Flow Control: Technology information & new products for fluid handling engineers

November 2008

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Portable Flow Calibration
Evolution of Secondary Standard Design

By Ron Madison

There are several ways for flowmeter owners to benefit from a portable calibration system approach, all depending ultimately on the accuracy requirements of the application. Correct calibration of flowmeters ensures accurate, safe and efficient fluid measurement and can result in significant cost savings for the end-user.

When seeking a calibration solution, flowmeter users have the choice of sending their meters to an outside primary standard calibration lab for service, or of employing a secondary standard portable calibration system that allows calibrations to be performed “in-situ” without removing the meter from service.

Dual-rotor master meter.

The following article concerns the latest developments in flowmeter calibration, particularly in the area of secondary flow transfer standards. Here we focus on how advancements in flow transfer standard design enable easy, onsite calibration of all types of flowmeter technologies.

Why Calibration Is Important
The flowmeter chosen for a particular application depends on many factors, including the process fluid measured, pressure, temperature, allowable pressure drop, density, conductivity, viscosity, pipe size and orientation, control system interface, accessibility, maintenance requirements, etc.

Flowmeter performance is ultimately dependent upon the sensors or other signal-producing elements, which have an active relationship with the flowing fluid. In order to be confident that a meter is functioning according to specification and providing accurate information, it must be tested for proper operation and recalibrated on a periodic basis.

A key incentive for flowmeter calibration is better management of the process or test operation, resulting in assurance of the volume dispensed or blended, the competency of the test result, or the quality of the end product produced. With time, flowmeter performance may degrade — negatively affecting product quality and/or costs. Measurement performance is affected by chemical buildup in the meter, wear surfaces, electronic drift or physical abuse. Timely calibrations help industrial plants and other facilities optimize efficiency, as well as avoid unscheduled downtime required for repairs or equipment maintenance.

Understanding Calibration Standards

Hand-held portable flow transfer standard

In the metrology world, a flow measurement is said to be traceable if it can be connected to a stated reference, usually a national standard, through an unbroken chain of documented calibrations with stated uncertainties.

The extent to which a flowmeter calibration is traceable to recognized standards, such as those established by the U.S. National Institute of Standards & Technology (NIST, www.nist.gov), depends on whether the calibration system used is a primary standard or secondary standard. NIST traceability means that the standards employed in the flowmeter calibration process have direct traceability to NIST or are linked back to standards that have a traceable chain to NIST.

A primary standard is only one step away from NIST, while a secondary standard is two steps away from NIST. In other words, it is really the degrees of separation from NIST and not the accuracy of the standard.

Primary standard values, painstakingly derived by metrology specialists, are based on a transfer system made up of various facilities, procedures and instruments. These steps assure the link between a relatively small number of highly accurate primary measurement artifacts and the nearly infinite number of lower accuracy day-to-day instrument measurements made in science and industry.

Generally speaking, a primary standard calibration is based on measurements of natural physical parameters such as mass, distance and time. A primary calibration procedure assures the best possible precision error and, through traceability, minimizes bias or systematic error.

Some common examples of primary standard flow calibration systems include positive-displacement (PD) calibrators, continuous-flow loop calibrators and time-weight calibrators.

Flow-transfer standard with extended-range, dual-manifold system

A secondary standard calibration does not utilize natural, physical measurements. Rather, it involves calibrating one instrument against a primary standard; in the case of flowmeters, this device is a "master meter" that has been calibrated on a primary standard. The flowrate is derived from the master meter and other application inputs (e.g., temperature and pressure). Secondary standard calibration uncertainty increases with the introduction of additional variances that derive the flowrate and repeatability of the master meter. In many applications, this uncertainty remains sufficient to meet the user’s accuracy acceptance specification.

Examples of secondary flow calibration standards include master meters, which typically include, but are not limited to, turbine flowmeters and sonic nozzles. These flowmeters are recognized for their precision and repeatability.

Advances in Portable Flow Transfer Standards
Unlike primary flow standards, whose most important characteristics are their traceability to physical primary measurements resulting in the minimization of absolute uncertainties, with less concern for usability or cost issues, the key criteria for secondary flow transfer standards are portability, low cost and the ability to calibrate the flowmeter in the physical piping configuration it lives in.

Instead of removing flowmeters from service for recalibration, flow transfer standards allow users to “bring the calibrator to the flowmeter.” These portable, documenting field flow calibrators are intended for in-line calibration and validation of meters using the actual process gas or liquid. The most advanced transfer standard systems incorporate hand-held electronics, thus eliminating bulky interface boxes and the need to carry a laptop computer into the field for calibration.

Helical dual-rotor turbine master meter.

Flow transfer standards utilize a master flowmeter that has been calibrated to a very high degree of accuracy. In the United States, master meters are calibrated at a primary standard flow laboratory that is NIST traceable. The best laboratories will have achieved NVLAP Accreditation. Good flow transfer standards also have the capability of measuring and correcting the influences of line pressure and temperature effects on flow.

