BACK PRESSURE CONTROL ON AN ALTERNATOR BASED SYSTEM
Backpressure control for an Alternator based system consists of an Inlet control Valve, which does the speed governing and a Vent Valve, which is used to control the backpressure to the process.
The inlet control valve takes the speed sense from the output shaft of the turbine and through the PLC maintains the speed of the alternator. If the alternator speed drops due to Load then the Inlet Valve gets a signal from the PLC to open and allow more steam. The Vent Valve takes signal from the outlet process line and through the PLC controls the back pressure to the process by opening or closing to vent out steam.
A schematic of the full operating system for an alternator based control for backpressure is depicted below for clarity.
The following cases are possible for such a system as shown in the table
Case Process steam reqmt. Power Reqmt. Remarks
1 High High Design condn. Valves normal
2 High Low Steam valve Bypass opens
3 Low High Vent Valve opens
4 Low Low Valves normal
Case 1: In this situation the process steam requirement is high and power requirement is also high i.e. design condition. In such a case the Inlet Valve would be open as per design i.e. 70-80% open and vent valve would be open.
Case 2: In case the process steam requirement remains high but the power requirement comes down, the speed would tend to go up, signalling the Inlet Valve to close. This would then go to a situation as in Case 4 and make up the balance steam through a bypass PRV
Case 3: In case the process steam requirement is low and the power requirement is high, in such a case the back pressure would increase causing the vent valve to open and let the steam out to maintain the back pressure.
Case 4: When the steam requirement as well as the power requirement is low, the steam would be throttled by the Inlet Control valve to maintain the speed due to lower power and the vent valve would therefore be throttled also.