TURBOCOMPRESSOR MODELING AND SURGE AVOIDANCE

The different types of aerodynamic flow instabilities are classified as follows (for a centrifugal turbocompressor, frequency ranges based on some case studies):

• Deep surge: Deep surge cycle is associated with reverse flow over part of the cycle. Frequency of deep surge is approximately 5-10% of the rotor rotating frequency

• Classic surge: Classic surge frequency lies around 10-14% of rotor rotating frequency

•Mild surge:Mild surge exists in a relatively narrow frequency bandwidth (roughly 14-16% of rotor rotating frequency). Mild surge easily transforms into other types of surge by throttling the compressor to lower mass flows

• Rotating stall: Rotating stall covers around 20-80% of rotor rotating frequency.

Surge is a phenomenon in which the gas current may reverse its movement periodically. It is characterized by large amplitude fluctuations of the pressure and by unsteady flow. At flows lower than the stability limit (surge limit), a turbocompressor initially shows a reduced capability to generate head.

This results in a backflow. During a surge, once the pressure at the discharge system is reduced sufficiently, the turbocompressor will recover and move gas again from the suction to the discharge side. Unless action is taken, however, the events repeat.

Ongoing surge can damage centrifugal compressor components (due to the massive change of thrust loads) particularly thrust bearings, seals, and eventually overheat the compressor. The speed of pressure variation (and the damaging effect of the surge) depends largely on the size of the effective gas volume (gas that participates in the surge event) downstream of the compressor.

The modeling of the compression system with a lumped parameter approach is widely accepted in literature and industry. The simulation target is generally to model rapid trips of turbocompressors (or similar emergency events, such as sudden closure of themain suction or discharge valve) in order to avoid energetic, potentially damaging surge events.

The anti-surge (recycle) valve, its controller, the suction and discharge piping, after-coolers, operating conditions, and various control- and valve-response times are important characteristics that govern whether a turbocompressor will reach its surge limit at a detrimental, high-head condition or at a more satisfactory low-head condition. Particularly the main factors include: Anti-surge (recycle) valve size, valve opening times, discharge gas volume and machine operating point (immediately before the trip).

The experience gained in various plants leads to the following practical notes:

1. The use of larger and faster opening anti-surge (recycle) valves will lower the energy of a surge at the time of a turbocompressor trip

2. The location of the compressor operating point on its operatingmap when a sudden trip occurs affects how rapidly and how energetic a shutdown surge will be. Operating points near the surge limit usually result in more energetic surge events compared to operating points away from the surge line

3. A more rapid speed reduction (or fast coast-down) for a turbocompressor can result in a more energetic shutdown surge compared to a slower coast-down

4. In high energy, complex,multi-section turbocompressor systems, vent valve(s) to remove high-pressure gas from the discharge piping system can be effective in reducing the surge energy

5. Although not a common practice, a short delay of 1 second (or sometimes 2 seconds) in removing power from a turbocompressor in the event of a shutdown condition allows anti-surge control actions (such as opening the recycle valve, vent valve, and so on) to begin in advance.

6. Discharge check valves could be effective in isolating upstream volumes of high pressure gas, thus allowing the energy of a subsequent surge to be reduced.

Dynamic simulation methods should be used to determine whether the combination of discharge volume and anti-surge valve size and selection can prevent the compressor from a surge during a trip.

Low-pressure-ratio compressors often do not require after-coolers (particularly low pressure ratio in pipeline services) or sometimes are designed with small and undercapacity coolers.

There are three primary strategies that could be employed to avoid overheating an under-cooled turbocompressor during a startup:Accelerate quickly; delay hot gas reentering the compressor; throttled recycle.

Usually the effort on all sides (vendor, contractor and client) to provide the input information for dynamic simulation is underestimated. The reliability of the dynamic simulation results most often suffers from incomplete and inaccurate information (particularly when simulation should be done at an early stage of a project). These effects of inaccurate data should be properly evaluated by a suitable sensitivity analysis.

The post-surge behavior is usually not important and only simple assumptions are introduced to keep the numerical dynamic model stable.

Author

Amin Almasi is a registered professional engineer in Australia and Queensland (M.Sc. and B.Sc. in mechanical engineering). He is a consultant specializing in rotating equipment, condition monitoring and reliability.