Density Control
The cement mixing system
can be operated in a manual or automatic mode. In manual mode the operator
uses a manually operated hydraulic control valve to control the cement
metering valve. In the automatic mode, an electro-hydraulic control system
operates the cement metering valve. The electro-hydraulic system is controlled
by an industrial process controller/computer. The slurry density can
be quickly reduced by the use of a bypass line (connected to the recirculation
pump suction).
Serva
Automatic Density Control System
System Design
Three component groups
work interactively together to produce the most consistent quality
cement possible. They are 1) cement metering valve, 2) high energy
mixer, and 3) the automatic control system. The Cement Control Valve
controls the incoming bulk cement delivery to the High Energy Mixer.
The Automatic Metering System consists of the Operator Interface Panel
and Microprocessor which controls the Cement Metering Valve position.
The control responds to measured mix water rate, slurry density and
slurry design. The Microprocessor receives measured data and input
commands and outputs corrections to the cement metering valve position.
The "Go-To" control algorithm will respond and adjust to
changes in inputs before errors occur in the slurry mix. The Control
Algorithm will respond to three possible changes during the job. (1)
Changes in the throughput rate, which are reflected in changes in the
mix water rate. (2) Changes in set point density (operators may "thicken" the
slurry at the tail end of the tail slurry). (3) And changes in cement
delivery due to irregular and/or declining cement delivery from the
bulk supply. The microprocessor, while in simulation mode, provides
systems function verification to test all lines, cables, flowmeter,
hydraulic system, feedback system and the microprocessor itself. This
simulation mode may also be used to train personnel in manual and automatic
density control at no additional cost.
Components
Bulk Cement Metering
Valve
The rotary actuated cement-metering
valve adjusts the rate of flow of bulk cement. It may be operated manually
or hydraulically. The valve is approximately linear and will provide almost
equal increases in bulk cement slurry delivery per increment of rotation.
A particularly important feature is the ability to meter cement at low flow
rates without bridging off (the phenomenon where cement stacks against the
opening and therefore prevents further passage of cement).
High Energy Mixer
This slurry mixer is
a recirculation type designed to provide high energy mixing. Reduced
maintenance requirements were incorporated into the design. Inlets include
one for mix water, two for recirculation flow, one for bulk cement and
one for slurry discharge. The operator may easily adjust the water flow
from zero to maximum. The valve is linear, providing equal increases
in water flow per each turn. Maximum water flow is a function of supply
pressure. The valve creates a high velocity jet, which intersects with
the incoming bulk cement. It provides excellent coverage even at low
rates. High recirculation rates result from four jets. This improved
recirculation mixer design increases mixing and homogenizing of the new
incoming slurry with the slurry already in the mixing tank.
Cement
Density Control System Operation and Features
Setting the Slurry
Design
The operator interface
panel and microprocessor allow the storage of three cement slurry design
settings. The operator enters the desired slurry density in lb/gal, the water
specific gravity (fresh or salt water), the design yield in ft3/SK,
mix water requirements in gal/SK and job rate in bbl/min for each slurry
design. The system defaults to the previously used value if unchanged (water
specific gravity seldom changes in a given job area). The microprocessor
calculates and displays the required mix water rate based on desired job
rate and displays actual water rate. The operator manually controls the mix
water. However, the microprocessor tells the operator the required water
rate to match the desired slurry rate.
Control Algorithm
The control system
responds to three possible changes during the job: throughput rate, density,
and cement bulk delivery.
The throughput rate
is based on the water rate that the operator has set manually. The water
flow meter measures this rate and provides feedback to the operator interface
panel and microprocessor. The microprocessor uses these data to set control
gain values and the cement valve nominal operating position. The result
is that for whatever reasons the water rate changes, the cement valve
will be repositioned appropriately without waiting for a density change
(maintaining slurry density before errors occur).
Variance in the throughput
rate may occur for several reasons. The operator may use the mix rate
as the tub level control mechanism rather than use down hole pumps. The
pump rate may have to be changed due to possible pressure limitations
on casing or for job design reasons. Also inadequate bulk cement delivery
may require the job to slow down.
In the case of such
changes, the Serva Auto Mixer system immediately recognizes the change
in water rate and moves the cement valve to a new position based on current
measured valve delivery rate. Thus the density is only minimally disturbed.
Correction is made before the density error shows up. Typically, less
than 0.012 g/cm3 0.1 ppg change in density would occur.
The control system
responds to density set points and densitometer measurements. The microprocessor
takes density reading every second and uses the data along with water
rate to adjust the cement valve position. The algorithm is based on calculated "Go-To" values
plus proportional, integral and derivative (PID) feedback, whose coefficients
vary with throughput rate and tub volume.
Some operators like
to change the set point density so that they may "thicken" the
slurry at the tail end of the mix. In this instance, the Serva Auto Mixer
system knows the current actual cement valve calibration. It calculates
the new value position based on the current calibration, the new density
and water rate. The new valve position is not based primarily on an error
signal but a known "Go-To" position. Once it reaches that position
and there still is some error, then the PID control takes over. This
error is normally minimal.
Corrections in cement
valve position may also be required due to irregular and/or declining
cement delivery from the bulk supply. The Serva Auto Mixer continually
calculates the actual cement valve calibration, which is the amount of
cement being delivered by the valve at its current position. With the
updated calibration and the known amount of cement required for setpoint
density (calculated by the Serva Auto Mixer microprocessor) and slurry
rate, the valve is again driven to a "Go-To" position. Errors
in density and "PID" control are not the primary control technique.
The Serva Auto Mixer works from measurement and then calculates values
for the system to "Go-To" and then uses "PID" to
make the final adjustments.
The Operator Interface
Panel displays the changing cement bulk system calibration value. Therefore,
the operator has an actual number on the ability of any given bulk supply
to deliver cement. A minimum bulk delivery calibration may be established
so that if a particular system falls below this value, then corrective
maintenance would need to be planned.
Data Storage
Density data
is stored in the microprocessor in one-second intervals for up to 50
hours of recording. Additional data replace the first data entered
(first in, first out). This data, which are stored in an ASCII format,
can be "downloaded" through an RS-232 computer port connection
on the front of the operator interface panel. Data may be imported
into a spreadsheet for plotting and analysis.
The microprocessor
is also capable of recording throughput rate and pumping pressure. Thus
all important job parameters are recorded with a single unit.
Control Function
Simulation
The microprocessor
may be placed in a simulation mode, which is useful for operator training
and system functional verification. This mode allows the operator to
control the complete system as if he were doing a cement job but without
cement. It will help insure that operators are qualified and confident
in operating the system for an actual job.
In the training mode,
the operator would circulate water through the mixer without mixing cement.
The operator can adjust the cement-metering valve manually or have the
Serva Auto Mixer system control it automatically. The microprocessor
reads the valve position and calculates an assumed bulk flow based on
a microprocessor-stored curve. The microprocessor calculates the theoretical
real-time density that would have been mixed in the tub if cement had
been used. This simulaltion takes into account the mix water flow (job
throughput rate), tub volume, slurry design yield, water requirement
and sack weight.
The systems functional
verification works the same as when the microprocessor is in the training
mode. All lines, cables, flowmeter, hydraulic system, feedback system
and microprocessor are tested during the simulation.
