Compressor Biannual Preventive Maintenance Inspection
This procedure outlines the biannual preventive maintenance and inspection of trailer-mounted compressor units, including trailer, grounding, fire protection, fluids, electrical panels, compressor components, engine mounts, regulators, and functional safety checks under load. This procedure helps ensure safe, reliable compressor operation by identifying and correcting leaks, mechanical wear, electrical issues, and configuration faults early, while verifying that protective devices, interlocks, and oil analysis are in place to support continued dependable service.
Envent Corporation
12/04/2025
Compressor Biannual Preventive Maintenance Inspection
This procedure outlines the biannual preventive maintenance and inspection of trailer-mounted compressor units, including trailer, grounding, fire protection, fluids, electrical panels, compressor components, engine mounts, regulators, and functional safety checks under load. This procedure helps ensure safe, reliable compressor operation by identifying and correcting leaks, mechanical wear, electrical issues, and configuration faults early, while verifying that protective devices, interlocks, and oil analysis are in place to support continued dependable service.
Unit Information
Compressor Unit
Date
Technician(s)
Starting Hours on Engine
Starting Hours on Compressor
DOT Inspection Completion Date
Power-Off Checks
Trailer Inspection
Visually check tires for proper inflation and dry rotting or damage. Inspect leg stands and grease all Zerk fittings. Inspect trailer air brake system. Inspect brake shoe thickness. Grease slack adjusters and S Cams on air brake system. Check hub oil, refill if needed. Inspect trailer lighting and mudflaps for damage. Inspect trailer suspension for damage.
Grounding & Bonding Inspection
Check all bonding and grounding cables for good wire, proper clamps, and connections. Each unit should have at least one ground and one bonding spool. Test continuity from ground clamp to trailer deck and from bonding cable clamp to trailer deck using ohms meter. Ohms reading should be less than 5.
Fire Extinguisher #1 Visual Inspection (Seal, UL, Damage)
Fire Extinguisher #1 Date Inspection
Fire Extinguisher #2 Visual Inspection (Seal, UL, Damage)
Fire Extinguisher #2 Date Inspection
External Leak Inspection
Without unit running, walk around and inspect the compressor frame, make-up oil (day) tank, engine, cooler and all associated line for fluid leaks. repair as needed.
Compressor Fluid Level Inspection
Without unit running, check the compressor frame oil sight glass and the frame oil level switch sight glass oil level. Both should be between half and 3/4 full when not running. Check the Force Feed lube pump oil box sight glass level. This should be a least 3/4 full.
Engine Fluid Level Inspection
Check the make-up (day) tank oil level. If less than half, tank must be refilled. Check the engine oil level sitch sight glass on the driver side of the engine. On McClung units, the oil level must be completely full. Check the oil dipstick on the driver side of the engine. The oil level on the dipstick must be at least 3/4 up the crosshatch on the dipstick. On the A/G units, check the engine oil level switch sight glass. On the A/G units, this sight glass must be between 3/4 full and full when not running. Check the engine oil dipstick on the driver side. The oil level on the dipstick must be 3/4 up the crosshatch on the dipstick.
Cooler Fluid Level Inspection
Check the coolant levels in the Jacket Coolant tank and Auxiliary coolant tanks on top front of the cooler. Coolant levels must be above the coolant level switches, but no more than 3/4 up the sight glass as to allow headspace for pressure expansion when the unit reaches operating temperatures.
Electrical Panel Inspection
Inspect Electrical Panels (on A/G-Main Panel, On McClung-CAT/Centurion panel and Murphy Panel) for loose wires, loose materials or paperwork in the bottom of the panel and loose or missing wire raceway covers. Perform pull-test on all wires and tighten terminals as needed. Open the CAT HMI panel and inspect for loose wires and materials. Perform pull-test on wires and tighten terminals as needed.
Battery Box Inspection
Open battery box and inspect for loose or damaged wiring and corrosion on terminals.
