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Control Valves

Difference between Quick Opening, Linear & Equal Percentage Control Valve characteristics

Each valve has a flow characteristic, which describes the relationship between the flow rate and valve travel. As a valve opens, the flow characteristic, which is inherent to the design of the  selected  valve,  allows  a  certain  amount  of  flow  through  the  valve  at  a  particular percentage  of the stroke.  This enables  flow regulation  through  the valve  in a predictable manner.

The three most common types of flow characteristics are:

  1. Linear
  1. Equal percentage
  1. Quick opening

Linear valve characteristics:

This characteristic provides a linear relationship between the valve position and the flowrate. The flow through a linear valve varies directly with the position of the valve stem. This flow- travel relationship, if plotted on rectilinear coordinates, approximates a straight line, thereby giving equal volume changes for equal lift changes regardless of percent of valve opening.

These  valves  are  often  used  for  liquid  level  control  and  certain  flow  control  operations requiring constant gain.

Equal percentage valve characteristics:

The equal percentage  valve plug produces  the same percentage  change in flow per fixed increment  of valve stroke at any location  on its characteristic  curve. For example,  if 30% stem  lift produces  5 gpm  and a lift increase  of 10%  to 40%  produces  8 gpm  or a 60% increase  over  the  previous  5  gpm,  then  a  further  stroke  of  10%  now  produces  a  60% increase over the previous 8 gpm for a total flow of 12.8 gpm.

These types of valves are commonly  used for pressure  control applications  and are most suitable for applications where a high variation in pressure drop is expected.

Quick opening valve characteristics:

A quick opening valve plug produces  a large increase  in flow for a small initial change in stem travel. Near maximum flow is reached at a relatively low percentage of maximum stem lift. Quick opening plugs are normally utilized in two position “On-Off” applications but may be used  in  some  linear  valve  applications.   This  is  possible  because  of  its  initial  linear characteristic at a low percentage of stem travel. The slope of this linear region is very steep which  produces  a higher  initial  gain  than  the linear  plug  but also  increases  the potential instability of the control valve.

CONTROL VALVE CHARACTERISTICS

Inherent valve characteristics:

An inherent flow characteristic is the relation between valve opening and flow under constant pressure  conditions.  The  inherent  characteristic  of  a  valve  is  obtained  when  there  is  a constant  pressure  drop  across  the  valve  for  all  valve  positions;  the  process  fluid  is  not flashing,  cavitating  or approaching  sonic velocity  (choked  flow); and the actuator  is linear (valve stem travel is proportional to the controller output).

Some  valves  have inherent  characteristics  that cannot  be changed,  such as full port ball valves and butterfly valves. For other valve types, such as the globe type, the inherent characteristics can be changed to suit the application.

Installed flow characteristic:

When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the valve travel changes. When the actual flow in a system is plotted against valve opening, the curve is called the installed flow characteristic and it will differ from the inherent valve characteristic which assumed constant pressure drop across the valve. When in service, a linear valve will in general resemble a quick opening valve while an equal percentage valve will in general resemble a linear valve.

Difference between installed and inherent characteristics:

The  inherent  flow  characteristics  do  not  reflect  the  actual  performance  of  the  valve  as installed. The ideal condition of constant valve pressure drop (∆P) is unlikely to be true and the  ‘operating’  characteristics  will  have  deviation  from  the  inherent  characteristics  and  is termed the “Installed Flow Characteristics”.

The  deviation  in  the  characteristics  depends  on  the  pressure  drop  variation  across  the control valve, as the control valve operates from minimum flow at its initial travel position to its maximum  flow at its fully opened  position.  The variations  in pressure  drop across  the valve can be attributed to two basic causes:

  1. The pump characteristic  which results in an increase  in pump head as the flow is reduced; and
  1. The reduction in line losses as the flow is reduced, causing more and more of the pump head to appear across the valve.

Control Valve DP Curve

In a pipeline carrying fluid, the dynamic system pressure (Ps) is made up of two components:

1)  the  pressure  drop  across  the  control  valve  (Pv)  and  2)  the  pressure  drop  along  the pipeline (PL), excluding any fixed static or elevation pressure head component. It is given by:

PS = Pv + PL

In the  pump  curve  above,  the  point  “A”  is the  point  where  the  system  resistance  curve crosses  the  pump  characteristic  curve  and  indicates  the  operating  conditions  (flow  and head). As the valve modulates to the closed position; the resistance to the system flow that the valve provides (valve pressure drop) will increase by shifting from point “A” towards point “B”. This increasing resistance will use more of the head in the system, as well as decrease system flow.

