MTS Model 286-30 Pore Pressure Intensifier - Manual (Page 24)

286.30 Pressure Intensifier 24
Pore Pressure System Operation Considerations
Operation
Closed-loop Control Modes Used for Pressurizing Pore Fluid
The pressure intensifier inside the PPI operates under closed-loop control
principles.
External electronic controller equipment receives signals from a pressure
transducer in the PPI, and also from a linear displacement transducer, which
indicates current pressure intensifier piston position relative to a reference
position. The magnitudes of the two (dc) signals are proportional to their
respective variables. Either one of these signals can be selected, as a feedback
signal for control purposes, at the controller; both are simultaneously available
for readout purposes.
Pressure control mode
Calibration of the electronic controller equipment is such that the signal from the
pressure transducer relates directly to pressure inside the triaxial cell, in MPa or
psi, depending on how the controller equipment was calibrated. When the output
of the pressure transducer is chosen as a feedback signal for the controller,
pressure becomes the controlled, or independent variable, and displacement
becomes a dependent, or uncontrolled variable. This is referred to as the pressure
control mode.
The pressure control mode is the preferred control mode for pressurizing pore
fluid.
Displacement control
mode
The signal from the PPI’s linear displacement transducer, however, is usually
calibrated to relate to the volume of fluid that is displaced as a result of the
pressure intensifier’s piston being displaced some linear distance from a
reference point. When the output of the displacement transducer is chosen as a
feedback signal for the controller, volumetric displacement becomes the
controlled, or independent variable and pressure becomes a dependent, or
uncontrolled variable. This is usually referred to as the displacement control
mode and calibration is usually in either cubic inches or cubic centimeters.
The displacement control mode is less desirable for use than the pressure control
mode because very small changes in displacement cause very large changes in
pressure. Also, if the displacement control mode were used while the triaxial cell
was being heated, the controller would be insensitive to fluid expansion due to
increasing temperature and very high pressures would rapidly develop.
If the triaxial cell is equipped with heaters, it is important to remain aware of the
effects of thermal expansion of the fluid when the Pore Pressure Intensifier is in
operation. This is described in
“Effects of Thermal Expansion of Pore Fluid” on
page 26.
The displacement control mode is most useful while performing PPI operations
such as filling the pressure intensifier or positioning the piston of the pressure
intensifier before starting a pressurization sequence in the pressure control mode.