General
Criteria for Using Fire Resistant Hydraulic Fluid (FRHF)
These criterias determine the usage of a FRHF and persons responsible for specifying its use.
Mineral oil can ignite and burn if sprayed, poured, or otherwise permitted to come into contact with hot
surfaces, open flames, molten metal, or electric arcs. The fire is a serious threat to property, jobs and the
lives of persons working in its vicinity.
FRHF is hydraulic media generally more difficult to ignite than mineral oil. They are less likely to continue
burning out of or beyond the area of intense heat. They normally will not continue to burn once the source
of ignition has been removed - i.e. - they self-extinguish.
They are not fire-proof and may ignite and burn if the ignition source is substantially higher than 705oC.
However, if burning occurs only in the areas of intense heat, it would have offered an advantage over
mineral oil. Employee must be protected from direct exposure to the heat and flame whether it be
momentary or sustained.
There are two main groups or four main types of FRHF. These are:
Group Containing Water
1. HF-A 95% high water based fluid
2. HF-B 35% water in oil emulsion
3. HF-C 35% water-glycol
and
Group Not Containing Water
4. HF-D phosphate-ester etc.
The use of FRHF however should not give one a sense of false security. One must use good design
practices, and common sense when making decisions regarding hydraulic systems in high fire danger areas.
Effort must be made to assure a leak-tight system. Hydraulic lines and hoses must still be located in the
safest position. When lines are located directly over or very close to a high heat source, they should be
guarded or shielded.
In most cases, FRHF require continuous maintenance and careful inspection on a regular basis.
Survey Guide for Fire Resistant Hydraulic Fluids
The degree of fire hazard must consider the type of FRHF, the distance to a sustained and recognised
source of ignition, and the consequence of a fire. Each situation must be judged individually. A
misjudgement can be forgiven; failure to make a judgement cannot.
Some factors to consider in assessing the use of FRHF follow:
Any heating or melting furnace, any metal-pouring station or sustained open-flame, high voltage arc,
open resistance-heater or any surface that is above 250oC should be considered a potential source of
ignition.
Any recognised and sustained source of ignition located in a direct line path within 15 metres of a
hydraulic circuit is of major concern. From 15 to 30 metres the probability of the fluid contacting the
source is less. Beyond 30 metres the probability of a contact is “almost” negligible.
A pumping unit with a stop-switch located between 15 to 30 metres of the pump is safer than a
system which does not have a stop-switch within that same distance. In those situations where the
question of safety is a marginal one, the installation of a remotely located stop-switch may be a
satisfactory precautionary measure.
A system with a dead-weight or an air-bottle accumulator is a greater risk than one without
accumulators because the flow cannot be stopped quickly. A system which is actuated by hand or
foot power may be a minor hazard unless substantial back pressure exists.
The probability of a serious fire is directly proportional to the capacity of the pump, the total capacity
of the reservoir and the system’s maximum pressure.
A hydraulic system which requires personnel to be in a location where they could be “trapped” by the
spray of a hydraulic fire is far more of a hazard than a system where people are located remotely the
hydraulic spray.
Responsibility for decision to use FRHF
The Engineering Manager at each location has the ultimate and final responsibility for using or not
using FRHF. He shall be responsible for the initial decision and the periodic review of that decision
making changes to the decision in line with the circumstances surrounding the use of a piece of
equipment.
Types of FRHF
HF-D Phosphate-esters
While the broad range of common seals are compatible with the new Phosphate-esters, for dynamic
seals, noeprene should be avoided. The use of appropriate hoses and paints need to be discussed with
the fluid supplier. The painting of the interior of reservoirs is not necessary.
Because of their greater specific gravity (1.15 compared with mineral oil at 0.85) and their relatively
poor Viscosity Index (V.I.), they require close attention to piping arrangements and temperature
control. They have better heat exchange properties.
Relatively speaking they are more expensive.
