Electric tank heaters present several possible dangers in process heating applications. Recent incidents include one at a manufacturing plant and another at an oil-and-gas processing facility.
The manufacturing facility incident occurred on the hydraulic oil storage tank similar to thousands in use all over the world. Because of the commonality of these systems, and the prolific use of electric tank immersion heaters, it is important to be aware of the potential danger associated with this setup.
The other explosion occurred at an oil-and-gas processing facility outside of Pittsburgh. There, a fire broke out at a complex of three storage tanks, each about 25' tall and 8' wide.
At the facility, the natural gas was pressurized within the tanks and moved along a pipeline to a processing plant, leaving behind a “briny fluid” made up of liquid hydrocarbon waste called condensate. The tank's electric heaters are designed to keep the liquid mixture from freezing.
The local fire chief explained that the tank designed to hold the liquid apparently was empty or nearly empty but contained gas vapors. When the liquid mixture is low, the heating element is designed to shut off. But in this case, the electric heater continued to operate, eventually warming the vapors to combustible levels, setting off the explosion.
Seeing such disasters, it becomes apparent that any electric tank heaters should be reviewed for design and installation issues that can lead to explosions so facilities can take action to avoid an incident.
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How would someone know that they might have a problem? In this article, several key electric tank heater design and installation issues - that, if left unchecked, could lead to fires or explosions - will be reviewed.
Know the Risks of Electric Bayonet Immersion Heaters. The electric heaters to be concerned about typically extend into a tank through the sidewall. They might be 12 to 36" long, or longer. Typically, they are in the form of a bundle of elements such as a coil, wire or other shape that resists the electric current, causing the bundle to give off heat. It is the same concept used in an electric stove or toaster.
In many cases, these elements work acceptably if they are covered with a fluid. If they are uncovered and the watt density is high enough, however, they can become an ignition source.
Consider Watt Density and Protection. Watt density -- defined as the number of watts per square inch of heating surface or element -- is what the designer planned for the unit selected. It makes sense that if you squeeze a whole lot of watts through a small heater, the heater elements can get very hot. The free air temperature of some of these elements can be more than 1,000°F (537°C). Remember, the flashpoint of many hydraulic oils is only 400 to 600°F (204 to 315°C).
It is not just oil tanks that are an area of concern. Any tank that can accumulate flammable elements must be considered. For instance, a wastewater tank that drains machining fluids may primarily contain water. But just a small amount of oil mixed in can cause a problem. The oil can be thermally cracked or broken down under certain conditions, filling the vapor space of the tank with hydrocarbon fumes. Then, when the level changes in the tank and air is brought in, a flammable mixture can be created.
Consider also the case of a wastewater tank that is part of a food process or sewage system. Whenever biological activity consumes organic materials, methane can be released. If the elements become exposed and there are no protections in place, you can create an ignition source directly in the tank.
Understand the Fire Triangle. The fire triangle concept is that when you bring fuel, oxygen and an ignition source together, you have all of the elements needed for a fire. When a flammable mixture is confined and ignited, you have everything you need for an explosion. The flammable materials try to expand to many times their volume in fractions of a second. When they cannot escape whatever vent is provided in a timely manner, the vessel or tank is pressurized and usually comes apart at some weak point. Often for cylindrical tanks, the weakest spot is at the top of the container. If an explosion does occur, the flying metal and resulting fireball can be deadly and destructive.
Strategies for Industrial Plant Safety
All users of electric immersion heaters must verify that they have minimized risks related to electric tank heaters wherever they are installed. Methods to minimize risks include the following.
Verify Watt Densities and Maximum Temperatures. Many options are available when buying electric heaters. You can purchase heaters that have features to minimize the maximum temperature, or you can specify lower watt densities.
Use Operating and High Temperature Limit Controllers. Operating temperature controllers are essentially like thermostats in your home. They seek to provide the right temperature on a routine basis. High temperature limit controllers are special, separately installed controls that have a manual reset feature once exceeded. In this case, the device shuts the unit off until a person manually hits a button or takes some action. This feature forces someone to investigate and hopefully understand that some unusual event occurred and that an unsafe condition was reached. If a high temperature limit is installed, the setting has to be well below whatever flashpoint could be reached.
Employ Level Controllers or Low Level Shutoffs. Level controls are available to help ensure that minimum levels are maintained in tanks for the heaters to operate. Level controls, however, are sometimes difficult to maintain. There are many things that can fool them. For this reason, in the case of boilers, there usually are two level controllers: one as a primary and one as a backup. It would never be a good idea to rely on level controls for the safety of your system, but it would be an extra layer of protection over and above a high temperature limit device.
Consider Tank Design and Practice Good Maintenance. Tank system design also needs to be considered. Several national safety code standards exist for fuel oil tanks that call out venting requirements. In some cases, relief valves also might be required. In most cases, the vents for oil systems are larger than one might otherwise consider. Remember that when replacing elements, care must be taken to consider watt densities and safety controls. There also needs to be consideration for how safety devices might be wired. In all cases, safety devices should be again checked whenever elements are replaced. The National Fire Protection Association's codes could be a good starting point for investigating tank issues. NFPA 22, in particular, has information about tank heating systems that might be helpful.
Practice Preventive Maintenance and Safety Device Testing. There was a tank explosion in an offshore facility in 2009, which was identified to have occurred because of the corrosion of a protective sheath on an electric heater. This incident illustrates the need to conduct periodic inspections of “at risk” electric heating elements and for testing of level controls and high temperature limits. Most combustion equipment codes and standards call for at least annual testing of safety interlock devices.
The implementation of a testing and inspection program for electrical heaters in tanks needs to be risk based for the application and should consider what is installed, the watt density, the materials in the tank and other factors such as operating levels vs. tank volume.
Note: This article was originally published with the headline, "Don't Be a Risk Taker," in the August 2011 of Process Heating magazine.