Gas Technology: Measure it, manage it

Flow metering a key to efficiency
By Plant Engineering December 20, 2011

Virtually every industrial operation requires flow metering of liquids or gases. For managers with concerns about energy efficiency, just minimal levels of measurement are not enough. Today’s efficient operations require extensive and precise process monitoring. Fortunately, today’s remote-reading flow meters and data collection systems allow that advanced level of measurement.

Actionable information

U.S. Dept. of Energy (DOE) publication titled “Metering Best Practices” emphasizes that metering is not an energy-saving tool in itself. However, accurate metering and monitoring can provide the information necessary to implement an effective energy management plan. The report notes, “Energy managers have long known the value of energy-use data. And with recent advances in energy-use metering – increased functionality at lower costs – obtaining these data in a cost-effective manner is now becoming a standard practice.”

Advanced meter types have been developed with a customer or end-use focus. These have the ability to measure and record data continuously or at preset intervals and communicate that data to a central location in a format that can be easily integrated into both process and energy management programs.

On the electric side, advanced digital meters are available to deliver information not only on kWh or kW, but also can include voltage, milliamps, power factor, power quality, and power use trends. Particularly if your operation involves large motor loads or other inductive loads, or sensitive computer components, or if you are operating on a time of use demand rate, you will want much more than a simple kWh number to identify potential equipment problems or opportunities to shift loads off peak.

Of special value in any industrial plant are flow meters, which can be used on a variety of lines including steam, hot water, heated fluids, natural gas, and liquid products. Often flow data is combined with temperature and/or pressure data to give a complete picture of continuous energy usage or product delivery.

Variety of flow meter systems

Flow meters are available using a wide variety of measurement systems. Mechanical meters use a propeller, paddle, disk or similar device with an analog output calibrated to match the actual flow in the pipe. Pressure-type meters use a venturi, orifice plate or pitot tube to indicate pressure differential across a constriction and give an analog or digital output.

Bob Griffin, a contractor to the Energy Solutions Center, has done extensive research and numerous presentations and papers on flow metering for the Center. According Griffin, orifice plate meters are the most common flow meter in use around the world. The standard orifice plate has a round concentric orifice which is supported by an approved flange.

Pressure changes indicate flow

The basic material of the plate is usually stainless steel but may also be carbon steel or even plastic, depending on the application. As the fluid flows through the plate, its velocity increases, and its pressure decreases. Pressure taps in the pipe or flange measure the differential pressure across the orifice. The pressure difference is used to calculate the flow volume or mass flow rate. More recent introductions to the flow meter field are electronic metering devices that use electromagnetic, thermal mass, optical or ultrasonic methods to calculate flow.

Griffin points out that both the accuracy and the repeatability of the measurement should be evaluated and known in a flow meter installation. He explains that for most energy management purposes, repeatability is the more important characteristic. This is because the operator needs to know trends or changes in the flow characteristics in order to identify equipment that my require adjustment or service. However in cases such as fuel mixing or ingredient mixing, accuracy is also critically important.

Understanding accuracy claims

Griffin notes that the flow-meter buyer needs to understand that the percent error specified by the seller may be stated in more than one way. For instance, error may be specified as a percentage of full scale reading over the calibration range, versus being specified as a percentage of actual reading over the calibration range. He uses the example of an orifice plate meter, where the accuracy of the flow reading of the primary element is specified as 2.5% of full scale reading, over the full range.

Here, if the maximum range of the orifice plate is specified as 4:1, the flow reading error at low flow would be four times that at full scale or 10% of actual reading. A meter with the same percentage error over the actual reading would be much more accurate at low flow.

Knowing Viscosity Important

Griffin also explains that for many liquid flow meter types, it is important that the viscosity of the fluid being measured is exactly specified, and that temperature corrections for variable viscosity fluids are used. Many types of meters are calibrated to operate within a specific flow range assuming a constant viscosity and density.  Outside the intended range, the meter has reduced accuracy. Thus, when selecting a meter, it is valuable to have an estimated flow rate for the fluid as well as a viscosity.

Data from the meter, regardless of type, is transmitted by wire or by wireless device to a central data gateway, where it is output to the user. In some cases there is also a local readout of the data at the measurement point.

Choosing the right meter

The DOE publication points out that it is critically important to select both the right size and type of flow measurement system. Certain flow meters are more effective for certain applications than others. Get help from a qualified engineer or system designer. DOE also points out that it is valuable to the extent possible to use a single provider of metering equipment.

In this way you can be assured that the equipment is compatible, and that the “blame game” for system problems is minimized. Complete system training can also be done more effectively by a single provider. A further advantage to standardization is possible dollar savings through volume procurement.

Standardize: Data transmission protocol

The DOE report also explains that electronic data transmission is done in a variety of protocols or formats, including BACnet, MODBUS, LonWorks, etc. All can be used for metering communication, but it is important that the metering equipment chosen all uses the same protocol, or can be “translated” into the standard protocol chosen. Again, choosing a minimal number of suppliers helps assure this is done correctly.

In some situations there is wide variability in flows between peak flow and a much smaller minimal flow, yet in both situations the operator needs accurate flow information. Most flow meters have a minimal flow rate, below which accurate flow information cannot be provided. The solution here is probably a compound meter, with flow metering elements for both peak and minimal flows.

Do it to code

Metering installations must meet various codes and standards, including those from ANSI, NEC, NFPA, IEC and FCC. In addition, local authorities, utilities and specific industries may have additional requirements. A new international standard, ISO 50001, is being introduced in Europe, the U.S. and Canada in 2011. This standard will focus on both equipment and management techniques for energy management, and will consider extensively the need for effective tools for manufacturing plant energy measurement.

A successful, accurate and repeatable meter reading requires a selection of the right type of meter, in a flow range appropriate to the situation. The meter must be installed in compliance with the manufacturer’s specifications, as well as all applicable codes. It must be calibrated at installation, and the calibration checked and adjusted regularly, again according to the manufacturer’s specifications. And the data as received should be compared with previous data for consistency. Data can then be applied to make rational energy management decisions.