Author: Melissa Westbrook, Instrumentation & Electrical Team Leader; Consulting Engineer I

There are several commercially available technologies for liquid, solid, and interface height measurements as well as a variety of options for liquid head measurement. The science behind these level measurement devices is straightforward, and for many level applications, one or more of these devices has a high likelihood of success. There are some applications where the process conditions are harsh, the material properties are extreme, or the infrastructure is limited, and implementing a level measurement becomes a formidable challenge. When approaching one of these difficult level applications, E²G recommends following a systematic approach to determine the best technology for implementation.

The first step is establishing the measurement objective and constraints. Although this may appear to be an obvious step, an inadequately understood objective or constraint is a frequent cause of an underperforming level measurement.

Some questions to consider when developing the measurement objective are below.

• What type of level information is needed (e.g., liquid height, liquid head pressure, volume, mass, etc.)? 
• Is continuous level needed or is point level detection adequate? 
• What are the data requirements: local, remote, both? 
• Will the instrument be used as the primary measurement for metering material into the vessel for mass balance?  For inventory tracking purposes?  To cross check with sending or receiving vessels’ level measurements?  For a custody transfer application? 
• Is the instrument needed for a level control application, such as maintaining adequate level in a feed tank, sump level control, level in continuous unit operations?   
• Will the level measurement be used for overfill protection or empty vessel detection?  Is it part of a protective system? 
• What are the accuracy requirements? 

The process conditions, material properties, and physical infrastructure—along with any safety requirements, management preferences, and site standards—establish the design constraints.  Table 1 summarizes aspects to consider when establishing the design constraints.

The documented measurement objective, along with the design constraints, is used as the basis for evaluating measurement technologies for the level application.

It is common for refineries and process facilities to follow a plant standard or a standard practice when implementing level instrumentation. The typical applications using the plant standard practice generally account for 80-90% of the level measurements on a site. Storage tanks of petroleum products and raw materials that represent the typical materials handled at a facility normally use the plant standard level instrument instrumentation (or there may be a standard technology used for storage tanks and a standard technology used for process vessels). The standardization is important for operations knowledge, maintenance consistency, and efficient spares storage.

The remaining 10-20% of level measurement applications are the difficult applications, the applications that are not able to follow the site standard for level measurement due to the measurement requirements (objective), material properties, process conditions, infrastructure limitations, or other constraints. It is likely that some of these are existing applications that have the standard level instrumentation installed and are underperforming because the standard does not fit the requirements and constraints of the difficult application. These underperforming devices are the ones known as the “maintenance headaches”: the devices that receive regular maintenance work orders for cleaning the sensor, reprogramming the electronics, readjusting the gain, etc.

For existing applications, it is assumed that there have been some iterations or maintenance activities that may have restored the measurement for a short amount of time but did not achieve long-term reliability. Iterations of the physical installation, control system logic, and instrument programming can continue, and sometimes this good work will result in success. But when a decision point has been reached that there are technology limitations that cannot be overcome, further iteration is not productive and it is time to move to a different technology.

The worksheet in Figures 1 and 2 are tools to document the difficult level application evaluation; it can be used as is or customized to reflect plant preferences. With a small, technology-focused group, review each of the level technologies and document the reasons why the technology meets or does not meet the measurement objectives and constraints. For reference, Table 2 summarizes the advantages and limitations of common continuous level technologies. This is a technology evaluation, so the physics of the measurement should remain at the forefront of the discussion.

After the first pass, eliminate the options that are not practical and do not warrant further consideration. Presumably, a small number of level technologies remain as feasible options. Repeat the review of the remaining technologies, but this time dive deeper into their viability for the application. As part of the evaluation process, the measurement objective and constraints may be refined or further developed. During this stage, it is appropriate to consult instrumentation vendors for their input as well. Iterate on this process until the team selects a level technology for the difficult application.

It is worth noting that nuclear gauges have the potential to be successful in most applications, but they are generally selected for only the harshest and most difficult of all level applications. Nuclear gauges are comparatively expensive to procure and have associated regulatory requirements that must be met throughout their life cycle, and some facilities are not comfortable managing nuclear sources. A typical requirement for choosing a nuclear gauge is demonstrating that all other technologies are insufficient.

This documentation and methodical analysis can be used to support the use of instrumentation technology that is new to a facility. The documentation and team review contribute to the continuous learning environment of the facility. Instrument specification sheets or data sets that are commonly used show the what, and the executed technology evaluation worksheet shows the why.

Engineers and technicians responsible for plant instrumentation strive to achieve high uptime and reliability for the systems they support. E²G recommends implementing the methodology established above to develop strong solutions for difficult level measurement applications. The best measure of success is getting a difficult application right the first time. Or, if this is a reevaluation of an existing application, use of the evaluation methodology can be used to support the justification of capital expenditure and builds confidence that the replacement will be successful.