Level Measurement and Inventory Tank Gauging
Level measurement devices determine the elevation of liquids and/or granular solids intanks, containers, silos, or any other suitable container. A recent, VDC market study found 2004's worldwide market for process-level measurement devices totaled $1.32 billion, with the global market for inventory tank gauging systems at $327 million.
Level measurement devices determine the elevation of liquids and/or granular solids intanks, containers, silos, or any other suitable container. A recent, VDC market study found 2004’s worldwide market for process-level measurement devices totaled $1.32 billion, with the global market for inventory tank gauging systems at $327 million. A large number of sensing technologies measure material level, a widely measured process variable.
Level measurement sensors are divided into two classes: point level switches and continuous level gauges. Some technologies are used for each. Point level is when a material is at, above, or below a certain point in a vessel; this device type indicates when a vessel is full, empty, or at an intermediate level. Continuous level gauges provide information about material level at all points in the vessel.
Inventory tank gauging (ITG) is the use of level measuring for inventory-storage and custody-transfer applications, as opposed to process control. Many ITG systems provide measurements over greater heights and with greater accuracy than process level measurement devices.
Shipments by technology
Process unit shipments include continuous and point-level measurement devices, except for the vibration products that provide only point measurements.
Hydrostatic (pressure sensing) represents by far the largest worldwide dollar-volume sales for process level measurement and the vast majority of these are in shipments for continuous measurements. The simplest of these are hydrostatic head instruments, where the level in an open tank can be determined by measuring pressure at the bottom of a tank, since the pressure at any point of a liquid of known density is determined by the height of the liquid above that point.
On the other hand, differential pressure devices are the most common means of continuous level measurement of liquids. In these applications, the high pressure side of a differential pressure sensor is connected to the bottom of the tank, with the low pressure side connected to the vapor space at the top of an enclosed vessel. Measured pressure difference is a true indication of level if the fluid density is constant. If not, changes in the liquid composition or operating temperature alter the specific gravity, creating false readings that likely require compensation.
Over the next five years, VDC forecasts a declining worldwide market-share for hydrostatic sensing devices, but not by much; pressure sensing is expected to remain the dominant technology for process level measurement. This technology has been used for a long time, and even major vendors worry about displacement, but it is not happening very fast.
Persistent dominance of hydrostatic level sensing can be attributed to a number of considerations, depending on the application. Attractive attributes of hydrostatic level sensing may include:
Low product and/or maintenance cost;
Ease of installation;
Broad media compatibility; and
Cost can be a major consideration. For example, among the continuous level measuring technologies accounting for =1% of that worldwide market segment in 2004, only sonic/ultrasonic products had a lower average price, and then not by much.
Non-contact microwave/radar dominates the worldwide ITG market, with over 60% of dollar-volume for 2004 shipments when both marine and non-marine applications are combined. Hydrostatic tank gauging accounted for less than 6%.
Since its introduction in 1976 by Saab Marine Electronics for use on tankers, non-contact microwave/radar’s use in ITG applications has expanded. For example, in 1991, these systems accounted for about 10% of the U.S. ITG market. The share reached about 14% in 1997, and some 22% in 2002.
Marketshare for this technology in the United States and in North America overall is much less than its worldwide share. This is principally because ITG system shipments for marine applications are chiefly for new ships, and shipbuilding now is mainly in other regions of the world, predominantly Asia/Pacific Rim countries. In 2004, ITG shipments for North American marine applications accounted for &2% of the worldwide total. Shipments to Asia-Pacific markets accounted for almost 73% of the total, with European markets seeing most of the rest.
Non-contact microwave/radar level-measurement devices are mounted at the top of a tank and transmit microwave signals down toward the surface of the material in the tank. Signals are reflected as an echo, which is detected by a receiver. Based on a difference between the transmitted- and received-signal, the system calculates the tank’s material level.
Two modulation techniques are used:
Pulsed systems measure the time for the echo to return from transmitted pulses. Time delay between the pulse transmission and reception is a direct measurement of level of the material.
Continuous wave transmission with frequency modulation (FMCW) is where altered frequency of the return echo is mixed with the transmitted microwave signal. Since these are at different frequencies—due to frequency modulation—the mixing results in a signal frequency proportional to the distance to the surface.
The microwave frequency used significantly affects the performance of radar level gauging. High frequencies, with their shorter wavelengths, are more sensitive to vapor, foam, and contamination. Around 24 GHz, even a small amount of water vapor can absorb microwave signals. Lower frequencies’ longer wavelengths and wider beam angles result in numerous interference echoes from the walls and agitators. Optimum frequency has been found to be around 10 GHz.
