Venturi burners backburning

[Brett]

Hello folks,

 

I have been trying for some time to fix a problem with a venturi burner that is burning gas back at the inspirator, and would appreciate any suggestions. The burners and regulator are capable of 250kpi and higher, but the two gas cocks are  each1060 kpi.

So far I have:

1. Replaced one entire burner from the same company
2. Had a licenced gas fitter check the nozzles and install the two burners.
3. Tried with air fully open to closed.
4. Tried at high pressure and low pressure.
5. The burners and regulator are capable of 250kpi and higher, but the two gas cocks are 1060 kpi.

It's got me stumped so any ideas welcome

Thanks for pondering,

 

Brett

 

[Bill Kielb]

Atmospheric burners contain orifices sized for a particular range of pressure and the fuel used to produce a certain amount of heat. No new news there, but for all that to work the velocity of the gas has be be greater than the combustion velocity of a fuel air mixture expected to enter let’s say at a 5:1 air fuel ratio or about 50%% of the required air. The remaining 50% of the required air must enter around the side of the burner through a sufficient overcut all the way around.

So:

• Wrong pressure
• wrong orifice
• Too much back pressure from the kiln
• Any chance you are feeding liquid to this?
• Not enough overcut around burner
• Adding a metering valve upstream of the burner and downstream of the main gas valve allows variability, how are you controlling the gas to the burners?

What size orifice, what is the burner rating (kilowatts, btuh - thermal output) , what pressure is this operating at (typically in the 2-3 kilo paschal range) not the maximum pressure ratings on the valves, what size regulator is installed on the tank, pictures of the installed burner, picture of it burning back ….. and never close the primary air 100%, it should likely sit at least 1/2 open or better and not really need adjustment.

Propane boils about -41c so at 21c there will be about 850 Kpa of pressure in the tank. Definitely way higher than your burner rating. So in the end sounds like these might be high pressure burners and need a first stage regulator to operate correctly.

This is fairly easy to solve but often folks, even licensed fitters confuse the pressures and installation requirements for kiln operation. Tell us a bit more

 

Edited November 15, 2023 by Bill Kielb

[Brett]

Thanks so much Bill! I will compile answers to all those excellent questions, and take some more pictures.

[Mark C.]

open up the air flaps in rear as they are not open enough in your photos to work well. Mine with natural gass are 100% bopen all the time. I never close them down.

[Brett]

On 11/15/2023 at 5:54 PM, Bill Kielb said:

Atmospheric burners contain orifices sized for a particular range of pressure and the fuel used to produce a certain amount of heat. No new news there, but for all that to work the velocity of the gas has be be greater than the combustion velocity of a fuel air mixture expected to enter let’s say at a 5:1 air fuel ratio or about 50%% of the required air. The remaining 50% of the required air must enter around the side of the burner through a sufficient overcut all the way around.

So:

• Wrong pressure
• wrong orifice
• Too much back pressure from the kiln
• Any chance you are feeding liquid to this?
• Not enough overcut around burner
• Adding a metering valve upstream of the burner and downstream of the main gas valve allows variability, how are you controlling the gas to the burners?

What size orifice, what is the burner rating (kilowatts, btuh - thermal output) , what pressure is this operating at (typically in the 2-3 kilo paschal range) not the maximum pressure ratings on the valves, what size regulator is installed on the tank, pictures of the installed burner, picture of it burning back ….. and never close the primary air 100%, it should likely sit at least 1/2 open or better and not really need adjustment.

Propane boils about -41c so at 21c there will be about 850 Kpa of pressure in the tank. Definitely way higher than your burner rating. So in the end sounds like these might be high pressure burners and need a first stage regulator to operate correctly.

This is fairly easy to solve but often folks, even licensed fitters confuse the pressures and installation requirements for kiln operation. Tell us a bit more

 

Here is a bit more thank you Bill:

The burner size is 50mm (2 inch) with capacity 400MJ (380,000 BTU) per hour @ 100kPa propane.

The images show the burners being tried at 100kPa, controlled by winding the knob on the 500kPa regulator attached to the changeover valve on the 45kg 3.3Mpa capacity propane cylinder.

Also shown is the burner installed, and burning back, plus the burner port as seen during building.  I have put them all in one image, plus a larger one of the burning back. If I should include larger pictures please let me know.   I also tested the burner in the open, off the kiln, on a small bbq propane tank with a regulator and also had back-burning. Thanks again for your help!

 

 

 

[Babs]

Back in the day , burner,shown is way too clise to kiln and is actually in the kiln, set it back out from the kiln a ways so air can be drawn in around the orifice to kiln and thus the flame. I'd start there, and flaps open as Mark says

[Brett]

Thanks Babs - it back-burns even when right away from the kiln with flaps open.

[Babs]

A possible danger where it is is that the heat sensor / gas switch off mechanism may be affected by the interior temp of kiln emanating from the orifice not the loss of flame from burner. Do you have another air conrtoller up the line?

Whats the flame path like inside and the suck of the stack/ damper at other end?

Will be a play between all of the above

Edited November 16, 2023 by Babs

[Bill Kielb]

25 minutes ago, Brett said:

- it back-burns even when right away from the kiln with flaps open.

Can you check orifice size for us. Wonder if one is in there, then what size. 3-5 Kpa would be typical sizing so at 100 Kpa unknown how that would react. We really want to know the orifice size but I am also curious if you drop this pressure down to single digits, does it operate.  I think it would be good to see this orifice in a picture as well - any chance a little critter crawled in and made a home? Often spider webs.