Component Checks
Compressor Valve Inspection
Remove compressor valve caps, valve retainers and valves. Inspect for visible damage to valve plates and springs. Replace with new valve if damage is found. If no damage is found, clean and reinstall valve as per Ariel procedure. Check valve passages for debris prior to reinstalling valves. Clean as needed.
Compressor Cylinder Inspection
Remove cylinder heads and inspect for cylinder wall damage and proper lubrication. Check for damage on the face of the piston and the face of the cylinder head. Reinstall cylinder head as per Ariel procedure.
Compressor Crosshead / Piston & Rod Inspection
Remove distance piece covers and inspect for debris and/or damage. Inspect crosshead, crosshead nut and babbit for damage. Inspect piston rods for deep scratches or grooving. Inspect piston rod packing for damage to seals and proper lubrication Any repairs must be performed by trained personnel only! If no issues found, reinstall distance piece covers as per Ariel procedure.
Flange Torque Verification
Check for bolt tightness on ALL flanges on the unit and torque as per flange torque specs
Engine Mount Jack Bolt Inspection
Inspect engine mounting jack bolts for tightness. If jack bolts are found to be loose, an alignment check is required before proceeding. Align unit as needed.
Grease VVCPs (McClung Only)
Loosen the lock nut on the VVCP shaft. Loosen VVCP threaded shaft until it stops. Count the number of full turns required to fully open VVCP and document in notes. Apply grease to the VVCP shaft threads and run the shaft in and out twice to spread grease. Retighten VVCP shaft (DO NOT OVERTIGHTEN!) and tighten the lock nut down.
Clean / Drain Y-Strainers
Remove all Y-Strainer caps and drain fluids. Clean Y-Strainer filter as needed.
Fuel Train Regulator Inspection
Remove regulator from base and inspect inside for buildup, corrosion and damage. Remove top cover and inspect spring, diaphragm and other internal components for corrosion or damage. Rebuild or replace if needed.
Power-On Checks
Setpoint Verification
Turn on main battery power. McClung Units-On the Murphy panel, go to the Setup Menu and verify all setpoints match the master setpoint sheet. A/G Units- On the Murphy panel, go to the Main Menu page, then the Timers And Miscellaneous Setpoints page and verify all setpoints match the master setpoint sheet.
Interlock Verification
On each of the scrubbers and both of the coolant tanks, trip the level switch interlocks and ensure the unit goes into a RED fault and can not start. On the oil level switches, trip the interlock and ensure the unit goes into a RED fault and can not start.
Refer to Test Fire procedure
When unit has reach operating temperature, take a frame oil sample using a oil sample kit from the pre-filter oil sample port on the front of the compressor frame. Give the oil sample to maintenance to send out for analysis. If oil is visibly dark or dirty, perform oil change.
While unit is running, use grease gun to apply grease to the auxiliary jack shaft and cooler fan shaft bearings. Inspect bearings for excessive grease sling and loose and damaged parts.
Post-Shutdown Checks
Once the test fire is complete, shut the unit down, but do not relieve the pressure in the unit. intall drain hose and fitting into each scrubber drain and have a bucket ready to catch any liquids. CAUTION! The unit will be under pressure, so you have to open the scrubber drain valve very slowly to avoid spraying and fluid spills. Drain the frame oil catch cans. On McClung units- catch can is located on the lower deck on the passenger side of the unit. On A/G units- the frame oil catch can/cans are located under the compressor trailer.
Using the blowdown valve, the rear suction and discharge manifold bleeders (McClungs only) and the bleeders on the recirculation tank, open the valves SLOWLY to relieve unit, hose and recirculation pressure HEARING PROTECTION IS REQUIRED! Monitor the unit pressure drop on the Murphy screen and the recirculation tank pressure drop on the tank pressure gauges. Ensure all gauges read zero before proceeding.
Source: Envent Corporation (Community Member)