4 replies to this topic

[NoobMi]

Hi guys,

 

I am a bit puzzled by the minimum flow during normal operations.

 

Based on my pump data sheet, the specified minimum flow is stated as 10 m3/hr.

The control method for min. flow is through the use of restriction orifice.

 

So

1) In the event that my downstream valve (to process side) is fully open, will the minimum flow back to tank still be at 10 m3/hr?

 

2) if my downstream valve is fully closed, the minimum flow will remain at 10 m3/hr?

 

From what i read so far, the minimum flow seems to be constant at 10 m3/hr, independent on the downstream valve opening. But i cannot understand the reason.

 

Shouldnt the difference in the pressure of the main line and the minimum flow line determine the actual flow rate? Can anyone enlighten me on this?

 

 

[breizh]

consider this resource to support .

 

breizh

Edited by breizh, 18 June 2013 - 08:00 AM.

[pavanayi]

Ofcourse the pressure differential across the RO drives the amount of flow through the min flow line.

 

When the downstream valve towards process is fully open, there is minimum backpressure at the pump discharge, and the pressure differential across the RO will be minimum. Therefore, the flow back into the suction of the pump will be minimum.

 

When the downstream valve is shut, the pump will virtually be at its shutoff head. The differential pressure across the RO will be maximum and the flow will also be maximum.

 

At any point, the flow of liquid (through the min flow line + that to the downstream process) should be atleast 10m³/hr.If no flow is going to downsteam, the flow through the RO should be 10m³/hr. If sufficient flow is going to the downstream process, then the flow through the RO can (will) drop.

[fallah]

NoobMi,

 

My comments:

 

Hi guys,

 

I am a bit puzzled by the minimum flow during normal operations.

 

Based on my pump data sheet, the specified minimum flow is stated as 10 m3/hr.

The control method for min. flow is through the use of restriction orifice.

 

So

1) In the event that my downstream valve (to process side) is fully open, will the minimum flow back to tank still be at 10 m3/hr?

No,

 

2) if my downstream valve is fully closed, the minimum flow will remain at 10 m3/hr?

Yes,

 

From what i read so far, the minimum flow seems to be constant at 10 m3/hr, independent on the downstream valve opening. Not right, But i cannot understand the reason.

 

Shouldnt the difference in the pressure of the main line and the minimum flow line determine the actual flow rate? Can anyone enlighten me on this?Yes.

 

 

[mansari]

Dear Noobmi ! Minimum flow for the centrifugal pumps is specified by the pump manufacturers. , Actually continuous operation of a centrifugal pump with discharge valve fully closed results in rise in fluid temperature (  which is governed by shutoff head, pump efficiency and casing volume.) as a result of continuous energy input from the driver. If the pump is operated in this manner continuously, pump seal may damage. Minimum flow is more critical for the pumps in high temperature and boiling liquid service. Specified minimum flow is recycled back to suction vessel of the pump.

 

When pump is delivering forward, and the flow is exceeding the minimum flow, there is no need to keep minimum flow valve open, because in this case, it is loss of energy. Depending on type of pump curve, minimum flow control philosophy is derived. If the pump curve is steep, a pressure control system is useful, where the valve closes as the pump operating head approaches toward the head corresponding to minimum flow.

 

In relatively straight and less steep curves, flow control philosophy is better where a reverse acting control valve operates on input signal of forward flow.

 

Fallah has already given you answers of the questions you raised in your post