Ratchet Arrangement in Vertical Pumps

Bear in mind that reverse flow through a pump will mean reduced flow and reduced pressure to the service user, which should raise an alarm. Eventually the downstream process will stop and then, or preferably before then, is the time to take corrective action.

Putting a ratchet mechanism on the pump does nothing for the process needs, which are, after all, why the pump was installed!

What make are these pumps, please?

Once again, Google is you friend. Try Google'ing Images on 'pump anti rotation device' and maybe 'pump anti reverse rotation device'. One of the main reasons these are used on deep (over maybe 20ft. or so) vertical pumps is that even if the check valve is functioning properly, the water will fall back down through the pump until the water level in the pump column is within about 30 ft. of the well level. Even a perfect vacuum can only maintain a column of cold water about 30ft. high. Deep well submersible pumps can use down-hole check valves just above the pump to prevent backflow and reverse-spinning, but of course you can't do that on a line-shaft pump.

Sounds like a submersible pump is called for here.

The ratchet assembly locks both the pump shaft and motor shaft if coupled. It prevents rotation of both. Finally, motor will trip on stalling, over current or instantaneous current high protection.

What - on a ratchet mechanism?

Dear Mr.VishalB,

The ratchet will and should stop the rotation and will NOT and should NOT lock. But if rotation is stopped, the water hammer problem should be taken care.

But the provision of ratchet is difficult. Ensure CHECK VALVE is perfect, even then, water hammer problem will have to be taken care.

DHAYANANDHAN.S

Thanks for all the responses.

By not providing a ratchet we can avoid a possible water hammer problem which can be severe in case of large vertical pumps. Then, is it advisable to have pump components designed for reverse rotation capability with some other protection device/option for motor?

Maybe in some cases the water will "coast forward", but not in all.

Pump and driver normally have a relatively low moment of inertia so that when the motor trips the pump slows rapidly. If the discharge pipeline is long, the inertia of the fluid will maintain forward flow while the pump internals act as a throttle valve. This causes a rapid decrease in the pressure in the discharge line and can cause vaporisation of the pumped fluid leading to separation of the liquid column. When this collapses (the columns rejoin) the pressure surge can cause severe damage to the piping system. Karasik in his Pump Handbook has a clearer and more detailed explanation.

The reverse rotation protection has little influence on this, but is intended to protect the driver and or driven from high speed reverse rotation.

I'm not so sure that not providing a ratchet avoids the water hammer issue. You haven't described your pump for us, but I think the basic cause of water hammer in a long (deep) vertical pump is the fact that the pump's discharge column is basically empty when the pump starts. Then the pump starts and the column of water is moving at full speed when it meets the static valve and mass of water in the discharge system.

Another issue is that the reverse flow of water could cause the pump and motor to overspeed (in the reverse direction), depending on pump and installation details.

These pumps shall be vertical turbine type to be installed for cooling water service with flow around 5000 m3/hr and head 50m. The sump depth around 7m.

How does pump speed in reverse rotation exceeds its rated speed in forward direction, as it is designed with backward curved vanes to get the best output?

What does the HazOp Study Report say about all this? It has been done, hasn't it?

Being a large vertical pump I would assume that this is an efficient high suction specific speed pump. Pumps of this sort, depending on head, length of discharge line etc can achieve 150% of normal speed in reverse direction. This may be a problem for the pump, but will almost certainly be a problem for the motor.

As the pump and motor shaft are coupled the reverse rotation protection mechanism (Sprag clutch or similar) can be fitted to either, but preferably to the driver.

This is how they work.