Do not implement a generic PI loop until the exact Grasso compressor capacity mechanism is identified. The available evidence does not specify the compressor type, controlled process, actuator, permissives, or whether capacity control is continuous or stepped. Use the SLC 500 as a supervisory sequencer only after the compressor documentation defines the approved commands and protections.
Establish the Control Boundary
First determine whether the compressors use infinite capacity control or step capacity control. Also verify whether the process is compressed air, refrigeration, or another service; the evidence mentions both air-compressor practices and ammonia-compressor experience but does not identify the actual application.
Obtain the manual for the exact compressor type and document:
- Available remote commands, including start/stop and load/unload.
- Whether a turn valve, poppet valve, inlet valve, or approved variable-speed interface exists.
- Command feedback, running status, capacity position, and fault indication.
- Required start, stop, loading, unloading, and restart restrictions.
- Protections that remain under the compressor controller instead of the SLC 500.
Do not infer these interfaces from the manufacturer name alone. The evidence describes possible screw-compressor methods, not confirmed features of either Grasso unit.
Select the Capacity-Control Method
| Method | Supported operating description | PLC decision |
|---|---|---|
| Motor start/stop | Stops or starts the compressor motor. | Use only within the manufacturer-defined cycling and restart constraints. |
| Load/unload | The motor continues running while the inlet closes and output falls to zero. The cited operating experience reports unloaded power near 25% of full-load power. | Account for delayed unloading; the cited range is about 20–30 seconds, although some machines may require minutes. Verify the actual Grasso timing. |
| Turn or poppet valve | Bypasses part of the flow toward the inlet to modulate capacity. | Use PI only if the exact actuator accepts a continuous capacity demand and provides the required feedback. |
| Inlet throttling | Modulates capacity through an inlet valve. | Confirm the permitted operating range and whether the local controller coordinates throttling with unloading. |
| Variable-frequency drive | Changes compressor speed. | Use only with manufacturer approval and defined speed, lubrication, cooling, and protection limits. |
| Demand-side valve | Regulates downstream pressure while receiver pressure varies. | This can decouple compressor sequencing from distribution-pressure regulation. |
The cited general operating ranges are 60–100% capacity for inlet throttling, 50–100% for a turn valve, and 30–100% for variable-speed control. They are not confirmed Grasso limits. The same evidence reports an approximately 5% efficiency penalty associated with a VFD and suggests turn valves can be more efficient above 50% demand, with inlet throttling becoming competitive above roughly 70–80%. Treat these figures only as screening criteria until the exact compressor performance data are available.
Decide Whether PI Control Applies
PI control is appropriate only when the controlled variable and final control element are both defined. A pressure or process measurement can feed a PI calculation, but its output must drive an approved continuously variable capacity command. A PI output must not directly pulse motor starts or load/unload commands.
For step-controlled machines, use pressure bands, staging logic, and timers instead of treating the discrete commands as an analog actuator. For continuously modulating machines, constrain the PI output to the manufacturer-approved capacity range and coordinate the lower limit with unload or shutdown logic. The receiver and distribution piping provide process storage and damping, so tune against the measured system response rather than an assumed fast response.
Sequence Two Compressors in the SLC 500
- Collect the exact interface definition for each compressor, including commands, feedback, trips, capacity method, and required timing.
- Assign one compressor as lead and the other as lag. Add rotation only after confirming that both machines can satisfy the same operating role; different sizes or pressure duties may require a fixed sequence.
- Start and load the lead unit when demand requires capacity. Add the lag unit only when the lead unit cannot maintain the target within its permitted capacity range.
- When demand falls, remove lag capacity before reducing the lead unit below its approved modulation range. Select unload or stop according to receiver storage, demand variation, and the manufacturer’s cycling restrictions.
- Prevent simultaneous controllers from independently correcting the same pressure. Use one supervisory demand calculation, then allocate capacity through staging and the supported actuator commands.
- Retain compressor-specific shutdown protection in the approved compressor control system. The SLC 500 should remove commands and report faults, not replace undocumented protective logic.
Commission and Verify the Sequence
Test each command individually before enabling automatic sequencing. Confirm command-to-feedback correspondence, correct lead/lag selection, and safe behavior after a fault or loss of feedback. Trend the controlled pressure, capacity command, running state, load/unload state, and fault state during demand changes.
Measure the actual unload response instead of programming the cited 20–30-second observation as a fixed machine value. Verify that staging does not cause rapid motor cycling, repeated load/unload transitions, or two capacity controllers competing against each other. Final acceptance requires stable control across the expected demand range and compliance with every timing and operating limit in the exact compressor documentation.
FAQ
Can an SLC 500 PI loop directly control a Grasso screw compressor?
Only if the exact compressor provides an approved continuous capacity command. Use staging and timed state logic for discrete start/stop or load/unload interfaces.
How long does a screw compressor take to unload?
The cited experience reports about 20–30 seconds, with some compressors taking minutes. Measure the actual response and use the exact Grasso manual before setting any SLC 500 timer.
How should two screw compressors be staged?
Run one unit as lead, add the lag unit when the lead cannot maintain the target within its permitted range, and remove lag capacity first as demand falls. Coordinate both units from one supervisory demand calculation.
