L.A.C.T. UNIT DESIGN
By Craig A. Francisco Welker Flow Measurement Systems,
Inc. 13839 West Bellfort Sugar Land, Texas 77478
The Lease Automatic Custody Transfer (L.A.C.T.) unit
is designed for the automatic transfer of ownership
of crude or condensate between the buyer and seller.
This can be on land or offshore, into pipelines, barges,
and tanker loading and offloading operations.
On land L.A.C.T. units are usually small, single run
systems with portable proving connections designed for
loading into trucks.
Because of the geographical area, this paper will focus
on the offshore L.A.C.T. units. These are dual run with
100% back-up and with mechanical displacement meter
provers at 150 ANSI working pressures.
The basic components and function
of a L.A.C.T. unit are:
1. Charge Pump and Motor - Largely overlooked
and undersized, special care should be taken into consideration
during sizing to ensure correct NPSH is available to
prevent cavitation, and discharge pressure is enough
to overcome pressure drop through the L.A.C.T. to allow
the required flow and pressure to the pipeline pump
2. Strainer/Air Eliminator - Strains solids
larger than the perforations in the removable basket
with liner. They should have differential pressure indicator
to show pressure drop caused by debris accumulation
and be cleaned periodically. Essential to prevent premature
meter wear or breakage. The air eliminator is located
on top of the strainer at the highest part of the system
to allow air to be discharged and not metered. This
should be piped with a soft-seated check valve to prevent
air from being introduced into the system during shutdown.
3. Sample System - Installed with an upstream
static mixer usually flow-proportional, isokinetic,
and tubed to a vapor tight storage vessel sized to allow
25 to 30 days storage. The vessel is provided with a
recirculation pump, the samples are mixed and then drawn
off to be checked for composite API Gravity and BS&W
during the delivery period.
Monitor and Probe - Installed downstream
of the sampler and upstream of the three-way divert
valve, this unit consists of a flanged probe that monitors
the flowing stream for basic sediment and water and
communicates with the "monitor" unit that
is normally installed in the control panel. The "monitor"
is usually set at 1.0%, and is wired to the solenoid
valves controlling the three-way valves on the bad oil
divert line. These will send oil to be treated if a
BS&W signal is received for a given time. When
a good oil signal is received for a set time, then the
valves will return to normal flow position.
5. Meter - Installed downstream of the three-way
valve and downstream of a properly sized thermal relief
valve. The meter measures the product stream and allows
totalization either through a local totalizer or electronic
pulses to a flow computer. This meter can be a positive
displacement or turbine meter.
The meter will provide signals to the flow computer
or PLC to allow:
- Sample Pacing
- Meter Proving
- Meter Fail
6. Meter Instrumentation - Downstream of the
meter a spool consisting of:
Temperature transmitter with platinum RTD installed
in a S.S. thermowell.
Pressure transmitter with a pressure gauge mounted
with a three-way gauge valve.
Test S.S. thermowell used to calibrate the temperature
The temperature and pressure transmitters are used
to send a live reading to the flow computer for compensation.
7. Check Valves - Downstream of the meter to
prevent backflow to the meter in case the downstream
block and bleed valve is left open and the opposite
meter train is running.
8. Block and Bleed Valves - Located downstream
of the check valves at the end of each run and as the
main line divert valve separating the to and from lines
to the prover four-way divert valve. This is to ensure
all fluid is being diverted to the prover during proving,
or a false meter factor could be obtained during proving
of the meter.
9. Prover Instrumentation - On the outlet of
the prover four-way divert valve a spool consisting
- Temperature transmitter with platinum RTD installed
in a S.S. thermowell.
- Pressure transmitter with a pressure gauge mounted
with a three-way gauge valve.
- Test S.S. thermowell used to calibrate the temperature
- Thermal relief valve properly sized.
10. Back Pressure Valve - On the skid outlet
to maintain pressure above the vapor pressure of the
fluid being metered and provide a constant pressure
and flow on the meter during proving.
11. L.A.C.T. Control Panel - This can be located
on the skid with PLC controls and local manual proving
connections would then be required, for a prover counter,
detector switch plug in, power for the counter, and
a portable pulse generator for P.D. meters.
An electronic temperature averager could then be used
in lieu of temperature transmitters; however, due to
their flexibility and relative cost, flow computers
are rapidly replacing them.
If located in the MCC room, the panel could then be
equipped with a PLC, flow computer, and printer to allow
for automatic proving and batch reports by pushing a
button, provided the prover's four-way valve is equipped
with a remote actuator, and pressure and temperature
transmitters were installed.
The control panel will have the following functions:
- Start and Stop Off High and Low Level Switches
- Hand-Off-Automatic Switch
- BS&W Divert Controls
- Meter Fail
- Monitor Failure
- Internal Battery Back-Up for Power Loss
12. Calibrated Bi-Directional Meter Prover - Because of the versatility of configuring a bi-directional
prover in tight offshore spaces and its' cost associated
with normal offshore flow rates; the bi-directional
prover has become the prover of choice versus the uni-directional
and small volume prover.
The continuous flow technique of meter proving is
accomplished by repeatable displacement of a known volume
of liquid in a calibrated section of pipe between two
signaling devices (detector switches).
A slightly oversize prover sphere inflated to normally
2% over the pipe inside diameter is used to displace
The fluid is run through the meter and the prover.
The metered volume is recorded by the electronic meter
proving counter (built into flow computers).
The known volume displaced is checked against the
meter's indicated output and a "meter factor"
is obtained after correction factors (Ctl)(Cps)(Cts)
and (Cpl) are applied.
The use of new work over chemicals now seen offshore
suggests internal coatings be exclusively a baked on
phenolic versus an air-dried epoxy for longer life.
The prover sphere most seen nowadays has gone from
the standard 53 Durometer hardness to a 58 Durometer
harder material for longer life and durability.
First and foremost, if your L.A.C.T. unit does not
meet your buyer's or shipper's standard specifications,
you may find yourself offshore with a tank full of oil
having to shut in your wells because your L.A.C.T. skid
has not been approved, or they just were notified that
you want to come on-line with a skid they have never
seen. Get hold of the specifications and get them involved
early in the fabrication of the unit.
Offshore space is a premium. Units can be built in
any number of configurations and footprints, but also
consider the serviceability of the unit. Viscosity
High viscosity can cause several problems.
1. Does the meter (P.D. type) require high viscosity
clearances and temperature trim on units above 150°
2. If heat traced and insulated, the instrumentation
may require new trim.
3. Check prover sphere operating temperature.
4. Is NACE trim required?
Is the L.A.C.T. unit only going to see one platform's
oil, or is there a chance that a new platform's oil
will be shipped over next year when it comes on-line?
Do you need to design the skid for an additional meter
The recent acceptance and development of new electronic
equipment for crude oil measurement in the past few
years has made them more reliable and accurate while
requiring less maintenance. However, the technician
needs to be more highly trained than ever before.
Many factors go into the design of L.A.C.T. units
flow rates, space limitations, temperature, viscosity,
corrosion, and customer specifications. As long as these
are taken into consideration, one can end up with a
quality measurement system.
The EESIFLO Series Watercut Monitor is a solid state electronic instrument that determines the percent watercut in an oil emulsion without requiring a physical separation of the fluids. The direct measurement of dielectric permittivity technique employed by the Watercut Monitor allows the emulsion stream to be continuously surveyed. The electrical feedback generated by the monitor oscillator is determined by the watercut of the emulsion stream in the probe.
article has been contributed by Craig A. Francisco
Welker Flow Measurement Systems, Inc.