Hoe Do Flame Failure Gas Shut Off Devices On Marine Cookers Work?

[Deleted member 36384]

The flame failure gas shut off device on my cooker's oven may be faulty as the burner will not stay lit.

It is a stainless torpedo shaped probe with a long, thin copper pipe terminating in the gas valve. The gas valve has three connections: main gas supply, burner gas supply and the flame failure device.

In so far as I can tell the gas valve end of the flame failure device is nothing more than a small mushroom type seal which is poked into the gas valve and a coupling made up to hold it in place.

I am at a loss as to how it works? If I compare the end with a working one the oven one looks as if the centre of the mushroom has slipped back inside the mushroom. However that could just be the way the mushroom was soldered on.

The gas valve is mechanically operated with no electrical devices controlling the gas valve position. I am wondering if heat travels along the copper pipe and pushes the mushroom offseat but I cant see how that is possible. I poked a working one into my coal fire and nothing could be observed. I did burn my fingers as the heat travelled along the pipe.

Any ideas?

Thanks,

BlowingOldBoots

 

[colvic987]

Here is some info, basic and technical info on thermocouples

http://www.garland-group.com/docs/uploaded/gar/products/a_geq_pa_flamefailure_16-031405.pdf

http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempThermCpl.html

 

[fireball]

It's a thermocouple - and you can normally get replacements ...

Doh - beaten too it.

 

[VicS]

It's a thermocouple - and you can normally get replacements ...

Doh - beaten too it.

And because there is no connection to the actual gas part of things there is no problem in fitting a replacemnt. No need to be a poodle.. sorry Corgi.. or a Gassafe fitter or even a rocket scientist.

 

[Deleted member 36384]

I came across the Garland Group site when trying to research how they work and they describe thermocouples for pilot light flame failure devices. I am struggling with what the electrical power does. The end of the thermocouple sits inside the gas valve and must make a gas tight seal.

Looking inside the gas valve I can see a spring and sleeve. The spring and sleeve move when I push the control knob in to allow gas to flow but there is no mechanism that I can see inside the valve that could be operated by an electrical signal, or lack off one.

I will look again at the gas valve in the morning. Thanks, BlowingOldBoots

 

[fergie_mac66]

I came across the Garland Group site when trying to research how they work and they describe thermocouples for pilot light flame failure devices. I am struggling with what the electrical power does. The end of the thermocouple sits inside the gas valve and must make a gas tight seal.

Looking inside the gas valve I can see a spring and sleeve. The spring and sleeve move when I push the control knob in to allow gas to flow but there is no mechanism that I can see inside the valve that could be operated by an electrical signal, or lack off one.

I will look again at the gas valve in the morning. Thanks, BlowingOldBoots

you can buy universal thermocouples about £6

 

[colvic987]

could be anyone of these type of thermocouples

http://www.radmidlands.co.uk/ControlsDom.pdf

 

[Searush]

I know this is "Noddy", but in my experience, they either work or they don't.

If they don't, the flame won't stay lit after you let go of the lighting device.

They are cheap & easy to replace, you've clearly removed it - just buy another one like it; D-I-Y shops & plumbers merchants stock them, usually under a tenner.

Why faff about trying to work out what it should do - just replace it.

 

[Giblets]

The tip of the t/couple in the flame is made of dissimilar metals which produce a very small voltage when heated, usually around 30mv. There is a small solenoid coil in the gas valve which is energised by this voltage, held open and allow gas to continue flowing to the burner when the button is released.

Contrary to VicS' information they are considered a safety device regardless of the fact they do not actually carry gas and, as such, should only be worked on by someone who is competent to do so. I leave the definition of "competent" up to the reader!

 

[Poignard]

Q. How Do Flame Failure Gas Shut Off Devices On Marine Cookers Work?

A. If it's a Plastimo - they probably don't

 

[Dave_Seager]

All of the suggestions so assume that a thermocouple is used on this cooker.

