- By Carl Kull
- February 17, 2003
A brief examination of the latest improvements to magnetic level gauge (MLG) systems will help bring any engineer up-to-date on the hardware that is now available to help improve plant operations.
Despite which direction the economy may be heading, for process engineers responsible for plant operations that handle fluids, these are the best of times. Continuous research and development in the area of level detection and its integration into process controls has garnered tremendous gains over the past couple of years. Spearheading the way, new improvements to magnetic level gauge technology have wrought increased accuracy, control, safety and reliability over older technologies such as sight glasses, differential pressure transmitters, and complicated pneumatic controllers. Recent innovations in float design, float chamber connections and transmitters have enabled the complete integration of leveling systems into plant digital control systems (DCS), while gains in quality control support extended warranties and performance guaranties that are unheard of in the industry.
A brief examination of the latest improvements to magnetic level gauge (MLG) systems will help bring any engineer up-to-date on the hardware that is now available to help improve plant operations. The inclusion of application considerations will further illustrate the benefits of today's modern MLG systems—benefits that include: greater control accuracy, improved reliability, lower installation and start-up expense, less maintenance, eliminated fugitive emissions, and a decreased risk of explosion.
MAGNETIC LEVEL GAUGE TECHNOLOGY
The foundation of any level measuring system rests upon the level gauge itself, and for many fluidprocessing industries the new platform of choice is the magnetic level gauge. Almost half of all MLGs sold to engineers today are used to replace older sight glass units.
In the past, a sight glass was felt to be simpler because it does not have a float. Yet, close scrutiny reveals that a slight glass can possess as many as 50 separate parts. Additionally, the continuous maintenance required to remove and clean the glass is quickly forcing them out of favor. Some users have indicated a six to twelve month payback in maintenance savings alone by replacing their sight glasses with magnetic gauges. The fact that the cushions and gaskets used to seal the glass can be permanently deformed by compression —resulting in possible leaks, stress points, and even potential breakage—is likely the ultimate reason for the demise of sight glasses. This older technology clearly represents a substantial risk for environmental and personnel safety.
A quick review of how MLGs operate helps to further point out the reason for their increasing acceptance. Whereas a sight glass indicates level by visually displaying fluid level through direct contact against a measuring grid, MLGs display level through a separate tube that does not contain the process fluid, hence they are sometimes referred to as bypass level indicators. An MLG is still a visual indicator of liquid level, but it utilizes magnetic transmission to couple the position of a float (housed within an external "float" chamber alongside the process fluid vessel) to a moving shuttle (indicator) housed in a closely approximated separate tube that is totally isolated from the fluid. As the fluid level is repeated in the float chamber, so is it represented in the indicator tube.
Since the visible shuttle/indicator avoids direct contact with process liquids, problems with coating, plating, fouling, fugitive emissions and hazardous material leaks are completely eliminated. This ensures safe leveling of liquids that are toxic, corrosive, or flammable. Magnetic coupling also makes it easier to determine the level of colorless fluids.
NEW REFINEMENTS TO AN ALREADY SUCCESSFUL PACKAGE
The current status of magnetic level gauges as the preferred choice of level measuring and control has been brought about by a series of incremental, but important, improvements. Today's state-of-the-art MLGs provide the highest levels of accuracy and longterm reliability, an achievement that can be attributed to treating each part within the gauge as an individually engineered design, and not as an off-the-shelf commercial product.
An engineered approach to float design
The float, which is also known as the buoyancy device, acts as the heart of any MLG. Ideally, each float is designed to the specifications of each particular application. The best representatives contain up to 15 rod-magnets, arranged around the circumference of the float and held in place with flux rings to evenly distribute the magnetic pull. Taking into account the specific gravity of the fluid, the ideal buoyancy—ranging from 75 to 200 grams—is incorporated during the design prior to the float being manufactured. This positive buoyancy ensures that the float stays at the level of the fluid despite the buildup of contaminants that would ordinarily cause a zero-buoyancy device to sink. Positive-buoyancy (maintained via a hermetic seal) floats also are proving superior to the old vented-tube designs, which sink to the bottom of the chamber when they eventually fill with condensed vapor Additionally, the float material often deserves application-specific consideration. For instance, for fluids with a very low specific gravity, titanium represents a good choice. If there is a heavy hydrogen sulfide or chloride content, then C-276 might serve better.
Combined, these engineering efforts provide an exact linear duplication of the fluid level onto the indicator. No offset curve is required for calibration, as is the case for some older-design floats that attempt to meet all applications with a single configuration.
