Liquid measurement field survey proves comprehensive.
Loss/gain may have exceeded allowable tolerances. Most pipeline operators monitor performance of their system in terms of loss/gain (also called over/short) which is a mathematical comparison of pipeline receipt quantities, delivery quantities and changes in internal inventories over a given period of time, for example, monthly.
Losses and gains are often plotted on a control chart with control limits which are derived mathematically from the normal loss/gain data. An example is shown on Fig. 1. If the loss or gain for a given month exceeds either the upper control limit or the lower control limit, as shown by the last data point on Fig. 1, a measurement survey is usually initiated to find the cause of the excessive loss/gain.
[FIGURE 1 OMITTED]
An team from the corporation auditing department may be coming out to the field to conduct an audit. In which case, it is a good idea for field personnel to conduct their own review of facilities and practices to be sure that things are up to snuff before the audit team arrives. This is particularly desirable because auditors are usually very good at accounting, but may have only limited knowledge of measurement practices. They usually will check to make sure that field operators are performing their measurement duties in accordance with written company operating manuals and/or industry standards. But, lacking actual operating experience, auditors may make recommendations that are not practical. Hence, it may be a good idea for field operators to make their own survey and make recommendations for changes in practices or procedures if needed.
Pipeline tariffs often include provision for a certain amount of normal operating "Loss Allowance." This means that normal operation with properly maintained equipment and proper measurement procedures will still allow for small losses. These normal operating losses are not actual physical loss of liquid--as would be the case with a leak--but are simply the result of small uncertainties that are inherent in any measurement system. These losses may be statistically definable, but usually cannot be changed significantly without making major--often expensive--equipment changes. Excessive operating losses can be costly if the tariff requires that shippers be compensated for losses that exceed the published Loss Allowance.
Owner companies of a joint venture pipeline system will often put together a survey team comprising measurement specialists from each company to review measurement operations and make recommendations to the operator of the pipeline.
Industry practices change and new equipment becomes available from time to time. A survey may be useful to determine if equipment or procedural changes would improve measurement performance.
Benchmarking is often used today to compare a company's performance against that of similar companies. This can be a useful exercise, but can be misleading when trying to benchmark pipeline systems against each other. Each pipeline tends to be an individual, and exhibits its own unique characteristics. It is difficult to compare two pipeline systems unless they are virtually identical in terms of physical configuration, measurement equipment and measurement practices. Benchmarking pipeline systems often leads to impractical recommendations that may work for one system but will not work for another. Nevertheless, surveys of two similar systems can yield some good ideas for consideration--as long as changes are not mandated just because someone else is doing something different.
Liquid measurement surveys often are conducted by a company's own personnel selected from those who are especially skilled in all aspects of measurement. A survey may be conducted by one expert or by a team of qualified people. A survey is also a good opportunity to train people who are moving into the measurement arena. Alternatively, an outside consultant can be hired to impart an arms-length quality to a survey, particularly if important business or legal ramifications are likely.
What Can Be Learned From Surveys
Liquid measurement field surveys can determine if measurement equipment is installed and maintained in accordance with manufacturer's recommendations and is operated and calibrated in accordance with industry standards.
Surveys can determine if field personnel are properly trained and are performing their measurement duties in accordance with company practices and/or industry practices. People tend to develop shortcuts when left to their own devices. Some shortcuts may be OK. Others, even though innocent in intention, can lead to measurement inaccuracies. Often, simple retraining and review of written practices can eliminate sloppy procedures and sharpen performance. A good training program for new measurement personnel is important. The apprentice system doesn't work very well in today's environment where excellence is imperative. The old guy that has been gauging tanks, for example, for 35 years may have been doing it wrong all that time or, at the least, may not be up to date on current practices, and he will teach wrong methods to the new guy. Similarly, periodic retraining programs for experienced personnel will make sure they are using current practices.
Surveys can identify equipment and instrumentation that should be considered for replacement.
The process of writing a run ticket, which documents a custody transfer liquid volume and quality, seems like a simple straightforward process. Yet, there is ample opportunity for errors between the initial preparation of the run ticket and the final liquid quantity and quality that is booked for a shipper by the accounting department. A thorough measurement survey will follow a run ticket all the way from its inception until the final booking to ensure that proper correction factors and current meter factors are used in calculations and that there is an established procedure for verification of Run Tickets.
