Surge and Compressor Choke in Compressor Control System

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Surge and Compressor Choke in Compressor Control System

What is Surge?

Since the fundamental purpose of any compressor control is to prevent or limit the effects of surge, it is appropriate to review the phenomenon itself. Surge occurs when the low flow operation limit of a compressor has been exceeded, resulting in flow reversal.
It is an unstable, pulsating condition that is usually evident by an audible boom, piping vibration, rapid increase in discharge temperature and oscillation of flow and discharge pressure. Violent surging may cause the following compressor damage:
• Open internal clearances which damage impeller seals and balance piston seals.
• Damage the compressor shaft end seals.
• Damage the compressor thrust bearings.
• Damage the compressor radial bearings.
• Cause impellers to rub against stationary diaphragm.
• Cause a shaft coupling failure.
• Possible shearing of drive shaft.
Along with compressor damage, the process flow and pressure can become very unstable contributing to upstream and downstream process upsets.

Surge and Compressor Choke in Compressor Control System

Figure shows a simple surge cycle at a constant speed and constant suction pressure. At point 1 of surge cycle we have low discharge point and high flow rate. As the system resistance increases (e.g. discharge valve closes, downstream processes shutdown or decrease load, series units drop off-line, or parallel units come on-line), the compressor flow decreases, and discharge pressure increases.

At point 2 of surge cycle is very near to surge limit and here flow countinues to decreas and discharge pressure will be increase. At point 3 of surge cycle finally stage comes where by due to system resistance compressor can not increase discharge pressure.

If the system resistance increases further, the discharge pressure becomes greater than the machine’s capability. This initiates a surge that spans between points 3 and 4. Flow may actually reverse through the compressor, as shown at point 4. A now reduced system resistance will allow increased flow back through the compressor that brings the operation back to point 2.

Maintaining flow above the compressor’s surge limit prevents these surge conditions. The compressor controller must monitor the operating point and compare it to the standard surge limit of the compressor. When we have minimum flow, then open anti surge control valve by controller, thus we can increase flow and decrease the discharge pressure and polytropic head. We get our operating point is getting away from surge limit.

What is Compressor Choke

Definition
Compressor choke is an undesired operating condition for centrifugal compressor. Choking of centrifugal compressor occurs when the compressor is operating at low pressure ratio resulting in high flowrates. Similarly to valves the phenomena is characterized by the fact that any further decrease inoutlet pressure will not lead to increase in compressor throughput. This phenomenon is also known as stonewalling of a centrifugal compressor.

How does a compressor choke?

Stonewall or choke point for a centrifugal compressor occurs when the resistance to flow in the compressor discharge line drops significantly below the normal levels. Due to low resistance, compressor overall compressor ratio across the compressor is significantly low. As suggested by the compressor maps for a fixed RPM value, compressor output increases as the backpressure at compressor discharge drops down. This leads to increased gas velocity in the centrifugal compressor. The increase in gas velocity can occur up to sonic condition. When the gas velocity in any of the compressor parts is about sonic velocity (MACH=1), no further gas speed increase is possible, hence resulting in stonewall (choke) conditions for compressor operation. On the compressor manufacturer performance curves the condition can be represented by a nearly vertical drop of compressor curves, catching the idea that once choke conditions are achieved any further drop in compressor ratio will not result in further increase of compressor flow rate..

How does a compressor choke?

Damages due to compressor choking

Prolonged operation of a compressor at its choke point can lead to damaging the compressor parts. Compressor choking is not particularly damaging to single-stage centrifugal compressors but can cause serious damage to the rotors and blades of multistage centrifugal and axial compressors.

How to prevent compressor choking

To prevent the compressor choke or stonewall it is required to enforce a certain level of flow resistance in the compressor discharge line. Anti-choke valves are usually placed to this aim on the compressor discharge downstream AS loop and associated check valve. When flow resistance in compressor outlet is low and flow approaches choking conditions, the anti-choke valves close to maintain the required minimum pressure ratio across compressor casing.

How to prevent compressor choking

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