The orifice size will tell us its intended operating pressure. Pictures might tell us if someone drilled it out.

Edited November 16, 2023 by Bill Kielb

[Brett]

 

There is one like this in the burner, not drilled as I have had since it was new. It is 0.75mm by my calipers, (and sliding a .5mm wire through). The gasfitter and I checked the orifices for critters etc and they are clear. I can't get the regulator to reduce the pressure to single digits though. Maybe I need another inline valve that I can use to reduce the pressure? Thanks for your help!

[Bill Kielb]

2 hours ago, Brett said:

(and sliding a .5mm wire through

Hmmm, pretty small actually I would  feel better if it was stamped. .5mm is .0280” which is a number 70 orifice which gets you  [5.9 kw] @ 34 Kpa so seems to be a high pressure burner. When burners burn backwards the combustion velocity is greater than the physical velocity so the flame travels back towards the orifice which is counter intuitive to having too much pressure. I assume it backfires in free air, so maybe an easy test I would suggest is to put a hand operating valve upstream of the burner and see if you can stabilize the flame slowly closing the valve ( in free air) - maybe to near zero flow.  The primary air needs to be fairly open, don’t close it. Again counter intuitive but nothing makes sense here really. If that works then I think we can safely assume it is low pressure or medium pressure burner and work from there rather than buying a bunch of stuff guessing.

Propane and natural gas have relatively slow combustion velocities which make this issue fairly rare for Venturi burners, stove burners, you name it. I think worth the test and observing the result.

Edited November 17, 2023 by Bill Kielb

[neilestrick]

The rule of thumb for burner placement is that is should be set back from the  burner port a distance of half the diameter of the burner tip. For example, a 3" burner tip should be set back 1.5".

Does it backburn when it's not at the kiln?

[Babs]

The image you show saying it is not backburning, the flame is orange and"woosy", not a fle I'd like to see when firing,  what makes you write it is backburning with the blue flame image, is it making sucking back and gurgling sounds in the burner head?

[Brett]

Hi Babs,

 

You can see in the image posted with the arrow to where the flame is at the inspirator. 

 

And yes Neil it backburns when not at the kiln.

[Brett]

5 hours ago, Bill Kielb said:

Hmmm, pretty small actually I would  feel better if it was stamped. .5mm is .0280” which is a number 70 orifice which gets you  [5.9 kw] @ 34 Kpa so seems to be a high pressure burner. When burners burn backwards the combustion velocity is greater than the physical velocity so the flame travels back towards the orifice which is counter intuitive to having too much pressure. I assume it backfires in free air, so maybe an easy test I would suggest is to put a hand operating valve upstream of the burner and see if you can stabilize the flame slowly closing the valve ( in free air) - maybe to near zero flow.  The primary air needs to be fairly open, don’t close it. Again counter intuitive but nothing makes sense here really. If that works then I think we can safely assume it is low pressure or medium pressure burner and work from there rather than buying a bunch of stuff guessing.

Propane and natural gas have relatively slow combustion velocities which make this issue fairly rare for Venturi burners, stove burners, you name it. I think worth the test and observing the result.

Thanks Bill - I will  put a hand operating valve upstream of the burner as you suggest, and take into account all the other information supplied by you and others. Thanks again!

[Babs]

17 hours ago, Brett said:

Thanks Bill - I will  put a hand operating valve upstream of the burner as you suggest, and take into account all the other information supplied by you and others. Thanks again!

Think that's a must.  I operated the first part of firing by using a pilot light, could be pretty vigorous but burner too vigorous for 1st part of my firing. After the cylinder regulator,  inside my shed, a shut off lever,  the gas control valve with pressure indicator was next, an air intake was next in line, then  down to the burner. Never touched the air intake at burner.

Just saying.

Good luck. Depending what neck of the scrub you are in there would be gas firers nearby who could twig what is wrong in the setup you have.

Have you been firing this kiln successfully and this burner issue has just arisen?

[Brett]

Thanks Babs. Fired once to cone 6 previously but struggled, so new burners and regulator. I start with  a small burner. 

[Babs]

Get a meter and regulator, ( which you can adjust at fractions of  psi ),  in line, trying to regulate at cylinder too "gross" if thats the right word.

Might be domeone who can send a pic of their set up.

Psst, move your burner back a tad.:))

[neilestrick]

7 hours ago, Brett said:

Thanks Babs. Fired once to cone 6 previously but struggled, so new burners and regulator. I start with  a small burner. 

Do you know for sure that the burners were the problem? More often than not what we see here on the forum is that there's a design problem with the kiln itself, or just user error in firing it.

[Brett]

5 hours ago, neilestrick said:

Do you know for sure that the burners were the problem? More often than not what we see here on the forum is that there's a design problem with the kiln itself, or just user error in firing it.

Hi Neil, yes both those are possibilities. It will take me a couple of weeks to install an inline valve and re-test, and then come back to the forum with success or failure. Thanks for all the suggestions.

[Mark C.]

As a gas potter for 50 years I see the burner is pushed into the kiln  to far-needs to be away from kiln a bit and the air flaps are closed down to much (open them up)

these are the most obvious ,but kiln design and orifice size are also concerns -I can see the 1st two issues in your photos for sure so they are easy to fix.

Edited November 20, 2023 by Mark C.