How to Increase Plant Efficiency and Safety with Level Measurement Technology |

How to Increase Plant Efficiency and Safety with Level Measurement Technology

How to Increase Plant Efficiency and Safety with Level Measurement Technology

By Patricia Mattsson, Marketing Engineer, Emerson Automation Solutions

Liquid level measurement applications can be particularly challenging within the chemical industry due to the huge variety of products measured, varying processes, vessel types and sizes. Blending tanks and reactor vessels often present difficult environments for level technologies due to internal devices, foam and condensation, all of which affect measurement performance significantly. A well-chosen measurement technology—ultrasonic, differential pressure, point level switches, guided-wave radar (GWR) or non-contacting radar—can enhance measurement and process performance.

For reactor, mixer and blending tank applications it is important to know exactly how much of each product is in the vessel to achieve the required final reaction or mixture. There are many ways to determine how much of each component is being added, but continuous level measurement using radar can provide complete control after the reaction. This is important because of all the things that can happen during the process. Temperatures and volumes can change, foam blankets form and vapors released. Non-contacting radar can see around obstacles such as agitators, avoiding the noise created by these devices which can affect the quality and reliability of measurements. Recent developments of non-contacting radars based on frequency modulated continuous wave (FMCW) technology, improve the reliability and accuracy of measurements, especially when applied to complex and demanding applications.

Traditionally non-contacting radar instruments have used a pulse modulation method. Microwaves are emitted towards the surface and reflected to the sensor, with the level being directly proportional to the time from signal transmission to reception. With FMCW, the transmitter sends a continuous signal sweep with a constantly changing frequency. The difference between the frequency of the reflected signal and the frequency of the signal transmitted at that moment (Figure 1) is proportional to the distance from the radar to the surface, which provides the means to calculate the distance. With FMCW, the process variable information is in the frequency domain instead of the amplitude-modulated or time-difference domain, which allows more accurate signal conversion. Most tank noise sources, such as agitators, are captured in the amplitude domain, so frequency modulated signal processing can ignore them, and accuracy is not affected.

Figure 1: FMCW technology provides a stronger measuring signal which can improve sensitivity while avoiding false echoes from internal tank obstructions.

The sensitivity of FMCW transmitters is more than 30 times higher than more traditional pulse-technology-based non-contacting radars. This maximizes signal strength, which produces greater measurement accuracy and reliability to provide a higher safety margin within demanding applications.

Although FMCW technology is not new, its power consumption has limited it to four-wire implementations which requires more costly support infrastructure. Users often settle for two-wire pulse technology transmitters to save installation costs. Fortunately, new FMCW transmitters (Figure 2) are now available as two-wire devices, enabling widespread deployment without costly infrastructure requirements.

Figure 2: New radar transmitters use FMCW technology but with low enough power consumption to operate in a two-wire installation.


Ease of Use

Technological improvements have been matched by efforts to simplify installation, operation and maintenance. In the past, a successful installation depended on knowledgeable technicians within the plant. Now, intuitive software interfaces guide users through the installation and commissioning phases along with maintenance procedures too, ensuring that all tasks are simple to perform. This supports greater worker efficiency, while ensuring that devices are working correctly and providing the information needed for safe and efficient operation.


Aggressive Media

Other challenges for instrumentation, including level measurement instruments, are the aggressive products commonly used within the chemical industry. This challenge is manageable when the situation doesn’t change, but units and vessels might contain vastly different products over the equipment lifespan. PTFE has emerged as an almost universal material able to withstand virtually anything, and many GWR and non-contacting radar transmitters are now available as all-PTFE solutions. The latest FMCW non-contacting radar designs even eliminate the need for wetted o-rings completely. If the corrosion characteristics change from batch to batch or over time, both common in the chemical industry, there won’t be a situation where the o-ring specified originally is incompatible with the new process. These features enable the latest radar to be installed and provide reliable operation independent of what the vessel may contain in the future.


Overfill Prevention

Monitoring the levels within tanks used to store raw materials, product at intermediate stages of a process and final product is another very common application for level measurement technology within a chemical plant (Figure 3). Measurements are used to manage supply and stock levels, support the control of media and prevent overfilling. Failure to identify and prevent the overfilling of tanks and vessels containing hazardous and flammable materials can have catastrophic consequences, as high-profile incidents such as the Buncefield fire in 2005 illustrate.

Figure 3: Storage tanks of all sizes can benefit from today’s improved radar level measuring technologies.

