High Level Switch vs. Transmitter

David W. Spitzer, P.E.

Some time ago there was interest as to whether a switch or transmitter should be installed in applications where either will serve the intended purpose. For example, a level transmitter on a tank may be necessary to manage its inventory so that the liquid does not overflow or become so low as to cause problems with its pump. Presuming that there are no other operational constraints, it may be acceptable for the level to vary between 30 and 70 percent of the tank contents.

Now assume the results of a hazard analysis determine that the probability of failure of the level transmitter and the severity of its consequences result in the requirement to install an independent level measurement system that alarms when the level is excessively high. Note that a high-level switch or a level transmitter with an alarm trip could be used to implement this function.

Installing a level switch addresses the goals of reducing costs and only installing the minimum equipment to perform a function. After all, level switches usually cost significantly less than level transmitters, so there is a definite cost savings. However, the analysis of costs should extend beyond the costs of the instruments themselves to include their installation and operation.

The installation of conduit/cable and programming are real costs that are associated with both installations. When these are considered, the percentage difference between the installed costs of the switch and transmitter becomes smaller. Further, if the cable for the new level transmitter can be routed in the existing conduit instead of installing a new conduit for the switch, the difference between the installed cost of the transmitter and level switch effectively becomes the difference between their purchase costs.

However, an important benefit of installing the transmitter is the information that it provides and the ability to verify its operation while the plant is operating. In one such application, an existing differential-pressure level transmitter was used to manage the level in an agitated tank that had some existing nozzles that were small in size. It was determined that an independent high-level alarm was needed in this particular tank to warn the operator of an impending overflow condition. A capacitance level transmitter with an alarm function was installed that measured between 60 and 100 percent of tank level.

With both level transmitters operational, the operator could readily compare the measurements from both level transmitters when the tank was above 60 percent level. Note that in this application, the existing nozzles in the tank limited technology options. Without these constraints, the installation of an ultrasonic or radar-level transmitter that measures the entire tank contents would allow the operator to continuously compare the two measurements. In contrast, a level switch would require periodic testing and do nothing until the level actually rises above the switch trip level. Is this improved diagnostic and resultant reliability worth the difference in cost?

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to... series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.

www.spitzerandboyes.com