Hot Oil Flushing

Hot oil Flushing or Flushing is the process of clearing contaminants in the piping system (which comprises the pipes and the system fluid)after installation. The sole purpose of this process is to avoid any damages caused by contaminants to the equipment connected to the piping system. The system is flushed by continuosuly recurculating the system fluid in a turbulent flow through filters until the desired cleanliness value is obtained. The cleanliness in the pipe is measured in terms of NAS/ISO/SAE Values.

Hot Oil Flushing Process

Hot Oil Flushing Circuit
Flushing involves continuous recirculation of the working fluid with turbulent flow through flushing filters until the desired cleanliness value is obtained.

The main components in the flushing circuit are
  1. Flushing Unit
  2. Contamination Monitor
  3. Flushing Filter
  4. Looping Materials
The Flushing Unit (FU) is basically a Hydraulic Power Unit (HPU) which drives the fluid throughout the circuit with a turbulent flow to get the lines flushed. The flushing filters in the flushing setup successively filters contaminants with each recirculation cycle of the fluid in the system.

The contamination monitor measures the cleanliness of the oil and gives a reading called cleanliness values (CV). These values are either as per ISO 4406 or NAS 1683. The looping materials are items to interconnect the piping system, materials like hoses, pipe fittings like tee, elbows, and manifold depending on the requirement.

The above Fig 1.0 shows a typical flushing circuit. Here, the fluid flows through the pressure line from the pump and branches from the tees through the tap-offs into the return and drain lines. The fluid flows back to the HPU through the return and drain line. In this process the fluid is filtered before entering the FU's tank and the cycle continues. The contamination monitor is connected to the return line just before the filter at the upstream side. The CV of the oil is checked using the cleanliness monitor. If the CV is not as per the requirement, the flushing cycle is continued until the CV is obtained. Once the CV is obtained, the flushing is continued for half an hour and the consistency of the CV is checked. If there is consistency in the reading, the flushing is complete.

Pre-requisites for Hot Oil Flushing

The main requirements for flushing are:

1. Fluid Velocity of the Flushing Oil
Flushing is effective when there is turbulent flow in the system. That is the fluid velocity should be sufficient to cause turbulent flow in the system. This required velocity can be found out by ensuring the system has a Reynolds No. of 4000 or above. Hence, the minimum velocity of the fluid can be found out using the formula:


Where, V= Fluid Velocity in m/s
Re = Reynolds No kept at 4000 (higher number will ensure a better flush)
γ = Kinematic viscosity of the flushing oil in m2/s
D = Internal Diameter of the pipe in m

Based on the velocity obtained, the minimum flow rate required to flush the system is calculated as below:

Q = A x V x 6000

Q = Flow Rate in lpm (litre per minute)
A = Area of the Pipe Bore in m2
V= Velocity of fluid in m/s

HPU Flow Rate Calculator (for Turbulent Flow)

2. Viscosity and Temperature of the Flushing Oil
Viscosity and temperature of the flushing oil plays a pivotal role in deciding the flow rate required for flushing. An ideal flushing oil should have low viscosity and a high specific gravity. On the other hand viscosity should also not be too low such that it is unable to carry loose particles which may settle down due to less gravity. Heating the oil to around 50 – 60 Deg Centigrade will help in loosening stubborn dirt or grease and hence drive them from the pipeline. It also helps in decreasing the viscosity which will in turn increase the fluid velocity.

Chase Resource (CR) Team is well versed with flushing and have executed large projects which involved flushing. CR uses in-house TMI Flushing Unit for all jobs.

Chase Resource technician operating flushing machine, right - image of flushing machine
Left: Chase Resource Engineer inspecting flushing operation. Right: Image of flushing machine

Chase Resource technician operating flushing machine, right - image of flushing machine
Left: Readings on a Contamination Monitor. Right: Pipes looped during flushing

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