If I would guess I would say BEP flow is propotrional to flow, but I'm sure someone else can answer on that.

2 - If you're problem is excessive torque and power requirement, one option is to decrease the system flow resistance (without changing speed) by opening throttle valves or opening parallel paths. On an axial flow pump, increasing flow generally decreases BHP (opposite of radial flow pump).





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[Artisi]

is this a new problem with these pumps or has it been an ongoing problem from start up and what size units are you talking about.



Naresuan University
Phitsanulok
Thailand

 

[ahg196]

There have been problems since pumps were first commissioned. The pumps are 1400mm diameter, 8000l/s with 330kW motors running around 300 rpm.

 

[Artisi]

Why not investigate the cause if you haven't already been done that track.

You say the pumps are generating a lot of torque and power, actually you mean that a lot of power and torque is being transmitted through the gearboxes.

Are the gearboxes in fact overheating - getting hot is a relative term - what is the operating temperature????
If the gearboxes ARE running "hot" this could be from a couple of causes, under-rated gearboxes, wrong lubrication, mis-alignment of the driver relative to the gearboxes (unlikely that all would be mis-aligned in a multi pump installation).

Are these units driven by fixed speed electric motors - if yes - how will you change the speed?

Reducing the through-put of the pumps will reduce -flow- power- head and torque - efficiency probably won't change much, it could reduce or improve depending on where the pupms are operating on the pump curve.

If fixed speed drives you could change impeller pitch if this option is available or reduce the impeller diameter and fit oversized casing rings if this is an option.

For me, I would be looking at the g/boxes initially before even thinking about changing the pump configuration. Maybe you can fit oil cooler or fans to the g/boxes or both- contact the g/box manufacturers / suppliers for advice.


Naresuan University
Phitsanulok
Thailand

 

[ahg196]

Thanks Artisi.

The motors are fixed speed, so no change will be possible there. I was thinking along the same lines as you.

1. Reduce the pitch of the blades which will reduce the flow and head and hence the torque required and power consumed

2. Change the gearing in the gearbox to give you a slower running pump.

Naturally no. 2 is more expensive than no. 1, so I'd see the effect changing the pitch has, first.

I suspect the problem lies in the gearbox. Inadequately rated, or perhaps a manufacturing problem/tolerance

 

[Artisi]

I still think you need a full investigation into the g/boxes, reducing the through put of the pump might still leave you with a g/box problem.
What is the operating temp?

Naresuan University
Phitsanulok
Thailand

 

[cbrn]

Hi all,
I can only say a little words about pump operating point: if you change the runner speed you change the velocities' triangles at inlet/outlet, and so you can dramatically affect efficiency. This is true especially if the pump runner has fixed-pitch blades: the efficiency curve of such a runner is extremely steep. If the pitch can be regulated/adjusted, then you may calculate the pitch that almost re-establishes the correct velocities in/out triangles (not completely, since the flow deviation depends on the pitch andother factors), and of course you won't be able to guarantee the same head / the same flowrate / the same NPSH, altogether. You can estimate the power required by the pump runner by P=rho*Q*g*H/etai, where:
- rho is water density in kg/m^3
- Q is the flowrate in m^3/s
- g is the earth gravity acceleration in m/s^2
- H is the head in m
- etai is the hydraulic efficiency of the runner.
Then you have to divide by the mechanical efficiency of the pump (bearing losses + ...) and you will find the power required at the gearbox "out" shaft. To find the power required at the motor, further divide by the gearbox's efficiency.
You will rapidly be able to see if the gearbox is rated for the power required. If P(required)>0.9*P(rated) then I personally think your gearbox is at high risk...
Instead of changing the pump's operating parameters, do you have the opportunity to cool down your gearboxes (simple "pipe coil" wrapped around the boxes, taking water from the HP side of the pump, for example)?