Providing the right level of protection
Deployment of level control technologies can prevent serious incidents
By Kevin Cullen
Measuring levels in tanks is important for inventory management, but when liquids are hazardous or potentially harmful to the environment, it is important to provide not only a level measurement but also an overspill protection system. The deployment of level control technologies can help prevent serious incidents.
In tank storage applications within the oil and gas, chemical, petrochemical, and pharmaceutical industries, overflow presents a risk to the environment, damage to the plant, and is potential harm to workers and people living nearby. Several high profile and serious accidents in the U.K. involving storage tanks have highlighted the need to increase the protection to primary containment systems. Maximizing tank capacity, while minimizing the risk of hazardous spillage and overfill, can be achieved with a combination of level measurement devices and level switches. Top-down radar transmitters, for example, offer a precise level measurement that is independent of the fluid properties, and installing level switches adds another layer of protection.
To improve accuracy and reliability and reduce maintenance costs, plant engineers are increasingly implementing non-contacting or guided wave radar devices (GWR) for measuring levels in storage tanks. GWR is based on a low energy pulse of microwaves being sent down a probe. When the pulse reaches the media surface, a reflection is sent back to the transmitter. The transmitter measures the time taken for the pulse to reach the media surface and be reflected back, and an on-board microprocessor accurately calculates the distance to the media surface using “time-of-flight” principles. GWR is not density dependent and is unaffected by high turbulence or vibrations; thus, measurement accuracy and reliability in such applications are improved. Because there are no moving parts to stick or wear, maintenance costs are reduced, and the problems of false readings, which can result in hazardous situations, are avoided.
Radar level data is often used to generate various alarms to prevent a potential overspill condition and, less commonly, used for low level alarms to prevent a possible tank overheating condition.
Radar level measurements are normally used for inventory management and to determine if there is capacity in a given tank to receive a new delivery. Radar transmitters are accurate and reliable. The level data from them is often used to generate various alarms to prevent a potential overspill condition and less commonly used for low level alarms to prevent a possible tank overheating condition. However, in the event of a malfunction of a radar device, or as a result of incorrectly setting up the radar, these alarms could fail to provide low level or overspill safeguards.
More stringent legislation covering overspill protection on critical applications now requires a level of redundancy to be installed. In addition, many companies are adopting multiple technologies to further minimize risk. It is recommended that independent high level switch devices should be fitted to all tanks. These provide a safeguard from spills in the event of a fault or problem with the contents measuring instrumentation, or in case there has been an error in the translation of level measurement into volumetric contents. This provides a secondary method of overspill protection. Alarms are derived from fixed position switches or probes within the tank, which operate when the liquid makes contact with them. These should be used in addition to the measurement-derived high level alarms.
Traditionally, displacers and floats have been used in about 80% of refinery tank level applications. These provide a very basic indication of when the contents of the tank have reached a specific level and trip a high level or extra high level alarm. However, the accuracy of these types of instruments is affected by varying density and temperature. These variations can cause inaccurate readings and affect control. Build-up on the displacer element also affects accuracy, and the mechanical nature of displacers means they are prone to wear and even failure.
Vibrating fork devices present the ideal solution for high or low level tank alarms. These level switches operate on the principle of a tuning fork, whereby an internal piezo-electric crystal oscillates an external fork at its natural frequency. The frequency changes depending on the medium in which it is immersed (the denser the liquid, the lower the frequency); thus, the frequency is different depending on whether the fork is immersed or dry. Changes to this frequency are monitored and used to trigger either a low level or high level alarm.
Intelligent vibrating fork devices, such as the Rosemount 2100 series vibrating forks, have no parts that can get stuck and therefore are less prone to failure.
The advantage of vibrating forks over displacers and floats is that they do not have parts that can get stuck and, therefore, are less prone to failure. With no moving parts, other than vibration of the fork (which does not cause any wear), this type of device requires very low levels of maintenance. Vibrating forks are therefore very reliable—exactly what you need for a device performing a measurement that prevents overspills.
Build-up can be a real problem with any liquid measuring device, as this may render a traditional level switch non-operational, with potentially serious consequences, such as spills. With a simple on/off signal from a hardwired displacer level switch, it is not possible to tell the difference between a stuck switch and an actual high-level condition. Similarly, it isn’t possible to tell if the level switch is damaged, or has failed, and the signal is therefore invalid. Technicians periodically have to go to the field to perform checks just to be sure, often to find nothing wrong.
With intelligent vibrating fork devices, however, changes in frequency are used to detect not only high or low levels, but also media build-up on the fork, external damage to the fork, internal damage to the piezo, and excessive corrosion. Build-up of material on the vibrating fork is detected in the early stages and flagged as an advisory alarm. This enables operations to schedule cleaning of the forks before build-up accumulates to the point where it causes a false process state indication.
This field intelligence eliminates the need to send a technician into the field to inspect on a hunch. Suspected problems can be verified remotely from the control room, and cleaning or service scheduled accordingly. Maintenance technicians can focus on the cleaning and repairs that are really needed, instead of inspecting a fork, which does not need to be cleaned.
One aspect that holds back the application of level instrumentation is a lack of existing cable infrastructure connecting the remote tanks. The cost of installing new wiring can include ducting, cable trays, and the digging of trenches, which can be significant and often cost prohibitive. There is also the inconvenience and issues of performing the work while the site is operating.
Wireless devices enable new instruments to be quickly and cost-effectively installed on remote tanks, with no existing cable infrastructure, using wireless networks such as IEC 62591 (WirelessHART®).
The availability and implementation of wireless networks at plants is reducing the cost of implementing new instrumentation. Using wireless adapters, which provide wireless connectivity for conventional devices, radar devices can now transmit level measurement data without the need for cabling. Wireless devices enable automated overspill protection to be applied to previously unreachable storage tanks.
Duplicate level technologies are recommended for tank level monitoring; however, it is “best practice,” and, in many cases, a legal requirement, to install a hardwired safety system to stop the filling of tanks before a safe level is exceeded. Tank overspill protection systems are designed to enhance existing safety measures, guarding against the potential hazards that would result from overfilling. Based on a safety instrumented system (SIS) consisting of sensor (level device), a state-of-the-art logic controller and control element (valve), the tank overspill protection systems will monitor the tank level and automatically shut off the feed to the tanks if the level reaches the high cut off limit.
The hardwired overspill protection is achieved by connecting the output from a separate level sensor to a safety relay system that automatically closes down any potential supply into the vessel. The safety integrity level (SIL), the level of safety required, is determined by carrying out a full safety assessment and review of the process. The protection system should then be designed to satisfy the SIL level determined by the safety review. Where risk needs to be managed using a SIS, level transmitters are available to support these with radar and vibrating fork devices suitable for SIL 2 rated applications. These devices are designed for high reliability in process grade applications and environments.
With the consequences of inadequate monitoring systems on storage tanks all too familiar, it is imperative that operators follow best practice guidelines and comply with the latest safety legislation. It is important to perform a full safety assessment, select and install multiple level technologies that provide redundancy and independent level measurements and safety alarms, and apply the correct instrumentation to meet the desired SIL rating. The latest level instruments support these requirements and increase the reliability of measurements, provide data about their own health, and perform measurements that were previously out of reach.
ABOUT THE AUTHOR
Kevin Cullen (firstname.lastname@example.org) is a senior product manager for Electronic Point Level Instrumentation at Mobrey Measurement, a business unit of Emerson Process Management based in Slough, U.K. He has more than 12 years of experience in sales and marketing with level and pressure instrumentation and control systems companies.