General gas safty questions

[Cheech]

Hi im studying for my gas test and been reading my viper book for weeks people i work with are not very helpfull and i have some questions i would really like some help on if anyone could help me out

1. I know standing pressure is to be no higher than 30mb but no were in my viper book does it say how low it can be I quess its 20mb but not sure

2. gas rate calculation's , i have calculations in book and can do them but not sure on them at all been tring to ask for help in understanding eg. turn boiler on max time for 2 min take first and second reading times then do calculation cant mind it off hand times by 30 then divide by 10.76 i think it is but i dont understand if its a 30kw boiler and the sum comes out 27.000 is that ok. Just need this all cleared up by someone who knows and can help me out

 

[kirkgas]

Hi im studying for my gas test and been reading my viper book for weeks people i work with are not very helpfull and i have some questions i would really like some help on if anyone could help me out

1. I know standing pressure is to be no higher than 30mb but no were in my viper book does it say how low it can be I quess its 20mb but not sure

2. gas rate calculation's , i have calculations in book and can do them but not sure on them at all been tring to ask for help in understanding eg. turn boiler on max time for 2 min take first and second reading times then do calculation cant mind it off hand times by 30 then divide by 10.76 i think it is but i dont understand if its a 30kw boiler and the sum comes out 27.000 is that ok. Just need this all cleared up by someone who knows and can help me out

the gas supplier must legally supply 15mb, so it can be as low as that without danger (this will be further debated as we all know 2 old appliances with no FSD on them, on together at this pressure could lead to flame outage at one of them, however it is the regs), anything less that 21+/-2 should be reported to the supplies as it isnt right, but dont expect them to come out and rectify it.
you are mixing up gas rate and heat input, simply put gas rate is gas used per hr so measure gas used in 2 mins and multiply by 30 this will give you gas used in 1hr and is noted as m3/hr, moving on with the previous gas measurement in 2 mins if you then multiply that answer by 321 this will give you the heat input, which is measured against the data badge, you wont see it written anywhere but it is widely accepted that if the answer is within 10% of badged heat input then all is well, my take on the 10% allowance is that the standard calculations we use have a standard calorific value (which is likely to be slightly different from the actual calorific value you have), and also it is a wee bit difficult to get an exact reading in exactly 120 secs so if you have a 30kw boiler and you multiply gas used in 2mins by 321 and get an answer of 27-33 then all is well, any wide variations from this and i always suggest the test is done again as it is quick and easy and usually wild results are caused by miscalculations, thats my take on it, others will give you there gas rate and heat input calcs, the best method to use is to actually find one you understand and stick to it, i put a link to a gas rate and heat input spreadsheet i made up spmewhere on here ( i think) on a post about BG guys and their laptops

 

[Cheech]

Thank you very much KIRKGAS for your quick help still all a bit confussed its all lots to take in. very hard to learn when your gaffer is an old git that does not like to explane anything. Just to clarify when doing a service on boiler im better to check the gas rate witch is your 2 reading's say 0.25 * 30 = 7.5 does that mean if the applince is 7.5kw its bang on and if it is you are allowed 10% +/- after reading your post again im thinking im getting mixed up again just really getting confussed thanks for the help mate. Think i need to see an exanple with what would be right/passible reading and what it means and what a wrong/unsafe reading would be

 

[REDSAW]

it says 5% tolerance on another ccn1 notes i have aquired, is this % acceptable in the answer given to assessor when asked if it is within range or acceptable during practical exam?.
23-30mb range on standing pressure and 25mb normally found. (just testing myself.lol.) pull of tube for a couple of seconds to allow for regulator to kick in giving a more accurate reading.

 

[Cheech]

on standing pressure i mainly see around 27mb when asked anyone how low can it be till its unsafe no one seems 100% sure so say standing pressure is 22mb would you have to call transco to say the standing pressure is to low ?

 

[tamz]

You are getting yourself all mixed up here so lets start at the beginning.
To answer your first question
1. I know standing pressure is to be no higher than 30mb but no were in my viper book does it say how low it can be I quess its 20mb but not sure
Look in section I page 4, 2nd paragraph.
It is not very concise so i'll explain a bit further.
A low pressure gas supply can have the pressure at the ecv up to 75mb although it is normally around 30.
The purpose of the regulator as you'll know is to do what it says. Regulate (govern)
The regulators are designed to lock up (close) at a pressure not exceeding 30mb so a standing pressure at the outlet of the meter of up to 30mb is acceptable.
A standing pressure test proves nothing other than the regulator is locking up before 30mb (most lock up around 25-26mb) and that you have gas.
So that answers the first question.

