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By Mo Javadi, Principal Engineer, and Thomas Olsen, Managing Director of Lagoni Engineering

Functional safety oriented compressor surge modelling

S tatic and dynamic modelling of large centrifugal gas compressors (> 10MW) has been used in the oil and gas and energy

industry to provide a higher degree of confidence in the design parameters. Models are often constructed with a certain aim such as process design assurance, venting calculation, speed controller tuning and such, thus seldom provide a true representation of the full dynamic process.

During an independent design review of a centrifugal compressor protection system, we have stumbled upon the age-old question of identifying the ‘worst-case’ compressor surge scenario for a typical gas transmission compressor. The evaluation of this problem

provides the means to calculate the process safety time for the compressor surge scenario. The process safety time must be captured in the safety requirement specification (SRS) as a requirement of the safety lifecycle design as per standard IEC/EN 61511. Protection of environment is in the scope of IEC 61511, however the environmental impacts of compressor surge are not considered in this study.

This paper aims to shed light on plausible surge scenarios for a gas compressor station and seeks to quantify which event requires the fastest surge protection system response time. Moreover, this model will serve to satisfy the functional safety requirements by evaluating the surge event process safety time.

COMPRESSOR SURGESA compressor surge is defined as the complete disruption of the flow to the compressor causing reverse flow and pressure pulsation in the compressor. Continuous surge pulsations will lead

FIGURE 1: Compressor’s operating envelope with speed lines

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to seal damage and can lead to a hazardous event.

Although compressor surge control and protection is not always a functional safety issue, depending on the outcome of hazard analysis and corporate tolerable risk criteria, compressor surges may be a process safety issue.

However, the application of the IEC 61511 lifecycle ensures formalisation of engineering the requirements around worst case scenarios, process safety time and safety response times. This provides an added level of assurance and aids in specification of the requirements for design, implementation and testing.

AspenONE HYSYS has been used to model an electric variable speed drive (VSD) compressor for this study. AspenONE HYSYS is a process modelling tool developed by Aspen Technology Inc that is used primarily in the oil and gas sector. While this study is based on a VSD, surge control will be considered for both VSD and turbine driven compressors. The simulation results have been validated using real plant data.

To better understand surge scenarios, it is essential to understand the turbomachinery affinity laws. The following three relationships are highlighted here to provide a basis for evaluating the surge scenarios:

1. Flow rate is proportional to the cubic root of the differential pressure across the compressor.2. Compressor pressure rise is proportional to the square of its angular speed.3. The flow rate at which surge occurs is proportional to the compressor speed.

The following scenarios are considered as having the greatest potential for causing surge in this compressor as process upsets upstream and downstream would have a lower impact due to the large volume of gas. Plant safety systems should prevent any of the following scenarios taking place. However, to calculate the process safety time, the surge protection is not modelled in this stage of the simulation.

Scenario 1 Undetected closure of the discharge valve: causes an increase in the compressor differential pressure. As the discharge pressure

builds up, as dictated by the formula above and the compressor’s polytropic behaviour, it results in surge and flow reversal inside the compressor.

In this model, the discharge valve has an equal percentage profile. This means that as the valve begins to close (typically at a nominal rate of one second per inch of valve) the flow does not change noticeably until the last few seconds, when the flow suddenly drops.

Scenario 2 Undetected closure of the suction valve: causes an increase in differential pressure across the compressor, eventually resulting in surge. In this model, the suction valve also has an equal percentage characteristic, meaning that closure of

the valve restricts flow significantly at the end of the travel.

Scenario 3 Rapid ramp-down of the compressor: The compressor’s maximum ramp down rate is fixed by the drive. Continuous rapid ramp-down of the compressor causes a sudden increase in the compressor differential pressure, forcing its operating point toward surge.

Scenario 4 Shutdown of the compressor drive: The outcome of this scenario depends greatly on the type of the drive. In electrically driven compressors, where the drive is coupled to the compressor, the high inertia of the drive will bring the compressor to a stop without causing a surge. In turbine driven compressors,

FIGURE 2: Modelled compressor train

FIGURE 3: Rate of change of flow vs valve closure

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The response time observed provides the basis for engineering the anti surge protection system

FIGURE 4: Comparison of surge scenarioswhere the drive and the compressor are not coupled together, stopping of the drive will cause a faster ramp-down of the compressor compared with a directly coupled machine.

STUDY RESULTSThe scenarios are developed based on ‘normal’ operating parameters, that is the compressor operating at a specific speed and efficiency deemed acceptable for this station. Data collected from the model are normalised and overlaid for comparison in Figure 4.

It is evident from the results that, for this modelled process, undetected closure of the suction valve would move the compressor’s operating point towards surge faster than all other scenarios. This would define the process safety time, measured at just over six seconds. This provides a basis for selecting the instrument for detecting the initiating event, determining the opening speed of the anti-surge valve (ASV) and help in tuning the anti-surge controller. The controller’s scan time (typically 50ms to 250ms for an industrial PLC) and the instrument detection time provide the least impact on the anti-surge system’s ability to prevent surge.

The process safety time will depend on the distance of the operating point from the surge line. Therefore, selection of the compressor and the operating regime are vital in maximising the process safety time.

The following variables are noted as having the greatest effect on the safety response time but will not have an effect on the process safety time. This is not an exhaustive list and all components affect this:● Flow coefficient Cv of the anti-

surge valve: the higher the value, the faster the surge could be avoided by opening the anti-surge valve.

● Volume of the recycle path: the higher the volume, the faster the process restores.

● Distance from compressor discharge to recycle path: the shorter the distance the faster the response of the process to opening of the ASV. The shorter the distance the less time the system has to recycle as the gas temperature rises.

● Opening speed of the ASV: the opening speed of the ASV (fast acting actuator at just under two seconds in this study) is the weakest link in the response time of the anti-surge system. A faster valve opening speed relates directly to the ability to avoid a surge scenario.

The following items must be considered in the design of an anti-surge controller:● The surge control line’s distance

from the surge line● Provision of an adaptive surge

control● The surge control backup line’s

distance from the surge line● PID tuning parameters● IEC61511/ IEC61508 conformance

(if required)

As part of these considerations, the design of the surge control line and algorithm is based on maximising the

available operating envelope, while ensuring the safety of the system.

CONCLUSIONThe design considerations discussed in this paper are seen as a good method for providing assurance in the design of an anti-surge system for a specific process. By taking the approach to analyse the control, process and mechanical engineering constraints within the same dynamic model, it is possible to understand each discipline’s impact on functional safety requirements.

The response time observed provides the basis for engineering the anti surge protection system. It also provides a better understanding of likely test cases and test methods that could be designed to validate the system.

Based on this study, it is deemed that the worst-case scenario for causing surge in this compressor is uncontrolled closure of the compressor suction valve. In conclusion, the variables impacting these results are plant-specific and driven by the process and mechanical design. ■

■ Lagoni Engineering is an engineering consultancy specialising in control system engineering. For more information visit www.lagoniengineering.co.uk

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Functional safety oriented compressor surge modelling

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