13 replies to this topic

[August]

In the project I am working on, a centrifugal pump is needed to pump out product from a vessel. But the vessel has only one nossel for product in and out which means that when the vessel is feeded, a reverse flow will go through the pump.(Discharge side to suction side). Of course, pump is not running, when the reverse flow goes in.

This kind of situation is acceptable in the real production? What are the risks ?

Thanks inadvance.

[shan]

In the project I am working on, a centrifugal pump is needed to pump out product from a vessel. But the vessel has only one nossel for product in and out which means that when the vessel is feeded, a reverse flow will go through the pump.(Discharge side to suction side). Of course, pump is not running, when the reverse flow goes in.

This kind of situation is acceptable in the real production? What are the risks ?

Thanks inadvance.

I have not heard a pump design with reverse flow. You should connect the inlet line to the pump suction line instead of pump discharge line to avoid the reverse flow situation.

[kkala]

Agreeing with shan, the pump has to be bypassed when the tank gets filled; see attached "bypass.doc". This practice can be found in local refineries, so it is acceptable. Evidently its disadvantage is that the tank cannot send and receive liquid at the same time.

Attached Files

•  bypass.doc   26.5KB   61 downloads

Edited by kkala, 08 August 2011 - 12:44 PM.

[August]

Agreeing with shan, the pump has to be bypassed when the tank gets filled; see attached "bypass.doc". This practice can be found in local refineries, so it is acceptable. Evidently its disadvantage is that the tank cannot send and receive liquid at the same time.

Thanks, the bypass is not suitable to our product as it is viscous and there is no enough space for a long bypass.
Does anybody know what will happen if the flow goes through the pump in reverse. What is the damage to the pump? From the engieer point of view, why we should avoid reverse flow?

[fallah]

Thanks, the bypass is not suitable to our product as it is viscous and there is no enough space for a long bypass.
Does anybody know what will happen if the flow goes through the pump in reverse. What is the damage to the pump? From the engieer point of view, why we should avoid reverse flow?

August,

It is strange for me that while bypass isn't suitable to your product how reversing the flow through your pump could be proper situation!

Anyway if the flow goes through the pump in reverse, the relevant motor and pump seal may subject to damage and normally this situation is not allowed at all.

Fallah

Edited by fallah, 09 August 2011 - 07:21 AM.

[August]

Thanks, the bypass is not suitable to our product as it is viscous and there is no enough space for a long bypass.
Does anybody know what will happen if the flow goes through the pump in reverse. What is the damage to the pump? From the engieer point of view, why we should avoid reverse flow?

August,

It is strange for me that while bypass isn't suitable to your product how reversing the flow through your pump could be proper situation!

Anyway if the flow goes through the pump in reverse, the relevant motor and pump seal may subject to damage and normally this situation is not allowed at all.

Fallah

Thanks Fallah,

Revese flow is not a proper way of course. but bypass is not proper way for us either. The problem is that some people don't think reverse flow is a problem, and the manufacure of pump doesn't think this is a problem either. I am looking for a very strong reason to persuade them not to let the pump face a reverse flow. But unfortuantely, there is no engieering reason for that so far.

August.

[S.AHMAD]

Agreeing with shan, the pump has to be bypassed when the tank gets filled; see attached "bypass.doc". This practice can be found in local refineries, so it is acceptable. Evidently its disadvantage is that the tank cannot send and receive liquid at the same time.

Thanks, the bypass is not suitable to our product as it is viscous and there is no enough space for a long bypass.
Does anybody know what will happen if the flow goes through the pump in reverse. What is the damage to the pump? From the engieer point of view, why we should avoid reverse flow?

Hi there..

If the viscous fluid can pass through a pump then it has no problem passing through a bypass. The bypass need not be long, it can be installed around the pump i.e discharge (downstream block valve) to suction (upstream of suction valve). Reverse flow will turn the pump into a turbine and hence increases the flow resistance and reduces the flowing capacity. However, you may try reversing the flow and please share with us the outcome.

Good luck.

Edited by S.AHMAD, 09 August 2011 - 07:50 PM.

[fallah]

Thanks Fallah,

Revese flow is not a proper way of course. but bypass is not proper way for us either. The problem is that some people don't think reverse flow is a problem, and the manufacure of pump doesn't think this is a problem either. I am looking for a very strong reason to persuade them not to let the pump face a reverse flow. But unfortuantely, there is no engieering reason for that so far.

