9 replies to this topic

[engg]

Is a minimum flow recirculation line necessary withe every centrifugal pump?

if so than the valve on the recirculation line should be fully or partially open?

Woudnt such a line reduce the total flowrate sent to the system.?

Thanks

[vvsastry66]

NOT NECESSARY:

• IF PUMP OPERATION IS INTERMITTENT;
• IF STANDBY PUMP CAN BE STARTED WITH DISCHARGE VALVE OPEN;
• IF PUMP POWER IS VERY LOW.

DIFFERENT TYPES OF MINIMUM FLOW LINES EXIST:

• CONTINOUS FLOW THROUGH AN ORIFICE. MINIMUM FLOW TO BE ADDED TO REQUIRED FORWARD FLOW TO GET PUMP RATED CAPACITY.
• FIC CONTROLLED, NO NEED TO ADD, WILL OPEN MINIMUM FCV IF FLOW FALLS BELOW MIN FLOW.

[breizh]

Hi ,

Let you read the attached document, it will support your query.

Breizh

[fallah]

IF STANDBY PUMP CAN BE STARTED WITH DISCHARGE VALVE OPEN.

Min flow control valve take inlet from common discharge line is needed.

Edited by fallah, 25 November 2010 - 03:16 AM.

[engg]

please see the attached schematic.

i was thinking if we are going to run the pump, then maybe i can use the 3" line (which is a bypass of the tank) as a min.flow, is it required?

I dont foresee any blockage at the discharge (so that csing temperature may rise) neither is there any orifice at the discharge of the pump....
Pump capcity is 30m3/hr while vendor specified min flow is 15 m3/hr

Attached Files

•  schematic.png   55.3KB   107 downloads

Edited by engg, 25 November 2010 - 07:47 AM.

[kkala]

Minimum flow is recommended, especially if the liquid is flammable. As a general practice we now install min flow lines (returning to tank / drum) in all local refinery pump discharge lines, even if operation is not continuous. One exception concerned a water pump, after Client's request.

In the attached schematic, let us consider the case of truck loading. When the truck gets filled, the valve near it shall close (manually or automatically). Besides shown valve on "truck connection" line can be inadvertedly closed. In these cases entrapped liquid around the pump will start increasing its temperature, so you have to stop the pump quickly. Operation of automatic min flow will get rid of such a care. Of course, min. flow line has to be installed downstream of first discharge valve; the min flow line has to have a FCV linked to flow measurement (e.g. orifice and flow transmitter) on the discharge line (additional instrumentation).

Connecting the min flow line to the existing 3" line has some drawbacks: all valves to the tank should be locked open and check valve is recommended of the line feeding the tank, upstream of the tie-in. How inconvenient can these be? A dedicated flow line (to a tank nozzle close to ground, as we do it) may be a simpler indicated solution.

[fallah]

A dedicated flow line (to a tank nozzle close to ground, as we do it) may be a simpler indicated solution.

Not to be a good configuration in all cases to conduct min flow line toward a tank nozzle close to ground.
In all cases i have ever seen,the min flow line have been routed to the nozzle on top (for vessel/drum) or to the nozzle on higher part of the shell (for tanks).

[kkala]

Not to be a good configuration in all cases to conduct min flow line toward a tank nozzle close to ground.
In all cases i have ever seen,the min flow line have been routed to the nozzle on top (for vessel/drum) or to the nozzle on higher part of the shell (for tanks).

Local practice for petroleum products here (to my knowledge from design) is to direct min flow line through a separate tank nozzle near ground, which requires a check valve on the line to prevent back flow.
If the line enters the top of the tank (or a nozzle at high level) you have to construct a downward pipe from nozzle to tank bottom to avoid splashing. An advantage of "top introduction" would be better protection against (gravity) backflow, but there are no piperacks on the tank basins here, so pipe should be elevated to tank top, too (and then routed down as said).
So generally(under the conditions considered) it may be simpler to route min flow line to a nozzle near ground. Of course other practices can be also technically acceptable and there may be specific reasons favoring them.
Above is limited to cone roof tanks, for floating roof tanks introduction of the min flow line below minimum possible level of floating roof is judged to be applicable (same local practice), not introduction through a guide pal (i.e.from up).
And all above concern min flow line in general. In the specific case of the schematic, the 3" existing line is used for other purposes too, so there may be drawbacks as mentioned.

Edited by kkala, 25 November 2010 - 04:09 PM.

[engg]

the whole operation is batch. The 3" line is a bypass to tank. If we are using the pump shown then 3" line has to be closed. however, my idea is not to install a new min.flow line but to use the 3" line as a min.flow bypass to tank top when pump is in operation. In case if we are filling the tanks and draining them at same time, in that case we cannot use the 3" line as a bypass..

thanks

[kkala]

the whole operation is batch. The 3" line is a bypass to tank. If we are using the pump shown then 3" line has to be closed. however, my idea is not to install a new min.flow line but to use the 3" line as a min.flow bypass to tank top when pump is in operation. In case if we are filling the tanks and draining them at same time, in that case we cannot use the 3" line as a bypass..

Attached minflow.jpg can indicate a min flow scheme more or less according to what you want - please consider the whole thread for more details. Valves CSO or CSC are so when pump operates and min flow control is active.

Attached Files

•  minflow.jpg   222.56KB   107 downloads