Most progressive chemical companies have implemented automated solutions to help improve not only the accuracy and reliability of measurements, but also to remove manual tasks requiring workers to climb on tanks and open hatches to measure levels. For many years, non-contacting radar, GWR and point level switches have provided far better measurement results in the vast majority of applications. Thousands of successful installations can already be found worldwide and the technologies continue to improve. Today’s level instruments feature powerful built-in diagnostics and can perform partial proof-tests remotely. Monitoring the health of devices remotely (Figure 4) ensures they will perform correctly within an overfill prevention system, while saving time and increasing worker efficiency.

Figure 4: The ability to monitor equipment condition, such as critical lubricant levels, continuously and remotely can capture problems before they cause an outage.

Boiler Efficiency

Boilers are a critical part of most chemical plants, providing heat and steam for production units. Efficient operation reduces fuel use, and level devices play an important role as well as helping prevent damage and potential safety incidents. Measuring level in high-pressure and high-temperature steam applications is challenging. A boiler will experience varying temperatures and pressures, especially during start-ups. The media changes density over the process and this can cause measurement errors of as much as 30 percent. Density-based measurement devices, such as displacers and differential pressure transmitters, depend on heavy compensation for these changes to show the true level. There are algorithms able to help the control system, but the operating pressure must also be known.

GWR has been adopted widely because it measures the surface of a liquid directly, independent of density, thus eliminating the need for any manually programmed settings for the compensation relating to density changes. This, along with beingable to withstand extreme temperatures of up to 750 ºF (400 ºC), and pressures up to 5000 psi (345 bar),makes GWR ideal for feedwater heater and boiler drum applications. However, the boiler water’s dielectric properties change during the phase change from liquid to steam. When under high pressure, the dielectric of the steam increases and the propagation speed of the microwaves down the radar probe will slow down. This can cause measurement errors of up to 20% without compensation.

Compensation can be performed in two ways: either using static vapor compensation, or in the case of the latest level measurement devices, dynamic vapor compensation (DVC). With static vapor compensation, the expected operating pressures and temperatures must be entered manually when the transmitter is configured. With DVC, the compensation occurs in the transmitter’s electronics continuously adjusting for changes in the dielectric constant in the vapor space so the transmitter sends the correct level to the control system.

DVC works by having a fixed target to provide a baseline. The transmitter knows where the target is (Figure 5) and will expect a corresponding pulse at this location when there is no vapor present. When vapor is present the pulse appears to move further away. The transmitter determines the difference between where the pulse should occur and where it actually occurs, to back-calculate the dielectric constant of the vapor space. The onboard compensation is therefore always performed in the same way and makes the compensation more accurate and repeatable. With DVC, error levels can be reduced to 2% or less. Even small deviations can have a major effect on efficiency and profitability.

Figure 5: Dynamic vapor compensation available with GWR level technologies can deliver more accurate readings in difficult boiler applications.


Plant Maintenance and Reliability

Around 5% of production capacity is lost each year due to unplanned downtime, and equipment failure accounts for approximately 40% of those unplanned outages. Equipment assets critical to production, such as blowers, fans and compressors, are increasingly being monitored to detect changes indicating potential health problems. Pumps play important roles within a typical chemical plant, with a reliance on them to move raw materials, intermediates and products as well as supply and recycle essential utilities. Failure to maintain and monitor the health of pumps can reduce uptime and degrade plant efficiency.

Pump lubrication system failures take out many installations. It is therefore important to monitor lubricant levels within the reservoir, but many companies still do this via manual rounds which is a waste of human resources because it is so easy to do automatically.

Vibrating-fork level switches, which identify low- or high-liquid levels through changes to the natural resonant frequency of the tuning forks, present the ideal automated solution for this type of application. They have no moving parts and are virtually unaffected by process conditions, which contributes towards extremely reliable level detection and control. When the reservoir level drops to a defined low point, maintenance can be alerted to address this before there is any risk to the pumps. Similarly, fork level switches can also be used to detect if there is any liquid running through a pump to avoid damage from running dry. Using the output from the switch, the pump can be disabled and prevented from overheating.

The challenge when implementing new automated solutions is often the cost of adding the necessary wiring. These instruments are mostly available with WirelessHART transmitters or can have WirelessHART adapters added to facilitate communication without wired infrastructure, thereby drastically reducing the installation cost.


Automation for Safety and Efficiency

Level measurement applications are just one area where effective instrumentation can improve plant performance as chemical manufacturers face challenges of recruiting and retaining trained operators. Automating routine measurements and functions helps make processes easier to control while saving key people for more important responsibilities. There are more and better technologies available now than ever before.

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