(157c) Automating the Depressuring of a Plant

Automating the Depressuring of a Plant

Gowthami Tamilselvan and Satyajit Verma, P.E.

The ideal option for depressuring a plant is to design the relief system to handle the simultaneous full depressurization load of the entire plant. Staggered or sequential deployment of depressuring valves could be considered for existing facilities that have undergone expansion and where simultaneous depressuring of several depressuring valves would overpressure the existing flare capacity. In such systems, flow control valves can be used as depressuring system which would allow controlling the total depressuring rate and maintain it below the flare capacity. Typically, steady state and/or dynamic simulation programs are used to develop flow and pressure characteristics of depressuring valves.

The staggered depressuring may be performed using on-off valves with subsequent exponential pressure decline. Alternatively the depressuring may be done using flow control, or pressure control or a combination of the two. Additionally, the staggered depressuring can be manual, semi-automated or fully automated. 

This paper will describe a fully automated depressuring method that can be utilized regardless of the control philosophy. Automation will be executed with a Supervisory Depressuring Program. The Supervisory Depressuring Program will contain the information for proper deployment of the depressuring valves, such as the order of the depressuring valves, the available capacity of the flare header, the flare header pressure, the depressuring profile of the depressuring valves and the electrical substation serving the depressuring valve. The depressuring valve deployment sequence can be developed in several ways, one of them is based on the life of the Uninterrupted Power Supply (UPS) system at the electrical substations serving the depressuring valves.