Apparatus for monitoring hydraulic plant for leakages

Apparatus is provided for detecting leakages in hydraulic equipment. A pressure supply line of the equipment has alternate fluid flow paths controlled by respective, oppositely directed non-return or one-way valves which are pre-loaded as by springs. A restricted flow transducer is located in a bypass line which bypasses the alternate fluid flow paths for sensing pressure drops in the system.

The invention relates to an apparatus for monitoring a hydraulic plant for 
internal and external leakages, such as occur from hose and pipe 
fractures, where the problem occurs frequently, namely when pressurized 
consumers pressurized operating fluids such as cylinders and hydraulic 
motors are in their inactive mode, i.e. in their inoperative position. In 
practice it has been found that losses of the pressure medium, from a 
hydraulic plant mainly occur in the inoperative phase, because the full 
pressure can build up at this time and also lasts for a longer period of 
time, since the working phase is generally substantially shorter than the 
inoperative phase. 
Flow monitors are known for monitoring streams of fluid or liquid flowing 
through a conduit, which as a signal-emitting transducer emit a 
measurement signal proportional to the flow, as in the case of transducers 
which operate according to the turbine or displacement principle for 
example. Differential pressure transducers are also known, which operate 
according to the throttle or restrictor principle. With this differential 
pressure method, the measurement signal, of digital or analog type, is 
proportional to the pressure drop which is caused by the flow of fluid 
through the reduced cross section of the throttle or restrictor. However, 
the pressure drop necessary for measurement is frequently undesirable, so 
that one can only work with small pressure drops and the measuring range 
and accuracy are thus limited. 
It is common to all known flow transducers that they emit a reasonably and 
sometimes very accurate measurement signal for their rated flow range, but 
that in the case of extremely small fluid or liquid streams, in relation 
to the flow therethrough, they emit only an inaccurate or no measurement 
signal. Added to this is the fact that the susceptibility to breakdown of 
flow meters operating according to the displacement or turbine principle 
is relatively great, in proportion to other hydraulic components, and that 
the breakdown of a flow meter in the main stream generally leads to the 
stoppage of the entire plant. 
Normal flow monitors used in practice as flow transducers, whose range of 
measurement is generally limited to a ratio of 1:10, therefore have an 
appreciable drawback when monitoring pressure medium losses. On the other 
hand, during the movement phase of the consuming device, they should allow 
the entire, i.e. the maximum possible stream of fluid or liquid to pass, 
without the signal emitted being evaluated for indicating leakage of oil 
caused, for example, by a pipe fracture. But on the other hand, in the 
inoperative position of the consuming device, they should monitor the 
pressurized consuming device such that both the smallest leakages of the 
consuming device as well as large leakages (for example, from a pipe 
fracture) record a useful fault signal. This is certainly not possible 
with flow transducers known and used hitherto. Flow transducers which 
fulfil this requirement should have an extremely large measuring range 
with simultaneous high resolution and high accuracy. 
It is the object of the invention to construct a flow measuring device 
which allows the rated flow to pass to or from the consuming device 
without an appreciable pressure drop at the measuring device, whereas in 
the case of very small streams of fluid or liquid, a measurement signal is 
emitted which is accurate and as far as possible proportional to the 
stream of fluid or liquid. 
This object is fulfilled according to the invention due to the fact that in 
the supply pipe to the consuming device, after or before a control valve, 
a measurement device or unit is installed that includes a flow transducer 
for a relatively small throughflow. The flow transducer is connected in 
parallel with pre-tensioned throughflow valves, preferably non-return or 
one-way valves connected in anti-parallel, which are sized to accommodate 
the rated flow of the consuming device. The specific flow characteristic 
of the non-return or one-way valves limits the partial stream through the 
flow transducer to any value which can be predetermined, and the 
arrangement is such that the maximum possible flow through the measurement 
device or unit is not limited by the size of the flow transducer. The 
fluid circuit arrangement of the measurement device thus makes it possible 
to allow large streams of liquid to flow through the non-return valves, 
and in the case of small streams of liquid to obtain accurate measurement 
signals by way of the flow transducer with a small throughput and high 
resolution. Thus, the maximum possible flow value can be determined 
optionally by the size of the two non-return or one-way valves, whereas 
the relatively small flow value, with which a measurement signal should be 
recorded, is determined exclusively by the size of the flow transducer. 
The supply of pressure medium to the consuming device thus takes place by 
way of one of the valves and the flow transducer, whereas at the time of 
the return stroke of the consuming device, for example of a cylinder, the 
quantity of liquid displaced by the piston can be discharged through the 
other valve, which is connected in anti-parallel to the first valve and 
through the flow transducer. 
It is important for the operation of the circuit of the invention that the 
non-return or one-way valves are pre-tensioned, which is normally achieved 
by springs, since to achieve the desired flow characteristic in the 
non-return valves connected in anti-parallel, depends both on the 
non-return effect as well as on the pressure-limiting effect due to 
pre-tensioning. On the basis of this fact, according to a further 
development of the invention, the interconnection of two directly 
controlled pressure-limiting valves connected in anti-parallel with a flow 
transducer connected in parallel is appropriate. 
When using the measuring device according to the invention with the 
interconnection of valves and a flow transducer, it is also possible to 
take advantage of the fact that even when the flow transducer, which is 
susceptible to trouble, breaks down, the plant can continue to be 
operated, namely solely by way of the valves, whereby in the short term, 
i.e. until the next repair, monitoring of small oil losses is no longer 
possible. 
According to the invention it is intended to use a flow monitor as the flow 
transducer, which monitor is dimensioned such that with a rated flow, the 
pressure drop at the pressure transducer is identical to the response 
pressure of the two pre-tensioned non-return or pressure-limiting valves.

