CONTROL VALVE HANDBOOK Episode 1 Part 5 ppt

67

Chapter 4

Control Valve Accessories

This chapter offers information on digi￾tal valve controllers, analog position￾ers, boosters, and other control valve

accessories.

Positioners

Pneumatically operated valves de￾pend on a positioner to take an input

signal from a process controller and

convert it to valve travel. These instru￾ments are available in three configura￾tions:

1. Pneumatic Positioners—A pneu￾matic signal (usually 3-15 psig) is sup￾plied to the positioner. The positioner

translates this to a required valve

position and supplies the valve actua￾tor with the required air pressure to

move the valve to the correct position.

2. Analog I/P Positioner—This posi￾tioner performs the same function as

the one above, but uses electrical cur￾rent (usually 4-20 mA) instead of air

as the input signal.

3. Digital Controller—Although this in￾strument functions very much as the

Analog I/P described above, it differs

in that the electronic signal conversion

is digital rather than analog. The digi￾tal products cover three categories.

Digital Non-Communicating—A

current signal (4-20 mA) is supplied to

the positioner, which both powers the

electronics and controls the output.

HART—This is the same as the

digital non-communicating but is also

capable of two-way digital commu￾nication over the same wires used for

the analog signal.

Fieldbus—This type receives

digitally based signals and positions

the valve using digital electronic cir￾cuitry coupled to mechanical compo￾nents. An all-digital control signal re-

Chapter 4. Control Valve Accessories

68

Figure 4-1. Modern Control Valves

Utilizing Digital Valve Controllers

W8861

W8119-1

places the analog control signal.

Additionally, two-way digital commu￾nication is possible over the same

wires. Fieldbus technologies benefit

the end user by enabling improved

control architecture, product capability

and reduced wiring.

Use of a single, integrated analog I/P

positioner or digital controller (figure

4-1) instead of a combination of pneu￾matic positioner and transducer (two

instruments) results in lower installed

cost.

The ability to embed software com￾mands into the memory of the device

represents the real difference be￾tween digital and analog I/P seg￾ments. This allows automatic configu￾ration and setup of the valve when

equipped with a digital controller. Most

importantly, it allows two-way commu￾nication for process, valve, and instru￾ment diagnostics.

Users purchase digital valve control￾lers for several reasons:

Reduced cost of loop commis￾sioning, including installation and cal￾ibration.

Use of diagnostics to maintain

loop performance levels.

Improved process control accu￾racy that reduces process variability.

Two aspects of digital valve control￾lers make them particularly attractive:

Automatic calibration and config￾uration. Considerable time savings

are realized over traditional zero and

spanning.

Valve diagnostics. Through the

Distributed Control System (DCS), PC

software tools, or handheld communi￾cators, users can diagnose the health

of the valve while it is in the line.

FIELDVUE instruments enable new

diagnostic capabilities that can be ac￾cessed remotely. This single element

requires a look at the potential impact

of the technology as it applies to con￾trol valves.

An in-plant person, with the aid of the

FlowScanner system, can diagnose

the health of a valve through a series

of off-line tests. The FlowScanner sys￾tem consists of a portable, ruggedized

computer and travel and pressure

sensors. The sensors are connected

to the valve to enable diagnostic tests,

which are conducted with the valve

off-line. A skilled maintenance techni￾cian can determine whether to leave

the valve in the line or to remove the

valve for repair.

Digital instruments allow an extension

of this service with added enhance￾ments:

Because sensors are part of the

instrument, tests can be run easily at

appropriate times.