With the ascent of the involvement of safety regulating
bodies in equipment manufacturers coupled with end users becoming more conscious
about the safety features in their equipment in the last few decades, the
design departments of OEM companies were in a tizzy to develop the safest and
most reliable operating equipment. The International Standards Organization concluded
that all excavators would have to function as cranes. This implied that the
earth moving equipment would now have to have a provision to hold the load in
place as well as have a system in place to prevent failure in case of a hose burst
situation.
One out of the many safety concerns were, what happens if
the hose connecting the cylinder to the valve was to rupture? Without any
failsafe in place, ruptured hoses would allow for the oil pressurized in the
cylinder to have a free path to atmosphere. This would bring the load down catastrophically
causing massive damage to man and machinery. Since there were not many options
to rely on, some Italian companies developed what was called a “Hose Rupture”
or a “Hose Burst” valve. It is, in fact, a “Velocity Fuse”.
How does it work?
The valve functions on one of the fundamental principles of
hydraulics, the pressure drop created across an orifice. The term “drop” is
used to imply that the pressure is always lesser downstream. More the flow
across the orifice, greater is the pressure differential between the inlet and
outlet.
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Figure 1: The simplicity in the design of the H.B.V. is
reflected in the crudeness of its function. The
metering of the flow through the valve cannot be
effectively controlled or even set with ease. |
The velocity fuse is composed of a body with a central stem
that retains a disk (Figure 1). The stem is secured on one side by a locknut
while the disk is held in place by a spring. The thickness of the gap between
the body and the disk is essential as it is this distance that regulates the
flow of oil out of the actuator and prevents failure in case of a hose burst.
Radial holes around the circumference provide a path for the oil from one port
to another.
The pressure drop relationship to the flow and gap spacing
is given in Graph 1 for various sizes of the hose burst valve. The graph is an approximation
of the setting to be achieved as the entire system is quite crude and cannot guarantee
that the valve will close at the required flow rates.
When the differential force caused by the flow across the
gap “T” reaches that exerted by spring, the disk closes and the flow stops. The
pressure on Port 2 then holds the disk closed to lock the load in place. This
is very similar to blow out of an electrical fuse that melts and closes the
connection when there is a spike in the current.
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Graph
1: The relationship between the flow and orifice thickness is shown for
Velocity Fuses of various sizes. This is one of the two ways a Hose Burst Valve
can be set to the desired flow rate. Although this method is not very accurate,
it is much simpler and lot less time consuming.
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Valve is generally set by adjusting the gap ‘T’ from the
graph above. This is a standard procedure setting and although quite easy, is
not accurate since variations in valve manufacture govern it mechanics more
than standardized graphs. The alternative is to set approximately to the
required flow and then iterate the gap using a flow meter to check the flow.
The latter method is time consuming although it gives a more accurate result.
Moreover, the setting may not be the same on the field as on the test stand as
the viscosity of the oil can alter the flow rate. The pressure drop in the
opposite direction (PoRt 1 to Port 2) will increase as the setting, and
consequently gap T, is reduced. It is best to locate the valve in the cylinder
port by using its cartridge form. Else, line mounted (male-male or male-female)
bodies can be used.
How safe is the Velocity Fuse?
The Velocity Fuse may not be the best way to do the job, but
it is cheap. It has carved out a niche so strong that customers do not want to
look past the valve for an alternative. It retains its popularity in Italy although in India it can still be found on some
archaic cranes.
The main issue with the Velocity Fuse is that what is needed
is the exact definition of the term “Hose Burst”. If it is set for very high
flow, small cuts in the hose and small opening in the fittings will not be
detected by the fuse and can be dangerous. If set for a very low flow so as to
be able to detect these problems, passing heavier flows through the valve will
be difficult. Hence judicious methods of flow setting are required.
Another problem with the hose burst is that the load,
although held in place, now has no simple means of being lowered back down to
safety.
Once the fuse is shut, the only way to open it would be to
apply pressure at Port 1 to reduce the pressure differential across the seal.
In case of the hose being ruptured, this would entail that
the load can be lowered only after fitting a new hose and completing the
hydraulic circuit. There is no way to bring the load down simply by draining
the oil to atmosphere.
What are the other options?
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Figure
2: In Tucson’s Overcentre valve the
poppet seals
the flow from the Actuator
Port (C) to the Valve Port (V). The free
flow check seat allows flow from V to C.
Pilot Pressure at Pilot Port (P) along with
load the induced pressure at C opens
up
the poppet and allows a reverse
controlled flow.
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Instead of Velocity fuses to control the load in case of a
hose rupture, the safer option is to install an Overcentre valves. These load
controlling valves are typically mounted on the cap of a linear actuator or
they can be line mounted with rigid piping between the actuator and the valve.
In the Overcentre valve (Figure 2), the flow is free from
Port V to Port C over the check valve, but reverse flow is blocked and can only
be opened by the pilot pressure acting on Port P along with the load pressure
on Port C.
The cylinder port of the valve is connected to one end of
the cylinder while the valve port is connected to the directional control valve
(Figure 3). The pilot port is connected to the oppositeend of the actuator and
serves as a pressure feedback. For retraction, the Overcentre valve restricts
the flow of oil out of the cap end which results in the pressure rising at the
opposite end.
This pressure is sensed at the pilot port of the Overcentre
valve which, assisted by the load induced pressure, compresses the spring
giving an outlet to the oil from the cap end.
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Figure 3: Rigid Piping is used between the valve and the
cylinder to prevent accidental hose burst in these lines.
The cartridge can even be mounted in a cavity machined
in the cylinder.
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Since the piping is rigid between the cylinder and the
overcentre valve, hose burst can only occur in Line A or between the overcentre
valve and the directional control in Line B. In this case, the flow out of the
cap end would not change since the load induced pressure would still act in
conjunction with the pilot pressure on the spring to allow the load to lower.
The only change would be the leakage from the burst hose.
In this way, hose burst protection can be offered safely and
securely to almost any hydraulic cylinder, without the use of archaic technology.
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