Gas pipe sizing

[Farey]

I fitted out my 60' narrowboat myself 13 years ago. I have a 4 burner gas hob and a gas grill and oven, in the galley, and no other gas appliances. I used 3/8" (external diameter) gas pipe and fitting, and took advice and had the installation checked by a boat safety examiner. It's also passed all its subsequent BSS checks.

 

Recently I've been thinking of replacing the hob, and looking into it, I'm now wondering if 3/8" pipe is adequate. The total KW rating is 9 (4 x 1.5 for the hob, and 1.5 each for the grill and oven). Looking at pipe sizing calculations, e.g. https://www.engineeringtoolbox.com/propane-gas-pipe-sizing-d_830.html it looks as if I need pipe with a 3/8" inch internal diameter, corresponding to 1/2" external, given that my total pipe run is around 5 metres, with several 90 degree bends.

 

Am I correct? If so, I would expect this to apply to most narrowboats to need 1/2" pipe, but 3/8" seems much more common in the chandlers, and 1/2" fittings seem few and far between.

 

If this is the case, then my preference would be to just replace the long, 4 metre pipe run and keep the rest as 3/8".

[stegra]

Are you ever going to have four hob burners plus oven and grill on full at any one time? Maybe redo the calculation to see what your current pipework will supply and decide whether that is sufficient for your realistic needs, rather than complicate things. 

[Alan de Enfield]

4 minutes ago, stegra said:

Are you ever going to have four hob burners plus oven and grill on full at any one time? Maybe redo the calculation to see what your current pipework will supply and decide whether that is sufficient for your realistic needs, rather than complicate things. 

It is irrelevant if you would ever have 4 burners + grill on at any one time. The rules of the BSS say to check the pressure / flame picture of the supply with everything on.

 

Section 8.8,.1

Light all LPG appliance burners and operate them at their maximum setting at the same time.

A satisfactory flame picture must be present at each LPG appliance burner when all burners in the system are operating at their maximum setting at the same time.

[alan_fincher]

9 minutes ago, stegra said:

Are you ever going to have four hob burners plus oven and grill on full at any one time? Maybe redo the calculation to see what your current pipework will supply and decide whether that is sufficient for your realistic needs, rather than complicate things. 

 

But when a BSS test is done, the examiner will light all appliances at once, and see if there is an unacceptable pressure drop.

So I would say no, you can't rely on not having everything on at once, if you wish to be sure never to have problems in passing a BSS.

[Farey]

I don't have a test point in the system, just a bubble tester, but was thinking of adding one.

[alan_fincher]

8 minutes ago, Farey said:

I don't have a test point in the system, just a bubble tester, but was thinking of adding one.

Lots of cookers and hobs actually seem to incorporat an inbuilt one.

 

However what the other Alan has cut and pasted seems to indicate that a test for all appliances on at once is visual, looking at flame patterns.

[Alan de Enfield]

4 minutes ago, alan_fincher said:

Lots of cookers and hobs actually seem to incorporat an inbuilt one.

 

However what the other Alan has cut and pasted seems to indicate that a test for all appliances on at once is visual, looking at flame patterns.

NOTE – in the event of a poor flame picture, take the actions described in Appendix A or A and B,

 

A APPENDIX –

PEOPLE OR PROPERTY IN ‘IMMEDIATE DANGER’ OR ‘AT RISK’

THE BSS WARNING NOTICE
A BSS Warning Notice will be issued in all cases where an examiner has cause to suspect that
people or property may be or are in ‘immediate danger’ or ‘at risk’ due to faults in installations or
appliances on a vessel being examined.
The purpose of the BSS Warning Notice is to alert the owner or person responsible for the craft’s
condition, that an immediately hazardous defect has been found and secondly to alert anyone
stepping aboard that they could be at risk and should take precautions.

 

B APPENDIX – 

‘IMMEDIATELY HAZARDOUS BOATS’

ADDITIONAL EXAMINER ACTIONS
Where the defects placing people or property in ‘immediate danger’ or ‘at risk’ relate to:
• leaking gas;
• leaking petrol;
• gas refrigerators with naked flames on petrol‐powered boats;
• heat damaged cables or other electrical fire risks;
• a.c. electrocution risks;
• significant quantities of fuel or other substances escaping into the watercourse.

…the following actions in addition to those in Appendix A are taken
• the owner (or representative) is informed about the hazards as soon as possible;
• the owner is asked for the hazard to made safe or neutralised immediately, such as by turning off the fuel or disconnecting the electricity supply;
• owners are advised that the systems, installations or appliances should not be used again until fixed;

NOTE: any repairs are best carried out by a competent person.
 

[Farey]

47 minutes ago, alan_fincher said:

However what the other Alan has cut and pasted seems to indicate that a test for all appliances on at once is visual, looking at flame patterns.

So that fact that it's passed 3 BSS inspections by 3 different examiners would seem to suggest that the flame pattern is ok.

