Preventing Tank Fires
Preventing Tank Fires
A tank fire is when the methanol fumes inside the tank catch fire and is caused by dry firing the heating element. Dry-firing is the act of burning out the heating element by exposing it to air when it’s operating. Water heater elements were designed to operate submerged in water so that the heat can be carried away quickly. Air is a good insulator and when the element is exposed to air, the temperature of the element rises to over 1400°F. That’s hot enough to melt the element. It’s also hot enough to ignite any methanol fumes present in the processor. Definitely a high adrenaline moment!
This drum was pressure tested with compressed air. At 70-75 psi it unrolled the bead holding the lid (closed head drum) and blew. The drum was launched into the air and landed 70 feet away. The lid flew off like a Frisbee into the woods and has never been seen again.
You can mount a power strip in a high visibility location to provide visual feedback of when the water heater element is plugged in.
Rated at 15A for 120V this illuminated switch can provide visual feedback of when the element is powered.
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When the element dry fires, it ignites the methanol. As the methanol burns it causes the air to heat up and expand, building pressure inside the tank. Appleseed Processors are the best equipped to handle a tank fire of all the processor designs. The tank is designed to operate at 150 psi and may be able to withstand higher pressures. We install a 30 psi relief valve in the Appleseed that will let most of the pressure escape. If the dry fire happens while draining the tank the vent valve is also open (It should always be open if you have one.) providing another avenue of escape for the pressure. If that is not enough, it will blow the hoses off the processor.
In line heaters with the heating element in a pipe are aslo susceptible to dry firing and tank fires if heating the oil directly. This style of heating is sometimes used on processors made from MDPE. It is possible to catch an MDPE processor on fire if the element dry fires. In addition if the pump fails it can heat the oil surrounding the element to tempratures that could ignite methanol laden oil. If the element is in a PVC housing, as I've seen on some MDPE processors, then pump failure could heat the oil hot enough to melt the PVC and spill the very hot oil on the floor before it dry fires and sends molten PVC everywhere.
Of all the tank fires that have been reported, there have been NO reports of the water heater rupturing! That is not the case for drum processors. Sealed 55 gallon drums are not pressure rated and will come apart at the seams at about 30 psi. There have been reports of drum processors blowing their lid through the roof in a tank fire. Drum Processors come apart during tank fires in ways that can injure or kill anyone around. Open head drums used as processors sealed by placing (not fastening) a lid on top are not able to build up enough pressure to rupture.
There are two situations that can cause a tank fire.
· Draining the tank with power applied to the heating element.
· Applying power to the heating element when the tank is empty.
These two situation require two different approaches for successfully preventing tank fires. If we forget to turn off the heating element we either need some sort of reminder that the heat is on or we need some mechanisim that turns the heat for us if we forget.
In the case of applying power to the heating element when the tank is empty, this is most often done by mistake. The operator thinks he is turning on the pump and turns on the heat by mistake. This can be prevented by making turning on the heat a combination of steps forcing the operator to think of the steps before applying heat.
To avoid the tank fire, never apply power to the heating element when it is uncovered. The basic Appleseed Kit uses visual feedback to let you know when your heating element is powered. If it’s plugged into the wall it’s powered up. Our process calls for you to unplug the processor after you have finished heating and before you add the methoxide. If the process is followed your not likely to dry fire.
There are a number of additional safety features that can be added to assist in preventing dry firing.
· Plug the element into a clearly visible location for a visual indication that power is applied to the element.
· Wiring a light in parallel with the heating element. If the wired correctly the light will be an additional indicator that power is applied to the heating element.
· Adding a relay that de-energizes the heating element when the oil is brought up to temperature.
· Adding a timer that only allows the element to run for so long.
· Making it more difficult to turn the heating element on in the first place.
· Using bottled nitrogen gas to slightly pressurise the tank when draining will prevent oxygen from being drawn into the tank a eliminate any possible tank fires.
· Using a washing machine level switch on the heating element to disconnect the power when the level in the tank is too low.
