6 replies to this topic

[von1987]

Hi there,

I am confused with the terms 'reverse flow' and 'reverse rotation' of a centrifugal pump. I think that reverse rotation means that the impeller is installed in the reverse direction,but it still delivers flow to the discharge outlet, and does not result in reversing the flow of the pump. Am I correct? I'm confused! I hope that someone can tell me whether they are the same, or if not, do explain both different cases for me.

Thanks!

[Art Montemayor]

Von:

When dealing with a centrifugal device – either a pump, fan, or compressor – the possibility of causing or provoking a reverse flow is sometimes present. And what I mean by “reverse flow” is that the normal direction of flow (i.e., from the suction piping – through the pump – through the discharge side of the pump and into the discharge piping) is reversed. In other words, the higher pressure existing at the discharge of the pump is allowed to overcome the lower pressure that is always present at the suction side of the pump. This is a situation that is always possible – if one allows it to happen. This is so because it is the primary reason for having a pump in the first place: we want to transport a liquid from a source of low pressure to a target that has a higher pressure. In the process of doing that, some unfortunate things can happen:

• The mechanical coupling that connects the normal electrical motor driver to the pump shaft may suddenly break and disconnect the pump from its driver;
• The electrical power source supplying the electrical motor driver may suddenly fail – causing a “black out”.
• The pump may suddenly lose its suction prime – such as sucking air or a vapor, or “cavitating” (a phenomena where the pumped fluid is vaporized because it reaches its vapor pressure temperature and then suddenly collapses). The net result of any of these things is that the pump suddenly stops pumping for a while – which means that it stops generating a discharge fluid pressure to overcome that which exists at the target of its flow.

The result of the above is that the high pressure fluid existing in the discharge side of the pump will seek a lower pressure. If there is no check valve (a device called a “non-return valve” in areas outside the Western Hemisphere) in the discharge or suction lines, there will be a flow through the pump in a direction opposite to the normal flow (reverse flow). That is why almost all centrifugal pump installations have a check valve installed in their piping configurations. Some install a check valve in both the suction and discharge sides of the pump.

A rare case where reverse flow can occur is when the pump’s impeller shears off or is disconnected from its shaft or key. A shaft key is normally used to contain the impeller in a fixed position on the shaft and not allow it to freely rotate around the shaft. Should this happen, the impeller rotates freely and is not capable of pumping fluid. Again, in the absence of a check valve device, reverse flow can occur.

I have used centrifugal pumps in reverse flow – but in a planned and engineered manner. All centrifugal pumps are capable of being used in reverse flow as work or power generators. You can use fluid hydraulic pressure (such as an elevated water source or a high pressure water source) to flow through a normal pump and turn the connected electric induction motor at its synchronous speed and generate electrical energy that is put back into the main power lines.

I hope this answers your query and resolves your confusion.

[srtools1980y]

hi!
i have attached an article on pump troubleshooting.
please study. it may be helpful.
goodday
srt.

article courtesy:PUMPS & SYSTTEMS_JULY 2004

Attached Files

•  Pump___Troublesh.03.pdf   289.81KB   209 downloads

[JLMONTREAL]

Hi

Mr. Art Montemayor has made good explanation. I would make more input regarding reverse rotation. For a centrifuge pump, the impeller has only one correct rotating direction, which can THROW liquid out of the pump case by centrifuge force, the correct direction can be observed from the case shape of most pumps. reverse rotation mainly caused by reversely connected electrical wires for the motor during initial installtion or maintenance. If this happened, pump cannot deliver fluid.

Hope helps.

joe

[srtools1980y]

hi!
always make the habit of bumping the motor,immediately after installation/maintenance of pump,for checking the direction of rotation.
goodday
srt.

[djack77494]

To further Joe's comment, it is possible to incorrectly wire your pumps' motor in such a way as to cause the motor to rotate in a manner opposite to what you intend. (In fact, this is not as uncommon as it should be.) This would produce what you are calling "reverse rotation". If this were to happen, a single pump would still be capable of generating flow in the forward direction, though the characteristics of the pump would significantly change. The nature of this change depends on the type of impeller and pump volute, though you can be certain that the change would NOT be an improvement.

Fortunately, the correct rotational direction is typically cast into the pump casing. As srt says, it is good practice to "bump" the motor and confirm proper rotational direction before putting a pump into service.
Doug

[von1987]

Thank you all for your comments, much appreciated