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Needs Of Pump Minimum Flow For Intermittent Pump


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#1 manchester

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Posted 10 February 2010 - 03:46 AM

Hallo,

Is it true that pump minimum flow line is not needed in intermittently operationed pump?
I'm talking about a centrifugal pump that pumping diesel oil to a day tank.

Thank you

#2 Art Montemayor

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Posted 10 February 2010 - 05:45 AM


Manchester:

This is a general question about a specific subject.

If your scope (or intention) is to pump all the liquid from one tank to another, it is obvious that you will use a centrifugal pump pumping at its maximum capacity until all the liquid from the source tank is in the target tank.

If you are not trying to pump ALL the liquid from the source tank, but only a portion until the target tank is full - then the pumping requirements should be the same: use the maximum capacity of the transfer pump to minimize the transfer time. Once the target tank is full, you shut down the transfer pump.

If either of the above specific scopes is the basis, then you obviously shouldn`t have any concern or worry for minimum flow. Your concern should be for achieving MAXIMUM flow in order to minimize the transfer operation.

I hope this helps to answer your question.


#3 fallah

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Posted 10 February 2010 - 01:26 PM

Hallo,

Is it true that pump minimum flow line is not needed in intermittently operationed pump?
I'm talking about a centrifugal pump that pumping diesel oil to a day tank.

Thank you


Pump minimum flow line,as general,is a facility for pump protection against pump discharge blockage and its requirement doesn't depend on type of pump operation (continuous/intermittent).

#4 kkala

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Posted 10 February 2010 - 04:10 PM

Pump minimum flow line,as general,is a facility for pump protection against pump discharge blockage and its requirement doesn't depend on type of pump operation (continuous/intermittent).

I agree with fallah. Working for an engineering company for refineries, we place min flow protection on every centrifugal pump nowadays as a rule, even for intermittent service (e.g. at offsites). This can be also concluded from the thread of 20 Nov 2009 by star@25 in Refining, Hydrocarbons,..forum (Centrifugal Pump - min circulation), even though not specifically mentioned.
One could think of not installing min flow return in case that: operator starts / stops pump locally, follows it locally during operation, can see its discharge line to the end (or see its flow by a local flow meter). It is very improbable that all three occur in a refinery. Very few old pumps from the past remain now without min flow.

#5 S.AHMAD

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Posted 10 February 2010 - 08:50 PM

Hi

The purpose of minimum flow line for centrifugal pump is to prevent pump overheating (potential explosion) and vibration (potential pump damage) due to too low the flow over a longer period of time (continuous operation). Since as Art mentioned, for intermittent pump operation, the purpose is to pump or transfer fluid maximum possible so the minimum flow situation does not arise since the pump operation under shut-off situation is very short time i.e. during start-up and shutdown. So, potential hazardous events due to minimum flow are very unlikely. However, the pump operator must be aware that too long pump under shut-off situation may result in hazardous events since minimum flow line is not provided.

#6 manchester

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Posted 10 February 2010 - 10:06 PM

Hi

The purpose of minimum flow line for centrifugal pump is to prevent pump overheating (potential explosion) and vibration (potential pump damage) due to too low the flow over a longer period of time (continuous operation). Since as Art mentioned, for intermittent pump operation, the purpose is to pump or transfer fluid maximum possible so the minimum flow situation does not arise since the pump operation under shut-off situation is very short time i.e. during start-up and shutdown. So, potential hazardous events due to minimum flow are very unlikely. However, the pump operator must be aware that too long pump under shut-off situation may result in hazardous events since minimum flow line is not provided.



Thank you all,
Especially for Ahmad,you really help me to understand Art's explanation.

#7 kkala

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Posted 11 February 2010 - 03:11 PM

