Actuator, control valve

Fluid-filled bulbs dehver enough power to drive controller mechanisms and even directly actuate control valves. These devices are characterized by large thermal capacity, which sometimes leads to slow response, particularly when they are enclosed in a thermal well for process measurements. Filled-system thermometers are used extensively in industrial processes for a number of reasons. The simplicity... 

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 schematic affixed to the valve includes the primary and secondary actuators used to control the valve. Figure 9.16 provides the schematics for three actuator-controlled valves ... 

Each stage of the process has its own inherent hazards and limiting conditions. Hence local process sensors are used to detect process hazards in each stage. Actuated control valves and electrical trip signals are then used to shut down the stage that has the problem. 

Following the invention of the hydrauHc press in 1795 (3), the use of hydrauHcs expanded rapidly during the nineteenth century. The weight-loaded accumulator, invented ca 1850, was used to store energy in hydrauHc systems. The elementary press circuit has several parts that are common to all hydrauHc systems a reservoir, a pump, piping, control valves, a motor, which in this case is a hydrauHc cylinder or ram, and the hydrauHc fluid. By ca 1860 hydrauHc presses were used for forging, and an adjustable-speed hydrauHc transmission was perfected in 1906 (2). The manufacture of hydrauHcahy actuated machines attained industrial importance after 1920. 

Once the desired control action has been transformed to an analogue signal, it is transmitted to the final control element over the transmission lines. However, the final control element s actuator may require a different type of signal and thus another transducer may be necessary. Many control valve actuators utilise a pressure signal so a current-to-pressure (I/P) transducer is used to provide a pressure signal to the actuator. 

Fig. 9. Control valve and actuator (a) nomenclature (b) fail-open (F/O) or air-to-close (A/C) action and (c) fail-closed (F/C) or air-to-open (A/O)...

Loop two or more elements where a signal passes between them for measurement and/or control, eg, sensor -i- transmitter -i- control + valve device two or more elements combined for a discrete state condition, eg, block valve -i- solenoid valve -i- actuator -I- limit switches... 

E. A. Mayer, Electro-Pneumatic Control Valve for EGRfATC Actuation, SAE 810464, Society of Automotive Engineers, Warrendale, Pa., 1981. 

A control valve consists ot a valve, an actuator, and possiblv one or more valve-control devices. The valves discussed in this section are applicable to throttling control (i.e, vviiere tlovv through the valve is regulated to anv desired amount betvv een maximum and minimum limits). Other valves such as check, isolation, and relict valves are addressed in the next subsection. As defined, control valves are automatic control devices that modify the tliiid tlovv rate as specified bv the controller. 

As most throttling control valves are still operated bv pneumatic actuators, the control-valve device descriptions that follow relate primarily to devices that are used with pneumatic actuators. The function of hydraulic and electrical coimteiparts are very similar. Specific details on a particular valve-control device are available from the vendor of the device. 

Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

Positioner Application Positioners are widelv used on pneumatic valve actuators, VIore often than not, thev provide improved process-loop control because thev reduce valve-related nonlinearitv, Dvnarnicallv, positioners maintain their abilitv to improve control-valve performance for sinusoidal input frequencies up to about one half of the positioner bandwidth. At input frequencies greater than this, the attenuation in the positioner amplifier netvv ork gets large, and valve nonlinearitv begins to affect final control-element performance more significantlv. Because of this, the most successful use of the positioner occurs when the positioner-response bandwidth is greater than twice that of the most dominant time lag in the process loop. 

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. 

Gas compressor anti-surge (GM-OFF) control circuit, comprising transmitters, computers and pneumatic control valve Reverse flow protection (on axnal compressors only) as supplementary protection device against surging, working independently of the control circuit Expander emergency stop valve with pneumatic actuator and solenoid valve... 

All valves are equipped with hydraulic actuators, electronic positioning controllers for precise positioning, and solenoid valves for rapid (trip) opening of bypass valves and emergency closure tripping of trip and inlet control valves. 

The strict requirements placed on the quality of the regenerator pressure control system necessitate complex control strategies that can only be achieved using modern freely programmable control systems, while the short actuating times of the control valves require controllers with ultrafast response. 

The control valves have a full-stroke actuating time of 600 msec thus, in 6 msec the valves are actuated through 1 % of their full stroke. Only controllers with a TPET of less than 3 msec are able to meet the strict requirements of this application. 

Figure 15-6. Typical single-port body control valve (left) and pneumatic actuator (right). Courtesy of Fisher Controls Internationa , Inc.]...

Data on spare and parallel equipment are often omitted. Valving is also generally omitted. A alve is shown only where its specification can aid in understanding intermittent or alternate flows. Instrumentation is indicated to show the location of variables being controlled and the location of the actuating device, usually a control valve. To help the reader better understand the process flow sheet, a list of commonly used symbols is presented in Fig. 5.9.1. 

A magnetic field due to an electric current can be turned on and off simply by turning the current on and off. A piece of iron attached to the end of a spring having the other end fixed can be moved with a magnetic field and returned to its initial position by the spring. The iron piece can then be used to actuate a switch or move a lever on a valve. Applications of this principle include electrically controlled valves in a washing machine and an electrically controlled switch for the starter in an automobile. 

Most reciprocating compressors have inlet and outlet valves (on the piston heads) that are actuated by a pressure difference. These are called self acting valves. There are some larger multistage reciprocating piston compressors that do have camshaft-controlled valves with rotary slide valves. 

Mechanical force can be more easily controlled using fluid power. The simple use of valves and rotary or linear actuators control speed, direction and force. The simplicity of hydraulic and pneumatic components greatly increases their reliability. In addition, components and overall system size are typically much smaller than comparable electrical transmission devices. 

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