Sometimes a survey will show that a pipeline operator is doing as good a job as possible with the facilities he has in his or her system. The senior management of a large terminaling operation that stores and delivers a variety of refined products for other companies hired an outside consultant to evaluate the overall measurement practices of the entire system. Most of the measurements at the terminal were made by hand gauging in more than two dozen tanks. Deliveries were made through a variety of customer-owned meters at a half dozen different locations.
Management was concerned that mixing tank gauge measurements and metered volumes might introduce errors that could be hidden in the mass of data accumulated every month. There also was some concern that customer meters might not be installed and operated in accordance with industry standards. The results of the survey determined that all tank gauge measurements at the terminal were well within statistical limits and industry practices. Metering at delivery points was judged to be adequate, although some valving arrangements at several delivery points could permit product to bypass meters if the valves were not properly maintained.
The operator of another large refined product terminaling operation was experiencing excessive losses. A survey identified a leaking valve in a stationary meter prover as the source of the losses.
A shipper on a crude oil gathering and transmission pipeline system complained that he was losing API gravity points on his crude oil shipments, and pointed to several measurement procedures that he thought could be responsible. A survey of measurement practices on the system revealed that the suspected measurement procedures could not possibly cause the magnitude of gravity loss. Rather, the loss in gravity was due to commingling several crude oil streams with different gravities. A method for compensating shippers for gravity loss was proposed to the operator of the pipeline.
Sometimes surveys uncover unexpected results. The operator of a truck-loading terminal requested a survey to determine how much product was lost from an underground piping leak. That objective was fulfilled, but a surprise surfaced. The survey showed that measurement practices on weekends, when the terminal was operated by relief personnel, were clearly substandard. Recommendations were made for training and monitoring relief personnel.
Conducting Measurement Surveys
There is no single best way to conduct a measurement survey. Each survey must be designed to fit the particular system and to achieve specific objectives. There are certain elements, though, that should be considered in the design of every measurement survey. Design and preplanning are essential to developing a survey that will yield useful results.
Don't be satisfied with the answer, "That is the way we do it." Ask, "Why do you do it that way"?" The answer to that question should be based on sound and practical guiding principles.
When conducting a survey to unravel a loss/gain imbalance, the best results will be obtained when new run tickets, for both meters and tanks, are cut at month-end and tank inventories are taken as nearly as possible at the same time at month-end. Tanks should be stilled when being gauged for month-end inventory. Line fill may be treated as a constant, or may be corrected for temperature, pressure and elevation if those data are available.
The first step in a field survey is to review applicable documents. This may include company measurement manuals, training manuals, and applicable chapters from the API MPMS (American Petroleum Institute Manual of Petroleum Measurement Standards) and ASTM (American Society for Testing and Materials). If measurement of light liquid hydrocarbons is included in the survey, it may be necessary to also review AGA (American Gas Association) and GPA (Gas Processors Association) standards. Other documents may include run tickets, adjustment tickets/reports, meter proving reports, tank strapping reports, tank gauge and thief reports, liquid inventory records, field gauger notebooks, laboratory records, maintenance records, marine vessel and voyage reports, pipeline receipt and delivery reports, batch reconciliation reports, loss/gain reports, and other applicable documents.
The next step is to talk with all personnel involved in the measurement process (for example, measurement specialists, supervisors, gaugers and meter techs) to learn their understanding of the measurement process and the equipment used. It is not unusual to find that a district supervisor believes his people are following the rules to the letter, but the people out in the field, miles away, are actually using somewhat different procedures. This often happens when formal training programs are not used.
Good formal training programs are important and should be conducted by, and periodically updated by, knowledgeable people. And don't just train the new guys. Periodically retrain those who are actively working in the process. Get them to help with the training program.
A good field survey will include trips out to the field to check all the equipment and facilities, and to observe gaugers and meter techs actually performing their jobs. If possible, try to spend time with every person--including relief personnel--who works in the measurement process. Sometimes this can require considerable time because (1) there may be a lot of field people to observe, and (2) enough time must be spent with each one to get him/her to loosen up and lapse back into normal practice, and stop putting on a "good show" for the reviewer. When time is a limitation, it can be sufficient to spend time with just a few selected individuals. Those individuals should understand that they were selected randomly and were not specifically singled out for some kind of a test.
What To Look For
Meters, samplers, meter provers, tanks and other stationary equipment should be sized and installed in accordance with applicable industry standards. In some cases, this includes specific piping configurations for some types of meters, notably turbine, Coriolis and ultrasonic meters.
