12 replies to this topic

[Prasant]

In a centrifugal pump, discharge valve is pinched. We all know that the pressure head will increase in the pump discharge. What will happen to the discharge pressure after discharge valve? Whether it will decrease or it will also increase?

[fallah]

Prasant,

When the discharge valve is pinched (system curve will move to left right of pump curve), fluid velocity decreases and causes a decrease in pressure loss in pipe and fittings respect to previous situation but an increase in pressure drop across the valve. It is obvious that the total pressure drop in discharge line would be increased due to valve pinch.
It is hard to say if the pressure after valve will decrease or increase. It strongly depends on degree of valve pinching, discharge line configuration, pump curve, , then it may increase or decrease and each case shouild be investigated as per its own characteristics.

Fallah

[S.AHMAD]

Prasant

1. The pressure downstream of the pinched valve is expected to be lower due to lower velocity and hence lower pressure drop downstream.
2. If you could draw the system curve (100% valve opening), superimposed with the pump curve, you will understand it better. The difference between the pump curve and the system curve is the valve pressure drop.

Edited by S.AHMAD, 23 October 2012 - 08:29 PM.

[a1986]

Prasant

if you analyze the functioning of the pump in relation with system, we can notice that the pump work where system tell its to work, in a definite operation point; infact when a pump is started, the discharge valve is open and slowly discharge valve is pinched.
Now therefore, if in the discharge pipeline is present a valve (in example a globe valve that allows a regulation) that could be pinched, it involves friction losses slowly the valve is pinched, and then system curve becomes so steeply and then oprating point is involved to the left.. when this happened it's necessary to analyze how much on the left side of the curve the operating point is, infact if the pump curve has a saddle this situation causes instability.

In the practice we can say that a pump is generally selected to have an operating point at left side of best efficiency point in the way to take into account any increases implemented by process, infact in the real running, operating point is shifted slightly on the right of the predicted operating point...but this is another matter.

best regards

[sheiko]

1. The pressure downstream of the pinched valve is expected to be lower due to lower velocity and hence lower pressure drop downstream.
2. If you could draw the system curve (100% valve opening), superimposed with the pump curve, you will understand it better. The difference between the pump curve and the system curve is the valve pressure drop.

Dear Ahmad,

1/ I agree.

2/ I don't believe the delta-head (or pressure) between the pump curve and the 100% open system curve corresponds to the control valve pressure drop.
The difference between the pump curve and one system curve decreases as the flowrate increases, so it cannot correspond to the valve pressure drop.
For example, the deltaP consumed when the valve closes from 100% to 50% is rather the pump's head when the valve is 50% open (intersection of pump's curve with system curve with valve 50% open) minus the pump's head when the valve is 100% open (intersection of pump's curve with system curve with valve 100% open).

Sorry I have not made a sketch to support my explanation.

Edited by sheiko, 25 October 2012 - 10:12 PM.

[sheiko]

It is hard to say if the pressure after valve will decrease or increase. It strongly depends on degree of valve pinching, discharge line configuration, pump curve, , then it may increase or decrease and each case shouild be investigated as per its own characteristics.

Fallah

Dear Fallah,

As Ahmad said, the pressure at the outlet of the control valve is only the backpressure of the circuit downstream the valve. So, as the flowrate decreases, so does the backpressure.

Edited by sheiko, 25 October 2012 - 10:13 PM.

[fallah]

Dear Fallah,

As Ahmad said, the pressure at the outlet of the control valve is only the backpressure of the circuit downstream the valve. So, as the flowrate decreases, so does the backpressure.

Dear Sheiko,

I would agree. Thanks.

Fallah

[fallah]

2/ I don't believe the delta-head (or pressure) between the pump curve and the 100% open system curve corresponds to the control valve pressure drop.
The difference between the pump curve and one system curve decreases as the flowrate increases, so it cannot correspond to the valve pressure drop.
For example, the deltaP consumed when the valve closes from 100% to 50% is rather the pump's head when the valve is 50% open (intersection of pump's curve with system curve with valve 50% open) minus the pump's head when the valve is 100% open (intersection of pump's curve with system curve with valve 100% open).

Sorry I have not made a sketch to support my explanation.

Dear Sheiko,

The DeltaP consumed when the valve closes, how much not important, corresponds to the part of vertical line between intersection of new system curve (valve in new closer condition) with the pump curve, and old system curve (valve in more open position). For this reason if valve to be more closer the DeltaP across it would be higher due to divergence of system curve from pump curve in moving to left side

Fallah.

Edited by fallah, 26 October 2012 - 01:33 AM.

[sheiko]

...corresponds to the part of vertical line between intersection of new system curve (valve in new closer condition) with the pump curve, and old system curve (valve in more open position)...

Fallah.

I would add the following to your statement:

"...corresponds to the part of vertical line between intersection of new system curve (valve in new closer condition) with the pump curve, and INTERSECTION OF old system curve (valve in more open position) WITH THE PUMP CURVE"

This will lead to a lower deltaP than with your previous statement.

If it's still not clear I'll upload a sketch to explain my point.

[fallah]

I would add the following to your statement:

"...corresponds to the part of vertical line between intersection of new system curve (valve in new closer condition) with the pump curve, and INTERSECTION OF old system curve (valve in more open position) WITH THE PUMP CURVE"

This will lead to a lower deltaP than with your previous statement.

If it's still not clear I'll upload a sketch to explain my point.

Sheiko,

In fact when you want to measure the pressure drop increasing across the valve in new position respect to previous position due to valve pinch, you should move vertically from new working point to a point at the previous system curve with the same flowrate corresponds to new valve position (or new working point) which naturally is a point at left side of old working point of the pump. While you did mention the pressure drop increasing across the valve would be the pressure difference between new and old working points that cannot absolutely be true. Actually by above changed statement you didn't specifiy how do you move vertically between old and new working points not to be in a vertical line, and if you say you meant moving from new working point vertically till intersects the horizontal line passing old working point; then how do you relate the second point to old system curve while it is not at that curve?

Anyway due to dynamic nature of pump operation i don't think a sketch can help some more in this regard but if it can i am ready to upload.

Fallah

[Nirav]

Attached file may help to understand the relation between system resistence curve & control valve pressure drop on a typical pump performance curve.

Please note that system resistance always follows the path of its own depending on the flow. However, due to introduction of additional resistance in terms of pinching a valve (manual / control), it gets altered.

@Prashant:
Theoritically, pressure in the downstream of the control valve will not be changed for the same flow rate if you introduce a valve on the system and pinch it to provide additional resistance. Because, pressure is back calculated from the fixed destination pressure.

When you are pinching the valve in pump discharge, what you are doing is adjusting the resistance in such a way that intersection of your required flow and resistance falls on pump curve.

Attached Files

•  Pump_Control Valve.pdf   44.66KB   79 downloads

[fallah]

Nirav,

Thanks for uploading the sketch. My explanations in post No. 10 is in line with your sketch. Am i right?

Fallah

[S.AHMAD]

Mathematically, the pressure downstream a vallve (e.g. control valve) is the back pressure that can be expressed as:

P_vdownstream = P₂ + 4f(L/d). densuty. (V²/2)/1000 in kPa consistent unit for all variables.

where P₂ is tge destinination pressure, assumunig negligible elevatation differential between valve and the destination point 2. and f is the friction factor (fanning).
P₂ is constant therefore the only variable is is the velocity which is depending on the flowrate. It is obvious that as V increases P_vdownstream is also increased and vice versa. When the valve is pinched, the flowrate is reduced. Hence, the P_vdownstram reduces.