[alan_fincher]

1 hour ago, Farey said:

So that fact that it's passed 3 BSS inspections by 3 different examiners would seem to suggest that the flame pattern is ok.

 

Yes, but I think you were talking about changing the hob?  If the new one were more powerful overall, I suppose it is possible it might not then be.

[MtB]

'Flame picture' tells us virtually nothing about the adequacy of the gas pipes and the gas pressure at the inlet to the gas appliance. 

 

Most gas appliances on boats have burners designed to run at a variety of gas pressures, i.e. the flame size can be regulated by the user. Consequently the flame picture of a gas appliance being supplied by an undersized pipe will usually have a perfectly acceptable flame picture. Only in extremis will the flame picture be corrupted, i.e. when the gas supply pipe is SO small or restricted that barely enough gas arrives at the appliance to keep the flame alight.

[Alan de Enfield]

 

37 minutes ago, Mike the Boilerman said:

'Flame picture' tells us virtually nothing about the adequacy of the gas pipes and the gas pressure at the inlet to the gas appliance. 

 

Most gas appliances on boats have burners designed to run at a variety of gas pressures, i.e. the flame size can be regulated by the user. Consequently the flame picture of a gas appliance being supplied by an undersized pipe will usually have a perfectly acceptable flame picture. Only in extremis will the flame picture be corrupted, i.e. when the gas supply pipe is SO small or restricted that barely enough gas arrives at the appliance to keep the flame alight.

Maybe the BSS should be made aware of this.

Edited August 15, 2018 by Alan de Enfield

[WotEver]

20 minutes ago, Mike the Boilerman said:

'Flame picture' tells us virtually nothing about the adequacy of the gas pipes and the gas pressure at the inlet to the gas appliance. 

 

Most gas appliances on boats have burners designed to run at a variety of gas pressures, i.e. the flame size can be regulated by the user. Consequently the flame picture of a gas appliance being supplied by an undersized pipe will usually have a perfectly acceptable flame picture. Only in extremis will the flame picture be corrupted, i.e. when the gas supply pipe is SO small or restricted that barely enough gas arrives at the appliance to keep the flame alight.

So what is the consequence of an undersized feed pipe if the burners all burn adequately with a good flame picture?  Is there actually a problem?

[David Mack]

24 minutes ago, Mike the Boilerman said:

'Flame picture' tells us virtually nothing about the adequacy of the gas pipes and the gas pressure at the inlet to the gas appliance. 

 

Most gas appliances on boats have burners designed to run at a variety of gas pressures, i.e. the flame size can be regulated by the user. Consequently the flame picture of a gas appliance being supplied by an undersized pipe will usually have a perfectly acceptable flame picture. Only in extremis will the flame picture be corrupted, i.e. when the gas supply pipe is SO small or restricted that barely enough gas arrives at the appliance to keep the flame alight.

So if the flame picture is OK then the appliance is operating safely, but perhaps at a reduced output compared with what it is supposed to have. So what's the problem - apart from it taking longer to boil the kettle?

[Boater Sam]

If the gas velocity leaving the jet is too low there is a risk of spillage of unburnt gas from the venturi into the appliance casing.  Propane being heavier than air it pools at a low point.

 

 

Eventually..........................................BOOM!!!

[MtB]

4 hours ago, Farey said:

I fitted out my 60' narrowboat myself 13 years ago. I have a 4 burner gas hob and a gas grill and oven, in the galley, and no other gas appliances. I used 3/8" (external diameter) gas pipe and fitting, and took advice and had the installation checked by a boat safety examiner. It's also passed all its subsequent BSS checks.

 

Recently I've been thinking of replacing the hob, and looking into it, I'm now wondering if 3/8" pipe is adequate. The total KW rating is 9 (4 x 1.5 for the hob, and 1.5 each for the grill and oven). Looking at pipe sizing calculations, e.g. https://www.engineeringtoolbox.com/propane-gas-pipe-sizing-d_830.html it looks as if I need pipe with a 3/8" inch internal diameter, corresponding to 1/2" external, given that my total pipe run is around 5 metres, with several 90 degree bends.

 

Am I correct? If so, I would expect this to apply to most narrowboats to need 1/2" pipe, but 3/8" seems much more common in the chandlers, and 1/2" fittings seem few and far between.

 

If this is the case, then my preference would be to just replace the long, 4 metre pipe run and keep the rest as 3/8".

 

 

That chart seems to give you the size needed for a maximum pressure drop of 125Pa, which equals 1.25mbar. 

 

ISTR older installations are allowed 3 mbar drop so your 3/8" pipe may well turn out to be compliant anyway. 

 

 

 

 

[blackrose]

I used the Calor tables for pipe sizing when I installed my gas system. They were very useful but disappeared from the website a few years later and were replaced with different tables that didn't seem as easy to use. Unless anyone knows where they are now? 