Thermocouples generate small electrical voltages when the two ends are at different temperatures. They are commonly used in domestic boilers and other gas appliances to detect whether the gas is burning correctly. There is an associated circuit which operates a solenoid valve.

However, it is unlikely that they would be used on a basic boat cooker since these do not usually have any electric power. I think that it is more likely to be a simple mechanical device, relying on the expansion of some medium inside the probe to operate a gas valve.

I have not been able to find any details of how they might work but if they use a liquid or gas it is unlikely that they can be repaired. You will probably need to buy the appropriate spare part.

 

[Giblets]

However, it is unlikely that they would be used on a basic boat cooker since these do not usually have any electric power. I think that it is more likely to be a simple mechanical device, relying on the expansion of some medium inside the probe to operate a gas valve.

I have not been able to find any details of how they might work but if they use a liquid or gas it is unlikely that they can be repaired. You will probably need to buy the appropriate spare part.

They do not need external electrical power as they produce their own voltage.

 

[Alpha22]

I am thinking hypothetically here of course, but if I had an older gas cooker on my boat that has no Flame Failure Devices, would it only require the replacement of the main valves and the addition of thermocouples to make it 'safer'?

Where would I be able to aquire such valves do you think???

 

[Searush]

(snip)

Contrary to VicS' information they are considered a safety device regardless of the fact they do not actually carry gas and, as such, should only be worked on by someone who is competent to do so. I leave the definition of "competent" up to the reader!

Bollix!

You can buy them in D-I-Y shops. Yes, it is illegal for someone unqualified to change them professionally, but you are perfectly legal to replace your own. It isn't hard & I think you already have it out. Ignore the "trade protectionistas" it is a perfectly safe diy job. After all, the valve is designed to fail safe when the couple dies, or if it isn't fitted right. All you can get wrong is sensor not in pilot light flame or other end not making good contact. Where's the danger in that?

 

[Searush]

I am thinking hypothetically here of course, but if I had an older gas cooker on my boat that has no Flame Failure Devices, would it only require the replacement of the main valves and the addition of thermocouples to make it 'safer'?

Where would I be able to aquire such valves do you think???

Probably easier to buy another cooker if you must. You will almost certainly have one on the oven anyway (you press a button in order to light it & release the button after a short delay or it goes out?)

IMHO, the oven is important because it has a large burner & could be left on (heating your FB pie) for a while unobserved - hence it could fill the bilges with gas

Hob burners are unlikely to be left unattended - if they go out you will spot it pretty quickly, the risk of an explosive concentration of gas in is minimal.

 

[fireball]

Hob burners are unlikely to be left unattended - if they go out you will spot it pretty quickly, the risk of an explosive concentration of gas in is minimal.

Hob at home doesn't have any failsafe - I have accidentally left a ring on unlit before - fortunately not for long and did smell it before operating any switches!!
It's ~10 years old ...

Anyway - you're quite right - it's easy enough to replace a thermocouple - we did so on the last boat when one failed - and if you don't get the installation right then the gas doesn't stay on.

Nice to hear how one actually works though - I'd pondered but not really thought too much about it ... and now I know !

 

[vyv_cox]

Here's a definitive post by Daveyw from last year. Seems to answer all the questions.

Ok, here's a free lesson on gas.
There are;
FFD's - Flame failure devices
FSD's - Flame Supervision Devices
and Thermostats.

FFD's are found on the burners on the top of the cooker, the grill and sometimes on the oven burner. They are indeed electrical devices. They generate a small voltage when heated and this voltage holds open a magnetic valve that you manually hold open by pushing in the control knob.

Problems with these are;
They burn out at the tip
They are incorrectly positioned in the flame(should be in the blue part of the flame.
The contact where the nut screws into the valve is dirty and need cleaning.
Grease and dirt is preventing the knob being fully pushed in.
These are cheap items(£2-3) and easy to replace.