  • Pressure drop across the control valve increases (∆Pv – ↑). The change in pressure drop across  the  valve  can  be  attributed   to  two  basic  causes:

1)  the  pump characteristic, which results in an increase in pump head as the flow is reduced, and

2) the reduction in line losses as the flow is reduced, causing more and more of the pump head to appear across the valve. The amount that the pump head will increase with a decrease in system flow will depend upon the operating characteristics  of the pump.  A pump  with a steep characteristic  will produce  a considerable  increase  in pressure head as the system resistance is increased. However, a flat characteristic pump will produce a relatively constant, high pressure head for any system flow. The relatively constant pressure would be preferable from a control standpoint.

  • Pressure loss in the pipeline  reduces  (∆PL   – ↓).   This is because  the decrease  in system flow will result in a decrease in pressure drop along the pipeline and is proportional to the square root of the flow rate.

This indicates that the pressure drop across the valve in the system is not constant and it varies with flow and other changes in the system. This has a significant impact on the actual installed  valve  flow characteristic.  The deviation  from the inherent  flow characteristic  is a function of a property called Valve Authority. It is defined as the ratio of the full flow valve pressure drop to the system pressure drop (including the valve)

Control Valve Equation

  • N = Valve Authority
  • ∆Pv = Pressure drop across the control valve
  • ∆PL  = Pressure drop due to pipeline friction losses
  • ∆PS  = System pressure drop = ∆Pv + ∆PL

When “N” approaches 1.0, then ∆PL  is almost zero and ∆Pv approaches ∆Ps. This satisfies the requirement for the definition of valve inherent characteristics.

Distortion  occurs  when  “N” falls from  1.0. This is the situation  when  the pipeline  system pressure drop (∆Ps) is not concentrated at the control valve alone but well distributed along the  pipeline.  An  inherently  equal  %  characteristics  control  valve  operating  under  such condition will behave like a linear valve and an inherently linear characteristics control valve will behave like a quick-opening control valve.

The effect of these system  variables  can be minimized  by keeping  the relative  change  in valve pressure drop as small as possible.

When the total flow is low, control valve pressure drop tends to be large fraction of the total system pressure loss; but at high flows this may not be true. A good design will respond well over the full range of conditions, hence it is important to pick the right characteristic for your system and size the valve for the right amount of pressure drop. For good control, it is nice to take a fairly large pressure drop across a control valve. This way it will have a big influence on the total system,  making  the operators  and control engineers  happy. However,  design engineers will worry that increasing pressure drop will tend to increase pumping and other operating costs. Compromise is necessary. As a rule of thumb, design the system and size the valve so that 25 to 33% (1/3rd) of the total system pressure drop (including the valve) is taken across the control valve, with a minimum of 10-15 psig.

General rules:

How do you decide which valve control to use? Here are some rules of thumb:

Linear Characteristics:

  • Used in liquid level or flow loops.
  • Used in systems where the pressure  drop across the valve is expected  to remain fairly constant (i.e. steady state systems).
  • Used when  the  pressure  drop  across  the  valve  is a large  proportion  of the  total pressure drop.

Equal Percentage Characteristics:

  • Used in processes where large changes in pressure drop are expected.
  • Used in processes where a small percentage of the total pressure drop is permitted by the valve.
  • Used in temperature and pressure control loops.

Quick Opening Characteristics:

  • Used for frequent on-off service.
  • Used for processes  where  “instantly”  large  flow  is needed  (i.e.  safety  systems  or cooling water systems).

Two rules of thumb for choosing the right flow characteristic:

  1. If most of the pressure drop is taken through the valve and the upstream pressure is constant, a linear characteristic will provide better control.
  1. If the piping and downstream  equipment cause significant resistance to the system, equal percentage will provide better control.

Typical application of quick opening, linear and equal percentage valves

i) Quick opening valve:

a) Frequent on-off service.

b) Used for systems where ‘instant’ large flow is needed (safety or cooling water systems).

ii) Linear valve:

a) Liquid level and flow control loops.

b) Used in systems where the pressure drop across the valve is expected to remain fairly constant.

iii) Equal percentage valve (most commonly used in valves):

a) Temperature and pressure control loops.

b) Used in systems where large changes in pressure drop across the valve are expected.

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