Hot Oil Flushing (HOF)

Hot Oil Flushing for Lubrication Systems

Hot Oil Flushing (HOF) is a vital process that eliminates contaminants from lubrication systems. This method ensures machine longevity and reliable performance.

 

Why is Hot Oil Flushing Necessary?

Skipping Hot Oil Flushing increases risks of contaminants entering your system during manufacturing, commissioning, or overhauls. These contaminants—like dust, water, and dirt—shorten equipment life despite clean tank oil. HOF removes debris from circuits, extending your machine’s lifetime and reducing long-term costs.

 

 

Key Moments for Hot Oil Flushing

Perform HOF at these critical times for best results:

• Initial Commissioning: Often overlooked, this stage is crucial. Hot Oil Flushing removes manufacturing debris like welding slag and sand.
• After Failures: Machine failures leave residues in lube systems. HOF cleans these to restore operation.
• Post-Event Maintenance: Planned maintenance or unexpected events often introduce contaminants.
• Oil Changes: Flush old oil to avoid mixing incompatible oils, improving system health.

 

How Does Hot Oil Flushing Work?

The theory of Hot Oil Flushing (HOF) is simple but can yield better results with certain techniques. Using a Reynolds Number above 4,000 ensures turbulent flow throughout the system. The OEM typically advises a minimum flow rate of 150% of the normal service rate.

The Reynolds Number (Re) is a dimensionless value, predicting flow patterns in various fluid scenarios. It relies on flow rate, viscosity, and orifice size. Increasing the HOF oil temperature reduces viscosity, which increases turbulence and helps remove contaminants.

The turbulent flow loosens and flushes contaminants from the system. If contaminants don’t dislodge at this high flow rate, they’re unlikely to during standard operations. Lower viscosity also allows oil to pass more effectively through fine filters, enhancing cleaning efficiency.

 

Bypassing Components During HOF

Bypassing sensitive parts of the circuit avoids damage by overpressure or contamination of the item.  This includes bearings, valves, or any other component that could break or fail if they come into contact with contaminants.  To bypass these points, jumpers, or short circuit hoses are connected to pass the flow around the area.  Often these are fabricated specifically for the application.

 

Essential HOF Basics

• Ensure supply and storage tanks are clean, dry, and odor-free.
• Avoid diesel flushing in oil circuits.
• Aim for Reynolds Number >4,000 using high-volume pumps.
• Bypass sensitive components for protection.

 

Should You Use Special Flushing Oil?

Typically, no special oil is required for HOF. Lower viscosity oils improve flow rates, but standard service oil works if it meets OEM cleanliness standards. Only use sacrificial oil if soluble contaminants remain in the system.

 

Best Practices for Effective Hot Oil Flushing

• Consider using a low-viscosity flush oil to increase velocity during hot oil flushing (HOF). A sacrificial oil effectively removes oil-soluble contaminants, preserving the new oil charge.
• Use strainers or flushing paddles to capture debris before it re-enters the tank. Apply Beta 1000 microglass filters in kidney loop filtration to improve oil polish in the tank.
• Reverse flushing dislodges particles accumulated in downstream, low-pressure areas.
• Implement thermal cycling to induce contraction in pipes, pinching scale off internal walls.
• Pipeline vibrators and hammers at pipe elbows enhance vibration and debris removal.
• Introduce nitrogen sparging to shake loose debris from the system. Filter the gas, and only inject it during operation in small, safe volumes downstream of sensitive parts. Ensure safety protocols for personnel by scheduling when other repair staff are off-site.
• Use diaphragm pumps in smaller systems for their pulsing effect.
• A Digital Particle Counter provides real-time ISO 4406 results for cleanliness verification.
• For final validation, install 100-mesh strainers, flush, and visually monitor until no particles over 25 microns are visible.
• Refer to ASTM D6439 for complete guidelines on cleaning and flushing procedures.

Note: Piping gaskets may continue shedding small carbon flakes during HOF. Identify these flakes as a potential root cause if visible.

 

Particle Removal in Hot Oil Flushing

HOF can remove particles down to 1 micron in size. A well-executed flush clears nearly all particles, leaving oil exceptionally clean.

No System has ever failed from being too clean!

 

Calculating the Reynolds Number for Hot Oil Flushing

Determine the minimum Reynolds Number for the slowest flow rate in your system to create turbulence. Contact us for a free Excel calculator.

Resources

Hot Oil Flushing & Filtering: https://biokem.com.au/service/hot-oil-flushing-filtering