To ensure the safe operation of turbocompressor units, the oil circuit must remain clean and free of contaminants. Even tiny impurities entering high-speed bearings, seals, regulating valves, as well as turbine bearings and governors sharing the same oil system may trigger severe failures, including bearing burnout and malfunction of regulating valves and governors, which will threaten the safe operation of the whole unit.

Therefore, newly installed oil circuits must be thoroughly cleaned to eliminate oil sludge, iron filings, welding slag, oxides, dust and other impurities trapped inside pipelines and fittings. This ensures unobstructed oil flow, precise and flexible action of regulating mechanisms and safe unit operation. The oil circuit system shall also be cleaned after every major overhaul of the machine.

The general oil flushing method is to circulate the unit’s operating oil within the system under normal operating pressure, subjecting the oil to repeated rapid heating and cooling within a specified temperature range. The temperature range for thermal cycling can be determined according to the specific oil system. For instance, one plant specifies heating the syngas compressor oil system to 75°C followed by rapid cooling to 50°C, while other plants adopt lower cooling temperatures. In general, shorter heating and cooling cycles are preferred. However, due to the large oil volume, extensive heat dissipation area and limited capacity of heating and cooling equipment, sufficient time is required to complete the thermal cycling. A typical cycle consists of heating the oil from low temperature to high temperature within 1–2 hours, holding for 2 hours, cooling back down to low temperature within 1 hour, holding for another 2 hours and then reheating. This process is repeated to generate thermal shock. During cooling, use a wooden mallet to tap the pipeline sequentially along the oil flow path, especially welds and elbows to create vibration, loosening and dislodging oxides and welding slag.

To achieve fast and effective oil flushing, the following measures can be adopted in addition to the basic method described above: intermittently start and stop the oil pump and open/close oil circuit valves to create swirling flow of flushing oil inside pipes; inject nitrogen into pipelines to generate turbulent flow and improve flushing efficiency; increase oil flow velocity and flushing volume by installing an additional oil pump of suitable size to boost pipeline flow rate.

Oil can be heated via heating coils at the bottom of the oil tank, yet these coils have limited capacity and slow heating speed, often failing to reach the target temperature. Alternative heating methods include using parallel oil coolers: one set supplied with hot water for heating while the other remains connected to cooling water for cooling, alternating heating and cooling cycles. If hot water is unavailable, connect a low-pressure steam line to the cooling water inlet of the cooler and heat the cooling water by introducing steam. The cooling water flow must be controlled appropriately so that the water can reach sufficient temperature to heat the oil. The steam feeding rate must be strictly controlled. Especially when all components are at ambient temperature, excessive steam input will create extreme temperature differences across the cooler, generating excessive thermal stress and damaging the equipment.

Strainers shall be installed at all main oil supply headers and oil inlet ports for bearings and seals to prevent impurity ingress and serve as inspection points during oil flushing. A debris basket shall be fitted at the return header inlet to the oil tank to collect contaminants and avoid massive impurities flowing back into the tank.

Adequate preparation before cleaning is critical to ensure smooth oil system flushing.

Major pre-flushing preparation works are listed below:

The flushing process is divided into multiple phases.

After every 8–12 hours of oil flushing, remove filter cartridges for inspection and collect filter residues for comparison. Repeat flushing cycles until the acceptance criteria are met. There is no unified industry standard for oil flushing acceptance. For this phase, the flushing is deemed acceptable if no visible residues or only sporadic fine particles are observed on inlet strainers, and temporary filter cartridges contain fewer than 2–4 impurity spots per square centimeter.

The flushing is finally accepted when no visible contaminants exist on permanent filter elements and all inlet strainers, and laboratory analysis confirms the oil inside the tank is free of acid, water and dust.