he very first recorded scientific measurement was the gauging of the rising and falling of the Nile using a level measurement device called a Nilometer several millennia ago. The Pharos used it as a process indicator for timing the planting cycle. Yet a recent CONTROL poll shows that level measurement is considered one of the two most vexing problems by process automation professionals. Another CONTROL poll indicated that the reasons advanced for this are either that the application was poorly designed, or that the level of the fluid (or solid) is itself very hard to measure, or that the measurement technology itself gives poor readings. If we’ve been measuring level for thousands of years, why do we still have so much trouble?
It’s About the Design of Processes
Certainly part of the problem is that most process engineers do not think of “level” as a control function. They think of mixing, batching, storing, surge control, and other process segments, which, to them, and strictly as a side issue, involve a level measurement. Many simulator programs for processes treat level measurement equivalently, and conventional loop tuning software is not always optimized for level loops. (See Sidebar below).
A process engineer in a chemical or pharmaceutical plant looks at the reaction happening inside a reactor vessel, and not the fact that because the reactor is sparged or agitated, it is exceedingly difficult to measure the level in the reactor. In fact, in the case of an agitated vessel, there is no level when the agitator is running. There is a vortex. What the operator wants to see is an inferred value: what the level measurement would be if the vessel’s contents were still. This is a very difficult value to present accurately, yet that is what the operator means when describing the “level” in that reactor vessel.
Many processes in the chemical industry, as well as in the pharmaceutical and even food processing sectors, are scaled up from pilot operations that may be as small as bench tops. In such cases, the controls are sometimes an afterthought, and not well integrated into the plant design. This, too, contributes to the difficulty of making a level measurement.
It’s About Vessel Design
Vessels are generally designed for the purpose of holding or processing a liquid or solid and not for the ease in measuring their levels. You find vessels built through floors of plants, vessels with only inlets and outlets and no measurement ports, vessels where there are so many internals that there are few ways to measure level. If the engineer who orders the vessel does not understand the process control loop of which the vessel is a part, it is likely that the vessel will be difficult to instrument for level measurement.
So what sort of things should our process engineer know to do when specifying the vessel? It is important to know about the purpose for which the vessel is to be used, as well as the characteristics of the fluid it will contain and the measurement technology being proposed.
But in the end, it is the performance of the vessel and sensor within the entire process loop that matters. As Expertune’s John Gerry says, “The performance goal of one level loop may be the opposite of another.” (See Sidebar below.)
Some vendors, such as Endress+Hauser have built online selection wizards that help end users and engineers select the correct level measurement technology. These wizards are helpful especially for the process engineer who doesn’t specify level measurement devices every day, and have the advantage that they are application specific, and not product-centric. Click on Product Selection Applicator to request a Free CD of the program, or just run it directly from the E+H website.
In his latest edition of the Instrument Engineer’s Handbook (CRC Press, Fourth Edition) BÃ©la LiptÃ¡k, devotes over 160 closely written pages to the measurement of level alone. This section of the IEH should be required reading for process and control system engineers. (See LiptÃ¡k’s answer to a difficult level application in Ask the Experts.)
It’s About the Fluid or Solid Itself
Most level measurement vendors will tell you that it is extremely important to know the physical characteristics of the solid or fluid you are going to be measuring. Not only temperature and pressure, but the actual physical characteristics are sometimes critical to the success of a measurement. One vendor, Magnetrol International, has for years provided the non-commercial “Technical Handbook” with selected physical constants, including dielectric constants, densities, viscosity tables, steam tables, specific gravity, and much more. This book, or one like it, is invaluable to the designer of a level measurement system.
It’s About Measurement Technology
Some vendors make only one technology, others make a wide selection. They do this because it is easier to make a successful level measurement if you don’t use a technology at the outer edge of its technical operating envelope. For example, if you need a non-contacting measurement, you can use an ultrasonic gauge, that is unless you are in a pressure vessel, or the vessel’s contents have foam or large amounts of vapor or dust. In that case, you should specify non-contact radar, that only if you can get the sensor into the tank at the correct location. If not, then you are limited to using a nuclear gauge, or perhaps, a load cell. The advantage to choosing a vendor with a wide range of products and technologies for measuring level is that there is a better chance of getting a “do-over” if the first selection fails to perform as advertised.