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Machine Design

Pneumatic Pressure Regulators

Pressure regulators, commonly called pressure-reducing valves, maintain constant output pressure in compressed-air systems regardless of variations in input pressure or output flow. Regulators are a special class of valve containing integral loading, sensing, actuating, and control components. Available in many configurations, they can be broadly classified as general purpose, special purpose, or precision.

General-purpose or utility regulators have flow and regulation characteristics that meet the requirements of most industrial compressed-air applications. Such regulators provide long service life and relative ease of maintenance at competitive prices. Precision regulators are for applications where regulated pressure must be controlled with close tolerances. Such regulators are used when the outcome of a process or the results of a test depend on accurate pressure control.

Special-purpose regulators often have a unique configuration or special materials for use with fluids other than compressed air. Regulator construction can range from simple to complex, depending on the intended application and the performance requirements.

However, the principle of operation and the loading, actuating, and control components are basic to all designs. Most regulators use simple wire coil springs to control the downstream pressure. Various size springs are used to permit regulation of the secondary pressure within specific ranges. Ideally, the required pressure should be in the center one-third of the rated outlet pressure range. At the lower end of the pressure range, the spring loses some sensitivity; at the high end, the spring nears its maximum capacity.

Regulators can use either a piston or diaphragm to sense downstream pressure. Diaphragms are generally more sensitive to pressure changes and react more quickly. They should be used where sensitive pressure settings are required (less than 0.04 psi). Pistons, on the other hand, are generally more rugged and provide a larger effective sensing area in a given size regulator. The functional difference between precision and general-purpose regulators is the degree of control accuracy of the output pressure. Output pressure accuracy is determined by the droop due to flow changes (regulator characteristics).

Pressure droop is most pronounced when the valve first opens. Factors contributing to droop are: load change with spring extension, effective area change with diaphragm displacement, and unbalance of area forces on the valve. The amount that output pressure changes with variations in supply pressure is called the regulation characteristic and is influenced by the ratio of diaphragm area to valve area and the degree of valve unbalance.

When selecting a pressure regulator, the important factors to consider are:

  1. Normal line pressure.
  2. Minimum and maximum regulated pressure required: Regulators can have a broad adjustment range and may require a specific spring or accessory to match the requirements. Also, minimum and maximum pressure should be within the middle third of the regulator range.
  3. Maximum flow required at regulated pressure.
  4. Pipe size: Not all regulators are available in all pipe sizes; note where adapters are required. Also, pipe size should be consistent with flow requirements.
  5. Regulator adjustment frequency: A number of different adjusting methods are possible. When selecting a regulator, consider the location, application, adjusting method, and user.
  6. Degree of pressure precision required.
  7. Accessories or options include gages and panel mounting.
  8. Environmental or fluid conditions that could be incompatible with materials used in the regulator.
  9. Special features such as high relief or remote control.
  10. The consequences of a regulator malfunction or failure: A damper or relief valve might be needed to protect personnel or equipment. Also, dead-end service or intermittent actuation may require positive valve shutoff, bleed units, or close control of pressure-relief points. Filters, lubricators, relief devices, and other system options should be considered in the selection process.
TAGS: Hydraulics
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