What is surging?
Surge is technically an unstable condition of compressor that causes the erratic compressor performance. Every axial and centrifugal compressor will experience the phenomenon called surge when the flow through the compressor drop below a certain level.
How Surging affects Compressor and it’s consequences?
During Surge momentary flow reversal takes place, This results in a very fast pulsating flow back and forth through the compressor interiors, resulting in severe and consequent damage.
Consequences:
- Unstable flow and pressure resulting in the process upset
- Damage in sequence with severity to seals, bearings, impellers, shaft
- Increased seal clearances and leakage
- Lower energy efficiency
What is antisurge control?
The surge control is to prevent surge. The task of the antisurge system is to detect and automatically compensate for the potential disturbance by maintaining flow through the compressor above the surge condition.
The anti-surge system consists of a surge controller, an anti-surge valve and a variety of additional accessories.
Working of an antisurge control:
The figure shows a typical anti-surge control schematic.
The surge controller consists of a temperature (TT), flow (FT) and pressure (PT) upstream of the compressor, the head across the compressor (DPT) and the compressor speed (ST).
The surge controller will operate the valve as close as possible to the surge line to maximize the efficiency of the compressor and gas turbine without compromising the compressor’s performance.
The anti-surge controller will send a signal to the anti-surge valve to open the required set point in case of a potential surge condition to pass a given amount of flow back to the compressor inlet. Depending upon the process upset, this may require a small amount of valve opening or full valve opening.
If the valve has to go wide open, in less than two seconds it has to do so. If the valve does not open quickly, additional surges may be exposed to the compressor.
The valve must have sufficient capacity to pass the amount of flow required to protect against an event of surge. Because the compressor inlet and outlet have a high differential pressure (1000 psi or higher).