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:
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.
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.
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.
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.
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.
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:
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.
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.
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)
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.
How do you decide which valve control to use? Here are some rules of thumb:
General applications of of quick opening, linear and equal percentage valves are :
a) Frequent on-off service.
b) Used for systems where ‘instant’ large flow is needed (safety or cooling water systems).
a) Liquid level and flow control loops.
b) Used in systems where the pressure drop across the valve is expected to remain fairly constant.
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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View Comments
Regarding the equal percentage discussion...
"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."
The lift increase following the stem lift should read "30% to 40%".
My comment is incorrect due to a misinterpretation of the original sentence. The sentence is correct as stated; my apologies.