[nicknorman]

3 hours ago, Boater Sam said:

If the gas velocity leaving the jet is too low there is a risk of spillage of unburnt gas from the venturi into the appliance casing.  Propane being heavier than air it pools at a low point.

 

 

Eventually..........................................BOOM!!!

What is the difference between a hob or oven burner on full but with some modest pressure drop in the pipework, vs a hob /oven turned down to a low flame by means of the knob? It’s hard to see that there would be any, making your post somewhat alarmist and scaremongering.

Edited August 15, 2018 by nicknorman

[blackrose]

5 hours ago, nicknorman said:

What is the difference between a hob or oven burner on full but with some modest pressure drop in the pipework, vs a hob /oven turned down to a low flame by means of the knob? It’s hard to see that there would be any, making your post somewhat alarmist and scaremongering.

Yes, sounds like bollox to me too. 

[MtB]

7 hours ago, nicknorman said:

What is the difference between a hob or oven burner on full but with some modest pressure drop in the pipework, vs a hob /oven turned down to a low flame by means of the knob? It’s hard to see that there would be any, making your post somewhat alarmist and scaremongering.

 

Correct. There is no difference. 

 

There is however an an argument to say once the gas flow through the jet falls VERY low (e.g. as the gas bottle runs out) there is not enough velocity for the gas to reach the Venturi in which case the effect boater Sam describes will happen. But at the same time the flame at the burner will extinguish as no gas is reaching it, and the flame supervision device will shut the gas off limiting this ‘dribbling’ effect to Just a few seconds, severely curtailing the risk.  

[BEngo]

14 hours ago, Alan de Enfield said:

 

Maybe the BSS should be made aware of this.

They were made aware of this bt the BSS technical committee before the scheme was imposed. The flame picture test replaced the earlier proposal for an operating pressure test .  As this would have required a gas test point at the end of the gas pipe(s) and the HSE declared that opening a  gas test point was 'work'within the GSUIR it was a non -runner.  BSS Manager of the day was not willing to hust drop the test altogether and did not have the competence to understand what he was being told so like many other things in the BSS we were stuck with a nonsense.

N

[WotEver]

29 minutes ago, BEngo said:

They were made aware of this bt the BSS technical committee before the scheme was imposed. The flame picture test replaced the earlier proposal for an operating pressure test .  As this would have required a gas test point at the end of the gas pipe(s) and the HSE declared that opening a  gas test point was 'work'within the GSUIR it was a non -runner.  BSS Manager of the day was not willing to hust drop the test altogether and did not have the competence to understand what he was being told so like many other things in the BSS we were stuck with a nonsense.

N

But haven’t we established above that if the flame picture is good with everything lit that there’s no problem? Even if the heat might be slightly lower than it might otherwise be? In which case where’s the problem?

[Alan de Enfield]

2 minutes ago, WotEver said:

But haven’t we established above that if the flame picture is good with everything lit that there’s no problem? Even if the heat might be slightly lower than it might otherwise be? In which case where’s the problem?

Hmmmm -lets find something else to test - "I know, lets test to see if water is wet - yes it is, - that's a pass then".

[MtB]

7 minutes ago, WotEver said:

But haven’t we established above that if the flame picture is good with everything lit that there’s no problem? Even if the heat might be slightly lower than it might otherwise be? In which case where’s the problem?

 

It is a hangover from when flame supervision was not mandatory on things like hobs and gas lamps. If a gas ring is set to low flame to simmer some spuds or whatever, and the supply pipe shared with the Morco is too small, when someone turns a hot tap ON then the pressure drop to the hob ca cause the flame to go out, then unburned gas will dribble out as described by Boater Sam. Further, when the water heater is turned OFF the 'low' hob flame gas flow is restored and un uncontrolled gas leak commences. 

 

All this is completely handled by the proper flame supervision now required nowadays. There is still a risk on those older boats with 'grandfather rights' to have hobs and cookers without flame supervision.  

[MtB]

 

There is also the risk that a non-variable burner appliance might be installed that is designed to run at a fixed 37mb (+/- 3mb) and nothing else. If the supply pipe supplies less that the permissible 34mb at the appliance inlet, proper combustion and possibly the flame picture might well be affected. This could be addressed by BSS bods using a gas analyser to test the products of combustion rather than opening up a test point.

 

 

Edited August 16, 2018 by Mike the Boilerman
Add a bit.

[alan_fincher]

24 minutes ago, Mike the Boilerman said:

All this is completely handled by the proper flame supervision now required nowadays. There is still a risk on those older boats with 'grandfather rights' to have hobs and cookers without flame supervision.  

 

And my experience on here, and elsewhere, is that many people do not realise that you can't simply buy (or otherwise acquire) any old Vanette or similar to replace a failed or failing hob or oven.

So people are putting in replacement units without FFDs, something the BSS does not allow even if those "grandfather rights" applied to what they are removing.

 

I suspect that in some cases units that had FFDs get replaced by older ones that do not.