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Apr-2014

Protecting compressors with dynamic simulation

Dynamic simulation supports reliable operation of compressor installations to deliver maximum plant availability and productivity

NICHOLAS BROWNRIGG
AspenTech
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Article Summary
Integrated dynamic simulation of gas processing and petroleum refining processes is vital in the prevention of catastrophic equipment occurrences. The protection of compressors from mechanical failure is essential to maximise operating time and ensure safer operations.

This article highlights how dynamic engineering software successfully meets tighter product specifications through improved understanding of plant operability issues and ensures faster, safer plant start-ups to help avoid unplanned downtime. The results deliver maximum plant availability and productivity.

Better understanding leads to better plant control
Understanding the process is the first step in the design of an effective control strategy. To prescribe a solution, the process control engineer must analyse all variables, process dynamics and unit interactions. First principle models (steady state and dynamics) provide a better understanding of the process dynamics and its interactions, enabling engineers to evaluate and tune strategies before implementation. Using dynamic simulation early in the design phase can help identify important operability and control issues and influence the design accordingly.

Compressors used to increase the pressure of a gas are necessary in many process designs. They are highly specific pieces of equipment and often custom designed and, therefore, expensive to purchase. Compressors operate with fast dynamics, meaning that a small disturbance can disrupt a compressor away from steady state performance very quickly.

When steady state performance is interrupted, a compressor can experience a phenomenon known as surge. This can lead to malfunctioning or, worse, induce major compressor damage requiring equipment replacement. In addition, a compressor surge can harm individuals working near compressors or release noxious gases into the surrounding atmosphere.

Using dynamic simulation can help to predict compressors’ deviation in behaviour from steady state that could cause compressor surge. Dynamic control schemes can also be modelled to limit steady state deviations if a disturbance is experienced in the plant.

The first step in any dynamic simulation is obtaining a steady state model. A steady state compressor model shows undisturbed compressor operation or, in other words, how the compressor would ideally operate if uninterrupted, given the process parameters. To generate the steady state compressor model in process simulation, the inlet streams of the compressor model must be defined, as well as one of three compressor specifications: the change in pressure across the compressor, the compressor’s pressure ratio or the energy supplied to the compressor.

Model based operations improve decision support and safety
Compressors are vital to gas processing and similar operations, so it is important that models include a built-in response to potential disturbances, such as a compressor surge. Dynamic compressor models can optimise design and operational performance to achieve the following:
• Analyse a range of process disturbance scenarios
• Add a proper control scheme to appropriately respond to process disturbances
• Perform process safety studies
• Finely tune process parameters to achieve improved production results.

Process engineers use AspenTech’s aspenONE solutions to achieve model based operations for decision support and safety. In particular, Aspen Hysys Dynamics extends Aspen Hysys steady state models into dynamic process models, enabling design and verification of process control schemes, safety studies, relief valve sizing and rating, failure analysis and development of start-up, shutdown and operating mode changes. Engineers can create steady state and dynamic models all within the same simulation environment, which saves significant time and effort (see Figure 1).

Extensive numerical and graphical results are generated for dynamic models in Aspen Hysys Dynamics, which allows for in-depth analysis to be performed on each completed simulation (see Figure 2).

Many compressor disturbances can be explored, including decreasing or increasing the feed rate to the compressor and modifying the composition or physical properties of the compressor feed stream, among others. Control schemes and parameters can also be modified, yielding more or less robust compressor control, depending on the user’s requirement. It is a powerful tool that can be utilised to ensure safe and proper functioning of a process’s compressors.

Engineers can broaden their ability to design safer, more operable plants, without over-designing, by learning to do the following:
• Use data visualisation to help optimise performance
• Implement and test control schemes
• Schedule events to study safety scenarios, start-up and more
• Switch steady state models to dynamic mode for greater accuracy and flexibility.

Reported benefits using dynamic modelling can generate $15 million savings through improved and faster start-up procedures. Also, the avoidance of over-designing relief systems can achieve $10 million in capital cost savings, improved safety through better operational procedures, better control system design and proper relief-valve sizing delivers enormous benefits. Operators can achieve better design decisions through detailed analysis of the trade-offs between process operability and process integration.

Safety and controllability mitigates equipment malfunction
By understanding dynamic plant behaviour, many production processes in the oil and gas, energy and chemicals industries will experience significant improvements in control and safety. Knowing the dynamic response of a compressor is important when designing or operating a process, and engineering dynamic software is a tool that allows users to perform dynamic simulations to obtain this information. Engineering modelling software provides multiple outlets to view and tabulate the dynamic response for equipment and controllers, including strip charts, tables and compressor curves. When Hysys and Hysys Dynamics are properly utilised conjointly, compressor surge can be avoided. Therefore, through better understanding of plant behaviour, companies will save equipment costs.
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