Hydraulic Test Equipment > Flushing Test Rig > The Importance of Flushing

Introduction to Contamination Removal

Particle contamination results in 70% of failures of hydraulic and lubrication systems

A study by Doctor E. Rabinowicz at M.I.T. observed that 70% of component replacements 
or “loss of usefulness” is due to surface degradation. In hydraulic and lubricating 
systems 20% of these replacements result from corrosion with 50% resulting from mechanical
Presented at the American Society of Lubrication Engineers, Bearing Workshop 1981

Most machines that fail during early service and commissioning fail because the systems
components were not properly flushed clean before fitment.

Sources of Contamination

1/ Built in contaminant from components: cylinders, fluids, hoses, hydraulic motors,lines and pipes, reservoirs, valves, et cetera
2/ External ingression: reservoir breathing, cylinder rod seals, and bearing seals
3/ Generated contaminants: assembly of system, operation of system, break in system and fluid break down.
4/ Contaminants introduced during maintenance: disassembly/assembly and make up oil.

Why Equipment is Susceptible to Contamination

Pumps, valves and components with fine dynamic tolerances do not recover if they are
initially subject to particle contamination during initial start-up.
The most common cause of failure of new machines results from debris left in the pipes 
from manufacture and assembly.
Debris can be flushed out of pipes, reservoirs and components before they are fitted or used.

Typical Critical Clearances for Fluid System Components


Typical Clearance Micrometres

Gear Pump (pressure loaded)

Gear to slide plate

0.5 - 5

Gear Tip to case

0.5 - 5

Vane Pump

Tip of Vane

0.5 – 1*

Sides of vane

5 – 13

Piston Pump

Piston to Bore Rą

5 - 40

Valve Plate to Cylinder

0.5 - 5



130 - 450

Flapper Wall

18 - 63

Spool-Sleeve Rą

1 - 4

Control Valve


130 – 10,000

Spool-Sleeve Rą

1 - 23

Disc type

0.5 – 1*

Poppet Type

13 - 40



Hydrostatic Bearings


Anti-Friction Bearings


Sleeve bearings


*Estimated for thin lubricant oil

ąRadial clearance

A micrometre (micron)µm = 0.000,039 inch or 0.001mm

Market Requirements

Original equipment manufacturers (OEMs) are setting ever higher standards of cleanliness for
all piece parts and pipes before they are fitted to machinery.
To achieve these levels of cleanliness very high flushing velocity is needed adopting very 
efficient test equipment, filters and fluids capable of removing and retaining debris extracted
from pipes and components tested cost effectively.
Flushing of pipes and components in a production environment in the shortest possible time has 
become a necessity for many suppliers of equipment that must be verified by instrumentation.
Bonaprene Fluid Power have designed and manufactured a variety of special purpose flushing and 
pressure test rigs that enable suppliers and OEMs to meet current cleanliness standards and 
reach the new standards being set.

The Effects of Contamination Left with in Pipes & Components from Manufacture

On initial start-up of the machine, when the oil is cold and viscose, only the large pieces of 
debris get shunted around the system. Providing that the system filter is not on “cold start by
pass”, these large bits start to get arrested.  
However, as the system works the oil warms up.
The oil now start to become much thinner and the static film of fluid on the walls of the pipes 
starts to release the higher concentration of debris that was sticking to the pipes.
The debris in the fluid starts to flow into the areas that are working at pressure and are 
commonly where fine dynamic clearances exist that need to be lubricated.
The debris initially left from manufacture then starts a chain reaction of wear. More debris is
created and the components are now going to have a shortened life expectancy and the higher the
amount of debris the shorter the life expectancy of the system.
The debris can actually be terminal for some systems within minutes of start-up.
The solution is effective flushing at very high velocity.