Learning From Catastrophe - Case Study of Heat Exchanger Failure

shell and tube heat exchangers in industrial plant
Shell and tube heat exchangers at industrial plant
Industrial accidents, whether minor or catastrophic, can serve as sources of learning when analyzed and studied. Operators, owners, and technicians involved with industrial chemical operations have a degree of moral, ethical, and legal responsibility to conduct work in a reasonably and predictably safe manner without endangering personnel, property, or the environment. Part of a diligent safety culture should include reviewing industrial accidents at other facilities. There is much to learn from these unfortunate events, even when they happen in an industry that may seem somewhat removed from our own.

The U.S. Chemical Safety Board, or CSB, is an independent federal agency that investigates industrial chemical accidents. Below, find one of their video reenactments and analysis of an explosion that occurred at a Louisiana chemical processing plant in 2013. A portion of the reenactment shows how a few seemingly innocuous oversights can combine with other unrecognized conditions that result in a major conflagration.

Check out the video and sharpen your sense of awareness for potential trouble spots in your own operation.

Continuous Liquid Level Measurement Technologies Used in Industry

Industrial pressure transmitter
Pressure measurements can be utilized to determine liquid level
Courtesy Azbil
Although continuous level measurement technologies have the ability to quantify applications for bulk solids, slurries, and granular materials, liquid level technologies stand out as being exceptionally crucial to many facets of the process control industrial sphere. Called “transmitters,” these continuous liquid level measurement devices employ technologies ranging from hydrostatic to magnetostriction, providing uninterrupted signals that indicate the level of liquid in a vessel, tank, or other container.

Hydrostatic devices focus on the equilibrium of dynamic and static liquids. There are three main types of hydrostatic transmitters:
  1. Displacer
  2. Bubbler
  3. Differential pressure
The displacer transmitters utilize a float placed within the liquid container. With its buoyancy characterized to the liquid and the application, the float, a connecting stem, and a range spring or similar counterbalance represents the liquid level in terms of the movement of the displacer (float). The displacement, or movement, of the assembly is converted into an electric signal for use by the monitoring and control system.

Bubbler transmitters are used for processing vessels that operate at atmospheric pressure. This method introduces a purge gas or an inert gas, e.g. air or dry nitrogen, into a tube extending into the liquid vessel. Precise measurement of the pressure exerted on the gas in the dip tube by the liquid in the tank is used to determine the height of the liquid.

Differential pressure (DP) transmitters rely directly on, in a basic explanation, the pressure difference between the bottom and top of the container. Precise pressure measurement is used to determine the height of the liquid in the tank. One of the most advantageous aspects of DP transmitters is that they can be used in pressurized containers, whereas displacer and bubbler transmitters cannot.

Other examples of level transmitter technologies––which are not hydrostatic devices––are magnetostrictive, capacitance, ultrasonic, laser, and radar.
magnetic liquid level indicator gauge with guided wave radar transmitter
Guided wave radar liquid level transmitter
joined with magnetic liquid level gauge
Courtesy Jerguson

In magnetostrictive level transmitters the measuring device, a float, has a series of magnets that create a magnetic field around a wire enclosed in a tube. Electrical pulses sent down the wire by the transmitter head product a torsional wave related to the position of the float, which moves with changes in liquid surface level. The transit time of the torsion wave back to the sensing head is measured and the depth of the liquid, as indicated by the float position, can be determined.

Capacitance transmitters are best applied to liquids that have high dielectric constants. Essentially, changes in the capacitance of the sensor / tank / liquid assembly will vary proportionately with the liquid level. The change in capacitance is measured and converted to an appropriate electrical signal.

Ultrasonic level transmitters emit ultrasonic energy from the top of the vessel toward the liquid. The emissions are reflected by the liquid surface and them time required for the signal to return to the source is used to determine the distance to the liquid surface.

Laser level transmitters operate similarly to an ultrasonic level transmitter. However, instead of using ultrasound signals, they use pulses of light.

Radar level transmitters involve microwaves emitting downward from the top of the container to the liquid’s surface and back again; the measurement is the entire time-frame. One variable radar level measurement echoes capacitance measurements: they both involve dielectric contact of liquid.

The precise measurement of transmit time for a wave or pulse of energy is employed in several of the technologies, the measurement of pressure in others. Each technology has a set of attributes making it an advantageous selection for a particular range of applications. Share your liquid level measurement challenges with an application expert, combining you process knowledge with their product application expertise to develop effective solutions.

Safety Cover For Magnetic Level Gauges

Magnetic level gauges provide visual indication of vessel and tank liquid levels. Their application advantage lies in their high visibility, isolation of the indicator from measured media, and options flexibility that permits use in many environments. While armored gauges are available, there is another level of safety that can be added to almost any existing or newly installed gauge.

The video demonstrates the use and toughness of the SafeView™ shield from Jerguson. It accommodates the company's line of magnetic level gauges, as well as those of many other manufacturers.

Your process measurement and control challenges are best solved by working in concert with a product application specialist. The combination of your process knowledge and their product application expertise will develop an effective solution.

Rotary Vane Actuators for Damper Control

pneumatic rotary vane actuator damper drive
One example of a rotary vane pneumatic damper drive
Courtesy Rotork
A rotary vane actuator is simply a part of an automated damper assembly: its role is to change the position of the damper, converting the motive force of fluid pressure into torque and applying it to a mechanism that will position the damper.

Vane actuators are widely used on quarter turn valves in industrial process automation, but their application also extends to dampers on all types of equipment and installations. Vane actuators are well suited for applications with operation requiring fully open or fully closed damper positions, although some do provide modulating service. A rotation of the actuator drive mechanism through a 90 degree arc, in combination with connecting linkage, quickly moves a damper between open and closed positions. A rotary vane actuator is well suited for driving this type of actuation, with its own 90 degree arc of movement.

A rotary vane actuator is specific to quarter turn opeartion. A pressure tight housing contains a movable vane which is sealed to the sides of the pressure chamber by means of a low friction gasket. Inlets and outlets to the chamber on opposing sides of the vane allow a controller to produce a pressure differential across the vane. The vane will move, in response to the pressure differential, in either direction. A shaft is connected to the vane and the vane acts like a lever to rotate the shaft as the vane is moved by fluid pressure. The torque produced by the actuator assembly is dependent upon the applied fluid pressure.

Hydraulic rotary vane actuators have the ability to handle large amounts of fluid and dynamic motions, exhibiting also qualities of durability and compactness. Pneumatic vane actuators use plant air pressure as the motive force. Both types generally provide fast operation, have few moving parts, and require little regular maintenance. A variety of typical automation accessories and options are available to customize a unit for a particular application.

More information is available from product specialists, with whom you should share your application requirements and challenges. Combining your process and facilities knowledge with their product application expertise will produce effective solutions.