Introduction
Several refining process units, such as Hydrocrackers and Hydrotreaters, include at least two parallel Makeup Gas Compressors (often abbreviated MGC) that boost makeup hydrogen (H2) from battery limits pressure to reactor pressure. The MGCs are positive displacement reciprocating compressors due to the low molecular weight and the high pressure ratio required. Two MGCs are usually needed to provide backup when one of the MGCs is taken out of service for repair. Each MGC can be operated while the other one is down, or both MGCs can be operated at the same time. Although the last scenario is rare and not desirable, the controls should be designed to support any of the three operating scenarios.
Each train consists of two stages with spillback valves for each stage. The spillback valve recycles discharge gas back to the suction to control throughput. Throughput can also be controlled incrementally with clearance pockets that allow the operator to set the capacity at various points, such as zero, 50, 90, and 100%. The board operator can change the capacity up or down via load and unload buttons.
The purpose of this functional specification is to describe the various controllers and strategies employed to control the MGCs. Figure 1 is a simplified diagram of the controls for a single MGC (Note: Intercoolers and suction drums are not shown).
FIRST STAGE CONTROLS
MGC First Stage
The suction pressure is an important variable for proper compressor operation. It can be controlled in three different ways:
- Bring H2 into the suction via controller PC-001A.
- Flare H2 via PC-001C.
- Recycle H2 from the 1st stage discharge back to the suction via spillback controller PC-001B.
Figure 1 shows the following 1st stage controllers:
- H2 makeup controller PC-001A.
- High Suction Pressure Controller PC-001C.
- Low Suction Pressure Override Controller PC-001B.
- Interstage Pressure Controller PC-002A.
- First Stage Spillback High Selector PY-001.
- First Stage Spillback Valve Controller HC-001.
H2 Makeup Pressure Controller PC-001A
Compressor operation is much more efficient when the spillback valves are closed, so the preferred method of controlling the 1st stage suction pressure is via PC-001A throttling the inlet valve.
Setpoint
- can be changed in any mode.
- does not track the measurement in manual mode.
Modes
- normal mode is automatic/local (locked into local).
High Suction Pressure Controller PC-001C
Pressure controller PC-001C adjusts the flare valve in the event of high suction pressure. Normally, the flare valve should be closed.
Setpoint
- must be set higher than the normal operating suction pressure to prevent opening the flare valve prematurely.
- can be changed in any mode.
- does not track the measurement in manual mode.
Modes
- normal mode is automatic/local (locked into local).
Low Suction Pressure Override Controller PC-001B
When demand for makeup H2 exceeds supply, the inlet valve will become wide open. In this event, the spillback valve must be used to prevent the 1st stage suction pressure from falling.
When the suction pressure gets down to the setpoint, the override controller PC-001B opens the spillback valve to dump higher-pressure H2 from the interstage into the suction. PC-001B output goes to high signal selector PY-001 before going to the spillback valve via HC-001.
Setpoint
- must be set lower than the normal operating suction pressure to prevent opening the spillback valves prematurely.
- can be changed in any mode.
- does not track the measurement when in track mode.
Output
- tracks the 1st stage spillback valve position when the spillback valve controller HC-001 is in manual.
Modes
- locked into automatic/local.
Note: If the operator wants to adjust the spillback valve in manual, then he must put HC-001 in manual and adjust its output.
Interstage Pressure Controller PC-002A
Interstage pressure controller PC-002A adjusts the 1st stage spillback valve through high selector PY-001 to control the interstage pressure.
Setpoint
- should be set to the desired interstage pressure setpoint.
- can be changed in any mode.
- does not track the measurement when in track mode.
Output
- tracks the 1st stage spillback valve position when the spillback valve controller HC-001 is in manual.
Modes
- locked into automatic/local.
Note: If the operator wants to adjust the spillback valve in manual, then he must put HC-001 in manual and adjust its output.
