Advanced automation technology and measurement problems

Ingemar Serneby from Emerson describes how the latest instruments meet level measurement challenges in typical chemical processing applications

The demands placed on level measurement instrumentation in chemical process plants can be extremely arduous, with applications typically involving toxic, corrosive, flammable or explosive materials. Wide ranges in temperature and pressure are commonplace, as are challenging process conditions, such as steam, dust, foam, turbulence and condensation. All these factors can affect the accuracy and reliability of level measurement devices.

To overcome this, new methods and technologies have been developed with enhanced functionality that helps to increase plant and worker safety, improve process efficiency and reduce operational costs. Basic technologies to monitor and measure the level of process materials in chemical applications range from viewing the material through a sight glass on the side of a vessel through to electro-mechanical devices, such as floats and displacer switches, ultrasonic technology, pressure sensors and load cells.

Advanced devices, such as guided wave radar (GWR) transmitters, non-contact radar transmitters, and vibrating fork switches, are increasingly replacing these methods. These offer better diagnostics, greater reliability and lower lifecycle costs, and are already widely used in chemical plant applications. This article describes some typical examples.

Mixing & blending tanks

In mixing and blending applications, level measurement devices must overcome a multitude of challenges. These include turbulence, foaming, rapid changes in level, and the presence of obstacles such as multi-level agitators. Non-contact radar transmitters provide a suitable solution.

Advances in this technology include dedicated software algorithms that can counteract the effects of turbulence, disturbing obstacles, foam and rapid level changes, so that measurement accuracy is maintained. They can also be used in chambers or stilling wells to isolate the surface from turbulence. These devices perform level measurements using either pulse or frequency modulated continuous wave (FMCW) technology.

Although the latter is more accurate and sensitive, it has traditionally used more electrical energy and has typically been deployed only within a four-wire device. These can require costly additional cable infrastructure, which has influenced some users to install two-wire transmitters based on pulse technology instead.

However, advanced FMCW devices, such as Emerson’s Rosemount 5408 (pictured above with process seal antenna), have greater energy efficiency, enabling power and communication to be provided by two wires and enabling quick installation. Consequently, these devices can now be cost-effectively installed in far more mixing and blending applications, helping to improve measurement accuracy and reliability.

Reactors

Vapours, foam and turbulence are usually present in reactor applications. The density of liquids can change as part of the reaction, whilst high temperatures and variable pressures are standard. All these challenges can affect level measurement accuracy.

Non-contact radar transmitters are, again, a very good solution as they do not need to compensate for changes in density, while pressure variation, high temperatures and reaction vapours will not affect their measurement accuracy. The latest devices feature all-PTFE process seal antennae, developed specifically for demanding applications such as reactors, and ideal for handling corrosive media and high condensation levels.

These antennae provide increased pressure and temperature ratings compared to older devices. Their advanced diagnostics can detect abnormal process conditions, such as a dirty antenna or the presence of foam, and enable preventative maintenance to be scheduled, thereby avoiding process upsets or costly unscheduled shutdowns.

Boilers

In high-pressure and high-temperature boiler applications, where the steam is saturated, media density changes can create measurement errors of up to 30%. Traditional technologies, such as displacers and differential pressure (DP) transmitters, must compensate for these changes to provide accurate measurements. These methods can require high levels of maintenance.

GWR transmitters provide a better solution, as they can withstand extreme temperatures and pressures, and do not need to compensate for density changes. In addition, they have no moving parts, which increases their reliability and minimises maintenance costs.

The dielectric properties of feedwater will alter during the phase change from liquid to steam. This affects radar technology, because the propagation of the microwaves used to perform level measurements will slow down, creating a level measurement error of up to 50% if the measurement is not compensated.

The latest GWR devices overcome this challenge by using dynamic vapour compensation to continuously compensate for changes to the dielectric constant in the vapour space. This allows error rates as low as 2% or less.

