Better Design of Systems Which Use Fire Resistant Fluids
FLUID POWER - Design Data Sheet 37
Design Problems. Fire resistant fluids are far
from being ideal fluids to transmit power from one part of the
circuit to another. Nearly always they are used for reasons of
safety; they have a far greater ability to resist combustion than
do petroleum base hydraulic oils. But their manufacturers do not
claim that they are 100% non flammable.
Almost everyone is aware that when using these special fluids,
careful consideration must be given to the type of seals in system
components, to the kind of paint used inside reservoirs and on the
outside of components, and to duty rating of components, especially
pumps, because of reduced lubricity of the fluid. However, the
problem of increased power loss and decreased power output of a
fire resistant hydraulic system is not so well known, and is the
subject of this issue.
Decreased Output. A hydraulic system presently
operating on petroleum base oil, if converted for use with a fire
resistant fluid simply by changing seals and fluid, will have less
power output for the same input power because of the higher
circulating losses of the heavier fluid. All fire resistant fluids,
whether water base, water/oil emulsion, or synthetic, have higher
specific gravities than does petroleum oil, and there will be a
greater loss of kinetic energy every time the fluid turns a comer
or changes its velocity. Pump efficiency will also be less because
of higher kinetic energy losses.
Energy in the Fluid. There is both potential
and kinetic energy in the moving fluid. Energy which is transferred
to the load is almost all potential energy. Kinetic energy is
supplied to the fluid by the pump and exists in the system as
momentum energy due to the mass and velocity of the fluid.
Practically all of this energy is used up in the circuit and does
not give any useful output. Therefore, we would like for as much as
possible of the input power to go into potential and as little as
possible to go into kinetic energy.
Kinetic energy to circulate the fluid is directly proportional
to its mass (or specific gravity) and is proportional to the square
of its velocity. So, in a system which must use a heavy fluid, it
becomes very important to keep the flow velocity lower than would
be considered acceptable in a similar system using hydraulic
Fluid Characteristics. Viscosity and specific
gravity are the two most important characteristics of a hydraulic
fluid. Fire resistant fluids are available in a wide range of
viscosities to meet the requirements of any pump. However, all of
them available at this time have a relatively high specific
gravity, and this becomes the No. 1 problem in system design. To
keep losses to a reasonable level, all flow passages must be made
larger than are acceptable in a similar system designed to operate
on hydraulic oil.
System Design. Rather than converting a
petroleum oil system to frre resistant fluid, it is far better,
when practical, to design the system specifically for the heavier
fluid to begin with. The key to efficient design is to keep fluid
velocity to a reasonably low rate throughout the circuit. This
includes not only the plumbing materials but valves, filters, and
possibly other components. This means that a fire resistant system
is more costly to build than an oil system.
Areas of Particular Concern
1. Suction Strainer Rating. Probably the most
critical part of the system is the strainer in the pump suction
line. We suggest a coarser strainer, of 60-mesh, with 200uM rating
rather than tne usual 100-mesh strainer with 150uM rating. It
should have 25 to 50% more straining area than would be used for
the same flow of hydraulic oil. This means about one pipe size
2. Suction Pipe. Also very critical is the
diameter of the suction pipe to the pump. We suggest increasing its
diameter to reduce flow velocity to no more than 2 feet per second
rather than 2 to 4 feet per second allowed on hydraulic oil. Use
the larger pipe for the entire distance, even though it may be
larger than the ports in either the pump or strainer.
Bush down when required. What counts is the length of pipe which
3. System Plumbing. Piping in the remainder of
the system should also be oversize. For example, where a flow rate
of 15 to 20 feet per second would be allowed for oil, increase pipe
diameter so the flow is 12 to 15 feet per second. The fewer elbows
the better. Every change in flow direction produces an additional
4. Suction Head Pressure. Flooded pump suction
should be used if practical, with the reservoir mounted at a higher
elevation than the pump inlet, and with oversize suction pipe.
However, flooded suction will not compensate for factors such as
overspeeding of the pump, undersize strainer or piping, etc. If
flooded suction is not possible, then the system should be designed
for minimum suction lift.
