Best-Practices for Level Measurement Device Selection

Submitted by Brian Sullivan
on Wed, 03/28/2018
In the chemical processing industry, there are a significant percentage of measurement devices that are not correctly matched to their application, leading to decreased quality and consistency of the operation. Often the source of this problem is the assumption that one level measurement sensor is suitable for multiple applications. For example, a float sensor may adequately serve its purpose in a completely liquid environment, but when particles or suspended solids are introduced to the scenario, the circumstances change and the accuracy of the original sensor is compromised. In order to maintain a high level of output quality in any chemical processing facility, the proper level measurement devices should be selected for each individual application. There is no “one size fits all” answer.

Based on the application, several factors must be considered to ensure the accuracy and effectiveness of a level measurement device. These factors include the design conditions, the specific media to which the sensor will be exposed, how the information gathered from the device must be transmitted, and what additional accessories are needed to complete the operation. Tank size affects some sensors more than others, but it is not the only contributing factor when it comes to selecting the appropriate sensor and instrumentation.

The first order of business when selecting the proper level measurement device is to narrow down the application for which it will be used. In other words, what will the sensor's job be? It could be as simple as a visual readout or as complex as a multi-stage automated response system. The complexity and versatility required of a sensor can be easily determined by considering this question first.

Secondly, one must determine the conditions of the application, otherwise known as design conditions, which are vital in choosing the correct option. Questions to ask at this stage include:
  •     To what material(s) will the sensor be exposed?
  •     Are solids or liquids being measured?
  •     Where and how will the level sensor be placed -- internally or externally?
  •     To what temperatures and pressures will the sensor be exposed?
  •     What is the material's density?
  •     Does the operation require multiple sensors?
  •     Does the sensor need to comply with any specific design codes?
  •     What is the liquid's boiling or flash point?
  •     What level of precision is desired in the measurement?
  •     Is there steam present?
  •     What is the tank's size?
Depending on these specific conditions, one measurement device may be suitable while another would be completely useless. For example, the distance between the sensor and the top of a “full” tank can determine whether or not an ultra-sonic sensor could be used. Furthermore, if a liquid has a film or layer of floating solids at the surface, this may disrupt the measurements of certain sensors. In short, there are a plethora of variables which can and should contribute to the proper selection of one level measurement sensor over another.

A specific concern at this point is an awareness of the material that is being measured and its potential impact on the level sensor over time. This is one of the first pieces of knowledge required to select the correct level sensor. For example, materials that are corrosive, viscous, or have suspended solids; materials that can transition from one state to another with changes in pressure or temperature; materials that are reactive or hazardous; all drive the selection of the materials of construction, sensor type, and other safety or durability features used to specify the correct level sensor.

The next step in the selection process is to establish what will be done with the information collected by the device. If an alert must be sent when a tank reaches a certain level, the measurement device must be paired with the correct switch or output that will transmit that information. If multiple points along the path of a float sensor are to trigger different operations, each switch must be properly routed to its corresponding destination.

The actions taken with this gathered information also necessitate certain accessories based on the process. This is the last step in fully integrating a level measurement sensor in a chemical processing operation. Any piece of hardware or software that the sensor interacts with to complete its desired task must be carefully selected based on its specific function.

If each of these steps is considered with a desired end goal for the level measurement device in mind, the dependability, accuracy and consistency of level measurement should increase in any given operation.
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