Storage & process tanks

Measurement challenges in general storage and process tanks include agitation, heating and condensation. Traditional technologies such as pressure sensors, mechanical devices, load cells and ultrasonic devices are density-dependent, sensitive to contamination, and require frequent maintenance.

Non-contacting radar or GWR devices can provide reliable level measurements to ensure inventory accuracy and maximise the use of storage capacity. Vibrating fork switches are typically used in conjunction with radar transmitters to perform essential safety functions.

Level measurement devices used as part of a safety instrumented system, such as an overfill prevention system, must undergo regular proof-testing to ensure they meet the required safety integrity level. This can take considerable time, effort and cost, and present a safety risk. The latest radar transmitters and point level switches enable proof-testing to be performed remotely from the control room with minimal process interruption, thereby increasing safety, saving time, and reducing complexity and costs.

Lube oil reservoirs

To help protect critical pumps, a continuous supply of lubrication oil is provided from a reservoir, and its level must be monitored accurately. If the level is too low, there is a risk of equipment damage leading to a costly process shutdown. If the level is too high, there is the potential for an overflow, constituting a safety hazard. High vibration and limited access in skid-mounted or smaller tanks can also make it difficult to get good level readings. Floats or DP transmitters with a wet reference leg system have high maintenance requirements, and floats are prone to sticking.

However, piezo-electric crystal-based vibrating fork switches present a reliable solution. When the oil level drops to a defined low point, the device sends an alert, so the issue can be addressed before there is any risk of pump damage. When the oil level rises to a defined high point, an alarm is issued to prevent the risk of an overflow.

Further supporting reliability, advanced diagnostics in these devices enable operators to continuously monitor their electronic and mechanical health. The transmitter detects any build-up, fork blockage, or excessive corrosion, indicating that maintenance may be required, and enabling it to be scheduled during planned downtime, thereby increasing efficiency and reducing costs.

Solids

When bulk solids are stored in small vessels, the filling rate can be high and surface levels can therefore change quickly. Radar transmitters are an appropriate measurement solution because of how quickly they respond to these changes in level.

GWR is especially well suited for vessels with a diameter of less than 10 metres containing powders and small granular materials and where the installation area is restricted. Non-contact radar transmitters have no restrictions with respect to the weight of the material, so they are suitable in applications where GWR may not be appropriate because of pull forces or concerns about probe breakage.

A case study

A good example of how advanced level measurement technology overcame a difficult chemical industry challenge can be seen at Perstorp Specialty Chemicals’ plant in Perstorp, Sweden. The company makes plastics, resins and synthetic lubricants for a broad range of industries, and operates the plant close to maximum capacity.

Accurate and reliable level measurements are essential for several reasons. These include preventing overfills, monitoring how reactions affect the level and volume of tank contents, and monitoring the amount of foam in a tank. Should a level measurement device fail or deliver false alarms, this can lead to a process shutdown and spoilt batches.

The production of each batch involves a four-hour process. The loss of even a single batch can have serious financial implications, in terms of lost product and production time, and the cost of cleaning tanks. The plant has reactors and mixer vessels that contain central shaft agitators, running at speeds of 30-80 revolutions/minute, at multiple levels. This makes the process material surface extremely turbulent and therefore difficult to accurately measure. Some of the vessels also contain a thick layer of dense foam and can be prone to condensation, adding to the measurement challenge.

Perstorp tested ultrasonic level measurement devices as well as both pulse and FMCW non-contacting radar transmitters on the vessels, but none performed as required, which resulted in costly downtime and lost batches. After discussions with Emerson, Perstorp solved the challenge by installing three Rosemount 5408 with process seal antennae. These were able to handle the foamy environment and measure the process material surface accurately, despite the extreme turbulence within the tanks, thank to advanced signal processing.

The antennae were also good for dealing with the heavy condensation in the tanks. Perstorp now has accurate and reliable continuous level measurement in these vessels, despite the challenging process conditions. This has resulted in fewer lost batches, and has also reduced downtime, maintenance requirements and costs.

CONTACT

Emerson

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www.emerson.com