In FIGS. 1 and 2, the circuit and co-ordination of the measuring device are 
represented by symbols conventional in hydraulics. FIG. 1 shows a 
measuring device 1 inserted in the supply pipe to a consuming device (not 
shown) of a hydraulic plant, with two non-return or one-way valves 2, 2a 
connected in anti-parallel, which are each pre-tensioned by a spring 3. A 
flow monitor 4 is located parallel to the non-return valves 2. In contrast 
thereto, the circuit according to FIG. 2 shows a measuring device 1, in 
which the non-return valves 2 (FIG. 1) are replaced by spring-loaded 
pressure-limiting valves 5, 5a connected in anti-parallel, which are 
likewise connected parallel to the flow monitor 4. 
The pressure/flow behaviour of the entire measuring device 1 as well as 
that for a non-return valve or pressure-limiting valve 2, 2a, 5, 5a and 
for a flow monitor 4 is shown in FIGS. 3a to 3c. The diagram according to 
FIG. 3a for a non-return or pressure-limiting valve shows that due to 
pre-tensioning by means of the springs 3, a flow through the valves is 
only possible when the pressure of a flow quantity Q of fluid exceeds the 
pre-set pressure p established by the springs 3, the valves having large 
dimensions and being designed for the throughput of large quantities of 
liquid when open, in order to facilitate desired good constant pressure 
behaviour. FIG. 3b shows the flow characteristic for a flow monitor 4, and 
it can be seen how in the case of relative slight increases in fluid flow 
quantities Q, the resulting pressure p increases very considerably. 
Finally, the diagram according to FIG. 3c shows the pressure/flow 
characteristic of the measuring device according to the invention, the 
curve resulting from the addition or superimposition--as is the case at 
the point A of the illustrated curve according to FIG. 3c, of the partial 
streams travelling through the non-return valve or pressure-limiting valve 
2, 5 and the flow monitor 4. That is, the point A indicates opening of one 
of the valves 2, 5, which then accommodates the major flow through the 
measurement device or unit. The flow monitor is thus designed such that 
the rated throughput can be kept in a very narrow range. When the quantity 
of fluid or liquid Q flowing through the measurement device or unit 1 
reaches a predetermined pressure p sufficient to open the valve, the fluid 
or liquid is thereafter supplied to the consuming device by way of the 
non-return valve or pressure-limiting valve 2, 5, the valves being 
pre-tensioned such that they only respond and allow a throughput on 
reaching the predetermined pressure. During the movement phase of the 
consuming device, the entire stream of liquid is thus guided by way of a 
valve 2, 5 and the flow monitor 4, but seen overall, with regard to the 
total stream, only a very small partial stream passes through the flow 
monitor, due to which it is possible to considerably restrict the 
measurement range of the flow monitor and thus to considerably increase 
the accuracy when monitoring the consuming device which is under pressure 
in the inoperative phase, so that even the smallest leakages of the 
consuming device produce a useful fault signal. Since it is only possible 
for liquid to flow though a non-return valve or pressure-limiting valve in 
one direction, at the time of the return stroke of the consuming device of 
a hydraulic plant, the second non-return or pressure-limiting valve 2a, 
and 5a connected in anti-parallel to the first valve 2, 5 opens and allows 
the discharge of the quantity of liquid, whereupon a new operating cycle 
can begin.