A surge control system should be implemented according to the user requirements and the manufacturer specifications. The surge control system performance should protect the compressor system over a range of “high risk” conditions, which may encompass the entire range or a subset of the operating range of the compressor.
1. Design to Avoid Surge: The philosophy requires control system design criterion based on a calculated allowable discharge system volume. The allowable discharge piping volume should be determined by simple or more complex transient models of the compressor system.
2. Design to Permit Surge Under Specified Conditions: The design philosophy acknowledges that due to operational changes to the compressor station or cost-based decisions, the compressor may not be fully protected by the existing surge control system. The criteria require that the surge control system function to avoid surge at the identified “high risk” or “high energy” conditions. Evaluation of the surge control system using a simple transient model is recommended to evaluate the occurrence of surge against the design criteria. An example of suggested evaluation criteria is provided in the guideline.
3. Design Based on Risk Evaluation: The surge control system is evaluated against a set of risk factors developed for a particular compressor and dynamic simulation is not necessarily required because of previous modeling efforts or experience. These type of economic and risk based analysis models can be used to determine the overall design and operational risk of the surge control systems within the compressor station. The models must be based on known risk probabilities, safety factors/requirements, and equipment repair/replacement costs. As this information is specific to an individual operator and site, the risk-based evaluation must be customized for the application.
Surge is defined as the operating point at which the compressor peak head capability and minimum flow limit are reached. The compressor loses the ability to maintain the peak head when surge occurs and the entire system becomes unstable. Under normal conditions, the compressor operates to the right of the surge line. However, as fluctuations in flow rate occur, or under startup / emergency shutdown, the operating point will move towards the surge line because flow is reduced. If conditions are such that the operating point approaches the surge line, the impeller and diffuser begin to operate in stall and flow recirculation occurs. The flow separation will eventually cause a decrease in the discharge pressure and flow from suction to discharge will resume. This is defined as the surge cycle of the compressor.
The surge cycle will repeat itself unless control systems are installed or operational changes are made to bring the compressor out of the surge cycle. The surge cycle may result in a small or large flow reversal period depending on the discharge gas volume and the pressure ratio. Chronic surge is characterized by intermittent periods of small flow reversal that may not cause severe damage to the machine. Acute surge is more pronounced, usually due to a rapid transition across the surge line. Any surge event can cause severe damage to the thrust bearings, seals, and the impeller. The extent of the damage due to surge occurrence is somewhat a function of the compressor design.
A surge control system should be capable of monitoring the operation of the compressor continuously. The function of the surge control system is to detect the approach to surge and provide more flow to the compressor through opening the recycle valve to avoid the potentially damaging flow reversal period and surge cycling.
The surge control system should be designed for the three surge environments (which may have competing demands) and the compressor operating parameters as well as manufacturer specifications.