Using a portable flow transfer standard requires a master meter be installed in series with the flowmeter under test. The readings from these instruments are compared at various flowrates or flow totals. A technician can install the master meter in the same system as the test meter, perform the calibration, and then remove the master meter and go about his business. This approach minimizes downtime and eliminates the need to purchase backup meters to replace units that are out for calibration.

Some flow transfer standards are designed for exceptionally wide flow ranges, which require a manifold control involving multiple flowmeters. The calibrator reads signals from the master meter, the flowmeter under test, and a fluid temperature sensor. It automatically selects the appropriate master meter based on the current flowrate. Flow conditioners are built into the manifold as well as the temperature sensor.

Automated flow transfer standards utilize advanced calibration software to compile flow data and save information of interest. Users can download reports showing data points for the meter under test and compare that information with output from a master meter. They can also generate calibration data sheets in volumetric or mass units, which can be stored for future reference. This capability enhances calibration management programs by providing a record of traceability to recognized calibration standards.

Choosing a Master Flowmeter
Flow transfer standards can be equipped with many different types of flowmeters for use as a master meter. Typical choices include turbine meters, positive-displacement (PD) meters, Venturi tubes, sonic nozzles and Coriolis meters. Master meters are selected to cover the entire flow range of the calibration application.

Turbine flowmeters are one of the most accurate flow measurement methods available, and therefore an obvious choice for duty as a master meter. In basic terms, a turbine meter creates a frequency signal that is linearly proportional to flowrate. The flowrate is the predominate effect on the rotation of the turbine. The K-factor is a common characterizing parameter of a turbine meter. The K-factor indicates pulses/volume per unit, such as pulses/gallon.

Portable flow transfer standard using single master meter. No computer is required to perform the calibration.

However, other fluid properties of the flow also affect the rotation of the turbine. These properties include kinematic viscosity and temperature. Most precision turbine meters can measure accurately over their repeatable flow range of 100-to-1 at a single reference viscosity and temperature using a flow computer to compensate for linearity. However, when multiple viscosity calibrations are required to develop a universal viscosity curve, the single-rotor flow range is reduced to 10-to-1 turndown, depending on the viscosity range.

Helical dual-rotor turbine meters represent the most advanced turbine technology currently available, achieving five times more flow range capability than a single rotor. The dual-rotor configuration often allows the use of a single turbine meter where two or more meters were once required.

A strength of the dual rotor is its ability to curve fit universal viscosity curves that yield 60-to-1 turndown flow range. The exceptional repeatability of .02 percent makes the helical dual-rotor design ideal for portable flow-transfer standards. This technology is used by NVLAP primary standard calibration labs for correlation testing in round-robin assessments.

Practical Installation Tips
Good engineering practices will take into consideration the future use of master meter installation. Generally, this approach is overlooked by meter users — making it extremely difficult to remove flowmeters, let alone calibrate them in place.

The following practical tips will ensure a successful master meter installation:
• Allow ample room for an additional meter in the piping and place a blind body in the pipe that can be replaced later by the master meter.
• Allow for power or use a portable battery supply.
• To minimize fluid loss, install valves in front and in back of the master meter.
• Be sure to add a vent to bleed-off air entrapped in the pipeline during removal and installation.
• As some fluid loss is unavoidable, provide a means of capturing the lost fluid so that it might be transported back to a proper disposal container.
• Leave ample clearance to access both the meter under test and the master meter while making sure that the lost fluid does not pour out on electronics or electrical pump motors.

It is also possible to leave the master meter in the piping if a bypass valve is installed to prevent the meter from measuring continuous process flow, thereby reserving the pristine condition of the calibration standard. Obviously, if the environment were not conducive to protecting the master meter, this would not be feasible. A manifold installation using diversion values could direct the flow into several installed flowmeters, thereby allowing several meters to be calibrated with the one installed master meter.

As the calibration data is the whole reason for this exercise, be sure to transfer the data onto a PC to both analyze and store it for traceable history. Master meters should be calibrated annually to maintain correlation history and ensure traceable accuracy. These few simple guidelines will win the favor of the calibration engineer and minimize downtime.

Modern portable flow transfer standards are an economical, user-friendly solution for flowmeter calibration when it is difficult to remove flowmeters from system piping or when onsite calibration is required. They allow users to either set up a flow loop with master meters in a manifold configuration and perform their own calibrations, or to install a master meter inline in their existing piping and calibrate meters under test based on actual process conditions.

Ronald G. Madison is a vice president and partner with Exact Flow. His career includes more than 37 years in instrumentation and process control, with senior positions for major corporations such as EG&G and Roper Industries. Previously, he was employed in technical positions with Motorola and Westinghouse ranging from operations and manufacturing engineering to development engineering. With Exact Flow, Mr. Madison provides product design concepts and management aimed at serving the customer base. Mr. Madison can be reached at rmadison@cox-instruments.com.

www.exactflow.com

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