Connecting Operations In Control: How to Implement Distributed Control During a Plant Revamp
Upgradation of the distributed control system at a urea plant – A revamp project for the DCM Shriram Group, which operates a fertilizer plant, was completed well before time without any downtime or process interruption at the plant. Though the project was difficult, the company managed to complete the task.
The DCM Shriram Group operates a fertilizer plant at Kota, Rajasthan. The urea plant was commissioned in 1967 with the latest available instrumentation and control systems of that time. In the year 2000, a small section comprising a hydrolyser unit was added along with a Distributed Control System (DCS) and microprocessor based smart field instrumentation.
Except for the hydrolyser, the process control for the plant parameters was being performed from a control panel in the centralized control room through individual standalone instruments such as controllers, recorders, monitors and field devices.
From Pneumatic to Electronic Transmitter
A majority of field transmitters and control room instruments were based on pneumatic signal communication. Some of the important control loops had been gradually upgraded by replacing the field and control room devices with the electronic smart transmitters and microprocessor based single loop controllers which experienced as better accuracy and control in comparison to the pneumatic control loops.
The safety interlocks was of relay based control circuits placed at different locations in the control room triggering alarm and trip commands from pneumatic switches in combination with relay based annunciators for visual alarm of process up-sets.
The Time for Change is Now
Having experienced the benefits of process control and maintenance with the new microprocessor based control system of the hydrolyser section (DCS) and single loop electronic controllers along with smart transmitters, the operation and maintenance engineers felt a number of limitations with the current system.
These included the obsolescence of measuring and control devices through pneumatic instrumentation and conventional control systems in the centralized control room (CCR), the non-availability of spares, frequent zero and calibration drift, lesser availability of process information to analyze up-sets and problems, more human interventions, less diagnostic features and lesser availability of trained manpower in the pneumatic system as old operators were retiring from service.