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Ratchet Arrangement in Vertical Pumps

05/15/2013 5:23 AM

Large vertical pumps in water service may at times face the problem of reverse flow causing reverse rotation. The reverse flow may arise due to failure of check valve or tripping of one pump in case of parallel operation.

Pumps can be provided with a ratchet arrangement to overcome the problem of reverse flow.

My question - whether ratchet stops pump rotor along with motor shaft or it just locks the motor shaft letting pump rotor assembly to rotate freely in reverse direction. If the latter is correct, is it that pump rotor assembly needs to designed for reverse rotation capability?

(If anyone can please share an animation of pump ratchet arrangement for better understanding of the mechanism, if available....)

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#1

Re: Ratchet arrangement in Vertical Pumps

05/15/2013 5:43 AM

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!

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#2

Re: Ratchet arrangement in Vertical Pumps

05/15/2013 7:10 AM

What make are these pumps, please?

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#3

Re: Ratchet Arrangement in Vertical Pumps

05/15/2013 8:47 AM

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.

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#4

Re: Ratchet Arrangement in Vertical Pumps

05/15/2013 9:35 AM

Sounds like a submersible pump is called for here.

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#5

Re: Ratchet Arrangement in Vertical Pumps

05/15/2013 11:45 PM

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.

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#6
In reply to #5

Re: Ratchet Arrangement in Vertical Pumps

05/16/2013 3:22 AM

What - on a ratchet mechanism?

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#7

Re: Ratchet Arrangement in Vertical Pumps

05/16/2013 5:07 AM

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.

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#8

Re: Ratchet Arrangement in Vertical Pumps

05/16/2013 6:02 AM

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?

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#9
In reply to #8

Re: Ratchet Arrangement in Vertical Pumps

05/16/2013 8:23 AM

Water hammer is the effect of water moving in a forward direction, and the driver (pump) stops. The water will coast forward and then start falling backwards. When it falls backwards, it will stop against whatever device is installed to stop it - a check valve, etc. The weight of this backward-traveling water is enormous in some larger installations. The non-reverse ratchet installed in many vertical pumps of this type, especially those using vertical hollow-shaft motor is to stop this backward rotation before it starts. The motor shaft can only turn in the forward direction. This is very important in pumps that have automatic start and stop components. If the water is allowed to free-flow in the reverse direction and the automatic contros start the pump while that is happening, the motor or pump shaft could twist off due to the motor starting against the reverse rotating flow. The damage can be very significant. Cushion check valves that have an air cylinder "piston" installed on the arm of the flapper will close slowly to reduce water hammer upon the cessation of flow through the pump. With the relatively low cost of variable frequency drives today, that is a very viable option as well. These drives normally are equipped with "soft start" capabilities such that the speed ramps up to the operating speed over a pre-set amont of time. Then, when the pump stops, the speed can be ramped down to allow the pump to stop without water hammer and without damage to check valves or other inline devises that may be sensitive to this phenomenon. One other problem that may occur with a pump allowed to "free-wheel" in the reverse direction is that the motor becomes a generator in that state and the current produced can be damaging to sensitive electronic devices that may be installed in conjunction with the system and it can be dangerous.

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#16
In reply to #9

Re: Ratchet Arrangement in Vertical Pumps

06/22/2020 12:59 AM

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.

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#10
In reply to #8

Re: Ratchet Arrangement in Vertical Pumps

05/16/2013 8:38 AM

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.

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#11

Re: Ratchet Arrangement in Vertical Pumps

05/16/2013 11:54 PM

In big pumps check valves are not generally provided. Motorized valves will take time to close. So, anti reverse rotation ratchet is a necessity to avoid reverse rotation. Reverse rotation may cause the pump to go beyond run away speed causing significant damage to the equipment.

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#12

Re: Ratchet Arrangement in Vertical Pumps

05/17/2013 1:20 AM

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?

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#13
In reply to #12

Re: Ratchet Arrangement in Vertical Pumps

05/17/2013 7:49 AM

When the pump stops the water will begin running back toward and through the pump if there is no check valve installed in the line. This will cause the impellers in the turbine pump to spin in the opposite direction than when it is pumping. The velocity of the downhill running water will increase as it drops. It the forcemain is long and the static elevation is rather high (yours seems to be moderately high), the velocity can reach speeds that will spin the impellers at a speed that may exceed the forward running speed of the motor. The non-reverse ratchet will prevent the motor from turning in the opposite direction than when it is pumping, hence the name "non-reverse" ratchet. It will not, however, stop the flow of the water in the oposite direction. It will only stop the pump from turning backwards. As I said before, a check valve will also prevent the water from running in the opposite direction. But, as has been noted, the water coming back against the closed check valve can cause water hammer that can damage piping and the check valve itself. For that reason, the check valve used should, at the very least, be a "cushion" check valve as I stated before. A check valve should be installed in the line even with the non-reverse ratchet because with the water coming back into the sump, you have to re-pump the same water on the next pumping cycle because the line has emptied. One poster suggested motorized valves. That is another viable option as is the VFD I suggested before. But with the VFD you should still have check valves, again, so the line doesn't empty.

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#14

Re: Ratchet Arrangement in Vertical Pumps

06/19/2020 7:20 AM

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

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#15

Re: Ratchet Arrangement in Vertical Pumps

06/22/2020 12:37 AM

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.

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