Reverse flow in vertical shaft pumps

[keyhanafshar]

Hi everybody

I've to dewater a reservoir to sea by using a wet pit mounted vertical shaft pump (axial flow), for watering the same reservoir i want to eliminate the extra pipe way and channel as there is positive head over the reservoir (sea level is 2.5 meter avove the reservoir)i am planning to use the dischrge flnge of the pump as the suction from sea to reservoir without runing the pump, Does anybody know what will happen to pump bearing wise?? Anyhow is it practible?? Please let me know about any remedies or cautions!!
Thanking you in advance.

 

[pardal]

If the pump motor is over the sea level, you have the first problem solved.

Normally the pumps have a check valve on the discharge side if it is so you must take it off.
BUT
The first to be aware is than when the water goes back the pump , the shaft will rotate in counter wise from original and the threaded parts could disassembly.

If you want to use the same pipe , use a check valve upward the pump and do a tee branch where to put valve , so when the pump is off you open this valve and can fill again your reservoir.
Dont worry about your english , mine is not better.








You can reach me at k281969@hotmail.com
but keep your post here for mutual benefits.
Pardal

 

[stanier]

You could fit a hold back on the pump drive so that it can only rotate in the desired direction. When the flow reverses the water will go through the pump. Be careful to check how the impeller is secured to the shaft. Sometimes they are simply screwed on and flow reversal can drive the impeller off the shaft and it will foul the casing.

Check with pump manufacturer.

Discharge check valves could be replaced with an actuated valve so flow could be stopped in reverse if required.

 

[315636]

If your pump has VHS (vertical hollow shaft) motor, you will not experience any problem, except removing or temporarily reversing the discharge check valve.
All VHS motors has non reverse rachets. motor will not rotate backwards and impellers will not come loose.

Regards

A. Ozden Ertoz

 

[Slagathor]

AARGH! The misconception persists!

Having the motors drive the pump in reverse WILL unscrew the couplings. Reverse rotation due to back flow WILL NOT!!!!

NRR's are an option, and they keep pump frop overspeeding. There are thousands of pumps installed without NRR's which go into reverse rotation evertime they stop.

When you reverse rotate, the force is applied at the impeller. As such, the toque direction on the shaft does not change. The couplings are still being tightened!

 

A pump by defination moves stuff from suction to discharge. My rule of thumb as an engineer is to make sure this is always the case. Pumps and motors are not designed to move fluid in reverse and therefore designs should make sure this doesnt happen. Non reverse ratchets are not standard on all motors, they are an option. For vetrical turbine pump applications, they should always be included in the design. Along with the forces on the impeller and shaft that was not intended to spin backwards, there are freakish types of power generated at the motor when it turns into a generator by spinning backwards.

I hope I helped somewhat.....

 

[yoel]

In my plant we use reverse flow to drain the outlet area when it should be washed.
We do not have threads on the shaft, but I agree with Slagathor that it makes no difference. In the same sense, the axial force in reverse flow, caused by the water flow, will act downward, like in the forward flow.
The major force on the shaft is due to the water column in the pump. In reverse flow, this force remain the same.
So, I see no problem to the thrust bearing of the motor in reverse flow.
But! Vibrations can occur and cause damage, so you should be near the pump and inspect this.

 

[sow]

Im just guessing here, but wont the shaft be in danger of over -torque, if the pumps are switched on during reverse flows?

 

[carlitros]

reverse rotation does not unscrew threaded shafts or impeller in vertical shaft pumps IF -underlined- it is due to reverse flow. the purpose of non-reverse ratchets is to prevent reverse rotation coming FROM THE DRIVER. (that would unscrew shaft or impellers).

the only problem i would expect from using the pump for discharging your reservoir would be overspeeding, which could cause excesive vibration. the speed on the pump could be up to twice nominal speed (if i remember well. it is called "velocidad de embalamiento" -sorry, i dont know the english word for it).
this wont happen if your motor has non-reverse ratchet. to keep speed within acceptable limits you would have to throttle the valve you have in the line (supposed you have one).

 

reverse engineering of a self priming petroleum pump

 

[vanstoja]

Bearing-wise, reverse flow-induced reverse rotation should not worry rolling element bearings. Fluid-film bearings will not be worried if the reverse rotating speed is not too low so as to prevent the formation of a fluid film. Speeds as low as 80 RPM usually suffice for vertical rotor shafts. Too high a reverse "runaway" speed might worry the rotor shaft stresswise and needs to be checked. With an axial flow impeller, the reverse runaway speed will not be all that high because resistance to reverse flow is about the same as for forward flow (not so for low specific speed radial impellers). If one deliberately, or inadvertently, starts the pumpmotor with the reverse rotating rotor running at high speeds, burnout of the motor windings is a prospect due to near-locked rotor current draw. Another possibility is that the rotor will not come up to full speed and if left in that state for a lengthy period may slowly cook the motor insulation to destruction. Reverse rotation-stopping pawls are indeed a solution to prevent high runaway speeds but I've read of some horror stories with commercial nuclear pumps that broke up their anti-rotation pawls into little pieces of destructive foreign material. Despite all this bad new...cheers! vanstoja