Automatic gauging technologies have advanced, but better accuracy is needed

Workers are installing a bottom-referenced, top-mounted digital tank gauge that measures both innage and ullage. Manufacturer MTS Systems Corp., Cary, N.C., says the Level Plus LDF gauge has an average-temperature sensor and is accurate to ?0.02 in.

This final article in a two-part series will detail current measurement techniques and describe the author's concept of an ideal tank gauging device. The comparisons in this article are based on his experience.

The first article traced the history of tank measurement accuracy, reviewed sources of measurement errors, and recommended procedures to increase measurement accuracy (OGJ, Mar. 3, p. 68).

Automatic gauges

Today's automatic tank gauges fall into six broad classes:

• Float-operated tank gauges

• Servo-operated tank gauges

• Radar tank gauges

• Hydrostatic tank gauges

• "Smart cable" tank gauges

• Hybrid tank gauges.

Table 1 [51359 bytes] outlines the significant features of each class.

It should be noted again that ATGs measure liquid levels by measuring either "innage" (the depth of the liquid) or "outage" (the height of the vapor space above the liquid). Gauges that measure outage are usually less accurate because, when they convert measurements to innage, no correction is made for vertical thermal expansion of the gauging well for floating-roof tanks, or of the tank shell for cone-roof tanks.

Float-operated ATGs

Float-operated tank gauges are the most popular. More than 90% of oil tanks in the U.S. have them. There are ten times as many float-operated ATGs as all other types combined. They are the least expensive and least accurate type of ATG.

Float-operated tank gauges have been used since the 1950s. They evolved from float-operated "gauge boards," which were developed to avoid the need to climb tanks for manual gauging.

In this type of gauge, a perforated tape runs from the float up to the top of the tank, over two pulleys, and down to the gauge head (Fig. 1 [14005 bytes]). The gauge head contains a "take-up" drum that winds up the perforated tape. The drum is mechanically connected to a direct readout and, in most cases, to a transmitter that generates a digital level signal. A negator spring compensates for the weight of the tape.

Float-operated ATGs measure outage. The accuracy of all outage ATGs suffers because of variations in reference height. Float-operated ATGs double this error because the gauge head is mounted at grade. In addition, maintenance requirements are high because of the guide pulleys and the moving parts in the gauge head.

Accuracy would be much better and maintenance much easier if the gauge head were mounted on top of a slotted gauging well, as is done with servo and radar ATGs.

In the last 5 years, level transmitters for float-operated ATGs have been significantly improved. They have far fewer moving parts.

Servo-operated ATGs

Servo-operated tank gauges were developed for custody-transfer level measurement in Europe at about the same time float-operated ATGs were developed in the U.S. They are more accurate than float-operated ATGs because they eliminate hysteresis and are mounted on the top of the tank.

The servo ATG uses a displacer suspended from a small wire. The displacer is lowered until it contacts the liquid. When contact is sensed, the displacer is raised.

The servo moves the displacer up and down, alternately making and breaking contact with the liquid surface. The wire wraps around a grooved drum, and the position of the drum is transmitted as a digital level.

Some servo-operated ATGs lower the displacer into the oil to determine the buoyant force at various levels. Buoyancy is a direct measure of oil density.

Servo-operated ATGs measure outage. For best accuracy, they should be mounted on properly supported slotted gauging wells to minimize the error caused by reference height movement.

Maintenance is relatively high because of the complicated electromechanical level-sensing mechanism in the gauge head. Performance has been improved in the past 5 years by the development of new gauge head mechanisms with fewer moving parts.

Radar ATGs

Radar tank gauges are a recent development. These gauges measure a microwave echo.

Like servo gauges, they are suitable for custody-transfer level measurement, and they measure outage. For best accuracy, they should be mounted on properly supported slotted gauging wells (Fig. 2 [112741 bytes]).

Maintenance requirements are relatively low because there are no moving parts.

Virtually all of the major ATG manufacturers now make a radar gauge. The major improvement in the last 5 years has been repackaged electronics to meet commercial rather than military standards.

Similar technologies are sonic or ultrasonic tank gauges, which measure a sonic echo. Their accuracy is affected by vapor above the product and they have not been widely used for tank measurement in the oil industry.

Hydrostatic ATGs

Hydrostatic tank gauges (HTGs) were developed about 10 years ago to meet an Exxon Co. U.S.A. specification. It was anticipated that the European Union would require its oil industry to convert from standard volume measurement to mass measurement. This has not happened, much to the dismay of HTG makers.

HTGs provide excellent mass measurement, limited only by the accuracy of the pressure transmitters and the tank strapping tables (Fig. 3 [36677 bytes]). But they provide only mediocre level measurement. Level problems occur if the tank contents are temperature or density stratified.

Because HTGs measure innage, they are not affected by variations in reference height. They need only a single temperature transmitter to determine standard volume.

A major improvement in recent years is the development of smaller, less-expensive hydrostatic interface units.

Smart cable ATGs

The term "smart cable" describes half a dozen types of ATG. Their common feature is a cable that runs from the bottom of the tank to the top. The cable contains a level sensor, hence the name "smart cable."

Some of these ATGs have a float that rides up and down the cable. Most of them have the following features:

• They measure innage, so their accuracy is not affected by changes in reference height.

• They do not require slotted gauging wells.

• They include a series of resistance bulbs in the cable to measure average temperature.

The main types of smart cable ATGs are:

• Capacitance ATGs, which measure the change in capacitance between the probe and the tank shell. The capacitance varies with level because the product has a different dielectric constant than the air or vapor. These usually are used to measure process vessel level rather than tank level.