FSD's are only found in the oven. These are mechanical expansion devices.
When you turn on your oven a small amount of gas is immediately emitted and hopefully ignited. This small flame heats the sensor bulb. The resulting expansion then open the FSD valve and allows full gas flow(this should happen within about a minute). The FSD is normally located at the rear and bottom of the oven.

Problems with these are they burn out.
You will normally need to take the cooker out to access this.
These can be fiddlely to change. They can cost anything from £20 to £200.

Thermostats are found in the oven and boilers and is another mechanical expansion device. The heat from the oven heats the the sensor bulb and the resulting expansion operates a valve inside the oven control valve.

Problems with these are they burn out.

These again can be fiddlely to replace and can cost £20-£100

A lot of boat cookers do not have thermostats but merely a high and low setting on the control knob.

Back to the original problem, if it's taking a long time for the burner to stay lit. Check position of the thermocouple(looks like a thick pin). Make sure it's in the flame. Doubt the thermocouple has moved so maybe the flame is wrong. Are all the holes in the burner cap clean. Maybe the flame is lifting off(the thermocouple is in the part of the flame with unburnt gas. Turning the flame to minimum is a temporary solution but then chances are that the unburnt bit will reappear when you turn the flame up again and trip the flame off again. Lift off is cause by excess gas pressure(unlikely if the other burners are ok), restricted holes in the burner cap(likely) or lack of oxygen(unlikely if this is the first burner you are lighting).

Best of luck!

 

[Deleted member 36384]

Thanks all for your replies. The cooker is a Force 10 American specification, which just means that it has 4 hob burners instead of the three.

I looked inside the oven gas valve today and indeed there is a device right where the flame failure coupling would seat.

Socal can supply a new pre 1995 model thermocouple at £39 but before I pay that I'll take the old one around some suppliers and see if I can get the same model.

My original post question was more to do with implementing a bodge if I could not replace what is really a 1998 obsolete model cooker. Simply removing the thermocouple would ensure no gas supply. If such a bodge was prudent I now understand that I would have to take the solenoid valve out to prevent it closing off the gas supply.

A new, current specification Force 10 American model cooker costs £1700.

Thanks again,

BlowingOldBoots

 

[Giblets]

Bollix!

You can buy them in D-I-Y shops. Yes, it is illegal for someone unqualified to change them professionally, but you are perfectly legal to replace your own. It isn't hard & I think you already have it out. Ignore the "trade protectionistas" it is a perfectly safe diy job. After all, the valve is designed to fail safe when the couple dies, or if it isn't fitted right. All you can get wrong is sensor not in pilot light flame or other end not making good contact. Where's the danger in that?

SR, I think you need to re-read my post. I deliberately used the word "competent" rather than "professional" as that is the word used in the Gas Safety Regulations. Agreed, you can do what you like to your own gas installation but if something goes wrong you may need to prove your "competency" either to your insurance company and/or a court of law. Best of luck if that happens, m8!

One further point - it is not illegal to sell 'em, you only have to be competent to fit 'em!

As for the OP considering "implementing a bodge" on a gas system in a boat (or anywhere else for that matter) I can't believe I just read that!

 

[DownWest]

In Vic's quote there was an important tip. Make sure that the end of the FFD is in a hot flame. Occasionally the one supposed to be heating the thermocouple is blocked.

Where I used to live, they passed a law that all rental properties must have FFDs on the oven. A common prob was after lighting the oven, shutting the door blew out the flame and a bit later the build up reached a top burner resulting in a spectacular bang.
Several houses had good British(New world etc) cookers that did not suffer from this problem and were prized by the owners. An agent asked if they could be fixed. And I ended up doing around 50 conversions. Quite simple, insert a spring loaded solenoid valve in the gas line to the oven regulater tap and route the thermocouple to the burner. The bits cost little and were easily available.
The valves rarely fail, almost certainly it is the thermocouple.

BTW, I am not, or have ever been, a registered poodle or a royal dog....)

Quite liked the cartoon showing the royal corgies scuttling down the airstairs from HM's plane. One is hesitating at the top and saying " Careful guys, this might be Korea"