Very few engineers are aware of the significant strides made in the design and development of magnetic level technology in recent years. The result is that this valid technology is often overlooked. Can it seriously compete with established methods of level measurement and is so, what possible gains can be had from it?
The common perception
The question is simple: How does one effectively control the level of a liquid? Many divergent answers can be expected and it depends mostly on whom you ask. Most traditionalists would probably immediately recommend pressure or DP transmitters while some would maintain that nothing compares with a displacer type level sensor for accurate and reliable process control.
Capacitance rods also enjoy a huge following while others would certainly recommend the use of noncontact measurement devices, from the relatively inexpensive ultrasonic sensor to the more expensive radar and microwave types.
What about the visual conformation of the level in a closed tank or vessel you ask? A chorus of a thousand voices answers: "Sight glass!"
The alternative solution
As magnetically activated level control instrumentation gets increasingly sophisticated, more and more engineers are becoming aware of this technology as a viable alternative to orthodox methods of level indication and control.
We have reached a stage in the development and implementation of the magnetic level that one can state with confidence that it can overcome problematic applications previously thought unsolvable using traditional methods. Due to the fact that all direct contact magnetic level devices are customer and application specific, the possible uses for this technology are almost limitless. After the installation and commissioning of a well-designed and constructed magnetic level indicator, its modularity permits upgrading by the end user almost at will.
What this means for the maintenance technician is that different functions and capabilities can be added at any time over the life of the device. This includes multipoint level control, 4 to 20 mA analog outputs, HART Protocol capability as well as Fieldbus compatible digital signals.
As all of these devices are externally mounted and not in contact with the process, upgrading can be done while the magnetic level is in commission and on-line. The writer believes that this inherent versatility is unsurpassed by any other product available on the market today.
Cost considerations
The current economic climate in South Africa and the financial constraints that are experienced by industry, are putting more and more pressure on plant designers to find alternative solutions to traditional instrumentation installations. These solutions are much closer and far more obvious than might be believed.
As an example, let us look at a typical level control and alarm/trip system for a pressure-containing drum. The bridle on the side of the drum usually contains the following:
* A sight glass.
* A displacer level controller.
* A minimum of two level switches, side mount float displacer type.
* At least 14 isolation valves.
* More than 100 threaded joints.
* Numerous pipefittings and plumbing.
The number of flanged nozzles on the drum itself depends on statutory requirements and customer preference but it can be as much as eight, just to facilitate level control.
All of the above can be achieved by one single magnetic level indicator/transmitter complete with multipoint trip switches, mounted on two flanges with two isolating valves. Where redundancy is required, two magnetic levels on separate sets of flanges will suffice.
Depending on process data and construction factors, the saving in cost can be as much as 70%. Add to this the low cost of ownership and ease of maintenance of a magnetic level installation; one ends up with an almost unbeatable package.
Resistance to change
No matter how good something works, technological advance inevitably renders the marvels of yesterday obsolete, however, process plant designers still insist on using the sight glass as a nonpowered level indicator on critical installations. Most modern plants being built today employ state-of-the-art control systems and the best primary measurement devices on offer but the archaic sight glass somehow still finds a place for itself in these process plants.
Although numerous magnetic level installations have been completed across the country on pressure vessels and boilers, the sight glass almost always has to remain in place due to statutory regulations regarding the visual observation of liquid levels in pressure containing drums. These regulations were determined many years ago using the best technology available at the time but this technology has now been made obsolete by the advent of the modern magnetic level indicator. It is time for a fresh look at these requirements and laws.
In many cases the magnetic level can solve level control problems previously not thought possible with, for example, a displacer type sensor. A good case in point would be on flashing liquid surfaces: the ordinary displacer is unable to cope with flashing or bubbling in the measurement chamber, whereas a purposely designed magnetic level takes these applications in its stride. This is especially evident on distillation columns or liquid ammonia installations with varying temperatures and/or pressures.
Modularity
Magnetic level detection really comes into its own with a large number of design and application variations possible and also the excellent modularity of its construction. It can be upgraded or downgraded at will by the user at any time in its life and maintenance can be done on the external components without any interference with the process.
Almost all magnetic level devices are customer designed, using set parameters and application criteria, making it one of the most versatile and flexible level control instruments available. What this really means to the customer is that the magnetic level is manufactured to suit the application and there is no need to modify the existing plant configuration, as is normally the case with standard, off the shelf equipment.
The big task at hand for the suppliers of magnetic level equipment, especially in the South African market, is to convince engineers that this technology is reaching maturity and that it should be placed in a category of its own.
Latest technology
For many years the only practical method of obtaining a 4 to 20 mA signal from a magnetic level indicator was through the use of a magnetically activated reed switch assembly. This method is now so common that these types of transmitters are available from most suppliers of magnetic level equipment, at relatively low cost.
Although reed sensors are very cost-effective and simple enough to manufacture in large volumes, they have reached the end of their development. The emphasis has shifted to the latest in magnetostrictive sensors, offering microprocessor based electronics with features such as HART Protocol capability, pushbutton calibration and on board diagnostics to name but a few.
At this stage in South Africa, there seems to be only two, possibly three suppliers competing in the market with this technology, rendering it fairly exclusive. By fitting one of these transmitters to a good quality magnetic level indicator, it is immediately transformed into a state of the art, highly accurate and versatile level control device.
Conclusion
This writer believes that the market for magnetic level devices is only now starting to develop properly and consequently most suppliers underestimate the value of the available market.
In the past, not much attention was given to this product and it was badly neglected by the suppliers and ignored by the buyers. Recently, specialised companies with international backing have opened the eyes of industry to the advantages of magnetic level Instrumentation insofar as cost effectiveness, reliability, modularity and ease of use are concerned.
Johan Steyn is the Managing Director of Kübler Africa and has been active in the instrumentation field for more than 15 years. He is also the current Chairperson of the Johannesburg Branch of the South African Institute of Measurement and Control (SAIMC).
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