Actuator systems control valves

Any suitable fire detection system can be utilized to actuate the control valve. Since any other fire detection device is likely to operate faster than a sprinkler head, it is common for waterto have reached the sprinkler by the time... 

The components of a hydraulic system are similar to those of the pneumatic system. Figure 11-14 shows the components of a hydraulic system. The pressure of the hydraulic fluid is provided by the system s pump. The hydraulic pump is a device that creates flow in the system. The reservoir stores the hydraulic fluid before it is pressurized by the pump. Transmission lines carry the pressurized hydraulic fluid to the control valve and actuators. The control valves regulate the flow and pressure of the hydraulic fluid. The actuators transfer the fluid power into mechanical power. 

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. 

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... 

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. 

Limit Switches and Stem-Position Transmitters Travel-limit switches, position switches, and valve-position transmitters are devices that, when mounted on the valve, actuator, damper, louver, or other throtthng element, detect the component s relative position. The switches are used to operate alarms, signal hghts, relays, solenoid valves, or discrete inputs into the control system. The valve-position transmitter generates a 4-20-mA output that is proportional to the position of the valve. 

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. 

In most fluid power systems, the motor is required to provide actuating power in either direction. In these applications, the ports are referred to as working ports, alternating as inlet and outlet ports. Either a four-way directional control valve or a variable-displacement pump usually controls the flow to the motor. 

Flow control valves are used to regulate the flow of fluids. Control of flow in hydraulic systems is critical because the rate of movement of fluid-powered machines or actuators depends on the rate of flow of the pressurized fluid. 

Sequence valves control the sequence of operation between two branches in a hydraulic circuit. In other words, they enable one component within the system to automatically set another component into motion. An example of the use of a sequence valve is in an aircraft landing gear actuating system. 

FIGURE 16.5 Schematic of instrumental setup for 2D micro-RPLC-CZE. A split injection/ flow system is used to deliver a nanoliter per second flow rate to the micro-RP-HPLC column from the gradient LC pump. The HPLC microcolumn has 50 pm i.d. and 76 cm length, and the electrophoresis capillary has 17 pm i.d., L — 25 cm, and/= 15 cm. The valve is air-actuated and controls the flow of flush buffer (reprinted with permission from Analytical Chemistry). 

Ideally, the axial velocity through the cross-flow unit should be greater than about 4-6 m/s to minimize the boundary layer of particles near the membrane surface. The wax permeate flow from the filter is limited by a control valve actuated by a reactor-level controller. Hence, a constant inventory of slurry is maintained within the SBCR system as long as the superficial gas velocity remains constant. Changes in the gas holdup due to a variable gas velocity are calculated... 

Systems are designed to function normally even when a single instrument or control function fails. This is achieved with redundant controls, including two or more measurements, processing paths, and actuators that ensure that the system operates safely and reliably. The degree of redundancy depends on the hazards of the process and on the potential for economic losses. An example of a redundant temperature measurement is an additional temperature probe. An example of a redundant temperature control loop is an additional temperature probe, controller, and actuator (for example, cooling water control valve). 

We have to make a distinction here between electric controllers (e.g. PID controllers) with a proportional valve as actuator and mechanical diaphragm controllers. In a regulation system w/ith electric controllers the coordination between controller and actuator (piezoelectric gas inlet valve, inlet valve A/ith motor drive, butterfly control valve, throttle valve) is difficult because of the very different boundary conditions (volume of the vessel, effective pumping speed at the vessel, pressure control range). Such control circuits tend to vibrate easily when process malfunctions occur. It is virtually impossible to specify generally valid standard values. 

One area where further development of fibre-optic transmission systems is required is in the design of suitable interfaces or converters to provide sufficient mechanical force to drive control valve actuators (Section 7.22.3)[Pg.550]

One important application of pneumatic transmission is in the operation of diaphragm actuators. These are the elements generally employed to drive the spindles of control valves (Section 7.22.3) and, if hard-wired transmission systems are employed, require devices which convert electric current into air pressure or air flowrate, i.e. electropneumatic (E/P) converters. The basic construction of a typical E/P converter is illustrated in Fig. 6.77. A coil is suspended in a magnetic field in such a way that when a current is passed through the coil it rotates. This rotation is sensed by a flapper/nozzle system (Section 7.22.1). The nozzle is supplied with air via a restrictor and its back pressure actuates a pneumatic relay. The output from the latter is applied to the feedback bellows and also acts as output from the E/P converter. Electropneumatic valve positioners employ the same principle of operation. 

A system controller sends out pump speed control signals to set flow rates and to generate solvent gradients. It should also be able to send 0- to 5-mV contact closure signals to actuate external switches and equipment. It also may be able to send out a 0- to 30-V powered contact closure signal needed to run solenoid valves used to control air pressure and liquid flows. Finally, the... 

Power actuated/assisted safety valves (CSPRS - controlled safety pressure relief system) A spring-operated safety valve actuated or assisted by an externally powered control device which can be hydraulic, pneumatic or electric (Figure 3.15). 

Trip Valves The trip valve is part of a system 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 below a preset level. Trip systems are used primarily on springless piston actuators requiring fail-open or fail-closed action. An air storage or "volume tank and a check valve 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 instability when the trip pressure and the reset pressure settings are too close to the same value. 

When the liquid makes contact with any of the electrodes, an electric current will flow between the electrode and ground. The current energizes a relay which causes the relay contacts to open or close depending on the state of the process involved. The relay in turn will actuate an alarm, a pump, a control valve, or all three. A typical system has three probes a low level probe, a high level probe, and a high level alarm probe. 

Because of the enormous diversity in components it is difficult to describe a straightforward design-path for components for the MCB concept. Here we focus on the modeling and the design of the fluid control modules and specific on the thermo-pneumatic actuated micropump used (twice) in the demonstrator. An elaborated model of this micropump is given by van de Pol et al. [21]. The main functions of the fluid control in micro analysis systems are the switching function and the direct flow and/or pressure control. Building blocks are hydraulic inertances, resistors, capacitors and passive and control-valves. Very often an active element like a micropump is needed. 

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