Two control valves for liquid transfer with pump 1

[sujins]

1)When we have the system of transfer liquid with using centrifugal pump which suction line connected from storage tanks and liquid is discharged to the process column, since the required flow rate is less than the point that performance curve meet the system curve.(Pump will operate at that point?) Then,during pump operation, we will use 2 control valves with one is the existing to control flow rate at the discharge line to column and the other one is the new pressure control valve which installed at the recycle line for the excess of flow rate to the storage tank. Since, we will not trim the impeller and will also not consider to use the variable speed drive to reduce the speed of motor. Is this concept of design able to control to achieve the desired flow rate and keep constant discharge pressure of pump. It seems like we have two control valves; one is for flow control and one for pressure control. If it's ok. Then, please suggest how to calculate to obtain the data to size the 2 mentioned control valves to get the desired flow and pressure. I mean pressure drop, flow rate of each control valve. Please help.
2) If the existing flow control valve is normally flow over from column to storage tank, but we will transfer liquid according to the above description back from storage tank to column which the flow of control valve will be flow under? Can we control that control valve with the reverse direction? Any other things that I might be forget and have to consider, please expert suggest me with the right solution.

 

[JimCasey]

1. Read the pump curve. It should have come with the pump, or it can usually be found in the literature.
The inlet pressure for the bypass valve is your pump head at the NPSH(Minimum flow) line. When you begin to close the flow control valve the bypass valve should open at the minimum flow line, then modulate so that the pressure does not exceed the pressure that corresponds to the minimum flow point on the pump curve. It would be even more accurate to measure the pump flow before the bypass valve and control the bypass valve to the minimum flow rate. This is because the pump curve is very flat at low flows, and a small change in pressure accompanies a large change in flowrate.

2. Reversing the direction through a control valve may or may not be possible. It depends on what type of valve you have, and what the dynamic conditions are. An unbalanced single-seat globe valve is not happy with the flow over the seat, but it can be done with a larger, stiffer actuator. A cage-balanced valve usually flows over the seat, but it doesn't really care, unless it's pilot-balanced. The pilot-balanced valve has to flow over the seat. An eccentric rotary valve is little-affected by flow direction.

 

[sujins]

Jimcasey Thanks!

I just wonder for the term of pump run-out,

1)How can it happenned? What situation that bring the run-out occured?
2)And what is the result of it? Is it harmful to the motor that drive the pump impeller? Please...

 

[JimCasey]

Pump runout is when you allow the pump to operate without back pressure. THis causes extremely high flowrates. I suppose that could lead to cavitation at the impeller, since with no pressure developed the backside of the impeller could go subatmaopheric and cause the pumped fluid to boil.

Stay within the published limits of the pump curve.

 

[sujins]

Dear Jimcasey!

Below detail is the spec. of flow control valve which I will be used in reverse direction that I mentioned to you previously. I don't know for the type (balanced, unbalanced or cage-balanced, etc) Should you see and give recommendation?

EXISTING FLOW CONTROL VALVE DESCRIPTION :

GLOBE BODY: MARK I, 3” 316 ST ST, 150# CLASS, ANSI
FACE TO FACE, SEP END FLNG 250/500 RA FACE FINISH,
STAINLESS STEEL END FLANGES & HALF – RINGS.
TRIM: =%, T/N 2.62, CV 104.00, 316 ST ST W/TFE SOFT SEAT.
BONNET: COLD BOX 18”, 316 ST ST, ST ST BONNET FLANGE &
ST ST BODY BOLTING,BRONZE GUIDES, STD TFE “V” PACKING,
TEFLON GASKETS, FLOW OVER.

LINEAR ACTUATOR: CYLINDER, 50, DUAL HD SPRING, AIR TO OPEN,
BETA I/P POSITIONER 4-20 MA W/AIR FILTER.

ACCESSORIES: LIMIT SWITCH MICRO EXAR(SPDT), INDICATION
CLOSE, SOLENOID ASCO EFHT8320G172, 3-WAY 115 VAC,
DE-ENERGIZED TO CLOSE, AIR FILTER.

Your answer will be highly appreciated.

 

[sujins]

Please anyone give answer or suggestion for the above mentioned detail. I really need help urgently!...

 

[JimCasey]

It depends on your pressure. I don't work for Valtek but I ran it through their sizing program. The sizing program says that thios actuator on this valve will handle flow over the seat or flow under the seat at up to 170 psi. Above that you will need a bigger actuator.