Using flowmeters to measure and manage plant energy resources

For industrial plants that want to minimize operating costs, remain competitive, and maximize profits, the need for effective control of energy use is paramount.

09/01/1999


For industrial plants that want to minimize operating costs, remain competitive, and maximize profits, the need for effective control of energy use is paramount. These concerns, combined with environmental protection regulations, mean that energy waste can no longer be tolerated.

As plant engineers and energy managers are well aware, plant modifications, regular maintenance, shutting off valves, etc., are all activities that help reduce energy costs. However, effectively managing energy demands that the flow of energy resources through the various plant media first be accurately monitored and controlled.

Monitoring energy use

Steam is the most widely used heating medium in the world today. Such a statistic clearly justifies the need for effective steam management. From a metering viewpoint, two of the most common instruments used for measuring steam are differential pressure (DP) devices and vortex shedding flowmeters.

DP devices

In the past, DP measurement devices were used almost exclusively for steam metering. The overriding reason for selecting this device almost exclusively has been the need to handle very high operating pressures and temperatures. Its primary element is simply a restriction within a pipeline across which a differential pressure is produced. The difference is measured by secondary instrumentation (DP cell) and the result used as an indication of flow.

A variety of primary elements exists, but the most common is the orifice plate. Other elements are typically only selected when pressure losses in the line need to be kept to a minimum. The orifice plate is a square law device: output is proportional to the square of the flow rate. This instrument is ideal for monitoring and controlling a constant steam flow. Unfortunately, its accuracy suffers with the wear of the sharp edge, and its range is limited to approximately 3:1. In its simplest form, then, it is usually unacceptable for steam accounting purposes unless the steam demand is fairly constant.

The need to increase the range of a DP flowmeter has led to the development of a meter with a variable area orifice. A contoured plug moves axially against a spring in relation to a fixed orifice. The geometry of the device lets a linear relationship exist between flow rate and DP. Some manufacturers claim a turndown in the region of 100:1 can be achieved.

The secondary instrumentation (DP cell) converts the measured DP to a signal suitable for driving recorders or counters. In practice, these instruments are connected to the primary element by impulse lines and manifolds. Impulse pipework must be installed with care to prevent blocked, leaking, or vapor-locked lines from causing significant system errors.

Flowmeters

The vortex shedding flowmeter (Fig. 1) is the latest instrument to be placed in steam lines to perform measurements. It offers a number of advantages over the DP device, including the cost effectiveness inherent in a modern meter that makes metering to distribution lines of different processes a viable option. The additional information this metering provides is definitely a bonus for the energy manager.

Vortex shedding flowmeters belong to a class of devices called fluid oscillators. A bluff body is placed into the flow stream and the fluid is forced to split into two paths around the object causing the vortices to shed from alternate sides. The rate at which the vortices are produced is proportional to the flow rate and independent of changing fluid properties.

When sized correctly, this meter accurately determines flow over a wide measuring range (up to 30:1) with a low pressure drop. The vortex flowmeter has no moving parts and mounts directly into a pipeline, simplifying installation. The signal, measured by the sensor, is generated as a frequency. It is not susceptible to drift, which ensures long-term repeat- ability. Modern electronics permit easy onsite range changing.

Achieving the accuracies claimed for the instrument requires that sufficient straight lengths up and down stream of the meter be available (Fig. 2). The same guidelines that are recommended for orifice plate installations should be followed. Sizing is important. The user must be aware that, when the flow rate falls below a defined minimum value, the meter cuts off and the output drops to zero.

Both DP and vortex metering techniques essentially use volumetric flow devices. Because steam use is almost always monitored in terms of mass (pounds or tons/unit of time), steam quality must be considered. If the pressure and temperature of the steam fluctuate, density compensation must be carried out.

If accurate steam accounting is necessary, this point is especially vital. Although accounting can be relatively easy to achieve using a suitable computer, the additional instrumentation greatly increases the installed cost of the system.

Measuring other media

These techniques can also be used to measure other industrial heating media, including hot water or oil-based heat transfer fluids. The electromagnetic flowmeter (Fig. 3) is a cost-effective alternative for water flow. This type of meter has no moving parts, creates no head loss, and does not restrict flow rate. Operating on Faraday's law of electromagnetic induction, the device simply generates a voltage that is monitored within the device and which is proportional to the average fluid velocity (Fig. 4). These meters assume a full pipe. Air or gas entrainment must be avoided or errors can result.

Today's meters are extremely accurate and reliable, and are ideal for heat metering duties. These devices measure inlet flows to a building or to another energy user. Two temperature transducers can measure the inlet, and feed return temperatures and respective outputs directly into an energy calculator, which indicates the amount of energy used within the building.

Gathering and analyzing data

Effectively using and managing energy require some form of datalogging. Information must be collected from the measuring points and presented in a format usable by the energy manager (Fig. 5).

Realistic energy targeting and management can succeed only when they are based on a solid foundation of known data acquired over a length of time. The large energy user will undoubtedly invest in a centralized data acquisition system. A variety of SCADA-based (supervisory control and data acquisition) systems are available with software packages suitable for formatting energy consumption trends. These systems are effective for large plants, but are typically cost prohibitive for smaller users.

For these users, a number of small, wall-mounted acquisition systems are offered. They accept data from a minimal number of measurement points. Data are held in the system until they are downloaded for analysis. Downloading can be achieved through hard wire or, in some cases, a telephone modem. The data can then be analyzed on a PC, formatted, and printed to provide hard copy records. This method of acquisition is ideal for small plants or for larger companies wishing to submeter specific areas.

The drive by companies to be more competitive and profitable will see even more investment in measuring technology for energy management. Even in seemingly well-run plants, effective monitoring and control can provide the key to improvement and optimization of processes. Phenomenal savings can be achieved.

-- Edited by Jeanine Katzel, Senior Editor, 630-320-7142, j.katzel@cahners.com

Key concepts

Before energy can be effectively managed, the flow of energy resources through plant media must be monitored and controlled.

Two of the most common instruments used for measuring steam are differential pressure devices and vortex shedding flowmeters.

Managing and using energy effectively require data to be collected, organized, and presented in a usable format.

More info

Technical questions about this article may be directed to the author at 01-61-998-1841 or the company at 800-428-4344, or visit the company's web site at www.us.endress.com

For a recent article on a related topic, see: "Using meters to measure steam flow" (PE, April 1998, p 99, File 3599).

For additional articles related to this topic, see the "Fluid handling" and "Instruments and controls" channels on Plant Engineering Online at www.plantengineering.com.





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