Piston actuator

Figure 8-74b is an example of a pneumatic positioner/actuator. The input signal is a pneumatic pressure that (1) moves the summing beam, w ch (2) operates the spool valve amplifier, which (3) provides flow to and from the piston actuator, which (4) causes the ac tuator to move and continue moving until (5) the feedback force returns the beam to its original position and stops valve travel at a new position. Typical positioner operation is thereby achieved. 

Figure 7. Piston actuator structure etched with a structural layer on top of the sacrificial layer (500 pm square piston area) general view and close-up.

In 1812, Pauly invented another percussion device. Here, a small piston, actuated by a spiral spring, struck a nipple upon which a few grains of MF were placed... 

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. 

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. 

The amplifier network provides signal conversion and suitable static and dynamic compensation for good positioner performance. Control from this block usually reduces down to a form of proportional or proportional plus derivative control. The output from this block in the case of a pneumatic positioner is a single connection to the spring and diaphragm actuator or two connections for push-pull operation of a springless piston actuator. The action of the amplifier network and the action of the stem-position feedback can be reversed together to provide for reversed positioner action. 

Requiring tight shutoff in large valves is particularly expensive in applications where spring and diaphragm actuators are preferred or where positioners normally would not be required. This is due to the necessity of changing to piston actuators in order to get the necessary thrust. 

A screw conveyor takes the comminuted raw material into a coaxial cylindrical chamber. On the left-hand side, this chamber features a reciprocating annular piston actuated hydraulically at a frequency of 120 strokes per minute to form a plug of compressed raw material serving as a pressure lock between the ambient atmosphere and the hydrolysis reactor. Due to its high frequency of operation, this feeder is likened to a machine gun. On the right-hand side, the chamber features a choke applying an adjustable pressure on the plug of raw material, and... 

The Eluent Delivery Pump. HPLC pumps deliver the mobile phase from the reservoir to the colurtm. Mainly double piston-actuated pumps accurately deliver solvent over a broad range of flow rates (0.1 pl/min-20 ml/min). Gradient elution (flow rate or composition) can be used in HPLC separations where the use of only one mobile phase (isocratic separation) would not have achieved adequate resolution. 

The positions of the flow restrictors inside both globe and rotary valves depend on the action of a piston actuator, either directly or through a gear mechanism. As noted in Section 7.2, the stroke of this piston is called the valve travel . We have already given this the symbol, x, with the convention that... 

Hydraulic and pneumatic vane actuators provide a direct translation from the vane to a partial turn actuator output without the additional conversion mechanism required for a diaphragm or piston actuator. 

A significant issue to address is the impact of "fail safe" versus "non fail safe" actuators. Some designs such as diaphragm and piston actuators with hydraulic and pneumatic power are readily available as spring return valves. The failure or release of control pressure permits the spring to drive the actuator to its safe position. 

The analysis can be done for valves, actuators or combination actuator/ valve devices. Table 11-4 shows an abbreviated FMEDA for a gate valve with piston actuator. This analysis shows that partial valve stroke testing... 

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