To further explain how it then works trying not to confuse you:-
When you turn on an appliance and gas flows through the regulator the pressure will drop to the working pressure, (it is no longer standing, it is moving (working)) which should be within the range 23 to 19mb (21+or-2). At low flow rates (relative to the meter size) it will tend to be nearer 23mb and high flow rates nearer 19mb so on a U6 meter if you were using 1m³ of gas per hour it would read nearer 23mb and if you were using 5m³/hr it would read nearer 19mb. Anything under 19mb and you should report it to the supplier.
You are allowed a 1mb pressure drop through the pipes between the meter and the appliance so you should take a reading at the meter and at the appliance and there should be no more than a 1mb difference.

This is where it may get confusing and you may want to disregard this bit for now.
At high flow rates there is an allowance of 4mb for absorption across the meter (pressure drop) so you "may" get a reading of 15mb working pressure at the meter outlet which would give you no less than 14mb at the inlet to the applaince. But. On say a boiler where the working pressure is taken at the gas valve, there is usually another 1.5mb allowance for pressure drop through the appliance pipeworks and valve which may give a reading of 12.5mb at the gas valve.

Question 2.
2. gas rate calculation's , i have calculations in book and can do them but not sure on them at all been tring to ask for help in understanding eg. turn boiler on max time for 2 min take first and second reading times then do calculation cant mind it off hand times by 30 then divide by 10.76 i think it is but i dont understand if its a 30kw boiler and the sum comes out 27.000 is that ok. Just need this all cleared up by someone who knows and can help me out

You nearly have the sum correct here but you need to multiply by 10.76 to get it to kw. The first part calculates m³/hr and the second part converts to kw/hr.
Say on a 30kw boiler you measure that it burns 0.1m³ in 2 minutes.
Multiply that by 30 to give the answer in m³/hr = 3m³/hr. This is how much it would burn in an hour and is the figure you should use to check against the MI's as that is what they will state.
Now it may say in the MI's that it should be 3.2m³/hr but you are allowed plus 5% and minus 10% to allow accuracy of reading and different calorific values of gas. The 3m³/hr that you calculated is within the limits so you are fine. If you want to convert it to kw/hr multiply the 3m³/hr x 10.76 = 32.25kw so all is well.

If you have an imperial meter it is a bit different. So using the above example.
You measure the time it takes for 1 revolution of the dial then divide 3600 (number of seconds in an hour) by your answer.
So lets say you measure the time for 1 revolution as 32 seconds.
Divide 3600 by 32 = 112.5ft³/hr
Now you look up the book but the gas rate is given as 3.2m³/hr???????????????
Just multiply the answer by 0.0283 to convert it so 112.5ft³/hr becomes 3.183m³/hr which is fine.

There are many ways to calculate this and each will give a different answer.

 

[PLUMBike]

G4 - E6 METERS (m3)​

GAS RATE(m3/hr)

First reading
0000789.345
After 2 minutes
0000791.678
The appliance burned ​

0000002.333


​

​

2.333m3 in two minutes.

We multiply by 30 to find out how much gas the appliance burned in one hour:

2.333 x 30 = 69.99m3/hr​

HEAT INPUT(kW/ hr)
​

GAS RATE X CALORIFIC VALUE
HEAT INPUT = ---------------------------------------
3.6 (conversion factor from Joules to kW)

U6 METERS (ft3)​

To find the gas rate of an appliance operating through a U6 gas meter the appliance MUST be turned on full and TIME the SECONDS TAKEN TO COMPLETE ONE FULL REVOLUTION on the test dial.

Ø 3600(The Number of Seconds in One Hour) must then be divided by this figure(TimeTaken to Complete a Full Revolution) to give the cubic feet per hour (ft3/hr).

To CONVERT Cubic Feet per hour (ft3/hr) in Cubic Metreper hour(m3/hr)
we MULTIPLY this answer by 0.028 (conversion factor).
​

 

[kirkgas]

let me try to explain again, for gas rate (which is simply gas used per hour and is measure in m3/hr) using your figure of 0.25 in 2 mins x 30 would give you a gas rate of 7.5 m3/hr NOT 7.5 kw, to confirm the heat input is correct and burning 7.5m3/hr is correct you take the original 2 min reading x 321 which will give you a figure of 80.25 (which is kw and should be measured against quoted heat input in MI) (it is unfortunate that you chose an original example of 0.25m3 in 2 mins as this means your boiler is 80kw therefore you or i shouldnt be working on it as it is over the 70kw net max haha)
i had a look on here for the post i put on linking to a spreadsheet i had made up, but i cant find it, not 100% it was on this site TBH, if you send me a PM with your email address i will send it to you, it is very straight forward to use and will give you gas rate AND heat input when you put in your 2 readings, i have also added a bit for U6 meters where you need to time how long it takes to burn 1ft3 which is one full revolution of the test dial
PS perhaps anyone who uses this and the other gas forums will remeber where i posted the link, it was titled along the lines of "BG/laptop/gas rates" but i cant actually remember, i also posted the link on someone elses request about gas rates but have no ides where, sorry

