Control valve gain, characteristics, distortion, rangeability

Control valve gain, characteristics, distortion and rangeability

The characteristics, rangeabilities and gains of control valves are interrelated and a good understanding of these is necessary to be able to relate to the 'personality profile' of a process control valve.

Valve and loop gain

Gain is defined as / Output Input ?? and for a linear (constant gain) valve, valve gain (KV) is defined as max /Stroke% F or the maximum flow divided by the valve stroke in percentage.

The loop gain of a process control system (KLOOP) should ideally be 0.5 to obtain quarter amplitude damping, an ideal and very stable state. Most process control loops consist of a minimum of four active units as listed below, each with their respective abbreviation indicated in [ ]'s as:

• A process control; P or proportional mode controller [KC]

• The controller output driving a control valve [KV]

• The valve effecting a process [KP]

• A sensor/transducer measuring the process and feeding this as an input to the controller [KS].

For the system to be stable all four components should have a linear gain and the overall product of their gains should equal 0.5 (for quarter damping).

When a linear controller and sensor are used, and the gain of the process is also linear, a linear (KV = constant) valve is needed to maintain the overall total loop gain constant at a value of 0.5.

However, if the process is non-linear (KP varies with load), while the gains of KC, KV and KS are constant, the value of KLOOP will also vary about the optimum value of 0.5 resulting in either a sluggish or unstable operation of the process. The only way to maintain stability is for another component in the loop to change its gain in the opposite direction and magnitude to that of the process gain change. This can be either the controller gain (KC) or the control valve gain (KV). Here we will consider changes in the valve gain (KV). When the control valve gain varies with its load (flow) it’s named according to its characteristics, these being:

• Equal percentage: KV increases at a constant rate with flow

• Variable rate: KV increases according to the profile; Parabolic, Hyperbolic, etc.

• Quick-opening: KV drops when the flow through the valve increases.

The theoretical valve gain invariably changes in actual use if the valve pressure differential varies with load, this is the case in most pumping systems where the valve differential drops with increasing flow rates thereby reducing the valve gain KV. This tends to shift the gain of equal percentage valves toward that of the linear type. In this case installing an equal percentage valve into the system often greatly assists in keeping the valve gain linear.

++++Inherent flow characteristics: quick opening, linear and equal percentage | % Lift or stroke % Lift or stroke:

The inherent characteristics of a control valve describes the relationship between the controller output as received by the actuator and the flow through the valve, assuming that:

• The actuator is linear (valve travel is proportional with controller output)

• The pressure difference delta_P across the valve is constant

• The process fluid is not flashing, subject to cavitation or at sonic velocity.

Selecting a valve characteristic can be a prolonged and complicated procedure; Driskell derived a general rule-of-thumb in selecting valve characteristics for the more common loops:

Required Service Valve delta_P

< 2:1 Valve delta_P

> 2:1 but < 5:1

Orifice type flow, Quick opening, Linear, Linear flow, Linear, Equal % Level, Linear, Equal % Gas pressure, Linear Equal %, Liquid pressure, Equal %, Equal %

List of common valve characteristics VV applications

In many cases the choice of valve characteristic has minimal effect on the loop parameters, and just about any type is acceptable for:

• Process with short time constants, such as flow control, most pressure control loops and temperature control when mixing a hot and cold stream

• Control loops operated by a narrow proportional band (high gain) controllers, such as most regulators

• Processes with a load variation of less than 2:1.

Valve distortion

Fluid flow through a valve is subjected to frictional losses, the consequence of this is shown in ++++ It can be seen from these curves that installation criteria can have substantial effects on a valve's flow characteristics and rangeability.

% Lift or stroke %, Lift or stroke, Linear, Equal percent

++++ The effect of the distortion coefficient (DC) on inherently linear and equal percentage valves, according to Boger.

The linear valve has a constant gain at all flow rates and an equal percentage valve has a gain directly proportional to flow.

Therefore, if a loop tends toward oscillation at low flow rates (indicating a loop gain =>1) and is sluggish at high flow (indicating a gain <0.25) one should switch from a linear to equal percentage valve. The opposite therefore applies if oscillations occur at high flow rates, and a sluggish performance at low flow rates, change from an equal percentage to a linear control valve model.

Valve rangeability

Traditionally, rangeability has been defined as the ratio between minimum and maximum 'controllable' flow through a valve. The term 'minimum flow' (FMIN) is defined as the flow below which the valve tends to close completely. Using this definition manufacturers usually claim:

• 50:1 rangeability for equal percentage valves

• 33:1 rangeability for linear valves

• 20:1 rangeability for quick opening valves.

This indicates that the flow through these valves can be controlled down to 2%, 3% and 5% respectively of their rated CV. However it can be seen that the minimum controllable flow rises as the distortion coefficient (DC) drops.

Due to the fact that at minimum valve opening, the pressure drop through a valve, delta_P , is at a maximum, the valve will proportionally pass more flow.

Rangeability should be calculated as the ratio of the CV required at maximum flow (minimum pressure drop) and the CV required at minimum flow (maximum pressure drop).

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Features and Applications:

Ball: Conventional Ball: Characterized Butterfly: Conventional Butterfly: High Performance Digital: Globe: Single Ported Globe: Double Ported Globe: Angled Globe: Exocentric Disk Pinch: Plug: Conventional Plug: Characterized Sanders: Sliding Gate V, Insert Sliding Gate: Positioned Disk Special Dynamically

ANSI class pressure rating Maximum capacity (Cd)

Characteristics --Corrosive service -- Cost (relative to single port globe)

Cryogenic service A High pressure drop (over 200 PSI) High temperature (over 200 ºC) Leakage (ANSI class) Liquids: abrasive service, Cavitation resistance, Dirty service G Flashing applications Slurry including fibrous service Viscous service, G Gas/vapor: abrasive/erosive

Listing of abbreviations:

A = Available C = All ceramic design is available F = Fair G = Good E = Excellent H = High L = Low M = Medium P = Poor S = Special design only Y = Yes X = Not available

Valve selection orientation table

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