It is an essential maintenance process that is designed to extend the life of machines and keep them

When do you need to Hot Oil Flush?


A Hot Oil Flush (HOF) is carried out to remove contaminants from your lubricating system.

It is an essential maintenance process that is designed to extend the life of machines and keep them operating at their optimal level.

Ask the HOF or HOF FAQ

What will happen if you don’t HOF?

Contaminants enter lubricating systems through many different methods during manufacture, commissioning, overhauls and when failure events occur.  Physical contaminants can range from free water, pipe scale, welding slag, dirt, dust, sand, rags, etc.  If left within the lubrication circuits the potential lifetime of the unit is compromised from the outset“ no matter how clean the lubrication oil is in the tank.

The commissioning process should include polishing the newly delivered oil and hot oil flushing at a velocity that will disturb debris from the pipe circuitry and then remove it.

By performing a HOF at the appropriate opportunity you can keep your medium and long term costs down, while increasing the useable life of your components.

When should we HOF?


The next question is when should you HOF?  Here are some critical moments in the life of your equipment when a HOF is recommended.

Initial Commissioning

Probably the most important time to HOF and one that can be often overlooked in the planning phases. A robust flushing plan should include all circuit components, spools, lube and run-down tanks and coolers. The HOF will remove manufacturing and service debris, casting sand, welding slag, drill swarf, and any introduced contaminants like silica.

Machine Failure

When components suffer failure there is always contaminant residue in the lube system. As part of the repair process a HOF should be carried out to restore the machine back to full operational capacity.

Post Event

There are a range of different events that result in contaminants getting into a lubricating system. For example, during planned maintenance or overhaul work, ingression events, or failure of components and filtration media.

Oil Replacement

This is probably least understood, but Best Practice is to conduct a displacement flush and remove any debris with the old oil, rather than risk blending two potentially incompatible oils.  A small volume of sacrificial flushing oil may be needed when transitioning.

How does HOF work?

The theory behind a HOF is straightforward, however there are various techniques to improve the results achieved. Normally a Reynolds Number of >4,000 is recommended to ensure a turbulent flow throughout the system and your OEM may recommend a minimum flow rate of 150% of normal service rate.
The Reynolds Number (Re) is a dimensionless quantity that is used to help predict similar flow patterns in different fluid flow situations.  It comprises of the variables of flow rate, viscosity and orifice sizes. So, if you increase the HOF oil temperature, this has the effect of reducing viscosity and increasing turbulence.
This increased turbulence picks up the contaminants and flushes them out of the system.  If the contaminants do not break free at the HOF higher flow rate, they are less likely to dislodge when the machine is in standard operation at service flow rates.  Reducing the viscosity also means the oil more easily passes through fine filter.

What components should we bypass during HOF?

Bypassing sensitive parts of the circuit avoids damage by over pressure 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.
BioKem Oil Services flushing on site; spools are connected and ready to be HOF’d

What are the HOF Basics?

  • Supply and storage tanks should be clean, dry and odour-free.
  • Diesel flushing of oil circuits is not acceptable.
  • Reynolds number >4,000 achieved with external high-volume pumps or Displacement Flush using standard service pumps
  • In very dirty applications the lube tank may be drained to permit manual cleaning of contaminants with lint free rags.
  • Bypass sensitive components with jumpers

Do I need to use ‘special’ flushing oil?

No, generally not, however a lower viscosity oil can achieve higher velocities improving the outcomes.  The proposed in-service oil can be used for the flushing application, then used for service once it is filtered (polished) so that it meets the cleanliness standard required by your OEM. The exceptions are when a chemical flush may be needed or the HOF removes soluble contaminants that cannot be removed from the oil.

What is HOF Best Practice?

  • You may consider the use of a separate lower viscosity flush oil to achieve higher velocities.  A sacrificial oil can also be used to remove oil soluble contaminants that may affect the fresh charge of oil.
  • Strainers or flushing paddles (screens) can be used to capture debris before it flows back into the tank.
  • Utilise quality Beta 1000 (or better) microglass filters in kidney loop filtration to polish tank oil.
  • Rapidly reversing the flow direction (reverse flushing) helps to dislodge particles accumulated in downstream low pressure areas of the circuitry.
  • Thermal cycling of oil temperature during the flush (as pipe cools and contracts, scale is pinched off the internal walls).  This is a very worthwhile effect to implement if at all possible.
  • Pipe line vibrators and dead blow hammers at pipe elbows to induce vibration.
  • Sparging – The introduction of Nitrogen can achieve significant vibration and rattle debris from the system.  The gas needs to be filtered, ONLY injected when the system is operating, and in small volumes as it can induce severe vibration.  Always introduce downstream of filters and any instruments that could be damaged by the vibration.  Safety procedures need to be implemented to protect staff, so best used when other repair personnel are off site.
  • Diaphragm pumps are excellent for smaller systems due to the pulsing effect.
  • Employ a Digital Particle Counter to give real time results of ISO 4406.
  • For validation of the clean, install 100-mesh strainers, flush and monitor and typically once there are no particles detectable by the naked eye (>25 microns) the system is flushed and ready for service.
  • Review the ASTM D6439 Standard which is a guide for cleaning, flushing and purification for more info.

Note:  Piping gaskets can continue to shed small carbon flakes throughout the HOF.  If you are seeing those little black specs then consider this as a root cause.

How small are the particles removed by HOF?

Particulate contaminant removed can be tiny, down to 1 micron.  A well conducted HOF can remove almost all particles down to 10 microns and leave circuitry and lube oil exceptionally clean.

No system has ever failed from being too clean!
Comparison of fine human hair at approx.50µm
Particles shown in image are 10µm
Smallest particle seen with naked eye ~25µm
Photo courtesy of Pall Pocket Book


100 mesh Flushing (screen) Paddle
Sandwich ply design made from gasket material
Contact BioKem for supply of these

system flush

How do you calculate the Reynolds Number?

system flush

You need to determine the minimum Reynolds Number for the slowest flow rate of the system so that turbulent flow is created through the entire circuit. Email us for a copy of a basic Excel Calculator.

Flow chart source: Noria Corp.



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