• Inductive ATGs, which measure level using a digital position signal generated by the inductive interaction with a transponder in the float.

• Reed-switch ATGs, which have a resistance running the length of the cable. A series of magnetic reed switches are spaced at close intervals. A magnet in the float closes the adjacent reed switch, which shorts the resistance. The resistance varies with product level. These ATGs have been used primarily for marine tank-level measurement.

• Resistive ATGs, in which a nickel-chromium helix is wrapped around a steel core covered with a Teflon jacket. The hydrostatic pressure of the product shorts the helix against the core. The resistance varies with the product level.

• Magnetostrictive ATGs, which measure the time of flight for a torsion wave to traverse through a ferromagnetic wave guide (Fig. 4 [22192 bytes]). The torsion wave is reversed by a magnet in the float. These ATGs have been available in long lengths for tank measurement for a few years. They provide high inherent accuracy because the design is almost immune to temperature and outage errors. Most users include multiple temperature bulbs in the cable.

Hybrid ATGs

Hybrid tank gauges are a recent development. The lower pressure sensor of a hydrostatic tank gauge is combined with a conventional level ATG. The level is measured by the ATG, and the mass is measured by the pressure sensor.

This design is still evolving, but it may prove to be a very attractive tank gauging system.

Hybrids can measure density without sampling or laboratory analysis. Because of the redundant level measurement, they provide a degree of error checking. API has approved a working group to write a standard for hybrid tank gauging systems.

Measurement purpose

When selecting an ATG, the intended measurement purpose should be considered. If the ATG is to be used for operations or inventory control, high accuracy and precise temperature determination probably are not required. The continuous measurement of tank density provided by hydrostatic and hybrid ATGs is a useful feature because it avoids the need for sampling.

If the ATG is to be used for custody transfer, initially or in the future, it should be properly selected and installed in accordance with the API Manual of Petroleum Measurement Standards (MPMS), Chapter 3, Section 1B.

In Europe, custody transfer with ATGs has been routine since the 1950s. In the U.S., custody transfer usually is done either by metering or by manual tank gauging. For custody transfer of small parcels from large tanks, metering is the only way to achieve reasonable accuracy.

Until about 5 years ago, the European standards for automatic tank level and temperature measurement were more stringent than the comparable API standards. The latest revisions of the API standards, however, provide criteria for installation and calibration that permit ATGs to be used for custody transfer. The API and International Standards Organization standards are now quite similar.

The decision to accept custody transfer by ATG rather than by manual gauging is made by the two parties involved in the sale. If ATGs are installed in accordance with the API standards, there should be no problem with U.S. customs.

The API MPMS standards describe current practice for level measurement with ATGs and temperature measurement with automatic tank thermometers (ATTs). The first article in this series outlined the most recent changes in API measurement standards.

Temperature measurement

Temperature is often the neglected tank measurement. Temperature errors have a more serious effect than density errors on the standard volume calculations.

It is difficult and time consuming to determine average tank temperature by manual methods. Manual digital thermometers, however, are faster and more accurate than cup-case thermometers.

Most oil tanks in the U.S. will have to be modified before they can be used for custody transfer by ATG. The key requirement is for automatic tank temperature measurement. On floating-roof tanks, this may require a separate temperature well for the average temperature bulbs.

MPMS Chapter 7, Section 4 ("Static Temperature Determination Using Fixed Automatic Tank Thermometers") was issued in 1993. It describes the requirement for accurate ATT measurement.

Innage vs. outage

There is a significant difference between ATGs that measure outage and those that measure innage. Float-operated, servo-operated, and radar ATGs measure outage. HTGs and most smart cable ATGs measure innage.

The volume of oil in a tank is determined by multiplying the innage distance by the tank area. Multiplying the outage distance by the tank area gives the "ullage," or volume of vapor space above the oil.

To determine the oil volume with an outage ATG, it is necessary to subtract the outage from the reference height. If reference heights were constant, outage ATGs would provide the same innage as innage ATGs. Reference heights are not constant, but there has been little published on the subject.

The author has observed reference height variations with oil depth, oil temperature, and ambient temperature. When the reference height changes, outage ATGs are inaccurate. If an outage ATG has an accuracy of 1/8 in. and the ATG mounting moves down 2 in. when the tank fills, then the accuracy of the ATG is 21/8 in.

The writers of API MPMS Chapter 3, Section 1B, struggled with the problem of field verification of outage ATGs. They finally decided that outage ATGs should be verified by outage manual gauging. The basis for this decision was that the ATG maker had no control over the tank, or over how the ATG was mounted.

The field verification procedure confirms the factory calibration, but is does not tell the user the true accuracy of the measured innage.

Properly supported gauging wells are the best way to minimize variation in reference height. If the gauging well is improperly installed, or worse, if the ATG is supported from the tank shell or mounted on a fixed roof, outage ATGs cannot deliver their high potential accuracy. MPMS Chapter 3, Section 1B, describes how to properly support gauging wells.

The ideal ATG

Based on 35 years of measurement experience, the author has devised a list of features for an ideal ATG:

• It measures innage.

• It does not need a gauging well.

• It provides average temperature measurement.

• It measures level to an accuracy of 1/8 in. or better.

• It has a minimum number of moving parts.

• It has low initial cost, installed cost, and maintenance cost.

• It includes a precision pressure sensor if the user needs a continuous on-line readout of tank product density.

The MTS Level Plus magnetostrictive ATG combines the majority of the features in this list (photo). For severe services that can cause fouling of the float, a radar gauge is the best choice.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.