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Pneumatic pressure controls fall in the category of pressure reducing valves, commonly referred to as air line regulators. It is also essential, once a system pressure has been selected to perform a task, that air be supplied at constant pressure to the actuator, regardless of variations in flow and upstream pressure. Thus, it is important to add to a pneumatic system a pressure regulator that:
• supplies air at constant pressure regardless of flow variation or upstream pressure,
• helps operate the system more economically by minimizing the amount of pressurized air that is wasted. (This happens when the system operates at pressures higher than needed for the job),
• helps promote safety by operating the actuator at reduced pressure,
• extends component life because operating at higher-than-recommended pressures increases wear rate and reduces equipment life,
• produces readily controlled variable air pressures where needed, and
• increases operating efficiency
Fig. 1. Simplest regulator incorporates an adjustment spring (that is, it is not pilot-operated) and an unbalanced poppet. It does not have a separate diaphragm chamber, and it is non-relieving.
Types of regulators
Unbalanced poppet, non-pilot operated — Figure 1 shows the simplest type of unbalanced poppet regulator. Normally, supply pressure enters the regulator and flows around the poppet, which is seated, blocking flow.
Turning the adjustment screw to compress the adjustment spring forces the diaphragm down. It pushes the stem down and the poppet uncovers the orifice. As downstream pressure rises, pressure air acts on the underside of the diaphragm, balancing the force exerted by the adjustment spring. The poppet throttles the orifice to restrict flow and produce the desired pressure. As downstream flow demand varies, the regulator automatically repositions the poppet in relation to the orifice. The spring under the poppet ensures that the regulator will close at no-flow. This regulator is non-relieving.
Unbalanced poppet, non-pilot operated with diaphragm chamber — The regulator in Figure 2 is larger (and more expensive) than the model in Figure 1. It also has a diaphragm chamber which isolates the diaphragm from the main air flow to help reduce the effects of the abrasive air on the diaphragm.
Fig. 2. Regulator has an adjustment spring and an unbalanced poppet. It has a separate diaphragm chamber which contains an aspirator tube connecting to the reduced-pressure port. It is self-relieving.
An aspirator tube connects the diaphragm chamber and the outlet chamber. As flow through the regulator increases, the tube creates a slightly lower-than-outlet pressure in the diaphragm chamber. The power pressure under the diaphragm deflects it downward, forcing the poppet farther away from the orifice. The adjustment spring extends to open the poppet orifice without significantly decreasing outlet pressure. The effect is the same as increasing the adjustment setting and thus reducing droop at higher flow rates.
This regulator’s much larger diaphragm area produces greater forces and thus displaces the poppet more with a given change in reduced pressure. Larger diaphragms increase regulator response and sensitivity.
Balanced poppet, non-pilot operated, with diaphragm chamber — This regulator, Figure 3, has the same general internal construction as the previous type. However, it has a considerably larger orifice to allow for greater flow. In addition, to maintain good stability, the poppet is pressure-balanced. That is, the poppet sees the same reduced pressure on both top and bottom surfaces. Thus, the effects produced by reduced pressure fluctuations cancel out, and sensitivity and response are greatly improved. This very-large-capacity regulator has low droop.
Fig. 3. Regulator has a balanced poppet and a separate diaphragm chamber with an aspirator. It is non-pilot operated.
With an exactly balanced poppet, the system-pressure/reduced-pressure ratio has no effect because the unbalanced resultant forces on the poppet caused by supply pressure are zero. However, these poppets are generally designed with a slight tendency to close. Therefore, a large increase in supply pressure forces the poppet closer to the orifice, throttling flow. This causes the reduced pressure to drop slightly.
Remote controlled, pilot operated, balanced poppet — In some applications, the regulator must be installed where it cannot be easily adjusted. The regulation and pressure setting mechanisms are then separated. A small air pilot line connects the regulator (in the air line at the point of use) to the remote setting mechanism, which can be mounted at any convenient location.
The remote setting mechanism is a small regulator that produces a control air signal. The signal is sent to a pilot-operated, balanced-poppet regulator, similar to the previous regulator except that the top is replaced with a short, pressure-tight bonnet to receive the control signal from the small remote setting regulator. Instead of working against a force created by a compressed spring, the pilot-operated regulator works against a force created by air pressure — that is, an air spring.
The air spring maintains a constant force on the upper side of the diaphragm of the pilot-operated regulator because the remote setting regulator holds the control signal at constant pressure. Thus, droop in this regulator is small.
Internal, pilot-operated, balanced poppet — This regulator also uses the pilot-operated principle to produce a precision regulator. Both the pressure setting regulator and the pilot-operated regulator are combined in a single housing. The same force-balance principle applies as in the previous regulator.
With this regulator, some supply air bleeds into the cavity over the lower diaphragm and escapes through the nozzle. As increased air pressure on the upper diaphragm opens the flexible seat, the pressure above the lower diaphragm drops and causes the poppet to approach the primary orifice, reducing flow, then pressure. A bleeding-type relief seat vents through the center of the diaphragm. This regulator also has a safety-type relief valve above the pilot mechanism, which rapidly exhausts any very high overpressure.