Instability refers to a condition in which the regulator is opening and closing too frequently instead of throttling to match the flow demand. This sometimes manifests itself as outlet pressure fluctuations as well as a noise described as “chatter” or “buzzing” due to the rapid movement.
Inadequate downstream volume - Small downstream pipe volume can cause the downstream pressure to rapidly change whenever the regulator opens or closes. Rapid changes in downstream pressure cause the regulator to receive incorrect flow demand signals, leading to erratic control. The regulator needs additional downstream volume so it can throttle. Restricted downstream piping (ex: orifice plates, block valves, etc) could also simulate a small volume system even if the total downstream volume is sufficient.
Poor sense line placement - If the sensed pressure is rapidly fluctuating, the regulator will likely be unstable as a result of responding to this erratic signal. Turbulent flow can occur due to swages, elbows, regulators, block valves, etc. Poor sense line placement is often at fault in instances where instability is present at high flow rates but not lower flow rates. Sense lines for external-sensing regulators should be placed in a clean run of pipe, six to ten pipe diameters downstream of any turbulence-causing components.
Oversizing - Oversizing a regulator causes the valve plug to operate very close to the orifice and therefore become very sensitive to phenomena such as changes in flow demand as well as the flow moving past the valve plug. When sizing a regulator, select the smallest orifice, body size, and model that will pass the application’s required flow. If the application has a very large difference between the maximum and minimum flow requirements a low-fire regulator could be used.
Obstructed venting- If air isn’t able to flow in and out of the spring case vent hole as designed, then the regulator can become unstable. A variety of issues can create this scenario including a plugged vent, inadequately-sized vent piping, or a bypassed vent assembly due to removal of the closing cap or vent assembly. To determine whether the instability is related to the vent, return the regulator to its original state. For example, if the construction has vent piping, temporarily disconnect it from the spring case to see if the problem resolves. The best practice for vent pipe sizing is to use next nominal pipe size larger than the vent connection for every fifty feet of total piping (elbows are equivalent to three feet). For regulators with internal relief, use the next nominal pipe size larger than the vent connection for every ten feet of total piping to minimize impact on the relief capability.
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