August 25, 2004
By Dennis Hall, Lead Metering Technician
Pump Station No. 1
Alyeska Pipeline Service Company
Prudhoe Bay, Alaska
Quick-Disconnect Cables Ease Workload for Instrument Technicians
The Alaskan North Slope may not be “the end of the world,” as some contend, but it is certainly an isolated place where workers face unusual challenges. Many work 12 hour shifts, 7 days a week, and anything that can be done to make jobs easier is greatly appreciated.
The Alyeska Pipeline Service Company operates Pump Station No. 1 at the northern terminus of the 800-mile long Trans Alaska Pipeline (TAPS), measuring and taking custody of about one million barrels of oil each day from five different North Slope producers. Alyeska’s goal is to measure as accurately as possible, the inflow of crude oil, a task complicated by the fact that each stream comes in through each line at different pressures, temperatures, and specific gravities (all within the TAPS specification). All raw measurements must be converted to gross standard volume so each producer can be fairly credited before the streams are combined and pumped south.
In order to assure metering accuracy, a number of requirements for custody transfer metering instrumentation that meet the American Petroleum Institute (API) standards, are observed by Alyeska’s Measurements Team. For example, Alyeska annually certifies the accuracy of some 53 temperature transmitters and their temperature sensing elements (RTDs) throughout the facility. Each transmitter and RTD must be removed from its field location, brought into a controlled environment, and placed in a temperature bath where technicians perform a five-point calibration.
The original transmitters were installed using rigid conduit, some with and some without explosion-proof flex connections. Later installations used what is called mineral insulated (MI) cable that was susceptible to damage despite its limited flexibility. None of these transmitters were easy to remove for annual certifications. Technicians had to loosen screws from terminals, remove the lugs, and pull wires through fittings and transmitter housings. Threads were easily stripped and electrical shorts and damage to the wires were fairly common. The rigidity of the wiring made things difficult. Consequently, Alyeska technicians began looking for a solution to the time-consuming removal and reinstallation of field devices.
Alyeska requirements specify a primary thermowell plus a second, separate verification thermowell in each custody transfer temperature measurement location. This redundant system is necessary to assure that accurate measurement of the temperature of the incoming crude that is important to the calculation of the volume of oil received is not lost. Originally, readings from the secondary RTDs at each producer skid were checked daily during meter proving by an operator in the field using a handheld monitor. These readings were then compared with the number continuously transmitted from the primary transmitter and RTD on the end device in the control room. Finding a more accurate and effective but less labor-intensive means of comparing the two temperature readings became another objective.
Application No. 1
To quickly remove and reinstall the temperature transmitters and RTDs without damaging the wiring, multi-conductor flexible plastic encased armored cordsets, with molded terminations on each end were installed. InterlinkBT cordsets, made by Turck, Inc., were used. These cables are instrument tray-cable rated and have locking devices that conform to the requirements of Class 1 Div 2 hazardous areas. Since Alyeska has both standard 4/20 MA analog and digital devices, it was also important to use cables suited for both, and be able to differentiate between them. This was accomplished with different colored cables and mated plugs and receptacles, making it virtually impossible to plug a cable into the wrong place.
All 4/20 MA and fieldbus temperature transmitters have now been equipped with these plug-in cordsets. Figure 1. When instruments are due for calibration, a technician can quickly unplug the transmitter and RTD without observed damage to the cables or connectors. Following calibration, the instruments are easily reinstalled.
Application No. 2
For existing locations, the solution was to install single enclosures containing two transmitters that could take readings from more than one RTD element. Rosemount transmitters were used to receive input from both the primary and secondary RTDs in one instrument. Again, Turck had a solution -- a multi-conductor “T” connector (Figure 2), compatible with the connectors described earlier. This enabled cables from the two RTDs to be plugged into the dual transmitter housing. This connector allowed Alyeska to eliminate the handheld temperature monitors and incorporate another, more accurate transmitter. Readings from the two RTDs are alternately displayed and compared at the transmitter, and observers are informed instantly if a significant variation exists.
This arrangement does not apply for the one metering skid equipped with fieldbus instruments, because a separate transmitter was installed on each thermowell. However, the original wiring has been replaced with Turck plug-in cordsets since these transmitters and RTDs have to be removed annually for calibration.
Application No. 3
Once flexible quick-disconnect cordsets had been installed on all temperature transmitters, Alyeska elected to consider using them wherever devices are frequently removed for calibration or maintenance.
One of the next applications was on the turbine meter spool pieces. These may be serviced several times a year. Figure 3. Before the spool piece can be rolled out to gain access to the turbine meter, two pre-amplifier boxes mounted on the side of the spool piece must be removed. These pre-amplifiers pick up magnetic pulses from the turbine rotor. These pulsed signals are converted to barrels of oil by a flow computer system. All these installations were originally wired with rigid conduit and were difficult to disconnect and move out of the way. Before the cordsets were installed, damage to the wiring was common, resulting in barrel count errors. This possibility has been eliminated through the use of the plug-in cordsets.
Ease of maintenance was the primary driving force behind all these changes. The flexible, removable cables and cordsets make the work quicker, neater, and much easier for the technicians. They save time, and time is money. A higher level of accuracy has also been achieved by eliminating potential wiring problems. As a result, metering skid operation has improved and is more efficient.
The Measurement Specialists who oversee custody transfer for Alyeska are very pleased with the results of the changes made to the Pump Station No. 1 metering skids.