Well I hate to admit this.... but I really don't know
either.
Maybe someone will chime in and explain how loop sheets
differ from ladder logic schematics?
John
A Loop Sheet, in the context I believe your are using it in, is associated
with individual, or multiple, control circuits. It can be represented in a
few different ways. It could a diagram of one individual loop, or the loops
of a complete system, or a number of interrelated systems. They could also
be presented in a few different formats. There is usually additional data on
the loop sheet relating to the loops, or circuits represented. That could
include, but not limited to, specs. on loop instruments, splice or
termination points, set points (temp, flow, and pressure, settings),
location of instruments, and other data pertinent to the loop, or system.
The loop sheet is necessary for tuning, trouble shooting, maintenance, and
changes to the process being controlled.
An example of a simple loop sheet for a system could be a tank with product
being drawn out for a manufacturing process. The factors needing control
might be the amount being drawn out and at what rates, the amount being
replaced and the rate needed to keep the tank at a constant level with the
tank influent being a mixture of multiple liquids each needing to be added
at a specific ratio while being supplied at different and changing
pressures, the possible need to keep the tank contents at a specific
temperature, and all these variables needing to be kept within a tight
tolerance to meet some standard, or risk substandard production of the end
product.
A loop sheet could be equated to a schematic, or print of an electrical
system, and would be just as useful, or useless, depending on if its been
kept up to date or not, and the ability to locate it at a moments notice
(it's 3 AM - all the managers and engineering staff are away from the plant,
liquid is spilling from the overflow of multiple tanks, the maintenance
staff can't access the loop sheets because the manager keeps them locked in
his desk in his locked office, and the process could become extremely
unstable, might be hazardous, be worth thousands per gallon, or take weeks
to restart production if it isn't shut down properly).
Working on a complex, or even some simple, control systems without loop
sheets would be like a doctor having orders to do an amputation, and not
being given the information as to which appendage needs to be removed, and
being told, "oh, he's in there", accompanied with an arm gesture towards the
hospital. :-] Hey don't laugh, it happens!
Louis--
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