Tanks should be free of dents and defects and not be tilted. If the tanks are not free of dents and defects or if they are tilted, then appropriate corrections should be made to the gauge tables.
Ancillary equipment (for example, thermometers, pressure gauges, gauge tapes, hydrometers, centrifuges, centrifuge tubes and thiefs) should comply with applicable standards.
Records and procedures should be reviewed to assure that equipment is calibrated properly and at recommended intervals.
Tank tables, volume correction tables (VCF), gravity correction tables, calibration certificates and records should be current and for the right equipment.
If control charts are used, they must be current and used as troubleshooting tools, not just stuck in a file and forgotten.
Review procedures used for reading meters, pulling and handling samples, and for proving meters. Sample pots must be cleaned regularly and properly.
Review procedures used for gauging, thiefing and measuring temperatures in tanks. Verify that volume corrections are applied correctly.
Review procedures for obtaining, handling and running samples for API gravity, S&W, or other contaminants in crude oils, and for vapor pressure and other specifications for refined products. Verify that laboratory equipment is calibrated and maintained in good order.
Check calculations for accuracy. Verify that run tickets and other documents are filled out completely and correctly.
New meter factors should be transmitted in a timely fashion to operators and accounting systems that need them.
Them should be a process for routinely checking and verifying measurements and calculations.
Check maintenance and repair records to ensure that equipment is properly maintained and that repairs are made in a timely fashion.
If there are deviations from API or other industry standards, there should be company standards and/or manuals that explain and justify those deviations.
When touring sites, look for evidence of losses due to spills, drips and excessive evaporation.
The general appearance of field facilities (in other words, clean and orderly as opposed to dirty and messy) will give an indication of the general attitude toward the importance of measurement.
Last, but not least, don't forget to check security. This includes locks and seals which are installed to prevent or detect tampering or theft of oil, and tampering with controls and ticket printers.
It is convenient to develop a comprehensive checklist that can be used for any survey within a given company. The checklist may contain things that do not apply to all operations or to all field locations, but can be used as a starting point for all surveys.
A checklist should include specific details regarding types of meters, proper installation. ancillary equipment, and calibration procedures for any given application. Installation, operation and calibration may be different for different types of meters, for example, positive displacement. turbine. Coriolis and ultrasonic.
There may be different requirements for meters that are used for custody transfer, for line balance, or for leak detection.
Some meters require flow conditioning upstream of the meter.
Meter proving procedures and frequencies should be detailed. Requirements for reproving based on changes in flow rate, temperature and pressure should be included.
Tank gauging should include gauging and sampling procedures, as well as details of datum plate location, reference heights and gauge well construction.
Liquid sample acquisition from meters or tanks should be detailed. Sample handling and integrity are equally important, as are details of how tests are run on samples. Calculation procedures and application of correction factors should be detailed.
Liquid measurement surveys, properly designed and properly conducted, can yield valuable information about the performance of pipeline systems, and can identify problems that need to be corrected. Failure to optimize pipeline operations can be costly.
Pipeline performance often can be sharpened merely by training and assuring that field personnel use proper procedures and that equipment is calibrated properly and often. A higher level of performance may require installing new equipment. A comprehensive survey can determine exactly' what equipment changes may be required and provide information that can be used for an economic study to determine if new equipment can be justified.
Although this article is specific to liquid pipelines, the principles of measurement surveys can be applied equally to bulk terminals. marine voyages, refineries, and any system that includes measurement of receipts, deliveries and inventory. The principles apply to any measurable liquid including crude oils. refined products and LPG.
The measurement survey principles reviewed here may also be applied to gas systems. but gas surveys will differ in equipment and procedural details, and often are not able to achieve the same level of accuracy as liquids due to the compressibility of gases and the extreme effect of temperature and pressure on gas volume. P&GJ
This article is based on a presentation at the 2004 international School of Hydrocarbon Measurement.
Author: Wesley G. Poynter retired after 35 years in various departments of the Standard Oil Company of California (later Chevron). His last assignment was Measurement and Oil Control Coordinator tie then spent I0 years as an independent consultant and Petroleum Measurement Analyst, specializing in measurement surveys and stock loss control. He served on several API working groups and chaired the API Committee on Measurement Education and Training.
Wesley G. Poynter, Special Advisor to the ISHM General Committee, Antioch, CA
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|Author:||Poynter, Wesley G.|
|Publication:||Pipeline & Gas Journal|
|Date:||Mar 1, 2005|
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