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ServaHEMI
High-Energy-Mixing-Injector System
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Parts List and Assembly Drawing
ServaHEMI
High-Energy-Mixing-Injector System
Our system was designed
for high energy mixing and ease of operation and maintenance. There
are many of this family of similar mixing systems working throughout
the world. The applications have varied from onshore to offshore and
dessert heat to cold climates.
The primary components
of the mixing system are
· ServaHEMI High-Energy Mixing Injector
· Cementing metering valve
Other components
which make up the rest of a complete system are:
· Mixing tub
· Recirculating centrifugal pump
· Mix water centrifugal pump
· Agitating system
· Air separator
· Transfer pump (optional for single pump cementers and required for double
pump cementers).
These items are discussed in separate documents.
ServaHEMI High-Energy
Mixing Injector
Introduction:
The latest in mixing
technology is the result of 15 years of cement mixing evolutionary
development. This latest mixer, the ServaHEMI, incorporates capability
of past systems but moves the technology two steps forward. Like its
predecessors, it is a high-energy mixer. However, its performance is
optimal over a wider range of operating conditions. It is equally as
good as a mixer in the recirculating batch mixing mode as it is in
continuous mixing (its predecessors were poor batch mixers). The mixer
also allows the use of the densitometer discharge for mixing. The densitometer
output can be connected to the central jet inlet so that there is no
longer a need for returning densitometer discharge to the mixing tank
directly. This will also apply addition backpressure on the densitometer,
which will provide better measurement (reduced effect of any air entrainment).
The bulk flow path is less restrictive and therefore has a larger capacity.
Additionally, the water control has an optional remote control to make
installations more flexible. The bulk control has always been available
with remote hydraulic control.
Description:
The slurry mixer
is a recirculating type mixer designed to meet the most demanding mixing
requirements. The mixer design provides easy cleanup and maintenance.
The mixer has a 5 ½" union thread half on the inlet for
attaching the dry bulk cement metering valve. In the center of the
mixing chamber is a high-pressure jet that wets the bulk cement from
the center of the mixing chamber. The recirculated slurry enters the
mixing chamber from an outer annular position surrounding the dry bulk
cement. The water jets enter the mixing chamber from alternating positions
with the recirculation jets. The force of the recirculating jets and
the mix water jets is directed inward and the force of the central
jet directed outward creates the energy to wet and mix the dry bulk
cement. This provides maximum wetting efficiency.
The operator can easily adjust the water flow from almost zero to maximum.
The valve is linear, providing equal increases in water flow per increment
of displacement. Maximum water flow is a function of supply pressure. The recirculating
mixer design improves mixing and homogenizing of the new incoming slurry with
the slurry already in the mixing tank.
View
Parts List and Assembly Drawing
Patent 6,749,330
Patent
6,749,330
The above drawing
illustrates possible arrangements for the ServaHEMI mixer and the cement
metering valve. The illustration on the left is the preferred arrangement
for several reasons. The first reason is that it has a significantly
lower assembly height (320 mm (12.6 in.) lower). The second reason
is that the metering valve will function more predictably when operated
at low bulk flow rates (less bridging). However, some operators prefer
the arrangement in the right illustration because the bulk inlet can
be easily turned to any position (to the right, center or left of the
cementing truck).
Bulk Cement
Metering Valve
Serva has developed
a new cement metering valve that overcomes common problems with previous
technology. Older technology valves worked reasonably well with clean
cement but would develop operation problems when dirty cement was encountered.
The new valve greatly improves the ability to handle dirty cement (rocks,
weld rod, sack material and other foreign debris) without damaging
the valve or becoming difficult to turn. Bearings further enhance ease
of operation and life. The body and metering sleeve are heat treated.
The rotary operated
cement-metering valve adjusts the rate of bulk cement flow. It may
be operated manually or hydraulically. In addition, if the hydraulic
system failed that is used to operate the valve, the valve is provided
with a means to insert a .75 in. steel bar for manually turning.
The valve is approximately
linear and will provide near equal increases in bulk cement 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).
FUNDAMENTAL OPERATING
PRINCIPLES
The high-energy mixer
provides excellent mixing, the metering valve provides good metering
of bulk cement, the diffuser eliminates bulk air and the agitator further
homogenizes the slurry. The slurry is continuously delivered to the
cement triplex pump and from there it is pumped downhole.
Its basic operating
principle is as follows:
The cement from the
bulk cement tank is delivered into the cement metering valve via a
bulk hose. The mixing water along with the recirculating flow power
the high-energy mixer to effectively mix the bulk cement.
The cement bulk flow
rate is controlled and adjusted via the proportional cement metering
valve.
The mixed slurry
is further homogenized by the tank agitator in the mixing tub.
The slurry densitometer
located on the circulating piping system detects the actual slurry
density.
The water flow meter
(optional) mounted on the mix water piping measures the water flow
rate.
Mixer,
Piping and Pump Arrangement Drawing
MIXING
OPERATING PROCEDURE
1.
