Electrical Resistance Probes (ER)
Retractable (Low Pressure under 1500 psi)
Retrievable (High Pressure over 1500 psi)
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The electrical resistance (ER) technique is an "on-line" method of monitoring the rate of corrosion and the extent of total metal loss for any metallic equipment or structure. The ER technique measures the effects of both the electrochemical and the mechanical components of corrosion such as erosion or cavitation. It is the only on-line, instrumented technique applicable to virtually all types of corrosive environments.Although universally applicable, the ER method is uniquely suited to corrosive environments having either poor or non-continuous electrolytes such as vapors, gases, soils, "wet" hydro-carbons, and nonaqueous liquids. Examples of situations where the ER approach is useful are:
- Oil/gas production and transmission systems
- Refinery/petrochemical process streams
- External surfaces of buried pipelines
- Feedwater systems
- Flue gas stacks
- Architectural structures
An ER monitoring system consists of an instrument connected to a probe. The instrument may be permanently installed to provide continuous information, or may be portable to gather periodic data from a number of locations. The probe is equipped with a sensing element having a composition similar to that of the process equipment of interest.
TYPES OF ELEMENTS:
Wire loop elements are the most common element available. This type of element has high sensitivity and low susceptibility to system noise, making it a good choice for most monitoring installations. Wire loops are generally glass-sealed into an endcap which is then welded to the probe body. The glass seal, which is chemically inert in most environments and has a good pressure and temperature rating, makes a good choice for most applications. Alloys commonly glass sealed are Carbon Steel, AISI 304 and 316 stainless steels. Where glass may be susceptible to corrosion problems, Teflon®-sealed elements are also available. Probes with wire loop elements are normally equipped with a flow deflector (or velocity shield) to protect the element from floating debris in the piping system.
Tube loop elements are recommended where high sensitivity is required to rapidly detect low corrosion rates. Tube loop elements are manufactured from a small bore, hollow tube formed into the above loop configuration. Carbon Steel is the alloy most commonly used. Tube loops sealed into the probe by a Teflon® pressure seal are also available. Probes using the tubular loop element can be equipped with a flow deflector to minimize possible distortion in fast flowing systems.
Strip loop elements are similar to the wire and tube loop configurations. The strip loop is a flat element formed in a loop geometry. The strip loop may be glass or epoxy sealed into the endcap depending on the required application. The strip loop is a very sensitive element. Strip loops are very fragile and should only be considered for very low flow applications.
Cylindrical elements are manufactured by welding a reference tube inside of a tube element. This element has an all welded construction which is then welded to the probe body. Because of this element's all welded construction, exotic alloy elements can be produced relatively easily. This probe is ideally suited to harsh environments including high velocity and high temperature systems, or anywhere a glass-sealed element is not an option.
Spiral loop elements consist of a thin strip of metal formed on an inert base. The element is particularly rugged and ideal for high-flow regimens. Its comparatively high resistance produces a high signal-to-noise ratio, which makes the element very sensitive.
Flush mount elements are designed to be mounted flush with the vessel wall. This element is very effective at simulating the true corrosion condition along the interior surfaces of the vessel wall. Being flush, this element is not prone to damage in high velocity systems and can be used in pipeline systems that are subject to pigging operations.
Surface strip elements are thin rectangular elements with a comparatively large surface area to allow more representative results in non-homogeneous corrosive environments. Strip elements are commonly used in underground probes to monitor the effectiveness of cathodic protection currents applied to the external surfaces of buried structures.
Information courtesy of Alabama Speciality Products
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