Oversizing is the most common regulator selection issue. When a regulator is oversized, the valve plug is forced to operate very close to the orifice. At this low travel, the forces from the fluid flow are large enough to cause the regulator to cycle: open, closed, open, closed… This high frequency cycling, known as instability, causes wear on the moving parts which can lead to component failure and loss of pressure control. This cycling can also produce significant noise and inaccurate pressure regulation.
Oversizing can sometimes be avoided by selecting the smallest product/orifice/body size that meets the maximum flow requirements at the minimum inlet pressure condition. However, many regulating stations have very large differences between the maximum and minimum flow requirements which make it impossible to accomplish with a single pressure regulator. Instead, a second regulator (hereafter called the low-fire regulator) can be added in parallel to handle the lower flow rates. The main regulator will then pass any flow beyond the low-fire regulator’s capacity.
Staggering the main regulator and low-fire regulator setpoints ensures that the regulators do not fight for control. The main regulator should be set at a slightly lower pressure than the low-fire regulator keeping it closed but prepared to open once the flow demand exceeds the low-fire regulator’s capability.
The load profile of the application must be considered when sizing and selecting a low-fire regulator. Because the main regulator will have to operate at any flow exceeding the low-fire regulator’s capacity, it is best practice to select a transition point at which the application doesn’t linger.Taking the load profile into account leads to some stations with a much smaller low-fire regulator than main regulator while other stations will be closer in size, sometimes even the same size.
Parallel runs are also utilized to keep the regulators in smaller, more easy-to-maintain sizes. The only limitation on the number of regulators placed in parallel is the downstream pressure variation due to the staggering of setpoints. It is not uncommon to see three or more regulator runs installed in parallel.
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