There are other methods as well, but most are very expensive or unreliable. The most commonly mentioned is using float switches. Float switches capable of withstanding the heat, methanol, caustic, and biodiesel are rare. Finding one that can withstand the harsh environment inside the tank and fit through an existing opening is literally impossible. Add to that the fact that float switches do sometimes stick and they become less attractive as a mechanism to prevent dry firing.
The simplest method is to mount an electrical outlet or power strip in such a way that you can quickly see with your eyes if the heating element is plugged in or not. If the outlet you use is hidden behind the processor, or you use a long extension cord, you won’t be able to easily see the connection and will have to remember to look before draining your oil.
Another simple method is to wire a light to the heating element so you know when the element has voltage applied. Wiring the female end of an extension cord across the hot and neutral wires in the junction box at the top of the water heater will do the trick. This can be as simple as a shop light. However if you are using 240V, you have to pay special attention to the wiring.
There are also light switches with indicator lights to indicate when the power is applied. Light switches have relatively low current ratings and are mostly SPST switches not applicable for 240V. To operate on 240V, you would need an illuminated light switch with DPST contacts and current rated high enough to handle your heating elements.
Another more complex solution uses feedback from the thermostat to disengage a relay once the oil is up to temperature.
120V Control Pannel
Bill Of Materials
| QTY |
Description |
Price |
| 1 |
Plastic Junction Box from Lowes |
$11.82 |
| 1 |
15A wall outlet (commercial duty) |
$6.49 |
| 1 |
On button, NO momentary contact green |
$9.25 |
| 1 |
Off Button, NC momentary contact red |
$9.25 |
| 1 |
30A relay SPST |
$16.00 |
| 1 |
Pigtail from 12/3 15A extention cord |
$15.00 |
|
Assorted crimp on wire terminations |
$5.00 |
| 1 |
Spool of 12ga multistrand wire
|
$5.99
|
| 1 |
Steel outlet cover plate |
$1.89 |
| 1 |
Wire Clamp for mounting wires to a pannel |
$0.79 |
 It is very important that your water heater be wired correctly for this control panel to work. The hot lead must be wired through the lower thermostat. Some people will have the neutral line wired through the thermostat, you will need to verify it is wired correctly by lifting one wire from the element and checking for electrical continuity between the narrow blade on the plug to one of the wire terminating screws on the lower thermostat. Double check by testing for an open between the wide blade and a wire terminating screw on the lower thermostat. |
This is the classic heavy equipment on/off button and relay combination with the added twist of feeding the coil voltage through the thermostat. The On switch is a momentary contact switch that when pushed bypasses the relay contacts. This power feeds through the lower half of the receptacle (the receptacle is one capable of wiring the upper and lower outlets with different power sources.) to the water heater pigtail and to the lower thermostat. If the oil is below the set point temperature the power will be applied across the thermostat and on to the element to provide heat to the oil. A voltage line (blue line in drawing) is run back to the control box from the switched side of the lower thermostat through a normally closed momentary contact button acting as the Off button. This voltage line energizes the relay and keeps the relay energized even when the normally on momentary contact pushbutton used as the on button has released. When the oil reaches temperature the thermostat will switch the power off to the element and remove voltage from the coil causing the coil to disengage. Pressing the stop button also has the same effect. The coil voltage can be tapped to provide power to the upper outlet while the coil is energized. A shop lamp can be plugged into this outlet to provide a strong visual indication of “Heat ON”.
This control panel also has the added benefit of requiring a manual restart after a power outage. By applying power to the heating element through an outlet on the panel, the option of unplugging the water heater when not is use becomes available. Unplugging the water heater when not in use prevents the possible accidental start of the heating cycle when there is no oil in the tank. The upper receptacle is hot only when the element is powered and can be used for a lamp indicator or used to power the pump. If the heating element is plugged into the upper receptacle, it will not power the heating element for more than a few milliseconds. This version limits the heating element to 1500W maximum to prevent the breaker from tripping when operated with the mixing pump on the same circuit.
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