To this topic I would like to add the following comments.
1. As S.Ahmad pointed out, purpose of minimum flow is to prevent pump from overheating or vibration due to low flow (e.g. lower than 25% of rated capacity). This can occur even in intermittent flow, if someone inadvertedly closes a discharge valve enough to result in too little flow (depending on the pump), or to shutoff.
2. Of course the catastrofic effect will not occur immediately, but overheating (e.g. to autoignition or evaporation) may be a matter of few minutes or less. Overheating time can be estimated by Process Engineers, vibration assessment is not known to me.
3. A hydrocarbon pump is improbable to accomplish its (intermittent) operation in such a short time (of few minutes or less). So it has a min flow protection as a rule in the refineries.
This protection is needed even if pump normally operates at max flow, as long as a discharge valve can be inadvertedly closed. This seems to be the main criterion of placing a min flow return or not.
On the other hand a control valve, demanding flow below "min flow" sometimes, makes min flow protection necessary.
4. Suppose loading ships with fuels, an intermediate operation working at max flow. Pump is usually near the tank, loading line is too long for visual check by operator, ship may suddenly abandon pier and then flow shuts off automatically. Min flow protection of loading pump seems necessary.
Intermittent transfer from tank to tank could assure no discarge valve closed, if tanks are near bye. But operator does not want to be always on field, and same pump is used for more distant tanks. Min flow protection is indicated.
5. I have not a feeling of critical time (according to 2) for pumps not transfering hydrocarbons, but it may be short since min flow protection seems to be widely applied there too, e.g. in firefighting pumps (intermittent operation).
6. So the cases where min flow protection (in pumps of intermittent service) is not considered necessary nowadays are quite seldom in my opinion. I had seen pumps of non flammable liquids stopping at shutoff after a determined time (motor protection?), but this was in 1979.
7.Is there any procedure/ standard about pump min flow protection? This could advise more specifically, in case of either continuous or intermittent flow. Hopefully somebody can inform on this matter.

#8 fallah

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Posted 12 February 2010 - 02:36 AM

To this topic I would like to add the following comments.
1. As S.Ahmad pointed out, purpose of minimum flow is to prevent pump from overheating or vibration due to low flow (e.g. lower than 25% of rated capacity). This can occur even in intermittent flow, if someone inadvertedly closes a discharge valve enough to result in too little flow (depending on the pump), or to shutoff.
2. Of course the catastrofic effect will not occur immediately, but overheating (e.g. to autoignition or evaporation) may be a matter of few minutes or less. Overheating time can be estimated by Process Engineers, vibration assessment is not known to me.
3. A hydrocarbon pump is improbable to accomplish its (intermittent) operation in such a short time (of few minutes or less). So it has a min flow protection as a rule in the refineries.
This protection is needed even if pump normally operates at max flow, as long as a discharge valve can be inadvertedly closed. This seems to be the main criterion of placing a min flow return or not.
On the other hand a control valve, demanding flow below "min flow" sometimes, makes min flow protection necessary.
4. Suppose loading ships with fuels, an intermediate operation working at max flow. Pump is usually near the tank, loading line is too long for visual check by operator, ship may suddenly abandon pier and then flow shuts off automatically. Min flow protection of loading pump seems necessary.
Intermittent transfer from tank to tank could assure no discarge valve closed, if tanks are near bye. But operator does not want to be always on field, and same pump is used for more distant tanks. Min flow protection is indicated.
5. I have not a feeling of critical time (according to 2) for pumps not transfering hydrocarbons, but it may be short since min flow protection seems to be widely applied there too, e.g. in firefighting pumps (intermittent operation).
6. So the cases where min flow protection (in pumps of intermittent service) is not considered necessary nowadays are quite seldom in my opinion. I had seen pumps of non flammable liquids stopping at shutoff after a determined time (motor protection?), but this was in 1979.
7.Is there any procedure/ standard about pump min flow protection? This could advise more specifically, in case of either continuous or intermittent flow. Hopefully somebody can inform on this matter.


KKALA

Thanks for your full explanations to verify need to minimum flow even in intermittent sevices!

I should add that in many applications minimum flow isn't final safeguard for pump protection and a FSLL will act to shutdown the pump in the case of relevant FCV malfunction/failure.

#9 kkala

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Posted 12 February 2010 - 03:58 PM

I should add that in many applications minimum flow isn't final safeguard for pump protection and a FSLL will act to shutdown the pump in the case of relevant FCV malfunction/failure.

This is true. Min flow is not taken into account e.g. in sizing a relief valve at pump discharge, apparently because min flow system is not considered reliable enoughby code. Nevertheless mechanical return valve (e.g Yarway type, or equivalent) must be safer than flow control valve receiving signals. I have seen mechanical return valves at boiler feed water or ammonia pumps, concluding these are for critical applications.
Another measure seen is a temperature switch HH to stop the pump, if min flow is connected to pump suction (instead of tank). Set point for diesel was set at 55 0C (flash point) in one case. Of course this system is less safe compared to min flow return to suction tank, it was applied due to space restrictions.

Edited by kkala, 12 February 2010 - 04:02 PM.





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