Howard Hinton
Diamond Shamrock Inc.
Sunray, Tex.
Harsh operating conditions and exacting measurement specifications guided Diamond Shamrock Inc.'s choice of a metering system for its new pipeline feeding crude oil to the company's Mckee refinery in the Texas Panhandle.
Following extensive research, the company chose Brooks Instrument's 8-in., P-Style, bi-rotor meter with automatic pressure lubrication.
Automatic pressure lubrication is a closed system which uses pipeline pressure to provide a lubricant bath in which meter bearings, timing gears, and counter-drive gears operate.
Such systems are used for abrasive, dirty, or dry liquid applications in which abrasion is commonly a problem.
ACCURACY; HIGH RESOLUTION
In 1992, Diamond Shamrock constructed a 14-in., 271-mile crude-oil pipeline from Wichita Falls, Tex., to the company's refinery at Mckee, 12 miles northwest of Dumas, Tex. (OGJ, May 24, p. 79).
The pipeline has two pump stations: Carey station has metering facilities while Groom station does not. This line can deliver 100,000 b/d.
The measurement equipment selected for this pipeline had to be both rugged and accurate. Engineering data specified that accuracy must be held to 0.10% while operating within a flow range of 750-2,400 bbl/hr.
The company further determined that to achieve desired results, the measurement system would require a meter which offered a high-resolution output performance (electronic pulse) and minimal mechanical gear-train slippage.
In addition, because of the distant location of maintenance personnel (2-4 hr from any of the measurement stations), the measurement system would have to operate with minimal maintenance.
Another important feature in Diamond Shamrock's selection of measurement equipment would be its capability for meter proving.
It was decided that Wichita Falls would have a stationary prover. Proving at the Carey station (midpoint check) and Mckee refinery would be accomplished on the company's Brooks small-volume prover.
Because of the crude oil being measured and the type of proving method to be employed, the meters chosen would need to have characteristics common to positive-displacement metering principles and the pulse-resolution capabilities associated with turbine meters.
METER SELECTION
These criteria guided extensive research and analysis.
Meters of several types and manufacturers were examined. Maintenance programs and problems, proving sequences and repeatability, and overall performance were considered.
One meter selected for testing was a positive-displacement type bi-rotor meter with an automatic pressure-lubrication design. Research and analysis of this meter involved a variety of locations including an Abilene, Tex., truck station known for problems caused by sand and dirt.
Following the research Diamond Shamrock decided to install the Brooks 8-in., P-Style, bi-rotor meter with automatic pressure lubrication.
Key design features leading to the decision included extended flow capabilities, available electronic-pulse resolution, high accuracy and extended record of low-maintenance performance.
The meter of choice, Brooks P-style (pulse type) bi-rotor, is a positive-displacement meter which uses the mechanical action of two helical rotors in conjunction with associated electronic engineering to produce an output signal used in the exact measurement of liquid product.
The signal is generated by passing a slotted rotating disc through the magnetic field of a reluctance-type "pick-off," a sensing device that responds to movement and produces an electronic signal.
It is mechanically attached to and turns in unison with the meter's timing gears.
The electronic signal is shaped by an amplifier mounted directly on the meter then transmitted to readout or control instrumentation. Dual-channel operation is achieved by use of two pick-off assemblies reacting to a common disc where the outputs are 90 out of phase.
AUTOMATIC LUBE
A prominent feature of the Brooks bi-rotor meter selected for the project is the automatic pressure lubrication (APL) option. It provides proper meter lubrication regardless of the characteristics of the product being measured (Fig. 1).
All measuring unit bearings, timing gears, and drive gears are isolated from the measured product and operate in clean lubricating oil.
This makes the APL bi-rotor suitable for liquids with entrained abrasive solids such as the crude oil being measured in the Diamond Shamrock project.
The principle components of the lubricating system include an hydraulic cylinder and piston with a differential pressure-relief valve, needle valve and lubricant fittings, and fluid isolating seals.
The hydraulic cylinder serves as a lubricant reservoir, segregates the flowing stream from the lubricant, and ensures that the lubricant pressure is slightly greater than the internal meter pressure during operation. This ensures continuous pressure lubrication.
OPERATION
The new pipeline began flow in September 1992.
Included were two Brooks meters installed at Wichita Falls and at the refinery and a single high-pressure meter installed at the Carey station (Fig. 2). Each location includes expansion capabilities for one additional meter.
As with any new system, some adjustment was required to perfect operation. The vast majority of "bugs" found, however, affected the meter's performance only slightly or not at all.
To date, 3.9 million bbl of crude oil have been delivered to the Diamond Shamrock refinery through this pipeline.
In February of this year, a measurement loss/gain performance was 0.01%.
All five meters have performed up to the expectations of Diamond Shamrock. The meter at Carey station has surpassed expectations in meter accuracy and linearity.
Most impressive was the linearity test conducted in March 1993 at Carey station.
The meter was calibrated at flow rates from 1,000 to 2,000 bbl/hr. Over this range, 16 provings were performed, producing a total factor deviation of 0.012% (Fig. 3).
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