First Stage High Selector PY-001
The 1st stage spillback high selector PY-001 allows either the low suction pressure override controller PC-001B or the interstage pressure controller PC-002A to adjust the 1st stage spillback valve, depending on which controller requires the valves open the most.
First Stage Spillback Valve Controller HC-001
In automatic mode, the 1st stage spillback valve controller HC-001 sets the 1st stage spillback valve to the output of high selector PY-001. In manual mode, the operator can set the valve position. PC-001B and PC-002A outputs track the spillback valve position when HC-001 is in manual.
SECOND STAGE CONTROLS
MGC Second Stage
Figure 1 also shows the following MGC 2nd stage controllers:
- Discharge Pressure Controller PC-003.
- Low Interstage Pressure Override Controller PC-002B.
- Second Stage Spillback High Selector PY-002.
- Second Stage Spillback Valve Controller HC-002.
Discharge Pressure Controller PC-003
Discharge pressure controller PC-003 adjusts the 2nd stage spillback valve through high selector PY-002 to control the discharge pressure.
Setpoint
- should be set to the desired discharge pressure.
- can be changed in any mode.
- does not track the measurement when in track mode.
Output
- tracks the 2nd stage spillback valve position when the spillback valve controller HC-002 is in manual.
Modes
- locked into automatic/local.
Note: If the operator wants to adjust the spillback valve in manual, then he must put HC-002 in manual and adjust its output.
Low Interstage Pressure Override Controller PC-002B
Override controller PC-002B adjusts the 2nd stage spillback valve to protect the interstage from low pressure. When the pressure gets down to the setpoint, this controller opens the spillback valve to dump higher-pressure H2 from the discharge into the interstage. PC-002B output goes to high signal selector PY-002 before going to the spillback valve controller HC-002.
Setpoint
- must be set lower than the normal operating suction pressure to prevent opening the spillback valves prematurely.
- can be changed in any mode.
- does not track the measurement when in track mode.
Output
- tracks the 2nd stage spillback valve position when the spillback valve controller HC-002 is in manual.
Modes
- locked into automatic/local.
Note: If the operator wants to adjust the spillback valve in manual, then he must put HC-002 in manual and adjust its output.
Second Stage High Selector PY-002
The 2nd stage spillback high selector PY-002 allows either the low interstage pressure override controller PC-002B or the discharge pressure controller PC-003 to adjust the 2nd stage spillback valve, depending on which controller requires the valve open the most.
Second Stage Spillback Valve Controller HC-002
In automatic mode, the 2nd stage spillback valve controller HC-002 sets the 2nd stage spillback valve to the output of high selector PY-002. In manual mode, the operator can set the valve position. PC-002B and PC-003 outputs track the spillback valve position when HC-002 is in manual.
ADVANCED REGULATORY CONTROLS
MGC Capacity Control
As stated in the Introduction, throughput can be controlled incrementally with clearance pockets that allow the operator to set the capacity at various points, such as zero, 50, 90, and 100%. Whenever the spillback valves, especially the 2nd stage spillback, get too far open, then the capacity setting is too high and should be reduced. Likewise, when the spillback valves are too far closed, then control is about to be lost and the capacity setting should be increased.
Normally, setting the capacity is the duty of the board operator. However, this task can be automated via advanced regulatory control. The logic must consider the possibility of “ringing”, which means rapidly changing the output based upon fluctuations in the input. A delay in the control action must be incorporated to prevent this undesirable situation.
MGC Capacity Feed-Forward
Any change in the capacity setting will disturb the MGC pressure controls. Therefore, feed-forward control should be implemented to automatically adjust the spillback valves to offset the effects of the capacity change. For example, when the capacity is increased, the spillback valve positions should be increased in anticipation of the imminent rise in both the interstage and discharge pressures. Likewise, when the capacity is decreased, the spillback valve positions should be decreased.
Feedforward techniques are discussed in a separate SAS Functional Specification.