August.

August,

Possibility of seal/lubrication system damage don't provide adequate reason for your thinking!

Anyway, if you would intend to use reverse rotation, by contacting to relevant vendor, be sure that the bearing lubrication system will operate in reverse and mechanical seal can tolerate short/long duration of reverse rotation without to be subject to damage.

Fallah

Edited by fallah, 10 August 2011 - 02:33 AM.

[August]

Thanks everybody,

Now I get ur point which is that the only problem of reverse flow through a pump is the damage on pump. If our pump vendor confirms that the pump can handel this, then there is no problem at all.

Our pump vendor has confirmed as I said in my last post, that the pump can handel the reverse flow.

I was thinking except the damage on pump, there must be some other disadvantages of reverse flow, Appearently, it is not true.

[kkala]

Now I get ur point which is that the only problem of reverse flow through a pump is the damage on pump. If our pump vendor confirms that the pump can handle this, then there is no problem at all.
Our pump vendor has confirmed as I said in my last post, that the pump can handle the reverse flow.
I was thinking except the damage on pump, there must be some other disadvantages of reverse flow, appearently, it is not true.

Intensionably reverse flow (i.e. not by wrong rotation of impeller, due to inadverted electrical connection of the motor) is not a recommended practice, even though extensive explanations on it has not been found; so the case should be looked into, to limit possibility of undesired consequences that may not be clear enough at present. A cognizant mechanical / electrical engineer can help to assess consequences. A few of them (thought of) could be as below.
-Excessive reverse flow (as it occurs at sudden pump failure and resulting water hammer) could burn pump motor; so reverse flow had better not exceed a value to be specified by vendor. Its statement of reverse flow handling may mean "up to a flow limit".
- Pump / impeller frictional pressure drop in reverse flow has to be specified. It is expected to be comparatively small in the present case, where pump induction motor does not supply power externally (during reverse flow).
- Mentioned risk of seal is understood to concern mechanical seals. But even a simple seal of rope can have leakages when shaft rotation is inverted.
- A similar topic previously discussed is at http://www.cheresour...verse-rotation/.
- In general considered reverse flow will have some unforseen risks to be faced in practice. Vendor could help by clearly answering relevant questions of yours, but could it be totally responsible for this matter? (doubtful).
Note: No info on such reverse flow in your project design practices is an argument against it; besides reverse viscous liquid flow will not be facilitated through the pump, compared to "bypass flow" (to my understanding, see also S.Ahmad's post); another pump may be needed for it at any case.

Edited by kkala, 12 August 2011 - 04:58 AM.

[vvsastry66]

Hi,

Reverse flow can occur, after pump trips, due to high pressure at discharge/check valve leaking. Some pump vendors design piping upto first isolation valve, with design pressure on discharge side.

Sastry

[GAURANG PATEL]

Hi August,

See U may go by reverse flow through pump, condition is that reverse flow & pressure should not too high say flow of 20-30 m3/hr at 4-5 kg/cm2 pressure. It is equally important that your pump should have similar capacity to pump in normal direction pumping.
I cant give u 100% surity but u can start with low flow - low pressure while trying in reverse direction.
The risk associated with reverse flow is that impeller may got dismantled or damaged.

Another better option is that provide one connecting spool from discharge NRV downstream to pump suction valve upstream with one isoaltion valve on that spool. This connecting spool will by pass the pump & of short length.

I do have experience of pump reverse flow & let me say it is working well.

[jpm.chem]

Hi August,
I have not come across a situation like that you have provided.. May be i have an alternate solution for this and have attached it.. Try it if your pump vendor fails..
Hav a good time..

In the project I am working on, a centrifugal pump is needed to pump out product from a vessel. But the vessel has only one nossel for product in and out which means that when the vessel is feeded, a reverse flow will go through the pump.(Discharge side to suction side). Of course, pump is not running, when the reverse flow goes in.

This kind of situation is acceptable in the real production? What are the risks ?

Thanks inadvance.

Attached Files

•  Alternate idea.xls   43KB   35 downloads

[demank]

An alternatif for pump reverse/forward flow, you may utilize screw pump, instead of centrifugal pump. It is also suit for your high viscous fluid.