Pneumatic piston actuator

Pneumatic Piston Actuator, severe service De-energize to trip 99.6%... 

Needle drives with pneumatic piston actuators that can be incorporated directly into the clamping plate of the mold have proven successful. Standardized designs, as shown in Figure 2.63, can be easily installed, and important functions such as radial compensation and needle adjustment are already integrated into the assembly. 

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. 

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. 

Hydraulic and pneumatic piston and diaphragm actuators provide a linear output. These may be integrated with scotch yoke or crank arm mechanism to provide a quarter turn output. These two mechanisms have variations in the output torque over the stroke of the actuator. When integrated with a rack and pinion mechanism they may provide up to a full turn output. 

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. 

Hydraulic The design of typical hvdraiilic actuators is similar to double-acting piston pneumatic types. One kev advantage is the high pressure (yvpicallv35 to 70 bar [500 to 1000 psi]), vvFich leads to high thrust in a smaller paclcage. The incompressible nature of the... 

Pneumatic actuators are normally used to control processes requiring quick and accurate response, as they do not require a large amount of motive force. However, when a large amount of force is required to operate a valve (for example, the main steam system valves), hydraulic actuators are normally used. Although hydraulic actuators come in many designs, piston types are most common. 

The output of the digester is pumped from the helix through a pneumatically actuated valve either into the neutralization vessel or to waste. A pump was designed and built for this purpose. Its action is based on an oscillating piston, it has no moving valves and the hot acid comes into contact only with Kel-F and PTFE. The maximum flow-rate is about 12 ml min. This pump has been described in more detail elsewhere [33]. 

Constant pressure pumps. Constant pressure pumps (Figure 6.11) deliver solvent via a small headed piston which is driven by a pneumatic amplifier. A gas acts on the relatively large piston area of the pneumatic actuator. This is coupled directly to a small piston which pushes the eluant through the column. Pressure amplification is achieved in direct ratio to the piston areas and thus for low inlet pressures (approximately lOOpsi (690 kPa)) it is possible to obtain large outlet pressures (lOOOOpsi (69 MPa)). 

In the DuPont 848 liquid chromatograph, a special Haskel mini-pump is used, derived from the Haskel Model M. The volume of the cylinder is small (about 2 ml). In the original Haskel Model M pump, the return of the piston is actuated by an air selector valve and a spring, so that the liquid pressure is not constant because the gas piston must compress the spring during its forward stroke. In the modified pump, the spring is replaced with a small counter-pressure. The main characteristics of commercial pneumatic amplifier pumps are summarized in Table I. 

The re-filling time is longer than with pneumatic pumps it takes one to several minutes, depending on the chamber volume and whether the chamber must be rinsed or not when a new solvent is used. To re-fill the chamber, the piston must be retracted. This operation can be effected manually with a crank after disengaging the motor-piston clutch, or can be made with a second, more powerful, motor actuated by a switch (Perkin-Elmer Model 601). Depression of the Fill push-button switch of the Varian Model 8500 pump actuates the downward piston movement and, at the same time, pressurizes the solvent reservoir, allowing the chamber to be re-filled. In the ISCO Model Sl l pump, the suction created by the retraction of the piston pennits re-filling from a reservoir. 

The tissues were Interfaced with the wearing surfaces as follows. The plugs were secured In stainless steel holders that were held by chucks attached to pneumatic actuators. The loads on the tissue were monitored by a load cell (BLH, Inc., Waltham, MA) positioned between the chuck and the piston of the actuators (Fig. 3). When loads were applied through the actuators, the... 

Fig. 6. Schematic view of the unit for low-temperature impact tests. Legend (1) outer cryostat shell, (2) internal cryostat shell, (3) nitrogen vessel, (4) screen, (5) siphon, (6) circular anvil, (7) crushed sample trap, (8) vacuum transfer, (9) dynamometer with strain gauges, (10) packing, (11) actuating power pneumatic cylinder, (12) hammer, (13) cutter, (14) piston, (15) upper test section, (16) lower test section, (17) rod, (18) cutter movement sensor shutter (19) cutter movement sensor, (20) lock crown, (21) lock, (22) support, and (23) control pneumatic cylinder.

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