Flow measurement is essential in a variety of businesses, including and especially in the liquid hydrocarbon (petroleum) industry. An ongoing challenge is to provide accurate flow measurement while anticipating and correcting problems in the measurement system.

An important objective of flow measurement is to control costs, particularly in flowmeter maintenance and unaccounted-for product losses. Measurement guidelines that follow focus on basic meter proving, but could help anyone wanting to reduce measurement or calibration costs:

• Select a flowmeter for high accuracy and increased reliability;
• Employ calibration methods with lower uncertainty;
• Acquire meaningful data of significant quality;
• Use data from calibrations to predict or prevent flowmeter failure;
• Use data to minimize product losses and increase uptime; and
• Implement software solutions to maximize operational efficiency.

Proving is a measurement term common in the hydrocarbon industry. It refers to multiple (typically 5) comparisons or calibrations at a single flow rate. These comparisons are performed on site, under operating conditions. The goal of proving is to develop a correction factor, also called a meter factor, which corrects for the influence of actual operating conditions (flow rate, pressure, temperature, etc.). Benefits of proving include:

• Verification of flowmeter accuracy;
• Documentation of flowmeter reliability;
• Reduced uncertainty;
• Lower incidence of product loss; and
• Enhanced prediction of flowmeter failure.

The number or frequency of provings required in any given circumstance must be determined by each operation according to its own priorities. Every situation should be assessed with regard to mean failure time of the flowmeter and the financial impact of the meter's inaccuracy. How often to prove is directly proportional to how much inaccuracy and profit loss a business is willing to tolerate.

Achieving benefits from proving requires good equipment and procedures. American Petroleum Institute (API) standards, published in the 'Manual of Petroleum Measurement Standards (MPMS),' provide guidance about proving. Although these standards were written specifically for hydrocarbon fluids, the underlying principles and procedures can be applied to any industry wanting to improve its measurement capabilities.

This example from 'Prove-It' by Flow Cal Inc. shows a meter factor control chart used to trend meter performance. Red lines are limits of meter accuracy. Low bars indicate something has changed the meter's accuracy or that a failure has occurred.

API standards are guided by the '1 in 10,000 rule' to provide greater uniformity of results for custody transfer of hydrocarbons. An accuracy rate of one part in ten thousand is the basis for:

• Collecting primary data (to 5 significant digits);
• Collecting secondary data;
• Calculating intermediate and final values (results); and
• Calibrating provers (the calibrator).

Differences in collecting primary data affect results. Consider this scenario: An operator uses his shop scale to calibrate a mass flowmeter. The chosen scale measures only in one-pound increments. If the shop scale indicates 100 lb compared to the meter's 100.00 lb, it would appear that the meter is 100% accurate. However, you must consider that proving has an uncertainty of 1%.

Since the scale lacks precision, mathematical accuracy of the comparison is plus or minus 1%. If the operator continues to measure 100-pound batches, the results would vary from 99 to 101 pounds. To lower the uncertainty of each proving, the scale and meter readings must include more significant digits (e.g., 100.01 lb). The table illustrates how the number of significant digits impacts results.

Good quality software simplifies the implementation of API standards. A number of programs are available to manage data effectively and thereby reduce measurement costs. Software typically improves adherence to standards, minimizes the operator's influence on results, and provides information for better overall management of costs. When choosing software for a particular operation, consider the following features:

• Automated data collection;
• Ability to acquire quality, meaningful data;
• Preprogrammed tolerances;
• Automated data verification;
• Historical evaluation of results;
• Data or result exceptions; and
• Predictive intelligence for meter failure or inaccuracy.

The graphic from 'Prove-It' by Flow Cal Inc., a proving software package popular in the hydrocarbon industry, shows how the meter factor control chart is applied.

ISO (International Organization for Standardization) and ANSI (American National Standards Institute) also have standards related to meter proving. Higher quality measurement and lower measurement costs can be achieved regardless of which standards and software tools are applied.