The Magnetic Level Indicator: A Technology Overview

Demands for increased safety and improved efficiency in processing facilities have made the magnetic level indicator an indispensable level control device. With the ability to perform reliably under extreme process conditions, as well as offer dual-technology redundancy for safety critical applications, magnetic level indicators, or MLIs, such as those manufactured by Magnetrol International and it’s daughter company Orion Instruments can make a smart alternative for a wide range of level measurement and control needs. Here’s a closer look at the applications and operating principle of MLI technology.

Overview
The magnetic level indicator, also called a magnetically coupled liquid level indicator or a magnetic level gauge, is in widespread use throughout process industries around the world. Originally designed as an alternative to sight glass gauges, MLIs are now commonly utilized in both new construction and plant expansions.

Typical applications/locations include:

  • Alkylation units
  • Boiler drums
  • Feedwater heaters
  • Industrial boilers
  • LNG facilities
  • NGL storage vessels
  • Oil / Water separators
  • Process vessels
  • Propane vessels
  • Storage tanks
  • Surge tanks
  • Wastewater tanks

Principle of Operation
Magnetic level indicators use the law of magnetism to provide liquid level information. They can activate a switch or provide continuous level data via a transmitter. Unlike a sight glass, magnetic coupling allows the MLI to measure liquid levels without direct contact between the externally mounted visual indicator and the fluid in the vessel.

A magnetic level indicator is mounted in-line with its respective process vessel.  It is exposed to the process media inside the vessel along with the process pressure and temperature. A float containing an array of magnets is sealed inside the MLI chamber. The float’s magnetic field interacts with the flags located in the visual indicator securely mounted on the outside of the chamber. As liquid rises and falls within the MLI, the float follows the changing level with the magnets remaining in the same plane as the liquid surface. The magnetic field causes the visual indicator flags to rotate, thereby revealing the liquid level inside the MLI.

In an MLI, the magnets within a float and an indicator are magnetically coupled. The float, located inside the chamber, dynamically tracks the surface of the liquid as it rises and falls. The magnet assembly inside the float generates a magnetic field that penetrates through the chamber wall to couple with the visual indicator.

Advantages of the Magnetic Level Indicator

A magnetic level indicator is often used in applications where a sight glass (or glass sight gauge) is either ill-suited based on process variables or is underperforming based on plant requirements. These can include enhanced safety for personnel; environmentally risky situations including media leakage or fugitive emissions; need for maintenance reduction; or need for high visibility from a distance. Typical shortcomings of glass sight gauges include:

  • High pressures, extreme temperatures, deteriorating seals/rings/gaskets, and toxic or corrosive materials may cause a risk of fugitive emission of dangerous substances.
  • The glass in a sight glass can become quickly discolored, thus decreasing level visibility, or it can acquire microfractures, which can become a personnel safety issue if left undetected.
  • Liquid/liquid interfaces can be very difficult to read in a sight glass particularly if the liquids are of similar color. Clear liquids can also be difficult to see in a sight glass.
  • Liquids that tend to coat or build-up on surfaces can hinder visibility by forming an opaque film on the glass.
  • To cover a large measuring span, sight glass assemblies typically must be staggered using multiple sections.

Common MLI Tank Configurations

The key reasons for selecting a magnetic level indicator over a sight glass are:

  • Improved safety due to the absence of fragile glass and a substantially reduced number of potential leak points.
  • Greatly increased visibility.
  • Reduced maintenance.
  • Easier initial installation and addition of transmitters and switches without interrupting the process.
  • Dual-technology redundancy, with the addition of transmitters or switches, for improved safety.
  • Lower long-term cost of ownership and legitimate return-on-investment benefits.
  • Single chamber measurement over 6 m without staggering chambers.

Safety
The obvious safety benefit of the MLI over a sight glass is reduced chance of breakage. If the process fluid is under extreme pressure or temperature, the likelihood of sight glass breakage is increased. The pressure boundary of an MLI is made of robust metal, frequently the same as the vessel piping, making MLIs as safe as the surrounding piping system itself. The indicators, transmitters, and switches are all mounted externally and, therefore, are unaffected by toxicity, corrosiveness, or other process fluid characteristics.

Another safety benefit is that the chemical compatibility with the fluid in an MLI is restricted to only three components, the metallic chamber, gaskets and float. With glass sight gauges, the process fluid may have chemical compatibility issues with any of the wetted materials—glass, metal, or sealants.

Maintenance
MLIs are virtually maintenance free once installed because the indicator never touches the process fluid. With sight glasses, the gauges must be periodically checked for leaks and cleaned on a regular basis. Scaling, etching and build-up on the glass from the process fluid can cause the sight glass to become unreadable.

Visibility
Visibility of the fluid level from long distances is another major reason for selecting an MLI over a sight glass gauge. Sight gauge level indicators are intended to be viewed at maximum distances of around 3 meters. However, the bright contrasting colors of the flags or a fluorescent shuttle on an MLI permit visible level indication at distances up to 30 meters or greater. Newer, more advanced visual indicators, such as those from Magnetrol International, have viewing ranges up to 60 m.

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