By Jack Sine and Izzy Rivera
Tom had a problem. As instrument chief for a Department of Public Works (DPW) in a major Northeast county, he was accustomed to solving problems. But this one had grown into something daunting.
“It started when the state mandated that all flow meters in a county wastewater system had to be recalibrated every year in order to receive state permits,” said Tom. “Our problem was that our county is the second largest county in our state with more than 1.5 million residents and we only had two trained technicians to calibrate meters at 135 waste stations that feed into 21 treatment facilities.”
As the demands on the waste stations increased, Tom explained, it began to get more and more difficult for the technicians to perform their normal duties along with the required annual calibrations. Fortunately for Tom, John O’Brien dropped in to see how his meters were performing for the county.
“Calibrating intrusive meters is always a challenge,” said O’Brien, general manager for FLEXIM Americas, a manufacturer of clamp-on ultrasonic flow meters. “Tom had purchased two of our portable meters a while back because, along with their accuracy, they don’t need to be physically inside the pipe to measure flow.” The meters are portable and clamp directly onto the pipe. They measure flow with an ultrasonic pulse from outside of the pipe. “These features make them ideal for calibrating intrusive meters including turbine, vortex, and magnetic meters,” said O’Brien.
Many types of permanent meters are beginning to offer devices that can be used to verify the diagnostics of the meter and provide a verification of the meter’s health. However, these devices are not really checking the accuracy of the meter. A meter can drift 5-10 percent and the verifier is unlikely to pick up this drift. Yet the alternative is very expensive and requires pulling the meter and sending it to a calibration laboratory. Calibration laboratories use standards that are referenced or traceable to NIST and are certified to the standard ISO/IEC 17025:2005, which specifies the general requirements for the competence to carry out tests and/or calibration.
For flow measurement, the three most common methods of calibration are: gravimetric (weight), volumetric, and master meter. With the gravimetric method, flow is diverted into a tank that is weighed. For the volumetric method, flow is diverted into tanks that hold a specific amount of volume. The master meter method is one in which a master meter that has been calibrated by one of the methods described above is used as the calibration reference source.
“The portable ultrasonic meter undergoes the same type of calibration as a master meter,” said O’Brien. The portable meter’s calibration is NIST traceable to an accredited ISO 17025 laboratory, making it suitable for use as a field master meter for calibration. “Compared to using a verifier device, the portable meter really checks the flow accuracy,” said O’Brien. “Unlike a verifier, with real measurement the ultrasonic meter detects any deviations in the accuracy of the permanent meter. Then it can be adjusted to bring it within one to two percent, which is within the regulatory requirements.”
How Ultrasonic Works
The technique most ultrasonic flow meters use is called transit-time difference. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium, kind of like a swimmer swimming against the current. The signal moves slower against the flow than with it.
When taking a measurement, the meter sends ultrasonic pulses through the medium, one in the flow direction and one against it. The transducers alternate as emitters and receivers. The transit time of the signal going with the flow is shorter than the one going against. The meter measures transit-time difference and determines the average flow velocity of the medium. Since ultrasonic signals propagate in solids, the meter can be mounted directly on the pipe and measure flow non-invasively, eliminating any need to cut the pipe. Or, using new technology, it can be isolated from an extremely hot or cold environment and still deliver accurate data.
An Ultrasonic Solution
“When I sat down with Tom,” said O’Brien, “the conversation moved from how the portable meters were performing to the difficult logistics of fitting in the permit calibrations with the technicians’ regular duties.” That’s when he told Tom about FLEXIM’s Flow Measurement Services division (FMS).
“Like all flow meter manufacturers, we offer field service, training, and maintenance. But our FMS division kind of developed a life of its own,” said O’Brien. “Where FMS is different from our normal service is that, with FMS, we are calibrating other manufacturers’ meters.”
According to O’Brien, it became apparent early on that field service in many cases was less than efficient across the whole spectrum of flow meters. “Plus, many manufacturers’ technicians know only the technologies of the products their company sells,” he added. With increasing regulatory requirements for annual calibration, O’Brien said there is a greater need for calibration, maintenance and repair services on a wide range of flow meter types.
