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Enhancing Reliability of Hydrogen Unit PSA in MRPL with Smart Technology
Positioners are crucial for ensuring the reliability of the control valves, and these valves are widely found in PSA system. The following case study highlights the significance of new smart technology in the positioners which enables the end user to either carry out modifications to their existing system or implement new system to improve the reliability of the PSA system as a whole.

To a large extent, unwanted process trips can be attributed to the malfunctioning of final control elements. Improvement in the reliability of a positioner contributes to the overall reliability of the control valves on which it is mounted, especially if these valves are operating very frequently from 0 per cent to 100 per cent as their normal operation. Such valves are found in the PSA system where purification of hydrogen rich gas is carried out in refineries. This case study presents one such practical approach in implementing the new technology in Smart Positioners by using their latest diagnostics features. This has helped in failure prediction and mitigation, thereby reducing the failure rate of the system.

Mangalore Refinery and Petrochemicals Ltd is a subsidiary of ONGC with a state of the art refinery at West coast of India with a refining capacity of 15 mmpta.
Hydrogen is produced in the refinery which acts as a catalyst for downstream units. There are many technology providers for producing hydrogen gas in a refinery. UOPs Polybed PSA technology is also one of them. This technology allows 99.9 per cent pure hydrogen to be produced from the hydrogen unit. The PSA unit uses adsorption process to purify the hydrogen produced in the reformers and produce high purity hydrogen and a fuel gas stream containing the impurities. The PSA unit is designed for 10 bed operation with 89 per cent recovery. The system is designed such a way that if there is a malfunction, the unit automatically switches over to 8 bed operation with 84 percent recovery. In case of maintenance the unit can be switched over to 5 bed operation and can process 65 per cent of design feed flow and 68 per cent recovery. The PSA system is controlled by a MODICON PLC, type 984-785E with a non-redundant architecture. The main field instruments include on/off valves and control valves from Neles Jamesbury and Valtek, Pressure transmitters from Emerson, and Solenoid va l ves from ASCO. Hydrogen produced in the unit is fed to downstream consumers like hydrocrackers.

Malfunctioning of PSA System
A survey of the total failures of PSA system from year 2000-2008 showed that 29.6 per cent of the failures were due to failure of solenoid coils, barriers, etc, which were unavoidable to an extent. The remaining 70.4 per cent failures were due to typical problems in PLC system, valve gland leaks, calibration failures of positioners, actuator air leaks, seat leakage, etc, which were avoidable with better maintenance management. Failures of PLC or control valves used to either automatically change over from 10 beds to 8 beds or along with pressure profile alarms trip the unit.
The operational switchover of the PSA from 10 beds to 8 beds or 8 beds to 5 beds results in reduced recovery or subsequently trip the PSA unit. The hydrogen production would either reduce or become zero. In addition, to an extent of 2000 to 3000 Nm3/h excess purge gas produced has to be flared causing a loss of 33000/hr. The effect of this is on the downstream receiver unit ie, hydrocracker. Either the hydrocracker feed has to be lowered by 4-5 per cent or bring down the entire unit in case of hydrogen unit trip.

System Notifications
Redundant Processor
The PLC to control the PSA system had a non-redundant processor. In case of a processor problem the PSA system is used to stop thereby tripping the entire hydrogen unit. PLC architecture is changed to incorporate redundant processor, thereby increasing the reliability of the control system as a whole.

Smart Positioners
The next item to focus for increasing the reliability was the control valves. The PSA valves operate in a specific cyclic nature. Due to frequent operations, the gland leaks was the area of main concern. Gland leaks were arrested during routine or preventive maintenance schedules once in two months or as and when the field operator identified. The failures or mal operation in pneumatics ie, I/P converter and positioner were undetected during preventive or routine maintenance activity. To overcome this issue, Smart positioners from Fisher and Metso with advanced diagnostic features were installed in all the control valves of the skid.

Diagnostics Software
The software remotely communicates with smart positioner through the HART multiplexers without disturbing the process. With the best available software both online and offline diagnostics features provides end user a lot of information regarding the healthiness of the valve. The main offline diagnostics includes valve signature analysis, step response analysis and dynamic response analysis.
The valve signature snalysis is used to evaluate friction, dead band and shut off capability. It also helps in identifying the packing problems and comparing the current condition (in-service condition) of the valve with that of the base line condition (overhauled condition). The signature analysis helps in managing the plant assets effectively by providing validation of valves after repairs.

Dead band and hysteresis are important nonlinearities, which affect the process control. These can be analysed using dynamic error band diagnostics. The step response test allows us to evaluate how well the valve tracks an input change. It also helps in validating tuning parameters. The software also provides real-time notification of current status and predictive future problems related to valve. It further provides the physical condition of instrument like I/P and conditions of pneumatic circuit including the volume of air consumed by the instrument. The software passively gathers data related to the valve while it is in service and provides information like valve friction, air leakage, air quality, etc. It also provides probable causes of the problems and recommends solutions. Based on past experience, actuator leak was one of the frequent causes of PSA system failure, which had to be predicted. Conventional methods of soap bubble test did not help in identifying the O-ring leak. Also, the air leakage noise was seldom heard due to high noisy atmosphere near PSA. Dynamic scan test helps in determining the leak when it is still small and take precautionary action well before the failure. Online monitoring of valves provides a lot of information about the valve performance and triggers alarms in case of deviation. Using these diagnostic alarms, the valve servicing can be planned and avoid unwanted failures thereby increasing the availability and safety of the PSA system. The system also sends an email whenever there is an alarm to the concerned engineer.
As an example (Refer Figure 2), the offline test on a 4 valve with double acting piston actuator provided the complete diagnostics of the valve. The test clearly showed that the valve was not opening fully. The probable cause was either due to the leakage in the piston or failure of Piston O ring. The test also gave information that the valve was not opening till 10 per cent of input signal and this led to faulty pressure profile in the system. Based on the finding, it was recommended to service the actuator. On opening, it was found that the aluminum piston had developed a crack which was the reason for not opening the valve completely.

Change Management
Replacing the PLC system was a herculean task as each instrument had to be loop checked before the final connection. Changeover from conventional positioner to new smart positioner was not a smooth transition either. During the re-commissioning each positioner needed to be tuned. The positioner had various modes of control ie, aggressive, fast, stable, moderate, and slow. Each valve needed to be tuned based on its condition. Secondly, the standard bracket supplied by the vendor was to be customised to suit the valve. Valve to positioner link design as well as MOC was also modified. Benefits of the System

Benefits of the system includes over all performance enhancement due to effect diagnosis, reduced breakdown due to predictive maintenance. (refer figure 3)
The study revealed that the predication of control valve performance and better controllability of the final control element led to decreased failure of PSA system. The graph shows the number of failures of PSA before and after installation of Smart positioner. The average unpredicted failure effecting PSA before installation of Smart positioner was once every 25 days. But after installation of Smart positioner with advanced diagnosis and early malfunction alerts, the unpredicted failure rate has decreased. There was no reported failure in the year 2011.

The authors would like to thank the management and colleagues at MRPL who have helped in making this paper.