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The use of aboveground storage tanks presents several concerns for both level measurement and leak detection. Aboveground storage tanks are subject to both U.S. Federal regulations and State and local regulations. Such aboveground storage tanks need to meet the U.S. EPA's Spill, Prevention, and Countermeasure (SPCC) requirements (40 CFR, Part 112). This particular regulation applies to any single storage tank with a capacity greater than 660 gallons or a multiple-tank site with a combined capacity greater than 1,320 gallons. In 1984 the U.S. EPA was authorized to develop regulations for underground storage tank systems as well. The EPA formed the Office of Underground Storage Tanks to carry out the mandate from Congress. Their federal regulations went into effect in December 1988. These regulations provide for the Office of Underground Storage Tanks to approve state programs. States with approved programs operate in lieu of the federal regulations. Currently 29 states (plus the District of Columbia and Puerto Rico) have approved underground storage tank programs. As of March 31, 2002, 422,573 releases had been reported from underground storage tank systems. These reported releases resulted in cleanup costs and, in some cases, required emergency responses. Such releases can be caused by leaks, spills, and overfills of the underground storage tank. One technology that has been used to accomplish both leak detection and overfill protection is float switches that use reed switch technology. With this technology having longstanding service reliability, the old adage of "an ounce of prevention being worth a pound of cure" is particularly relevant.
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| A suspendible/submersible float switch. |
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There is no one leak detection system that best suits all tank sites, or a particular sensor technology that is the most appropriate for all tank installations and their unique characteristics. Identifying the best leak detection choice for your storage tank depends on a number of factors including cost (both initial installation cost and long-term operation and maintenance cost); facility requirements for piping runs and an interior or exterior tank site; temperature ranges; and availability of experienced installers, to name a few.
The basic liquid level indication can be accomplished with a float switch design, usually mounted on the tank top, extending down into the tank to the lowest point where the empty indication is desired. This would be the simplest configuration for obtaining a low-level signal. This could also be accomplished by mounting a single-point float switch horizontally into the side of the tank. Such single-point devices are 2-wire electrical hook ups, and it is easy to select the proper position on the tank for installation. The only consideration that needs to be made about the tank construction is that the wall or top must not be constructed of carbon steel, since this material would cause interference with the magnet and reed switch within the float switch. Float switches have been successfully used for both overfill and leak detection with petroleum products, gasoline, jet fuel, motor oil, kerosene, diesel fuel, and ethanol and/or methanol blends. Other successful installations have included the sensing of liquids stored in tanks constructed of various steels that do not affect the magnetic functioning of the switches. Additional indication, such as high level and overflow level, can be achieved by the use of a vertically mounted float switch. With a multi-point signal, we can then interface to relays and pump controls to maintain the desired liquid level in the tank. The level indication for HIGH HIGH level limit (overfill alarm – stop filling), HIGH level limit (tank full), LOW level limit (reorder delivery), and LOW LOW level limit (stop dispensing) are all typical liquid level switch configurations.
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| Continuous level sensors with a self-contained, compact alarm display. User can program set points. |
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Continuous level indication, at a higher cost, can also be provided with reed switch devices. If this type of level indication is required, specify a 5mm resolution. (Although grosser resolutions are available at lower cost, leak detection regulations may not allow their use.) This specification should take into consideration tank capacity, tank configuration (linear or non-linear) and the cost benefit of the material being monitored. With this type of device, logging of the tank's inventory may help in detecting leakage if the operator begins to record inventory loss between refills. This would not be considered a highly reliable method of early tank leak detection, however. A float switch can also be used to monitor the interface level between two different liquids, such as water and gasoline. This type of application requires the use of different float configurations on the switch. For example, a low signal can be obtained when the float designed for a water specific gravity (SG) is at the desired level. With the float that travels the majority of the float switch span having its SG set for gasoline, the desired level indication for the fuel will also be sent. This type of float switch is also used in oil/water separator systems.
Any tank storage system that can allow water to accumulate in the tank through loose fittings or condensation should have a water detection method installed, if the water would have a negative impact on the system or its process, A float switch installation that takes into consideration the liquid being stored and water is a low-cost way to monitor for undesirable water.
Each of these types of liquid level switch configuration can also be designed to have an integrated temperature sensor such as a thermocouple or Resistance Temperature Detector (RTD) embedded inside. This provides both liquid and temperature indication with only a single installation hole required in the tank.
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| The low profile and small size of the Interstitial Switch makes it ideal for leak detection in double-walled tanks. |
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Leak detection in tanks is another obvious use for reed switch technology. An interstitial float switch having a very low profile can actually be placed between the walls of double-walled tanks, at the tank bottom. Furthermore, liquid leakage between the tank walls would activate the interstitial switch's float, thereby causing a signal to indicate a leak. With this use of the float switch, an early warning can be given prior to leakage through the exterior tank wall. Such an early leak indication would help eliminate the costs associated with site cleanup and regulatory compliance.
Indication of leakage from a tank that is installed above a sump can be accomplished by suspending in the sump a liquid level switch designed for this submersible application. Leakage into the sump would activate the float switch, which would send its signal. This style of level switch can also be used in dispenser pans, double-wall tanks or other locations where the presence of liquid would indicate that a leak has occurred.
Not as common, but also used in aboveground tank installations, are bottle switches, which are reed switch indicators housed in a metal "bottle" that is plumbed to piping that leads in and out of the tank. The liquid passing through the pipe and bottle switch provides a level indication based on the relation of the liquid in the pipe egress from the tank to the liquid passing through the bottle switch, activating the float switch housed inside.
The use of reed switch technology in each of these applications provides a highly reliable, low-cost method to send an input signal to a tank control console. With the use of a liquid level switch, both accidental spills and leaks can be indicated at an early stage, with an inexpensive remotely monitored system having a simple alarm in the wiring circuit. With its low cost compared to the more expensive continuous monitoring systems or the delayed detection of manual methods such as sticking, a redundant indication can be considered.
Retrofit projects are easy to deal with, since a float switch is adaptable to a wide variety of mounting configurations. With only minor wiring modifications and slight additional costs for an indicator and/or alarm device, such configurations can include field-replaceable designs with integrated indication and alarm points. These points can be set by the user without the need for skilled technicians. By adding a basic indicator to the level switch, a visual indication can be given when the fill point is reached, and another setting can be used as the alarm for an overfill condition. For leak detection, the same type of alarm can be set so that the first low level of liquid creating the float activation will trigger the visual (or audible) alarm, providing early warning for possible system shutdown or planned maintenance for a tank leak.
In summary, float switch technology represents a field-proven method for both tank overflow protection and leak detection. These versatile sensors, off-the-shelf or with minor modifications, provide the reliability required by government programs for virtually any aboveground or underground tank.
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