“Those are not critical for ultrasonic meters because they are calibrated during manufacture and have no moving parts that could cause them to go out of calibration,” he explained. But intrusive meters have moving parts and are exposed directly to the flow material. Therefore, they must be checked regularly for accuracy, recalibrated, or repaired. If they can’t be repaired, they have to be replaced.
O’Brien developed a plan to create service centers at key locations across North America manned by certified technicians who had the skills to calibrate and repair all types of flow meters.
“Ideally, we wanted people who already had knowledge of all varieties of flow meters,” he said. “If the technician was highly skilled but lacked some knowledge, we would hire him and teach him. We don’t necessarily have to have extensive knowledge of the meters we calibrate, just enough information to be able to adjust a meter to bring it within specification.” O’Brien said their expertise comes from understanding flow measurement and the vast types of applications they encounter.
According to O’Brien, FMS services a wide range of industries - water and wastewater, HVAC, processing plants, semiconductor, aerospace, refineries, and offshore facilities - wherever there is a need for flow measurement calibration services, or for a measurement service in general. “Recently we were hired by a water utility to help them quantify a water leak that they suspected they had in a pipeline that traversed a river,” he said. “By putting a meter on the pipeline on either side of the river we were able to quantify the leakage rate for the utility.”
O’Brien currently has teams strategically located and can service operations throughout North America. “We typically work off a service contract and try to group the instruments we service so we don’t have to travel to service just one meter,” he explained. “In Tom’s case, he saw the value of the service immediately.” Since management was aware of the problem, Tom was confident he would get approval but would have to put it out for bids.
Specifications for Maintenance Program
Tom immediately went to work on a spec detailing what the contract would entail. The following would be required of suitable vendors:
- The calibration flow meter must be of portable clamp-on design that mounts externally on the pipe. The meter must use the transit-time flow measurement technique and have the ability to monitor flow channels simultaneously.
- In the case of limited straight run of pipe (less than 10 pipe diameters upstream of metering location) the meter must be capable of being configured in dual-beam mode to minimize the effects of flow profile uncertainty.
- The portable clamp-on meter must have a NIST-traceable (5-point minimum) wet flow calibration certificate proving accuracy of better than one percent of actual rate. The NIST calibration certification’s expiration date must be less than 1 year from the date of use in the field.
- Confined space: The vendor must meet OSHA Confined space certification as per 29CFR 1910.146.
- Vendor must supply technicians with significant experience in the calibration and repair of the listed equipment and needs to supply references of experience. The vendor shall have at least three full-time, fully qualified service engineers and vehicles.
- Vendor shall be currently involved in the calibration, repair and replacement of flow- and height-based instrumentation. The vendor shall demonstrate that it is a responsible service and maintenance organization possessing adequate financial resources to accomplish the various services described herein and that has a satisfactory record of performance and integrity. With this bid, the contractor shall provide the county with a list of similar work currently being performed in the local area.
- Report Data: Vendor will provide a new calibration certificate for each of the existing flow metering systems serviced. Each certification will include the following: Serial number, size, manufacturer, location, environmental conditions, and all valid NIST-traceable documentation concerning the measurement and test equipment used in instrument calibration.
Winning the Bid
“We won the bid not only because we met all of the criteria but also because of the depth of our technicians’ knowledge, which we were able to document with work we had done at other establishments,” said O’Brien. After the contract was signed, FLEXIM technicians began the tasks of calibrating meters according to DPW’s priorities. As requested, Tom was able to group meters so the techs could do several meters per trip. If a meter was in need of repair, they would fix it and calibrate it. If it was beyond repair, the county would usually replace it with a permanent ultrasonic meter. The technician would simply install the new meter next to the failed one.
“We found out early on that some of the flow meters were out of spec or damaged,” said O’Brien. “But we were able to get them in compliance and assure that the county qualified for its state permits.
O’Brien said he’s certain Tom is pleased with FLEXIM’s service work. “Not only have there been no complaints, our contract has just been renewed for another year.”
Jack Sine is a freelance writer specializing in the flow measurement, power, chemical, and HVACR industries. He can be reached at [email protected] or at (845) 831-6578. Izzy Rivera is service manager for FLEXIM Americas. He can be reached at [email protected] or (631) 492-2300.