Non-contact microwave/radar devices are completely or relatively immune to measurement problems associated with many other technologies, such as:
Dielectric constant dependence;
Non-operational under vacuum;
Sensitivity to the atmosphere between the medium and sensor, such as foams and variable-density vapors;
Sensitivity to scaling; and
This technology’s non-contact nature particularly suits it for measuring harsh or corrosive substances. By far, most ITG systems are used for gas- and petroleum-industry applications, marine- and land-based, including refineries. In 2004’s worldwide ITG-system market, these applications had >87% share, with the largest portion of these shipments using non-contact microwave/radar technology.
Non-contact microwave/radar devices are expensive—an obstacle to even more widespread use. This is particularly the case for process level-measurement applications, where this technology accounted for about 6% of worldwide shipments in 2004. However, prices have been coming down for process- and ITG-system applications; the sharpest declines are expected in the average prices for products employing this technology over the next five years.
Contributing factors include use of lower-cost components, offering of devices with less functional capability, and expanded offerings of low-priced products. Expanding shipments will lead to greater economies of scale. Also, more firms are entering these product markets, creating further price pressures.
Non-contact microwave/radar products are forecast to be among the fastest-growing product categories for process level measurement and ITG applications. Even higher growth rates are forecast for contact/guided microwave/radar products, and for sonic/ultrasonic systems in the ITG market. Sonic/ultrasonic technology, however, accounts for an insignificant portion of the worldwide ITG market (&1%).
In 2004, contact/guided microwave/radar products accounted for about 4% of the worldwide process level-measurement device-market, and about 3% of the worldwide ITG market. This is an even more recently introduced level-measurement technology. VDC first became aware of these product shipments for process level-measurement applications by Bindicator and Krohne in 1997, and for ITG applications by Barton Instrument Systems in 2002.
Contact microwave/radar level measurement devices can measure levels of liquids, pastes, slurries, powders, and particularly granular materials. Worldwide, this technology accounted for 8.8% of 2004 shipments for solids process level measurement applications versus 3.1% for liquids.
These products—some of which are referred to as RF- and time-domain-reflectometry—are mounted at the top of tanks. They transmit radio- and low-microwave-frequency signals along transmission lines or wave guides extending into the material in the tanks. As with non-contact-type microwave/ radar level-measurement devices, pulsed and frequency-modulation contact-type devices are available.
Contact-type microwave/radar gauges operate at considerably lower radio and microwave frequencies than the non-contact types. These devices cost less than non-contact types, yet offer many of the same advantages. Additionally, these typically have more installation flexibility and are less expensive to install. Use is not subject to site licensing by the U.S. FCC or similar government regulators elsewhere.
These all-electronic units can measure over short or long distances and are impervious to dust, air movement, temperature, or pressure fluctuations.
On the other hand, heavy material build-up on the transmission line or wave guide can cause false readings. These products also have difficulty in sensing plastic materials with extremely low dielectric constants such as EPS beads. However, the technology works very well with plastic pellets and flakes.
Search level sensors at www.cesuppliersearch.com for a list of technologies with links to vendors.
|Jim Taylor is group manager at Venture Development Corp. (VDC);|
Level measurement technologies
Mobrey magnetic level gauges from Emerson Process Management are said to provide reliable level measurement of all liquids, including aggressive or toxic, without an external power source. The gauges also provide basic or interface-level, lower-cost redundancy. They can operate in temperatures from -160 to 400 °C with pressure ratings from PN6 to PN420 (ANSI 150 to 2500). Chamber is sealed; the only moving part of the instrument in contact with the liquid is the stainless steel or titanium float, minimizing maintenance requirements and increasing product life.
Modular design of Soliphant M permits the Endress+Hauser vibration limit switch to be adapted to the respective application. It has a variety of electronic interfaces, sensor designs, housings, process connections, and certificates. The new short fork geometry facilitates small tank applications. Intrinsically safe electronic inserts offer sensitivity adjustment, switching delay, and diagnosis of build-up and abrasion. The unit is available as a compact version (FTM50), with tube extension (FTM51) and with rope extension (FTM52).
Liquid level sensors family from Honeywell Sensing and Control incorporates the principle of total internal reflection to promote speed, reliability, and cost-effectiveness in a solid-state sensor. The sensor uses an optoschmitt trigger, which provides a digital output that indicates presence or absence of liquid. They are IP67 and designed for extremes in temperature, pressure, vibration, and shock.
Siemens Milltronics offers a variety of level measuring instruments for liquids, solids, slurries, and interfaces. Depending on the measuring task, a first distinction can be made between point-level measurement (a binary signal is output when the level falls below or exceeds set limits) and continuous measurement (dynamic processes are constantly monitored and transmitted as an analog signal or a digital value). Various technologies are available, such as ultrasonic, radar, capacitance, and hydrostatic methods.
EJX110A Differential Pressure Transmitter from Yokogawa Electric can be used to measure liquid, gas, or steam flow as well as liquid level, density and pressure. It simultaneously and accurately measures differential pressure and line pressure and displays this information on the multi-function LCD. Digital communications allow remote monitoring. It’s best used for presence/absence detection of liquid.