Inspite of these shortcomings, they offer a wide spectrum of usage. Under proper design and
operating conditions, they will perform better than mineral oil. This is due to the very good oxidation
stability and long operating life of the product - fill for life. It has extremely good lubricity (used as
ashless EP additives in modern hydraulic oils) and can operate at all operating pressure ratings.
HF-C Water-glycols
These products are readily available from a large number of sources. They vary in their make-up, but
generally contain above 35% water.
They are moderately expensive. They require special attention to the type of paint used, but do not
require seals or o-rings different from those used with mineral oil. Their V.I. is as good or better
than mineral oil, and specific gravity more nearly equal to mineral oil.
They perform well in vane pumps, gear pumps and some axial piston pumps.
Their loss of water with increased system temperature may require addition of make-up water.
While they should always be a first consideration in the selection of a fire-resistant fluid, they have a
somewhat lesser spectrum of usability than the phosphate-ester type fluids.
Water in Oil Invert Emulsion
This fluid consists of over 35% water dispersed as small droplets throughout the bulk oil phase.
Visually it looks like mineral oil. In many respects it handles like mineral oil as it flows through
pumps, valves, etc. This type of fluid has been popular in the coal mining industry.
It is not appreciably safer than petroleum oil with respect to the sustained sources of ignition
commonly found in the aluminium and steel industry.
Currently this type of fluid is not commonly used due to its limited functional capability.
Polyol Esters
These are products formulated from synthetic fluid that claims to be FRHF. Although these fluids
passed the old Factory Mutual Standard as FRHF, it is now reported by Factory Mutual that these
are not appreciable safer than mineral oil in their New FRHF standards. They are unsafe with respect
to sustained sources of ignition commonly found in the aluminium and steel industry.
HF-A 95% high water based fluid
This type of fluid contains about 95% water and 5% additives, such as emulsifiers, wetting agents,
lubricity improvers, rust inhibitors, bactericides, defoamers and thickeners. Two types, micro-emulsions and synthetic solutions, are currently available. They are generally less costly than mineral
oil and have excellent fire resistant properties.
Hydraulic System Component Material Compatibility
All dynamic seals should be evaluated and may require changing before the fluid change is made.
Static seals seldom require change-out until such time as the equipment is later disassembled. Upon
reassembly, new seals, etc., will have to be used. Viton seals and o-rings are generally suitable for
both mineral oil and synthetic fluids. Butyl and ethylene propylene seals are acceptable for use with
phosphate-ester and water-glycol fluids, but not mineral oil. Buna-N is unacceptable with phosphate-esters.
Cork seals or non-water proofed fabric seals and gaskets and non-treated cellulose filter cartridges
are completely unacceptable for use with water-containing fluids.
The existing pump must be evaluated before the fluid change is made. Be sure the pump and the
system design can accommodate the special properties of the fluid.
SELECTING THE APPROPRIATE FRHF FOR THE CONVERSION OF AN EXISTING
MINERAL OIL SYSTEM
Operating fluid bulk temperature >65oC?
Type of pump(s), valves
Operating pressures?
Positive head on pump inlets?
Elastomer compatibility? - ease of change over of elastomers?
Low temperature requirement?
V.I. requirement - system response critical?
Toxicity, disposability considerations?
Volume of the system
Cost considerations?
Ongoing maintenance plan? (Change out every X’mas shutdown, or leave unattended in remote
location?)
How critical is hydraulic reliability? (Aircraft, building elevators, braking systems, motion heave
compensators)
What is the fire hazard? Potential for disaster?
SELECTING THE APPROPRIATE FLUID WHEN DESIGNING A NEW SYSTEM
Make use of current proven technology to maximise safety and hydraulic performance
What is the prime consideration for this system - fire safety or hydraulic performance?
What is the fire source?
Reliability requirements?
Any limitations on available space?
Working environment temperature?
Cost/performance analysis of low operating pressure vs. high operating pressure.
Maintenance considerations?