Preliminary
- Check that the
inside of the mixer is clean
- Connect the water
supply hose and the bulk cement hose. The rated working pressure
of the bulk cement supply system should be 20 to 30 psi
- Connect slurry
discharge line if not part of unit
- Check that there
is adequate water supply
- Open the water
supply valve
- Check that the
centrifugal pumps are working normally
2. Operation Procedures
(Step by Step)
Start Mixing Process:
- Bring on water
with the mix water pump and fill all lines. While doing this, set
the water metering valve to 120 to 150 gpm
- Fill the mixing
tank to approximately 1/3 full and then close the master water valve
- Prime triplex
pumps
- Prime recirculation
pump
- Close the bulk
cement master valve
- Open the cement
metering valve to a position between one and two. After starting
the recirculation pump to full speed and having the bulk delivery
system pressurized to about 25 psi, open the master bulk valve slowly.
The color of the mixing tub should be changing into gray and some
dust may be present in the tub. As soon as the slurry level reaches
the agitator, turn on the agitator to the maximum speed without causing
excessive splashing. The idea is to keep the agitator at its maximum
speed to improve mixing and homogenization of the slurry without
causing vortexing or splashing
- If you are not
going to immediately start the continuous mixing process, close the
bulk master valve when the density reaches within .2ppg of the desired
density
Continuous Mixing
Process - Manual Control
(This
process will be started immediately after the tub is full or the density
has reached within .2ppg of the target density.)
- Start the continuous
mixing process after the "Start Mixing Process" is completed.
Start the mix water pump, open the water metering valve approximately
20% to obtain 120 to 150 gpm (It should be already in this position)
and open the master water valve. The bulk cement valve should still
be open and therefore cement should be flowing. If it is not open,
open it.
- Adjusting the
tub level: Start pumping down hole when the level reaches within
about 18" from the top. Adjust pump speed (triplex) to maintain
tub level (about 18" +/- 6"). The tub level can also be
adjusted by changing the mixing rate. When adjusting the rate, note
that both the water and cement flows will require adjusting to maintain
a constant density. This is why it is better to start the mixing
process at approximately the required rate and adjust the tub level
by only changing the triplex downhole rate. This will provide a smoother
density chart while manually mixing.
- Maintaining density
as the bulk unit pressure declines during the delivery of cement
from an individual tank will require opening the bulk metering valve
gradually. Open the valve only one quarter to one half (1/4 to ½)
of indicated increment (i.e. 2.5 to 2.75 or 3.0) and observe the
response. If the density is still falling, open the valve again in
small increments.
- When changing
bulk tanks it will be necessary to reduce the bulk metering valve
position to deliver the correct amount of cement. The new tank will
probably be at a higher pressure (pressurized before the job started)
which will deliver too much cement relative to what is required.
The amount the valve position will need to be reduced will depend
on the job rate (the higher the rate, the more the pressure will
decline in a given tank). This is also a function of the ability
of the compressor to keep up. The larger the compressor, the more
stable the pressure and therefore the more stable the bulk cement
delivery.
- When ending the
mixing process, allow the bulk delivery system to "blow down" while
the metering valve is in full position.
Cautions & Warnings
Before the job:
- Read the operation
manual carefully before starting the skid.
During the job:
- Watch for the
possibility of the densitometer becoming plugged.
If during the job, the density reading increases quickly to higher than required
values, the densitometer may be plugged or be partially plugged. This condition
can be verified by looking at the densitometer measurement reading, which
has a flow measurement. If the flow is zero or below 2 BPM, the meter is
plugged or partially plugged. To solve the problem, manually close and then
open the recirculation valve (below the mixer). This should clean the blockage
out. A blockage is an inclusion or poorly mixed cement, which may have several
causes. This may be caused by not enough mixing energy due to not operating
the circulating pumps at maximum RPM. Also if the system is operated at rates
that are too high for difficult to mix slurries, the mixing energy may be
insufficient. The solution is to slow the mixing process by 20 to 30%.
After
the job:
- When washing the
unit, do not wash the densitometer external transducer with high-pressure
water.
- Clean the cement
metering valve after each job by blowing out with air and physically
emptying dry cement. Do not flush the valve with water.
- While the cement
metering valve is removed, wash out the inside of the mixer (the
area through which the bulk cement passes) with the wash-up hose.
- After each cement
job it is critical to immediately flush the mixing tub, the piping
system, the densitometer and the slurry centrifugal pumps.
- Drain the centrifugal
pumps, the mixing tank and all equipment that may contain water after
each job, especially during freezing weather.
Maintenance:
Welding:
When welding is required, extreme care must be employed to prevent damage to
electronics.
- Disconnect starting
battery
- Disconnect any
computers and any other electronic equipment
- Be careful not
to allow welding leads to come close to electric cables and wires
associated with electronics including transducer wires.
- It is best to
remove or totally isolate electronics before welding.
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