A dual manifold pressure reducing station, complete with reducing valves, alarm, filters, pressure gauge, drains, and safety valve, should be installed as shown in figure below for panels requiring pneumatic service. These stations are employed to filter and reduce pressure to a common header supplying instruments in the control house or field panel.
Each reducing regulator should be sized to supply existing and future instruments using a pilot air capacity of 0.50 scfm for each instrument. The bottom regulator should be set for 20 psig and the top regulator should be set for 17 psig.
A low air pressure alarm set for 18 psig should have an individual supply line with a block valve.
Minimum pressure/temperature rating of filters should be 125 psig and 360°F. The pressure/temperature should specify minimum design requirements for mechanical integrity.
The pressure relief valve should be sized to handle the volume of air through one reducing valve, thus specifying the limiting condition for safety relief. Assumes only one regulator in normal operation.
Pressure relief valves for control panel instrument air supply headers should be set at 30 psig, thus specifying the limiting condition for safety relief. Normal maximum instrument supply requirement is 20 psig.
Air piping to panel mounted instruments should not interfere with possible future instrument installation or accessibility for maintenance. Nitrogen or other inert gases should not be piped into the control center located panels, or walk-in field located panels. Panels containing inert gas should be properly ventilated and provided with warning labels on all access doors specifying the presence of Nitrogen or other inert gas within the panel.