Pneumatic position controller

The symbol for the control valve in Fig. 8-47 is for a pneumatic positioning valve without a valve positioner. 

Fig. 3 shows the experimental apparatus. The feed tank had a 50 gallon capacity and was equipped with a variable speed mixer. The feed pump was a flexible impeller, positive-displacement pump to minimize shearing of the feed emulsion. The pumping rate was regulated by a Graham Variable Speed Transmission. Each flotation tank was 11.5 in. ID with 6.5 in. liquid depth maintained by a CE IN-VAL-CO conductometric level controller with a pneumatically actuated control valve in the effluent line. The fourth cell was not equipped with an air inducer. The outer diameter of the air downcomers was 1.5 in. The rotor in each air inducer was a turbine taken from a 2 in. turbine flow meter. Each rotor was belt driven by a 10,000 rpm, 1/30 hp motor and all three motors were governed by the same variable transformer. Another pulley on each rotor shaft was attached to a non-powered belt connecting all three shafts to ensure that each rotor turned at the same speed. 

The state variables that provide valve position control are used to diagnose the health of the final control element. In addition, some digital valve controller designs integrate additional sensors into their construction to provide increased diagnostic capability. For example, pressure sensors are provided to detect supply pressure, actuator pressure (upper and lower cylinder pressures in the case of a springless piston actuator), and internal pilot pressure. Also, the position of the pneumatic relay valve is available in some designs to provide quiescent flow data used for leak detection in the actuator. 

Level instrumentation can be a compact pneumatic controller. Self-contained pneumatic instruments provide a positive level readout, a level sensor, a control point, and a pneumatic output control signal all in one small housing. 

Pneumatic cloth control In this system the movement of the control roller (swivel roller) is activated by a sensor paddle and a solenoid valve controlling the flow of compressed air into one of two bellows (one bellow on each roller end), which inflates to move the roller in one or the other direction, which aligns the cloth by moving one end of the roller forward or backwards to track the cloth back to its centred position on the filter. 

The output signal of a controller is sent electrically or pneumatically to a valve positioner that modulates the supply pressure to the control valve actuator and must make the stem position proportional to the input signal from the controller despite loads on the valve [6, 25], There the electrical signal must be converted to a pneumatic positioning pressure or the pneumatic signal must be brought to a higher pressure in order to move the stem of the actuator reliably and to adjust the position of the valve accurately. 

Various accessories can be suppHed along with the control valves for special situations. Positioners ensure that the valve stem is accurately positioned following small or slowly changing control signals or where unbalanced valve forces exist. Boosters, which are actually pneumatic amplifiers, can increase the speed of response or provide adequate force in high pressure appHcations. Limit switches are sometimes included to provide remote verification that the valve stem has actually moved to a particular position. 

Manually adjusted screw or ratchet take-ups that adjust the position of the tail pulley to control belt tension can be used on relatively short, light duty conveyors. Automatic take-ups are used on conveyors over about 25 to 30 m long. The most common is the weighted automatic gravity take-up (see Fig. la). Other types of automatic take-ups have hydrauHc or pneumatic powered devices to adjust a snub pulley position and maintain a constant belt tension. The requited take-up movement varies according to the characteristics of the belt constmction and the belt length. Typically, take-up movements for pHed belts are 2% to 3% of the center distance between head and tail pulley, and about 0.5% for steel cable belts. The take-up movements requited for soHd woven belts are usually shorter because of the lower elastic stretch. Take-up requirements for a particular situation should be confirmed by the belt manufacturer. 

Solenoid Valves The electric solenoid valve has tw o output states. Wlien sufficient electric current is supplied to the coil, an internal armature moves against a spring to an extreme position. This motion causes an attached pneumatic or hvdraiilic valve to operate. Wlien current is removed, the spring returns the armature and the attached solenoid valve to the deenergized position. An intermediate pilot stage is sometimes used when additional force is required to operate the main solenoid valve. Generallv, solenoid valves are used to pressurize or vent the actuator casing for on/off control-valve application and safetv shutdown applications. 

Trip Valves The trip valve is part of a system that is used where a specific valve action (i.e., fail up, Fail down, or lock in last position) is required when pneumatic supply pressure to the control valve falls befow a preset level. Trip systems are used primarily on springless piston ac tuators requiring fail-open or fail-closed acrion. An air storage or Volume tank and a check v ve are used with the trip valve to provide power to stroke the valve when supply pressure is lost. Trip valves are designed with hysteresis around the trip point to avoid instabihty when the trip pressure and the reset pressure settings are too close to the same value. 

As microprocessor-based controls displaced hardwired electronic and pneumatic controls, the impac t on plant safety has definitely been positive. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator s awareness of the current conditions in the process. Process operators are expected to exercise due diligence in the supervision of the process, and timely recognition of an abnormal situation reduces the likelihood that the situation will progress to the hazardous state. Figure 8-88 depicts the layers of safety protection in a typical chemical jdant. 

In a similar incident, a furnace damper was closed in error. It was operated pneumatically. There was no indication on the control knob to show which was the open position and which was the closed position. 

Small leaks from the glands of a carbon monoxide compressor were collected by a fan and discharged outside the building. A man working near the compressor was affected by carbon monoxide. Itwas then found that a damper in the fan delivery line was shut. There was no label or other indication to show whether the damper was closed or open. In a similar incident, a furnace damper was closed in error. It was operated pneumatically, and again there was no indication on the control knob to show which were the open and closed positions. 

Figure 12-31. Capacity control suction valve unloaders are available in either of two designs with pneumatic operators (A) direct-acting (air to unload) (B) reverse-acting or fail safe (air to load), which automatically unloads the compressor in the event of control air failure (C) an innovation in manually operated unloaders. Here, the lever cam arrangement provides positive loading or unloading, eliminating the requirement to turn a handwheel completely in or out. (Used by permission Bui. 9-201B, 1991. Cooper-Cameron Corporation.)...

Most ventilators are powered in one direction with spring return. Thus, a pneumatic ventilator might be described as pressure to open or pressure to close. With the pressure to open type the ventilator will fail to the closed position under the influence of the return spring, ensuring the building remains weatherproof. Where ventilators are installed mainly as smoke ventilators it is important that they fail to the open position, so pressure to close ventilators should be used. A fusible link is normally fitted into the controls so that in the event of fire, affected ventilators will open automatically to release smoke and heat. 

The output meter is the horizontally positioned meter below the deviation and setpoint indicators. It indicates controller output signal in percent. This particular controller ranges from zero to 100% current. However, this will correspond to an air signal for pneumatic controllers. 

By themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators. Actuators may be pneumatic, hydraulic, or electric solenoids or motors. 

A simplified diagram of a pneumatic actuator is shown in Figure 35. It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem. 

A positioner is a device that regulates the supply air pressure to a pneumatic actuator. It does this by comparing the actuator s demanded position with the control valve s actual position. The demanded position is transmitted by a pneumatic or electrical control signal from a controller to the positioner. The pneumatic actuator in Figure 35 is shown in Figure 36 with a controller and positioner added. 

The U.S. Army Defense Ammunition Center and School is employing the use of intrinsically safe electrical circuits in equipment designed to demilitarize and renovate munitions - from small arms to large projectiles. This is accomplished by using pneumatics and hydraulics to provide the power, while using position switches and solenoid valves linked to programmable controllers to direct the total machine process. 

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