14 replies to this topic

[ankit2206]

I am really confused.

Does it controls (increases / decreases) Pressure? Does it controls volumetric flowrate of the fluid flowing through the pipe?

 

As far as I know, according to the 'equation of continuity', for an in-compressible fluid flowing through a pipe, whenever cross-sectional area decreases, its velocity should increase.

The flowrate essentially remains constant.

 

Why then, when the control valve position is lowered i.e. its cross-sectional area decreased, does fluid flow-rate decreases? Shouldn't its velocity increase with the flowrate remaining unaffected?

Isn't this contradicting the continuity equation?

 

Let us suppose that a fluid is flowing through the pipe with the flowrate -> 450 m3/hr initially.

On closing the control valve a little, we start getting the flowrate => 350 m3/hr

Where has that 100m3/hr gone. Has the pump which was pumping the fluid at 450 m3/hr suddenly suffered a loss in flow rate? because I don't think that's possible unless we decrease impeller diameter or decreased the rpm.

 

Please help me clear the confusion.

Thanks.

 

 

[thorium90]

Control valves reduce pressure. It also allows the operator to control the flowrate downstream of the valve.

 

You describe your understanding of continuity by stating "whenever cross-sectional area decreases, its velocity should increase", that implies you put

a1*v1=a2*v2

a*v=flowrate

 

If you think of a valve closing as having less space for the fluid to squeeze through, then with the same pressure upstream, the smaller valve opening means lesser fluid gets through. The amount of fluid entering the control volume becomes lesser so the flowrate out of the control volume also decreases. You have assumed incorrectly that the flowrate into the control volume is always constant. The extra work becomes lost. You can think of Bernoulli equation here.

Edited by thorium90, 20 February 2013 - 08:29 AM.

[kkala]

Probably some examples and notes can reduce confusion.

1. Placing a control valve (CV) at the discharge of a positive displacement pump, flow shall not decrease. This pump gives constant flow rate irrespectively of discharge pressure. Discharge pressure will increase, since CV will create additional ΔP.

Note: This example is not applicable in practice, you risk breaking the CV if totally closed (it is possible in branching).

2. Placing a CV  at the discharge of a centrifugal pump is a different story. Additional ΔP will move operating point to lower flow, according to pump performance curve. Assuming constant suction pressure, the flow where pump discharge pressure (per its curve) equals discharge piping pressure drop (friction+static)+destination pressure will be lower than before. That is flow decreases when CV is closing. The centrifugal pump has suffered a loss of flow rate and has increased its head. Head is usually maximum when valve is 100% closed (sutoff).

3. Continuity equation expresses just a mass (or even volume for incompressible fluid) balance among two sections of pipe at steady state. A centrifugal pump introduces fluid to the pipe at a rate Q depending on the pressure developed at its discharge / pipe start. That Q remains constant all along the (non branched) pipe.

4. Looking into a detailed CV process data sheet (including ΔP at minimum flow, ΔP at maximum flow), along with related process data, could clarify the matter more.

Edited by kkala, 20 February 2013 - 12:25 PM.

[shan]

A Control valve is just able to increase pressure drop. Higher pressure drop results less flow from the source if the inlet and outlet pressures are kept constant. Therefore, the flow is reduced at the source (pump, compressor, tank, vessel) and not disappeared at the valve.

[narendrasony]

Continuity equation is true for unrestricted pipes. It may hold good at a reducer but not necessarily at a valve. A control valve trim is designed (e.g. globe valve with tortuous path ) to create pressure drop.
If you close a gate valve or butterfly valve, you will not see considerable pressure drop even at lower openings say 25-30%. It creates pressure drop only at low openings say 10-15%. Effect of this pressure drop is reduction in flow through valve (compare with electric circuit), upstream system may or may not be ready for that as explained by Kkala .
Regards
Narendra

[thorium90]

Narendra, continuity is like a mass balance, it always holds true. The misconception of the OP is that the mass flow into the control volume is still the same even at different control valve openings which is not true

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Edited by thorium90, 20 February 2013 - 11:36 AM.

[narendrasony]

Thanks Thorium90 for correcting me.

[sheiko]

Ankit,
Your question is very good.
I suggest you to use the "search" button of the forum to find other posts on control valve.

[Art Montemayor]

Ankit2206:

You are confused – and you are probably confusing a lot of your readers because you jump from one type of fluid phase to another, without taking the proper, applicable reservations and applying the corresponding principles.

 

For example, you write about the continuity equation and its application to incompressible flow (liquids) and then make the general (erroneous) statement that the velocity should increase because a control valve upstream is throttling the flow.  You fail to consider the fact that the velocity does increase – but ONLY AT THE VALVE’S REDUCED SEAT AREA.  THE CROSS-SECTIONAL FLOW AREA OF THE PIPE IS CONSTANT AND,  THEREFORE, THE VELOCITY OF THE LIQUID FLUID DECREASES BECAUSE THE FLOW IS THROTTLED WHILE THE FLOW AREA IS KEPT CONSTANT.  You have failed to take into consideration that the pipe diameter (and its flow area) remains the same.

