Emerson Exchange 365

A regulator’s lockup pressure refers to the pressure increase in the regulator’s outlet pressure above setpoint required to shut the regulator off tight. Most product documentation does not list expected lockup pressures because many of the factors that affect lockup are installation-dependent. Lockup pressures could be elevated when exposed to low temperatures, dirty process fluid, rapid on/off flow demand, small downstream volume, and aged elastomers. When troubleshooting an installation that appears to have higher than expected lockup pressure, first evaluate whether any of the before mentioned conditions could be causing the concern.

Common Causes

1. Excessive lockup can occur if the valve plug and orifice are not fully sealing. The valve plug or orifice could be scratched or debris could be keeping them from tightly sealing. In pilot-operated regulators, both the pilot and the main valve should be inspected unless you can isolate which one is leaking by closing a block valve on the pilot bleed line. 
2. If setpoint was made at high flow, higher than anticipated lockup pressures could result. Establishing setpoint at a small, nominal flow is the best practice for setting the regulator. Because self-operated regulators are less accurate than pilot-ops, the flow rate at which setpoint is made makes a bigger difference. The figure below shows the increase in lockup pressure associated with a regulator set at a high flow rate.    
3. If the maximum pressure drop or maximum inlet ratings are exceeded, the lockup could be elevated. The force from the outlet pressure on the regulator diaphragm provides the force needed to achieve shutoff  unless overpowered by the force from the higher than recommended inlet pressure on the valve plug.
4. If any of the regulator’s moving parts are galled, bent, or damaged, the regulator could have high lockup pressures.