SURGE CONTROLS FOR CENTRIFUGAL COMPRESSORS
IN LOAD SHARING MODE
- Client has highly varying flow requirement of Nitrogen at @ 30-32 bar pressure in their process. The
varies from 25% to 100% and yet did not want any stoppages in their process with a possibility of going
to 130% as well at times.
- After discussion with the Compressor OEM, they decided to purchase 3 compressors of equal capacity.
- The compressors will be started, stopped, loaded and unloaded as per the process flow requirement.
- To have more flexibility for future modifications, the client requested to have a PLC based control for
Compressor rather than an OEM standard controller.
- Criticality of the process in the plant demanded a separate control system for each compressor to avoid
plant stoppages arising out of non availability of the control system.
- The client requested for a SCADA system common between the 3 Compressor Controls and placed in the
DCS room. They also requested for a common HMI operable from near the compressor for operator
- Generally a separate controller is required to execute the common load sharing logic.
- Due to space constraint and to avoid investing in and maintaining one more control system, having this
separate control system was not acceptable to the customer.
- How to execute a common load sharing logic amongst all the compressor control systems?
- Which one of the control systems should be the one to decide the execution of load sharing logic?
- What if the system which is executing the load sharing logic develops a fault?
- If all the systems are to execute load sharing logic, which one’s output should be considered?
- After a lot of brain storming within the team, a separate detailed SOP was formed specifically for the
operation of the load sharing logic.
- All three systems needed to have the capability to execute the load sharing logic since any one of the 3
could be taken out for maintenance.
- One of the 3 systems was to be selected as master while the others shall have a dormant load sharing
- The master controller decides which controller should increase / reduce the inlet valve opening so that
all the compressors share equivalent opening of the bypass valves as well as inlet valves.
- All the three control systems needed to be in constant communication with each other to know the health
of all the three control systems as well as the compressors.
- Our team developed the logic to select the master controller for load sharing considering the
availability of each compressor, its related control system and the number of running and loaded hours
each compressor has clocked.
- The transition from one master to another was developed to be so smooth that the operator does not feel
any shift in the operation.
All the three compressors were commissioned with the only operator input required being the start button.
The complete start up, stop, loading and unloading of the compressor was automated and presented to the
client without glitches.
All compressors shared the load equally all through out the 72 hour run maintaining the process flow to the
A great hand over work by our team members to the client with necessary trainings and briefings for their
further operation without any further assistance required from our engineers.