 

You are further getting more confused because you obviously don’t know how a centrifugal pump operates.  It is a dynamic, centrifugal device that follows the flow rate depicted on its performance curve.  If you spend some time studying centrifugal pumps you will learn that when you throttle the discharge of a centrifugal pump, the discharge flow is reduced appropriately.  It behaves unlike a positive –displacement type of pump.

 

You are totally wrong when you state “Has the pump which was pumping the fluid at 450 m³/hr suddenly suffered a loss in flow rate?  because I don't think that's possible unless we decrease impeller diameter or decreased the rpm.”  It is not only possible, it is an engineering fact.  You need to spend some time studying the Unit Operation called pumping of fluids.  That will resolve your confusion

[S.AHMAD]

1. I have no choice except to totally agree with Art Montemayor that you are confused.

2. The basic reason is that you do not understand the centrifugal pump characteristics and system characteristics.

Mathematically, the upstream system pressure (let say P_d) could be expressed by:

 

P_d = Po + DP_line + DP_valve

 

3.  When we close the control valve opening, the pressure drop across the valve increases and this will increase the upstream pressure namely the pump discharge pressure. In other words, the pump discharge pressure is determined by the system, not by the pump. The function of the pump is to supply the flowrate that corresponds to the system pressure according to the pump's characteristic curve. If you study the pump's characteristic curve you will notice that at higher pump discharge pressure, the flowrate is lower. This explains why the system flow rate is lower.

4. I hope the above explanation does not make you more confused.

[narendrasony]

I think Ankit is confused, why constant flow is not maintained across a control valve even when it is throttled. Or in other words why there is pressure drop across the valve.
Most of pressure drop is created by vortices formation and and flow separation (some of forward flow motion is lost and even backward flow starts) immediately as the flow area starts increasing after passing through the (reduced ) valve seat area. In case of globe valve, Some drop is also created by change in flow direction .
Net result will be increase in upstream pressure (Assuming downstream pressure as constant). Now as Kkala, Art and S. Ahmed have mentioned, pump (centrifugal) will deliver lesser flow since it knows its characteristic curve only. Can you call it a loss in flow? No. Can you expect a (given) centrifugal pump to deliver the same flow at higher discharge pressure? No.

Regards
Narendra

Edited by narendrasony, 22 February 2013 - 03:02 AM.

[kkala]

In addition to post no 12 by naredrasony: continuity equation means that mass flow rate Q (and volume flow rate V for incompressible fluids) is same over any section of (not leaking, not branched) pipe at steady state, including that of any fitting / valve. The  centrifugal pump had better be considered out of this pipe system. If it should be, steady state would mean invariable suction and discharge pressure to result in constant V.

Arithmetic examples can make it clearer.

Confusion over basic principles is common, I have found myself trapped in it; a  way out is to  express it specifically and ask for clarification, as it occurs in this topic.

No queries on basic principles may not mean that everything has got clear. I remember a classmate comment, true though exaggerating, after new lessons of modern mathematics in high school (1966). All of us are supposed to have understood these "simple" meanings, but  even our instructor has not. Well, through effort and exercises the gap was rather adequately filled  in a year's time.

[Lai.CY]

Hello Ankit,

 

Since I'm not an expert, my simple understanding is that the fundamental balance is 'In = Out' (forget accumulation for the time being)

1000kg/h enters the pump = 1000kg/h exits the pump

 

Note that volumetric flow = mass flow / density

Mass is fixed, regardless of condition, but volumetric is not. At different conditions, volume is governed by the density, which changes at different conditions. Especially when liquid flash across control valves. But mass is still the same.

 

For centrifugal pumps, you can look at the pump curve. The discharged fluid is dependent of the backpressure. Higher discharge resistance = lower flowrate. 

Meaning, you have less flow entering the pump when the control valve partially opened, not that the flow disappeared.

 

For Positive Displacement (PD) pumps, liquids are pushed out of the pump chambers by pistons, delivering a fix flowrate regardless of backpressure.

[vikramltv]

Dear Ankit

 

Continuity equation is valid for steady state flow. But as soon as Control valve opening starts increasing  or decreasing, steady state flow breaks and a new steady state established at the control valve final position. So there is no steady state while control valve is operating so continuity equation does not apply during this inter-phase.

[ankit2206]

Thanks all the experts and especially @kkala and @s.ahmed for clarifying the doubts.

After studying pump performance curves, reading your replies and doing a little thinking, I think i understand where I was going wrong.

 

Thanks once again.