Troubleshooting Burner Instability

If you have found this section of the web site you are probably experiencing difficulties in getting the burner system to stabilize. Or perhaps you are experiencing burner pop-back. Others are prudently looking ahead to ward off any possible difficulty, "to head them off at the pass," so to speak. This is the page where we talk about a seemingly puzzling situation called "pop-back" or "pre-ignition" or sometimes called "burn back". We begin by showing what is a stable situation of ongoing combustion.

Pictured here is an environment (glory hole or furnace) which is burning in a stable manner. The flame has a blue core, the environment is at working temperature, and the burner head and mixer are cool and could pass the touch test. Even the face of the head is relatively cool because of all the combustibles (cool air and gas) which are being pushed through it. The flame is burning quietly, but steadily. This is a happy burner system.

happy burner system

In contrast, this next image shows a burner system with burner pop-back. This is a condition where the mixture of gas and air are burning in the mixer and head area of the burner system. If this condition goes on for a while, the head and pipe work may become cherry red. When the system is shut off and naturally left to cool for a few minutes, it most often can be re-lit and everything is fine. But sometimes there is damage to the pipe and head. An inspection of the head will reveal if there are any serious cracks, and if so the head should be repaired or replaced. If this furnace had a safety system it would have shut down immediately when the flame began to rumble (or flutter). Such a safety system could be hooked to an alarm to notify you of any burner or furnace abnormality.

burn back

Here is a list of conditions that help identify this condition.

  • First, you should always be able to touch the pipe work behind the burner head. Use the old spit on the finger technique. If it really sizzles, it most likely indicates the fire is burning in the head. If this condition is allowed to continue unchecked the entire pipe work can become cherry red as shown in the above image. Not a good situation. To rectify the problem, turn the gas off and wait for head to cool for a few minutes. Then attempt a re-light. If the head is cracked, you will most likely need to replace the head with a repaired or new one.
  • The paint should not be burnt off the back of the head. If it is, this indicates there is or was a burn-back condition.
  • The flame should burn evenly (not "rumbly").
  • There should be no popping noise.
  • Sometimes, a novice using a Giberson Head will not recognize when it is burning incorrectly, usually at a first start-up. Check using the "spit on finger technique."
  • Another indication of "burn-back" is the flame will not go out of reduction, coupled with a "rumbly" burn. Call Dudley (603) 456-3569 and ask for help.
There are two main causes for this condition:
  1. The gas and air mix is moving too slowly through the system so that the speed at which this mixture burns is greater than the speed of the mix being pushed through the system. It is like in the movies where the bad guy is going to blow up the bank using his trusty drum of black powder. Not being very organized that morning he's forgotten his roll of fuse material so he pours out a line of powder for some feet, lights it and runs. For the sake of argument and demonstration this highly explosive material does in fact burn with a calculable rate of speed. Gas and air, when properly mixed, have that same property. The small holes in the burner face serve this interesting purpose, they provide the "speedifier effect" where the unburned mixture speeds up through the hole structure and keeps the burning gas/air mix on the far side. What this means is if you turn down your burner system to a point low enough, you will reach this pop-back point. This exists in every burner system.
  2. There is a second reason for burner pop-back: A given mix of gas and air has a calculable temperature at which it ignites. If any part of the inside of the burner head reaches that ignition temperature, "pop, bang, rumble!" It is totally predictable. You could stake your life on it. It is a very simple proposition: the gas/air mix comes through the burner head keeping it cool. When this system of cooling the head gets disrupted, such as by turning the system down off a high temperature condition too quickly, the problem can occasionally show up.

Here is a probable scenario­

One day a glassblower makes a new melt and really gives it the business, gets the furnace really hot, and on the down side he is in a hurry and wants to plane it off because he has a hot date in a couple of hours. He turns it down to a setting which normally works when the furnace is a bit cooler and off he goes to visit his 'sweety.'

When he comes into his shop the next morning a sense of panic overcomes him. The furnace is roaring and the pipe work near the burner is red hot. The head is perhaps broken. (He did not have a safety system.) And he is indignant!

"That god dammed head! I knew it wasn't any good before I put it on! I saw some air bubbles in it. I think it was defective."

That's when I get a call from Mr. X. It takes me awhile to get Mr. X. back to the real problem and that is how to successfully turn down a burner after a melt and go on a date at the same time. I don't know about the date thing, but we can get the furnace turned down ok.

The idea of the red line (the 1000°F heat line). redline idea Both of the burners shown below are functioning correctly.
The cool air/gas mix is keeping the inside of the head in the black, whether the unit is fired to a modest temperature or is blasted.
An example of a low heat situation redline in low heat situation low heat head
An example of a high heat situation redline in high heat situation high heat head

If we think about it, we know at the height of the melt, the one thousand degree (° F) red line, moves to some point closer to the outer wall of the furnace. The burner and the burner port are affected by this heat increase, meaning they get hotter. And things are generally fine until the turn-down. That's when the red line can move nearer to the inside of the head. This heat increase is caused principally because after the turn down, there is less cooling going on throughout the inside of the burner system because there is less cool air/gas mix being pushed through the system. It is sometimes a thin line. I have learned only through experience how to keep the heat out of the head. Here is my technique:

During normal working conditions when I blow glass I have the pressure set on 8-10 psi to hold the glass to a good working temperature during the day. At the end of the day I simply shut off the burner for 3-4 hours and then re-light at 5 psi. By morning the furnace is back to working temperature, ready to go to work. Every few days I do a melt when I frequently push the furnace to 25 psi to get the furnace temperature over 2400° F. Coming off this melt profile I drop down to 12 psi, which I have found sufficient to keep the head cool enough. In the morning I plane the glass by dropping the pressure to the 8 psi level and at times I open the door to let off some of the heat. This settles the glass and it is shortly ready to blow. What I practice is a steady, careful lowering of the heat from the high temperature of the melt cycle. I keep the gas/air mix flowing through the head which keeps the inside of the head from getting too hot. This system works well for me.

To give your burner the best possible chance of working trouble-free please study the Proper Installation Paper. And for additional information on other possible trouble areas please read the "Don't Blow 'Um Up Or Wack 'Um With A Hammer Page"