Pressure spool-valve

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 4.19 shows a spool-valve eontrolled linear aetuator. When the spool-valve is moved to the right, pressurized hydraulie oil flows into ehamber (1) eausing the piston to move to the left, and in so doing forees oil in ehamber (2) to be expelled to the exhaust port. 

Fig. 4.20 Pressure-Flow-rate characteristics fora spool-valve.

Alternatively, the pneumatic pressure can be held constant and a valve used to control the fluid flow. Needle valves, spool valves, and rotor valves are commonly used in this type of device. In all devices of this type, the fluid flow rate (0 is directly proportional to the material viscosity. Those factors that change the viscosity should be controlled carefully or the equipment must be able to compensate for the changes in order to achieve consistent results. 

The system has a hydraulic fluid source that is supplied at a constant pressure to the system. The spool valve can be moved back and forth. When the spool valve is moved in either direction, a port that leads to the hydraulic ram is opened. At the same time on the other side of the ram a port that leads to a hydraulic fluid return tank opens. In actuality, there is really only one port on each side of the hydraulic ram. Each is used as either a supply or a return port depending on the direction the... 

Spool valve is displaced. Figure 11.47 shows how the flow is diverted into the ram or the flow from the low-pressure side of the ram is diverted away from the supply and into the return tank. 

A number of features common to most sliding-spool valves are shown in Figure 9.10. The small ports at either end of the valve housing provide a path for fluid that leaks past the spool to flow to the reservoir. This prevents pressure from building up against the ends of the pistons, which would hinder the movement of the spool. When these valves become worn, they may lose balance because of greater leakage on one side of the spool than on... 

Spool valves are connected so that the safest position is set this means that, for instance, the pressure relief valves are connected so that they open if solenoid failure occurs. 

As inlet pressure rises above set pressure, dome pressure reduction will be such as to provide modulating action of the main valve piston proportional to the process upset. The spool/feedback piston combination will move, responding to system pressure, to alternately allow pressure in the main valve dome to increase or decrease, thus moving the main valve piston to the exact lift that will keep system pressure constant at the required flow. Full main valve lift, and therefore full capacity, is achieved with 5% overpressure. As system pressure decreases below set pressure, the feedback piston moves downward and opens the inlet seat to admit system pressure to the dome, closing the main valve. 

Check for large pressure drops such as those that can occur at control valves. Determine if pressure drops will induce corrosive flashing. Flash spools or splash plates may be required downstream of the affected control valve. 

At this state, the spool of the control valve in the power steering gear does not act and is at the central position. The hydraulic oil supplied from the oil pump does not flow into the oil cylinder, while flows baek to the oil reservoir through the control valve via the loop. This is because that the pressure at both sides of the piston of the steering gear is the same and the piston does not move. 

The conventional method to provide double block and bleed isolation is to provide two valves with a spool piece between them. The spool piece has a small bleed valve attached to it so that when the two main valves are closed, the small bleed valve can be opened to bleed off pressure between the valves to determine if the two valves are sealing properly. 

One potential problem with the use of rupture disks below relief valves concerns the possibility of pinhole leaks in the rupture disk. If such a leak occurs, then the pressure on the downstream side of the disk will be roughly equal to the process pressure. Therefore, the absolute process pressme required to rupture the disk may be almost twice as high as expected. Moreover, the existence of the pinhole leak may lead to the development of the problem that it was intended to avert, e.g., the relief valve could be plugged with polymer without anyone being aware of the situation. Because of the dangers associated with pinhole leaks, it is good practice to install a pressure gage in the spool piece between the rupture disk and the relief valve. 

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