MTS Model 286-20 Confining Pressure Intensifier - Manual (Page 25)

Confining System Operation Considerations
286.20 Pressure Intensifier Operation 25
Use of Error Detectors and Limit Detectors While Pressurizing
Information under heading Effects of Thermal Expansion of Confining Fluid
describes two situations in which confining pressure can be, or can become, an
uncontrolled variable. These situations, and actually all “normal” pressurizing
procedures, warrant the use of whatever devices are available, on the electronic
controller equipment, that enable system conditions to be monitored and system
operation to be shut down in the event abnormal or undesirable operating
conditions occur.
In the first situation described under heading Effects of Thermal Expansion of
Confining Fluid , the controller is controlling pressure but is unable to
compensate for additional pressure increase caused by further thermal expansion
because the piston of the pressure intensifier has bottomed out and cannot retract
any further. In this case, two monitoring circuits, common to most controllers,
would afford some protection, if properly pre-adjusted.
Error detectors Error detectors monitor the amplitude of the controller’s error signal
(proportional to the difference between command and feedback) and are
therefore always associated with the controlled variable. In the situation where
the controller becomes unable to compensate for further thermal expansion
(because the pressure intensifier is unable to retract any further), the error signal
will begin to increase in magnitude as soon as the controller can no longer
compensate, if further thermal expansion occurs. If the error detectors had been
adjusted to be sensitive to small error levels, the pressurization procedure would
be terminated very soon after the problem occurred.
Limit detectors Limit detectors can be set up to monitor the level of variables, whether the
variables are independent (controlled) or dependent (uncontrolled). In the first
situation described under heading
Effects of Thermal Expansion of Confining
Fluid , the controller’s limit detector could also have been set up to terminate the
pressurization procedure in the event that pressure exceeded some pre-
established limit. With both error detectors and limit detectors in use monitoring
the controlled variable, the pressurization procedure would be terminated by
whichever detector reacted first.
In the second situation described under heading Effects of Thermal Expansion of
Confining Fluid , with volumetric displacement the controlled variable, the error
detector could be adjusted to react to some unusual level of error (associated with
the controlled variable, volumetric displacement) and the limit detector could be
set up to actuate if the dependent variable, pressure, exceeded some undesired or
unanticipated level.
Since an unanticipated high pressure level is nearly always of greater concern
than is high volumetric displacement (although a displacement limit could be
caused by a leak, which could be extremely dangerous), pressure should always
be monitored by either limit detectors or error detectors, or both. Limit detectors
provide some advantage over error detectors in that precise limit levels are easily
established. Error detectors actuate when feedback (the actual level of the
controlled variable) deviates from command (the desired level of the controlled
variable) by some presettable amount and they can detect such errors regardless
of the level of the controlled variable. Both detector types offer unique
advantages and should be used simultaneously and judiciously according to the
test situation at hand.