Chemical and petrochemical industry applications can prove extremely demanding for level measurement technology. This article explains the challenges presented and compares the ability of traditional and modern measurement methods to perform accurately and reliably. It also provides some examples of how modern automation technology is delivering increased safety, reduced costs, and improved product quality and consistency. Article contributed by Emerson.
SIGNIFICANT advances in the capability of level measurement technology are helping devices to overcome these challenges and meet operators’ stringent accuracy and reliability requirements. This advanced functionality is helping to ensure the safety of the industry’s processes, plants, and personnel, however harsh the demands of the application in question.
Due to the nature of the chemical process industry, there is a necessity for many different technologies to fit its many needs. During the years there have been a number of ways to measure level, but the oldest and most basic is the manual method of viewing a liquid through a sight glass to determine the level within a storage vessel. Other traditional methods of level measurement include electro-mechanical devices such as floats and displacer switches, ultrasonic technology, pressure sensors, and load cells. However, chemical and petrochemical plants are increasingly replacing these devices with more modern electronic technologies that offer greater diagnostics and reliability, and lower lifecycle costs. These technologies include vibrating fork switches, guided wave radar (GWR), non-contacting radar, and acoustic phased-array antennas.
There are numerous level measurement applications found within the chemical process industry. The following selection provides a broad overview of some very common application types and explains how the latest level measurement technology can help overcome the various challenges each one presents.
In applications where the mixing of chemicals takes place, level measurements are needed to monitor the addition of fluids to help achieve product consistency. However, there are several challenges that measurement technology must be able to overcome. These typically include tank interiors containing agitators with a central shaft, and blades typically rotating at a speed of 20-80rpm, as well as baffles installed on the side of the tank walls. Process conditions often include heavy turbulence and whirlpooling, and it is also common for foam and condensation to occur, and for build-up to affect antennas, all of which significantly affects measurement performance.
The primary function of reactor vessels is like that of blending tanks, except that a chemical reaction is required to produce the intermediate and final product, and these vessels also contain the reactions. While the components themselves can create an exothermic or endothermic reaction, sometimes external heat is required. Vapours, foam, and turbulence are often present in these applications. Density can change as part of the reaction, and other challenges include high temperatures and variable pressures. As with blending tank applications, current best practice is to use non-contacting radar transmitters. These devices do not need to compensate for the changes in density that can occur during the reaction. Also, pressure variation, high temperatures, and reaction vapours will not affect their measurement accuracy.
Boilers have an important role to play in chemical plants, as they convert water into steam, which is then used to provide heat or power. Accurate level control in boilers is critical as a means of ensuring safe and efficient operation, as inaccuracy can cause damage to the steam drum, the boiler, and even the turbine. If the drum level falls below a set point, the boiler may run dry and explode. If the level exceeds a set high point, wet steam may be carried into the turbine, which can damage the turbine blades. Redundant measurements are therefore required for safety purposes.
General Storage Tanks
General storage tanks can contain a huge variety of liquids for use in a plant’s various chemical processes. The accurate and reliable measurement of these liquids is important as it enables operators to ensure continuous production. Agitation, heating, and condensation are all common challenges associated with these tanks, which level measurement technology must overcome.
Measuring the level of solids is complex, first and foremost because the surface of solid materials within a tank is rarely flat. Instead, the angle of repose, or surface inclination, will constantly change as the vessel fills and empties. Further challenges include the amount of dust that is created during fill cycles, the low dielectric value of many solids, and the fact that heavier materials can create a pull force that can break cables. This last point is more likely to be an issue in larger tanks, but even in smaller vessels, care must be taken to guard against this possibility.
Pumps play an important role within the harsh operating conditions of chemical plants, and it is important that they are kept running 24/7. Failure to correctly maintain and monitor the health of pumps can seriously impact plant uptime and efficiency. To protect a plant’s pumps, a continuous supply of lubrication oil is typically provided from a lube oil reservoir, and it is crucial that the level in this reservoir is monitored accurately. If the oil level falls too low, the pumps will not have enough lubrication to run properly and there is a risk of equipment damage, which could lead to a costly process shutdown. If the level rises too high, there is the potential for an overspill or leakage, which would constitute a safety hazard.
Solving Challenges With New Technology
As advances are made in the functionality of the latest level monitoring and measurement technology, it can be applied to an increasing number and type of applications.
An example of this can be found at a US manufacturer of customised chemical solutions. The company required a solution to detect the level and interface of acid/water in an acid decant tank. The tank has a layer of water on top of the acid for stability purposes, but when the acid is decanted into containers, any water included in the acid could result in a costly scrapped batch. Top-down radar was unsuitable because the microwave signal could not pass through the water layer on top of the acid. The company also looked at magneto-restrictive devices, but these were complex to configure and their flexibility in different fluids was not good.
Operators were therefore tasked with observing the visual indication of the phase change/interface through a sight glass in a pipe spool piece. When the interface was observed by the operators, they radioed the control room to turn off a valve to stop decanting into the container. However, this resulted in a huge variation in batch consistency because some operators were more vigilant than others. The company wanted to eliminate operator interaction to increase personnel safety and allow staff to concentrate on other tasks. It also required accurate indication of the acid/water interface to retain batch consistency but needed a method that would be able to measure through the layer of water on top of the acid.
The site already had a wireless network, so a wireless device requiring minimal installation was ideal for the application. A Rosemount 2160 Wireless Vibrating Fork Level Switch was installed in the acid tank. The frequency of the fork’s prongs changed depending on whether they were in acid or water, and this frequency change gave a reliable indication of the phase change from acid to water. The wetted material options – in this case Alloy C-276 – solved any concerns regarding the acid corroding the device. The result was that product quality improved and throughput became more consistent throughout. And most importantly, eliminating human intervention in areas with health and safety hazards increased the safety of the plant. Cost savings were made due to the elimination of scrapped batches, and because the Rosemount 2160 is a wireless device, the installation was quick and easy. The company was very happy with the result and decided to equip a second tank with the same solution.
Level measurement applications within the chemical and petrochemical industry can be extremely challenging. Devices must provide repeatable, reliable, and accurate measurement of hazardous materials, in wide temperature and pressure ranges, and with process variables such as steam, dust, foam, turbulence, and condensation. Advanced functionality in the latest level measurement technology is helping to meet these challenges and helping manufacturers to become safer and operate more efficiently.