New strides in chamber design
Since the float chamber must house a small amount of the process liquid, it must be almost as strong as the vessel it is measuring. Today's leading MLG chambers are manufactured from a variety of materials including PVC, alloy, titanium, and stainless steel, allowing them to withstand process extremes from -320 to 1000 °F and full vacuum to 4,500 psig. Sturdy materials, such 316 SS allow MLGs to operate accurately within highly corrosive environments, such as in offshore operations.
However, the most beneficial gains in chamber design stem from a recent breakthrough in connecting the float chamber to the process or storage vessel. Currently, only one North American gauge manufacturer utilizes this new connection technique, referred to as extruded-outlet. With extrusion outlet welding, the bead is placed well outside the lumen of the float chamber. Because there are no internal distortions within the chamber, the float can move freely and accurately reproduce levels without hanging up.
The previous use of saddled and beveled outlet pipes often left an unusable float chamber because of sink-in and bowing after welding, which ultimately interfered with float movement. Attempts to re-straighten the chamber after welding often resulted in unnecessary bends and occasional code violations. Fillet welds, weld-o-lets and butt weld tees often cause additional distortion and alignment problems of their own.
The new standard of extruded outlets eliminates all distortion problems by providing a smooth radius on the inside, while also guaranteeing a stronger connection. For example, a 2" outlet on a 2" schedule 10 pipe has a pressure rating 1000 psig. Extrusion welds meet ANSI B31.1, B31.3 and Section VIII standards.
Improvements in Indicators
Nowadays, the best indicator tubes are hermetically sealed. Some manufacturers nitrogen-purge a glass or polycarbonate tube and then put ryton (to avoid corrosion) caps on each end to ensure air-tightness. This hermetic seal eliminates the possibility of any moisture entrapment and build-up that could negatively impact the indicator movement and travel. This allows a smooth path for the indicator, which tracks the float to represent the exact level against a calibrated scale.
In some instances, a bar graph is required. Even here, gains have been made. In the past, many bar graphs were housed in an aluminum channel with a flat glass front. However, metal and glass expand and contract at different rates, so it is not possible to maintain a hermetic seal. Moisture laden air then reaches the metal pivot points, leading to corrosion that causes the flags to hang up and yield inaccurate readings. But newer designs take advantage of ryton pivot points that do not corrode, and hermetically sealed tubes that prevent moisture from entering in the first place.
Quick Deliveries are a Reality
One MLG manufacturer is providing MLGs in sixteen (16) different custom configurations that can be ready for shipment in fifteen (15) working days – with no expedite fees. These custom configurations can be supplied with a measuring length up to 18 feet with optional transmitter and switch accessories.
NEW OPTIONS FOR UNIQUE APPLICATIONS
In some cases, two separate fluids are contained within the same vessel—as when oil floats on top of water, for instance. Understandably, measuring the interface could ordinarily prove very difficult. Modern magnetic level gauges cope with this situation through floats that are specifically designed to float with half their volume in the upper fluid and half in the lower fluid. By dialing in the specifications of the fluids at the MLG factory, interface differences as low as 0.03 SG can be accurately measured.
To go one step further, the use of two floats— and a third connection point from the float chamber to the process vessel—allows some MLGs to also measure the total level of upper fluid, in addition to the interface level (between the upper and lower fluid).
Top mounted applications
When levels need to be detected in underground tanks and sumps, the gauge must be located above the vessel instead of at the more traditional location alongside the vessel. In the case of top mounted MLGs, rods are attached to the floats. Again, accurate engineering of the float, magnets and the rod can allow grade level observation of tanks that span up to 14 feet underground.
The latest MLGs can provide accurate leveling of fluids at temperatures up to 1000 °F through the use of factory-supplied high temperature insulation jackets. The most effective jackets consist of two layers: an inner ceramic fiber insulation blanket, and an outer fiberglass cloth jacket that is silicone impregnated for weather resistance. This not only helps the gauge withstand high temperatures, but it provides moisture resistance and increased operator protection. At the other end of the spectrum, many low temperature fluids are clear, or create excessive frost, and cannot be reliably indicated by a sight glass.
To counteract this dilemma, some manufacturers use foam-glass insulation with mastic and an openweave fiberglass cloth vapor barrier, along with a smooth aluminum jacket for protection. This cryogenic insulation is then furnished with a clear frost extension (made of Lexan) on the indicator so that the level is visible on the outside of the thick insulation. To maintain accurate tracking of the indicator through all of this insulation, even more powerful magnets are built into the float, allowing some modern MLGs to accurately indicate levels at –320 °F.
