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Dirty Flame Scanner Lens
Contaminants are accumulating on the lens of the flame scanner on the coal-fired burner even with purge air being sourced from the core air fan


Contaminants are accumulating on the lens of the flame scanner even with purge air being sourced from the core air fan. The flame scanner is intended to detect the flame quality and pulsating intensity of combustion happening in the burners, through the sighting tube.

There has recently been a lot of dust accumulating on the lens of the flame monitor. While the purge air system we have installed is supposed to blow away the contaminants, it has failed to do so - resulting in manual cleaning.

This accumulation of contaminants has also resulted in boiler flame loss tripping and thus the mill tripping.

Is the problem the insufficient flow rate / pressure of the purged air from the core air fan?

Could using other technologies (say a compressor) be helpful?

How else could this problem be solved since it is very grave.


We have had many issues with our flame scanners as well. We have had some luck relocating the scanners to point down toward the flame. They were originally installed point up which would allow for dust to collect. Unfortunately, some of the time we could not get the scanner to pick up the flame due to the way the port was installed.

We have attempted to adjust or scanner air, but have not had success. We are considering using a more advanced flame scanner. Right now our engineer is doing a project to improve the reliability of our flame detectors. I am not directly involved in the project, but I think he is looking at a fiber optic flame detector that does not used scanner air.

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1. Observation
On average, I drive about 100 miles (160km) daily to various plants that are not construction sites - they're operating plants.

I need to wipe the transparent lens on the front of the car radio at least twice a week to clear it of dust buildup. The vehicle is 2 years old and has a passenger compartment air filter for the A/C/heater/ventilation. Even so, the dust build-up is heavy.

So I have to ask, what effort goes into making your purge air really, really clean?

2. Not that long ago, the same topic cropped up and I discovered Google produced links to patents that discuss the topic and

provide a long list of prior art.

Flame scanner collimator body

Asymmetric purge air system for cleaning a lens

Purge air system for a combustion instrument

Hello David

Thank you for your detailed response. These do seem to be interesting methodologies indeed and I will further research them.

I would like to ask whether applying a filter to the fan would be feasible. Would it be the insufficient pressure or the back-flow of dirty contaminants that is causing the problem Are there any other alternatives (innovative or not) than the three patents you had suggested, as the dirty lens can be a grave problem.

2 out of 2 members thought this post was helpful...


My experience is as an commissioning/service engineer for a gas/oil (supplementary) firing system within conventional boilers and HRSG's. I don't know what you are firing (fuel), and under what conditions (fresh air, recirculation or exhaust gast), and at what angle. However, I hardly have hardly had any issues experienced with our costumers. So I can tell you how we did it.

Construction wise:
Flame scanner mounting tube through the furnace wall, in an downward angle toward the furnace (for condensate/dust reasons).

flame scanner tube never perpendicular on the combustion air/turbine exhaust gas. Always slightly towards the furnace. Or directly from behind the burner into the furnace.

Assembly wise from instrument to furnace:
Flame scanner, ball valve, concentric pipe 1/2" to 3", pipe 3" approx. 50 cm long. Just downstream the concentric piece and elbow 1/2", thread, flexible to and air header with a gate valve 1/2" inch. Gate valve fully open.

They always used auxiliary air from an auxiliary air fan.
(30 mbar (while gas firing) or 120 mbar (while oil firing)
Dedicated to burner stabilization air and auxiliaries (sight glass, scanner and igniter)

Instrument air could do the trick as well. Just regulate the pressure with a needle valve.

We never used any available cooling air connections on the instrument itself.

Flame wise:
Gas fires (lighter carbon fuels) UV-scanner.
Oil fires (heavier carbon fuels) IR-scanner.

Adjustable settings: flame sensitivity and discrimination.
Our used brands: Lamtec, Fireye, BFI.

Perhaps this gives you any additional insight, like how other parties do their stuff.

Again. With this setup, once a year cleaning was more than sufficient. Even the boilers of the 60/70/80's

*Elbow should be pipe nipple (threaded)

Dear The BurnerMan

I found your reply to be very relevant to what I am searching for.

I would like to ask the following:

1. usually what is a good value for the downward angle? I have heard it being between 4-6 degrees, but I am not sure how this figure is derived.

2. Sorry I am not sure what the difference between auxiliary air and instrument air is. I also have heard of 30mbar being used as an industry standard but am not sure why.

3. Lamtec/BFI. Are you aware of Forney? Are there other brands more prominent or having presence in Asia that you are aware of?

Many thanks again

1 out of 1 members thought this post was helpful...

1. Angle we didn't consider leading. Aiming at the flame leading. As long as the angle is downward tilted.

2. Auxiliary is sometimes called burner air or stabilization air. From a burner dedicated fan. 30 mbar is a value based upon experience/calculation for proper atomizing of the gas (120 mbar for oil), auxiliaries (sightglass, scanner and igniter just need positive flow from fan to furnace).

3. Yes. Aware of Forney. We mothballed them haha. And No. Even in Asia we brought majority time Lamtec F200K of F300K series

Alright thank you very much

We want to find the flow rate form a burner dedicated fan because there is currently insufficient flow.

We know the maximum and minimum flow rates but not the operating flow rate.

We have a core air fan curve which shows flow rate versus pressure difference and assumed 30mbar.

Can you advise how to measure operating flow rate?

Haha, so Forney is not pretty good.

Is 200K or 300K better for just natural gas monitoring?

Our boilers, Utility and CO, have always used purge air blowers with an instrument air backup.

I would suggest that you look at a Fireye installation manual for their flame scanners. Their scanners have a Y connection for purge air connection. Or they use a Y fitting to connect the purge air to the scanner view tube so that the purge air blows towards the boiler and away from the scanner.

They also use a union in the scanner tube between the scanner and Y fitting. The union can hold a quartz window that blocks heat from the scanner and also dirt, putting the quartz window close to the purge air inlet.

good luck

Currently, we are using a 'T' connection from the instrument air.
Not sure if the type of valve/connection has a huge implication to the dust accumulation.

The main issue for us is that the flow rate is insufficient currently. I am looking for other technologies such as a sootblower, air compressor. Any others you could suggest which would be a good component for purging?


As many as two separate purge connections are required for particularly dirty gases, with a gate valve between the two purges to permit cleaning if necessary.

If the installation nozzle is long enough, purge may not be required unless condensibles are involved.

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I think that a Y fitting will tend to create a suction at the scanner end of the fitting, doing a better job keeping the scanner clean than a tee.

Do you have 2 scanner air blowers? Does switching between them make a difference? Is one of them weak? What happens when you run both?

good luck

I would recommend a dedicated fan. Differential pressure switch Low (alarm) and a Low-Low (trip) between fan discharge and furnace pressure to safeguard positive flow.

And don't use the cooling air connection at the instrument it self as it will give a lot of turbulent flow at the lens. Make the connection further downstream the instrument to create a cooled relatively calm air pocket between instrument and cooling air connection. The majority flow will go directly to furnace.