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System requirements
With a level switch having maximum and minimum
contacts only, there is control over two points of liquid level
but no information or control in between. What is needed is a
fluid level monitor which gives a continuous signal related to
the level between the maximum and the minimum. This continuous
level monitoring should provide a standard 4-20mA analogue signal
output.
Figure 2 shows an example of the changes in
reservoir fluid level taking place over a single production cycle.
In many modern plants the analogue signals representing the fluid
level could be fed into an electronic data processing (EDP) system
and it may possible to "teach" the level movements to
the EDP over the complete cycle, so that these represent the datum
against which unexpected changes become apparent.
Figure 3 shows how this applies to a portion
of the cycle from Figure 2. A defined span of acceptable values
is allowed either side of the datum but movement outside it is
cause for investigation. Depending on the size of the reservoir,
this might indicate a loss just a few litres of fluid, but leakage
of even that amount can cause disruption and environmental damage
that may be costly to rectify.
Clearly this degree of control is possible only
where the fluid demand cycle is repetitive. This applies to a
large and growing number of hydraulic systems powering automated
processes (such as injection moulding).
A number of surveys have shown that the greatest
potential for fluid loss occurs when the reservoir is being filled
or topped up. An effective solution is to employ a motorised pump
set and use the "reservoir full" signal from a level
control unit to switch off the pump motor. If something along
these lines was made compulsory throughout the industry, a great
deal of fluid would be saved and much expenditure on cleaning
up would be avoided.
The
Buhler level control unit The
unit providing the analogue signal output has been the subject
of much development work. Initially it was intended that this
should provide a completely smooth variation of output in response
to level changes, but this entailed the use of a larger float
and heavier magnets. The combination of mass and viscosity effects
led to an unacceptably slow response to changes in level.
The system now in use is based on a series of
closely spaced reed switches in a low voltage circuit that produces
the 4-20 mA output signal. This is unaffected by cable length
and electromagnetic disturbances. The unit is flange mounted with
the same fixing dimensions as a standard filler/breather unit
and is thus can be conveniently combined with other tank mounted
facilities. It is offered with various combinations of filler/breather,
sampling port and an electronic temperature sensor with a 4-20mA
output and up to five setpoints.
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