CONTROL VALVE HANDBOOK Episode 1 Part 3 ppt

Chapter 2. Control Valve Performance

27

nal changes as great as 5% before it

begins responding faithfully to each of

the input signal steps. Valve C is con￾siderably worse, requiring signal

changes as great as 10% before it be￾gins to respond faithfully to each of

the input signal steps. The ability of

either Valve B or C to improve pro￾cess variability is very poor.

Friction is a major cause of dead band

in control valves. Rotary valves are

often very susceptible to friction

caused by the high seat loads re￾quired to obtain shut-off with some

seal designs. Because of the high

seal friction and poor drive train stiff￾ness, the valve shaft winds up and

does not translate motion to the con￾trol element. As a result, an improper￾ly designed rotary valve can exhibit

significant dead band that clearly has

a detrimental effect on process vari￾ability.

Manufacturers usually lubricate rotary

valve seals during manufacture, but

after only a few hundred cycles this

lubrication wears off. In addition, pres￾sure-induced loads also cause seal

wear. As a result, the valve friction

can increase by 400% or more for

some valve designs. This illustrates

the misleading performance conclu￾sions that can result from evaluating

products using bench type data before

the torque has stabilized. Valves B

and C (figure 2-3) show the devastat￾ing effect these higher friction torque

factors can have on a valve’s perfor￾mance.

Packing friction is the primary source

of friction in sliding-stem valves. In

these types of valves, the measured

friction can vary significantly between

valve styles and packing arrange￾ments.

Actuator style also has a profound im￾pact on control valve assembly fric￾tion. Generally, spring-and-diaphragm

actuators contribute less friction to the

control valve assembly than piston ac￾tuators. An additional advantage of

spring-and-diaphragm actuators is

that their frictional characteristics are

more uniform with age. Piston actua￾tor friction probably will increase sig￾nificantly with use as guide surfaces

and the O-rings wear, lubrication fails,

and the elastomer degrades. Thus, to

ensure continued good performance,

maintenance is required more often

for piston actuators than for

spring-and-diaphragm actuators. If

that maintenance is not performed,

process variability can suffer dramati￾cally without the operator’s knowl￾edge.

Backlash (see definition in Chapter 1)

is the name given to slack, or loose￾ness of a mechanical connection. This

slack results in a discontinuity of mo￾tion when the device changes direc￾tion. Backlash commonly occurs in

gear drives of various configurations.

Rack-and-pinion actuators are particu￾larly prone to dead band due to back￾lash. Some valve shaft connections

also exhibit dead band effects. Spline

connections generally have much less

dead band than keyed shafts or

double-D designs.

While friction can be reduced signifi￾cantly through good valve design, it is

a difficult phenomenon to eliminate

entirely. A well-engineered control

valve should be able to virtually elimi￾nate dead band due to backlash and

shaft wind-up.

For best performance in reducing pro￾cess variability, the total dead band for

the entire valve assembly should be

1% or less. Ideally, it should be as low

as 0.25%.

Actuator-Positioner Design

Actuator and positioner design must

be considered together. The combina￾tion of these two pieces of equipment

greatly affects the static performance

(dead band), as well as the dynamic

response of the control valve assem￾bly and the overall air consumption of

the valve instrumentation.

Positioners are used with the majority

of control valve applications specified

Chapter 2. Control Valve Performance

28

today. Positioners allow for precise

positioning accuracy and faster re￾sponse to process upsets when used

with a conventional digital control sys￾tem. With the increasing emphasis

upon economic performance of pro￾cess control, positioners should be

considered for every valve application

where process optimization is impor￾tant.

The most important characteristic of a

good positioner for process variability

reduction is that it be a high gain de￾vice. Positioner gain is composed of

two parts: the static gain and the dy￾namic gain.

Static gain is related to the sensitivity

of the device to the detection of small

(0.125% or less) changes of the input

signal. Unless the device is sensitive

to these small signal changes, it can￾not respond to minor upsets in the

process variable. This high static gain

of the positioner is obtained through a

preamplifier, similar in function to the

preamplifier contained in high fidelity

sound systems. In many pneumatic

positioners, a nozzle-flapper or similar

device serves as this high static gain

preamplifier.

Once a change in the process vari￾able has been detected by the high

static gain positioner preamplifier, the

positioner must then be capable of

making the valve closure member

move rapidly to provide a timely cor￾rective action to the process variable.

This requires much power to make the

actuator and valve assembly move

quickly to a new position. In other

words, the positioner must rapidly

supply a large volume of air to the ac￾tuator to make it respond promptly.

The ability to do this comes from the

high dynamic gain of the positioner.

Although the positioner preamplifier

can have high static gain, it typically

has little ability to supply the power

needed. Thus, the preamplifier func￾tion must be supplemented by a high

dynamic gain power amplifier that

supplies the required air flow as rapid￾ly as needed. This power amplifier

function is typically provided by a

relay or a spool valve.

Spool valve positioners are relatively

popular because of their simplicity.

Unfortunately, many spool valve posi￾tioners achieve this simplicity by omit￾ting the high gain preamplifier from the

design. The input stage of these posi￾tioners is often a low static gain trans￾ducer module that changes the input

signal (electric or pneumatic) into

movement of the spool valve, but this

type of device generally has low sen￾sitivity to small signal changes. The

result is increased dead time and

overall response time of the control

valve assembly.

Some manufacturers attempt to com￾pensate for the lower performance of

these devices by using spool valves

with enlarged ports and reduced over￾lap of the ports. This increases the dy￾namic power gain of the device, which

helps performance to some extent if it

is well matched to the actuator, but it

also dramatically increases the air

consumption of these high gain spool

valves. Many high gain spool valve

positioners have static instrument air

consumption five times greater than

typical high performance two-stage

positioners.

Typical two-stage positioners use

pneumatic relays at the power amplifi￾er stage. Relays are preferred be￾cause they can provide high power

gain that gives excellent dynamic per￾formance with minimal steady-state

air consumption. In addition, they are

less subject to fluid contamination.

Positioner designs are changing dra￾matically, with microprocessor devices

becoming increasingly popular (see

Chapter 4). These

microprocessor-based positioners

provide dynamic performance equal to

the best conventional two-stage pneu￾matic positioners. They also provide

valve monitoring and diagnostic capa￾bilities to help ensure that initial good