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.
Keeping in view the limitation of old instrumentation vis-à-vis benefits of the latest systems, specific special provisions were made during finalization of the technical specifications of the entire system such as high fail safety, redundancy and optimization of complex control loops.
Since upgrading a DCS can be a monumental and expensive project, it was important to evaluate whether it makes more sense to migrate the entire system at once or conduct a partial or phased approach, which most DCS suppliers offer. A phased upgrade can minimize the downtime, be less risky and the cost can be spread out.
Upgradation of DCS
Most of the new versions of DCS can be configured on top of the legacy control systems. This can be done on a running plant with no impact to production. By upgrading the DCS, you ensure that operators know the new technology before the complete system is installed. Changes in presentation and interaction with new controls are some of the biggest hurdles that operators face.
Many new DCS packages also support extra connectivity, such as OPC. In view of the above limitations, it was planned to migrate from a conventional pneumatic instrumentation and process control system to the latest state-of-the art DCS along with microprocessor based smart field devices to run the plant without shuting down which was a challenge.
A Team of Specialists is on The Task
A cross functional team (CFT) of process, instrumentation and electrical engineers was formed to successfully complete the project in phase manner in mid 2012. This team did a marathon exercise that included consolidating and studying old drawings and P&IDs, interviewing plant operators, deciding the operational philosophy, preparing wiring connections and JB details, etc.
Go Ahead for the Project
After receipt of the management approval in April 2013, the process of indenting was initiated. A close monitoring process for timely indenting and delivery of field devices, erection hardware and DCS system was managed.
An extensive factory acceptance test (FAT) of the new DCS was carried out with active participation from the process and instrumentation side. All activities such as cable tray and junction box installation, cable laying, termination and loop testing were carried out simultaneously to expedite the work.
Only Small Space in the Control Room
There was space scarcity in the control room to accommodate four operator consoles because of the already placed operating consoles of the existing DCS through which
the hydrolyser unit was running and shut down was not possible in the running plant.
The extra margin in the length of the existingcables (communication, power and earth) helped us to lift and shift the old consoles at a closer distance in the ‘power on’ condition without interrupting the hydrolyser process operations and continued till the hydrolyser unit operation shifted from the old DCS to the new DCS system.
How to Keep Operations Running During Upgrade Processes
It was a challenging job to continue the hydrolyser unit operations parallelly from both the DCS systems while switching almost 250 inputs and 16 outputs (close loop) to the new DCS marshalling for operation/control. At first, the multi pair cables of monitoring parameters were shifted and established a new DCS system.
Sixteen closed loops were switched one by one taking the control valve operation through hand jack. It was also tuned and commissioned through the new DCS system.
After completion of the hydrolyser work, the team moved towards commissioning of the Urea plant. A new maintenance free earth pit was also installed for the purpose of electronic grounding and safety of the system.
Bulk of the commissioning activity was focused on the DCS and field instrumentation, which was to be replaced with SMART transmitters. After FAT in end September 2013, a system was dispatched to Kota by a vendor. The process of installation was initiated and completed with civil work. The system was commissioned in all aspects in December, 2013.
Connecting operations for Better Control
During commissioning of the urea plant, almost 70 closed loops and 110 open loops were shifted on the DCS including the urea reactor pressure interlock. Daily meticulous planning was done to take up open loops and closed loops. Plant operators were kept at strategic locations whenever pneumatic to electronic conversions were being carried out in order to avoid any mishap. The system is in operation since December 2013, and no operational problems have been faced at the plant till date.