Differences Between Direct & Pilot-Operated Regulating Valves
What is the difference between direct & pilot-operated valves?
Regulators, both pressure and temperature, can be classified as either “direct” or “pilot” operated depending on their design characteristics. The purpose of this primer is simply to explain the differences between these two types and highlight the benefits of each for proper selection per application.
Below is a simple comparison highlighting the basic variations and characteristics of direct and pilot-operated regulators. Additional detail can be found in the following FCI publications:
Tech Sheet CVR #402
Fundamental Principles of Self-Operated Pressure Reducing Regulators
FCI 86-2-2000
Regulator Terminology
FCI 95-1-1995
Pressure Regulator and Temperature Regulator Specification Sheets
ANSI/FCI 99-2-2004 (This has been replaced with FCI 99-2-2015)
Pressure Reducing Regulator Capacity
COMPARISON OF GENERAL CHARACTERISTICS OF DIRECT AND PILOT-OPERATED REGULATORS
This Tech Sheet was developed by the members of the Fluid Controls Institute
(FCI) Control Valve / Regulator Section. FCI is a trade association comprising the leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment. 2/23/10 Page 1 of 5 This sheet is reviewed periodically and may be updated. Visit www.fluidcontrolsinstitute.org for the latest version. Tech Sheet #CVR 404
Operation – Direct-Operated
It is easiest to begin with our understanding of the differences between direct and pilot operated regulators by focusing on direct-operated. This is because they are the simpler of the two in terms of both design and operation (Figure 1).
Direct-Operated Regulators respond directly to variations in downstream pressure (P2) to provide the necessary flow and pressure to satisfy the system set point. P2 opposes the balance force provided by the spring to determine the plug position. Therefore, for direct-operated regulators, P2 is also considered the loading force.
FIGURE 1
Schematic of a Direct-Operated Pressure Reducing Regulator
If P2 increases, the spring is compressed and the plug moves upward towards the closed position, further restricting fluid flow. Conversely, any decrease in P2 allows the spring to move the plug downward, opening the valve further and allowing more fluid to pass. The plug position in a direct-operated regulator responds immediately to any change in P2.
Operation – Pilot Operated
Pilot-Operated Regulators consist of the same essential components of direct-operated units (plug and seat, diaphragm, spring) with the addition of a pilot which separates P2 and the diaphragm loading force (PL). By doing this, PL can be controlled in relation to changes in P2, meaning that PL can be increased from P2 (Figure 2). This allows the regulator to be more responsive to condition changes.
Furthermore, the pilot requires an external sense line connected to the downstream piping. Measuring pressure farther downstream of the valves allows for more accurate sensing of the true outlet pressure conditions as it moves the sense point away from the flow turbulence generated by the plug and seat. For all intent and purpose, the pilot can simply and effectively be considered a second regulator, providing additional control to the main regulator, improving overall sensitivity and, ultimately, accuracy.
FIGURE 2
Schematic of a Loading-Style Pilot-Operated Pressure Reducing Regulator
When flow demand increases, P2 will decrease and the pilot valve will open and increase PL to the main valve diaphragm allowing the main valve to open and supply flow and pressure to the application. When there is a reduction in demand, P2 will increase and the pilot spring will compress moving the pilot plug towards its seat, closing the pilot and reducing PL. This resultant reduction in PL under the main valve diaphragm will allow the main valve plug to modulate towards the seat, closing the valve.
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Control Valve / Regulator Section. FCI is a trade association comprising the leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment. 2/23/10 Page 1 of 5 This sheet is reviewed periodically and may be updated. Visit www.fluidcontrolsinstitute.org for the latest version. Tech Sheet #CVR 404