In some cases, precise temperature control of the process fluid is necessary for "freeze" protection. The idea is to heat liquids above their pour point, and to prevent paraffin build up as seen in oil production applications. In the past, gauges have been heated using steam or electric heat tracing within narrow trace tubes which parallel the gauge. However, this does not provide a uniform distribution of heat. Instead, the latest MLGs employ a steam or electric heat jacket that completely surrounds the entire float chamber to ensure the maintenance of a constant temperature.
Highly volatile fluids
Of course, when highly volatile fluids—encountered during ethylene production and also in propane chillers, for example—create their own "steam," a different approach must be taken. If an old-style gauge is used in an application where the fluid is operating near its vapor pressure, the level indication may be erratic because vapor from the boiling fluid cannot pass the float quickly enough. Modern MLGs sidestep this hindrance by building an oversized float chamber that allows the vapors to pass around the narrower float. Built-in guide-rods within the chamber itself help to keep the float pressed close to the indicator tube, thus insuring no loss in accuracy.
Oversized float chambers are also increasingly indicated for use in measuring fluids with suspended particles.
When the particles are metallic, even more specific measures can be taken. To illustrate: rusty piping systems and vessels will occasionally bring about faulty readings when the rust particles find their way to the float, causing it to stick. However, this problem is now solved through the installation of magnetic traps that catch the rust.
ADVANCEMENTS IN SYSTEM INTEGRATION
One of the major advantages of today's MLG technology is that since the vessel contents are totally contained within the float chamber, the same magnetic leveling can be used to actuate limit switches or continuouslevel transmitting devices without having to disturb existing piping or vessels. Installation and modification can be undertaken while the plant is on-line. This paves the way for easy integration of leveling technology into plant digital control systems (DCS). For instance, level transmitters can pipe continuous level readings via common industry protocols such as 4-20 mA, HART, Honeywell DE, Foundation Fieldbus, while deriving their source of power from the control loop.
Electronic innovations in transmitter technology are quickly replacing older mechanical means such as reed-switch transmitters and pneumatic controllers. Leading-edge magnetostrictive transmitters, for example, feature only one moving part. The transmitter detects the position of the float within the float chamber by using "time of flight" technology to calculate distance. The resulting, non-contact, level measurement is accurate to 0.01% of full scale or 0.050 inch. Guided wave radar transmitters (no moving parts) are also very popular devices used in conjunction with MLGs to achieve a redundant level measuring system particularly harsh conditions such as parafin wax build up in crude oil applications.
This same technology allows for the hand-in-hand integration of control switches. Alone, or in combination with a magnetostrictive transmitter, high and low tier switches can easily be configured to shut a pump off or close a valve, for example. For some of these switches, the trip point can be adjusted by simply loosening the clamps and repositioning the switch.
It should come as no surprise that all of these technological accomplishments have been accompanied by improvements in quality and reliability. At a minimum, ISO 9000 certification is becoming more common. Fully assembling, function testing and calibrating all gauges and accessories at the gauge factory further optimize quality control. Continuing on, though, one MLG provider even has on-site ASME certification, permitting an "S", "U" or "UM" certification stamp on its gauges. Most of these gauges, transmitters and switches are FM, CSA and Cenelec certified. Along with a PED and ATEX certification, this equipment can be used anywhere in the world. This progress in quality culminates in the offering of a five-year warranty and two-year performance guarantee by one manufacturer.
A SUCCESSFUL OUTLOOK FOR PROCESS ENGINEERS
A few installation examples serve to illustrate the outcome of all of these accomplishments. At one processing plant, the replacement of the an old sight-glass instrument bridle—with a new MLG, three electric switches and two pneumatic switches— eliminated 15 valves, 42 pipe fittings, 103 threaded joints and two sight glasses including 16 U-bolts and nuts. The difference in replacement costs resulted in a savings of $2,795 while decreasing maintenance costs.
One level system for an industrial boiler originally resorted to an inconsistent sight glass for local indication, an unreliable conductivity "spark plug" system for remote indication, and separate float-operated switches for safety shutdown. The fitting of an easy-to-read local MLG indicator, with reliable noncontact switches to operate the remote indication panel lights and non-contact switches for safety shutdown, resulted in a much more integrated system that proved far easier to maintain.
And finally, at a Pulp Mill in South Africa, an MLG was fitted with a magnetostrictive transmitter to monitor boiler drum level that operated at 600F @ 1500 psig. The combo level systems were successful in replacing an unreliable sight glass and bridle system. Such successful installations of new magnetic leveling technology are increasingly being recreated throughout all manner of industries that process and store liquids. For those engineers that reap the benefits of these innovations, the future is bright indeed.Click Here for More Information
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