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1 June 2006

Indian Acid Plant Gets Turbo Power

Interrupted production leads to integrated turbo revamp, ends manual intervention during load changes

By Keyur G. Vora and Krishnan Narayanan

With new construction and expansion projects springing up in Asian countries, it's not a good time for plants to experiment with unproven control system vendors. Unplanned shutdowns could put production commitments at risk. Due to increasing global demand for pure terephthalic acid (PTA), the opportunities from continuous operation of a PTA plant with uninterrupted productions are greater than before.

Two process air compressors (steam- and electrical power-driven), having unequal capacity and operating in parallel, provide the main air duty for a PTA plant at India's Reliance Industries' Patalganga unit. During fouled rotor conditions, the compressor surged due to rapid grid frequency dips or at low frequency periods unloaded the motor-driven compressor. Sometimes reductions in air supply to the reactor interrupted the plant production. Plant operators considered several main factors to justify the OEM control system replacement: projected annual savings (based on gain in PTA production under compressor-fouled conditions) and eliminating unscheduled shutdowns during low frequency periods of operation. After trying out various available options with the OEM supplied control system, the end user decided to implement an integrated turbo-machinery control system (ITCS) solution.

ITCS is a multivariable control system that provides antisurge control, coordinated header pressure control, and parallel load sharing for energy efficient operation. The end user overcame a few problems by retrofitting process air compressors with ITCS in January 2004.

The compressor network consists of two parallel compressors delivering air into a common header: a steam turbine-driven compressor (~2/3 plant capacity) and motor-driven compressor (~1/3 plant capacity). The larger capacity compressor is a steam-driven unit with a variable speed 7 MW steam turbine. The steam-driven compressor consists of three compression stages and two intercoolers with a design capacity of 64,000 Nm3/hr. The smaller capacity compressor is a motor-driven unit with a constant speed 4.5 MW induction motor. The motor-driven machine has five stages of compression and four intercoolers, with a design capacity of 36,000 Nm3/hr. In addition, the motor-driven machine has inlet guide vanes for capacity control.

The previous control system consisted of OEM-supplied, single-loop antisurge and master/capacity controllers. A surge limit line, which an antisurge algorithm defines, must not vary with operating conditions. Using a surge limit line instead of a surge limit surface in the antisurge controller results in loss of turndown or operating in the unstable zone, which leads to surge.

We employed a unique surge detection algorithm for the motor-driven compressor and implemented fail-safe fallback strategies in the antisurge controller in the event of transmitter failures.

Antisurge control algorithms

An antisurge control algorithm must be energy efficient for all operating modes of the plant. Automated vent valve response ensured the compressor is automatically loaded and unloaded in an efficient, safe, and controlled manner during start-up and normal shutdown. In case of major process upsets or emergency conditions, the compressor is unloaded rapidly by quick opening of the vent valves.

Antisurge control systems at the plant operated in automatic mode but with problems when grid frequency dropped. The electrical grid frequency at the end user varies from 47.8 Hz to 51.2 Hz during evening time. The motor-driven compressor's trip set point is at 47.7 Hz, and the frequency low alarm setting is 47.9 Hz. The drop in electrical grid frequency occurred every several minutes on average and occasionally every few seconds. The antisurge control was unable to protect the motor-driven compressor from surge and keep it online when the grid frequency dropped below the 48.5 Hz. The drop in electrical grid frequency results in a reduction of motor speed, causing a drop of flow and pressure in accordance with compressor fan laws.

The opening of the antisurge vent valve—as the motor-driven compressor's operating point reaches the antisurge control line—magnifies the resulting decrease in flow and pressure delivered to the reactor. This causes an even larger process upset. The inadequate surge control algorithm along with the conservative surge control margin we used resulted in excessive opening of the vent valve, causing the motor-driven compressor to go offline, thus magnifying process upsets.

Exacerbating the above problem is the effect of compressor fouling. The motor-driven compressor's stability range reduces as the compressor experiences fouling from particulate matter deposited in the air path from the ambient air drawn into the compressor. Each issue tends to reduce the stable operating range and further reduces the stable operating envelope. During the fouled rotor condition, it was difficult to keep the compressor online during frequency dips or at very low frequency periods due to surging. The result was production losses due to the PTA plant's unscheduled unloading of its oxidation section.

Originally, the compressors operated independently in manual mode for controlling the system header pressure. The master discharge pressure controller never worked as designed.

For more than a year, the plant experienced production loss of 2,711 MT due to unloading of both compressors and load reduction during the low-frequency period of motor-driven compressors.

Control system revamp objectives, solution

End users raised capital expenditures for retrofitting existing controllers on process air compressors to fully automate header pressure control, enhance reliability of compressor performance during low frequency and fouled rotor conditions, and eliminate unscheduled plant-load reductions or process shutdowns, and maximize the availability of air compressors. They also wanted energy efficient load-sharing of dissimilar compressors to minimize utility costs.

The control system vendor's task was to analyze and check the sizing and adequacy of suction flow transmitters, blow off control valves, and offer a suitable solution to the end user's problem. The ITCS solution involved a multivariable control system providing antisurge control, coordinated header pressure control, and parallel load sharing for energy efficiency. The objective of ITCS is to operate the turbo machinery within a safe operating regime (envelope) by properly integrating the compressor antisurge and capacity controls. This envelope is defined in a compressor map figure for a variable speed turbocompressor.

The ITCS is versatile enough to adapt to diverse turbo machinery configurations, such as driver (fixed versus variable speed driver), inter-stage coolers, and stage designs (parallel and series configuration) for both process air compressors. To ensure optimum performance, we executed control algorithms in a maximum of 40 ms for an antisurge system.

ITCS Benefits

After implementing the new compressor control system, the master pressure controller is maintaining steam-driven and motor-driven compressors discharge pressure by load sharing between compressors, eliminating manual intervention during load changes.

We reduced standard deviation of master pressure fluctuation compared to pre-shutdown operation from 0.4 barg to 0.1 barg. Now, the air flow to reactor is precise and steady.

The motor-driven compressor operates at a safe distance with a wider margin from surge control line due to advanced surge control algorithms in ITCS, making production losses a moot point.

The new antisurge curves fitted on both compressor controllers increased safety margin between design point of compressor and surge limit line by about 10%, maximizing throughput from compressor and avoiding possible machine surge.

About the Authors

Keyur. G. Vora is senior manager of instrumentation (PTA/PX) at Reliance Industries Limited Patalganga Unit, Maharastra India. Krishnan Narayanan is director at Advanced Application & Research Compressor Controls Corporation, Des Moines, Iowa.