5. Suction By-Pass Valve. If the suction
strainer has an internal by-pass valve, a spring with lower
cracking pressure will be helpful, a 2 PSI spring instead of the 3
to 5 PSI spring which would be used with hydraulic oil.
6. Pump. The pump will cavitate more readily on
fire resistant fluids, not because of viscosity but because of
their higher specific gravity. Therefore it may not be acceptable
to rotate a pump at the maximum speed rating in the catalog, if
this rating is for petroleum base hydraulic oil.
Fire resistant fluids have less lubricity than oil, and since
the pumped fluid is used as a lubricant in most pumps, some pump
manufacturers de-rate their pumps to a lower maximum pressure,
sometimes as low as 50% of the oil pressure rating, when they are
to be operated on fire resistant fluid.
7. Fluid Viscosity. As a rule, the viscosity of
the fire resistant fluid should be the same as recommended for
hydraulic oil. Excessive slippage in the pump will be the effect of
reducing viscosity for the purpose of reducing flow losses.
8. Magnets. Magnets may be used inside the
reservoir or incorporated in the suction filter to help offset the
loss of filtering efficiency caused by using a coarser mesh
Suggested Circuit for Saving Air on the Return Stroke.
Important power savings can sometimes be made on selected
applications by reducing the air pressure on the return stroke of
an air cylinder. Most air cylinders do their heavy work on the
forward stroke, and return unloaded. When the piston stalls at the
end of its return stroke, air continues to flow into the cylinder,
and as much air is used on the return stroke as on the power
stroke. Reducing the return pressure will reduce air
Usually, the most practical way to reduce return air pressure is
to use a 5-way rather than a 4-way valve for directional control. A
5-way valve is similar to a 4-way, but has two inlets and one
exhaust port. Full pressure for the forward stroke is connected to
one inlet and reduced pressure from a pressure regulator is
connected to the other inlet. The second regulator not only reduces
return pressure, it also serves as an excellent speed control for
the return stroke, and limits the stall force exerted by the
The amount of power saved by reducing return pressure can only
be estimated when the lowest pressure which gives sufficient return
speed can be experimentally determined. As pressure is reduced, the
In making estimates on air savings, Table 1 from Design
Data Sheet 58 may be used. Computations can be made at
both full pressure and reduced pressure, and the difference is the
amount of air saved by return pressure. There will be some small
additional losses through Regulator 2 due to over-compression which
are difficult to estimate. So, air savings calculations should be
regarded as approximate.
Figure 1. Regulator 1 reduces shop air pressure
to the level required for the power stroke of the cylinder. This
pressure is connected to inlet port Pl of the 5-way valve.
Regulator 2 then further reduces the pressure for the return stroke
only. This pressure is connected to inlet port P2 of the 5-way
valve. Only one flow control valve is needed, for the forward
speed. Regulator 2 controls return speed, and should be
experimentally adjusted to the least pressure which will give
sufficient return speed to obtain the desired cycling rate.
Example: Find the approximate SCFM and HP saved
on an air cy nder by reducing return pressure to 30 PSI from a
system pressure of 100 PSI. The cylinder has a 5-inch bore and
30-inch stroke. Desired cycle rate is 8 cycles per minute.
Solution: Use Table 1 from Design Data
Sheet 58 and compute the SCF used by the cylinder
operating at 100 PSI both forward and return on a 30-inch stroke.
Then compute the SCF used when operating at 30 PSI both forward and
return. The air saved should be approximately 1/2 the difference
between these two computations.
SCF at 100 PSI both directions =
0.1737 × 30 = 5.2
SCF at 30 PSI both directions = 0.0677 × 30 = 2.0
5.2 - 2.0 = 3.2 SCF difference
One-half the difference = 3.2 ÷ 2 = 1.6 SCF saved on each cycle.
If the cylinder is able to make 8 cycles per minute on a return
pressure as low as 30 PSI, the savings will amount to 8 × 1.6 =
12.8 SCFM. This is equivalent to about 3 HP of compressor
Download a PDF of
Fluid Power Design Data Sheet 37 - Better Design of Systems Which
Use Fire Resistant Fluids.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this