 

[kirkgas]

it says 5% tolerance on another ccn1 notes i have aquired, is this % acceptable in the answer given to assessor when asked if it is within range or acceptable during practical exam?.
23-30mb range on standing pressure and 25mb normally found. (just testing myself.lol.) pull of tube for a couple of seconds to allow for regulator to kick in giving a more accurate reading.

depending on who you lissten to the figures usually quoted are 5% or 10% for tolerance, i have never been convinced either/or is right or wrong, although in tamz post he has 5% higher and 10% lower which kinda makes more sense from a safety point of view, not really sure if there is a definative answer to this anywhere

 

[CMairiD]

With regards to standing pressure, my course notes say no more than 30mbar and no less than 19mbar. So if we are allowed as little 19mbar standing, how the flip am I supposed to get the required 20mbar for a tightness test?

Had this happen yesterday. Carried out the tightness test using 19. WP @ the meter with just the boiler dropped to 18.5 and with the cooker on as well, went to 18. Called the Grid, but the engineer I was with said in all likelihood they'd do nothing about it.

 

[kirkgas]

tut tut, easy peasy
if there is less than 20mb for TT, attach air pump and top it up to 20mb for TT, THEN remember you must then purge it, IF it is existing and pressure isnt right it can be noted and left, if it is a new install you test with air/gas topped up to 20mb, then you purge and you can only leave it on if the new appliance will work with the pressure you have, ie 19mb at meter, 18mb at appliance and gas fire requires 17mb so do as above then commission fully as you have more than 17mb at the appliance, if it is a boiler and requires 19mb at the appliance you can top up and TT but as you only have 18mb at the appliance you cannot fully commission it so you need to isolate and cap, (now we will move away from the training centre into the real world, where boiler installers getting 18mb at meter inlet will TT at 18, cause we all know if it is ok at 18 it will be ok at 20!!!!!, then they will fire the boiler up check the rads are heating and there is hot water coming out of the tap and off they will go to the next job) PS forgot to add they will do a hot flush on the system when they see a yorky fitting leaking and have to drain down to fix

 

[tamz]

PS forgot to add they will do a hot flush on the system when they see a yorky fitting leaking and have to drain down to fix

Your spot on with that one for a lot of them.

 

[Cheech]

listen thanks very much for everyone helping me out found these all very helpfull cleared up a lot for me gonna print some off just to take to work as i forget easy.

Thanks again everyone to ask something else similer my friend got gas safe inspection done to get his personal ticket. During this the guy told him if you are getting more that 1mb drop from the meter to the applince you have to upgrade the pipework, also he said if the customer if not happy for you to start disturbing the house to fit bigger gas pipe you can pass it just as long as the bigger drop is noticed on your paper work

does anyone know more on this if its true.

 

[kirkgas]

basically if you are attending an existing system then you can very much leave and report tons of things, depending on results of tests etc, but the scenario you mentioned is ok to leave if just under the MI requirements and safe to use however the difference is when you are fitting new, it has to be bang on, so if you install a new boiler and it isnt right you need to upgrade the pipe etc to ensure that your new appliance is fitted as per MI

 

[metaldust9]

its 28mb or it was in my acs and the 27,000 as you put it you need to devide by 1.11 to give you the net rate when they came out to a house we was at it was 14mb and they didnt do owt except say they were going to change the main but they never did

ps its not supose to be more than 1mb drop across pipework but in reality they dont do out if it is

 

[kirkgas]

its 28mb or it was in my acs and the 27,000 as you put it you need to devide by 1.11 to give you the net rate when they came out to a house we was at it was 14mb and they didnt do owt except say they were going to change the main but they never did

ps its not supose to be more than 1mb drop across pipework but in reality they dont do out if it is

the gas supplier has no interest in the pressure at the appliance, only the inlet pressure at the meter, any other issues are down to the customer to get someone in to fix it, but you are right if the main pressure is low they hum and haw and bale out

 

[Cheech]

I have 1 more question i dont know if i should post new thing but will just ask anyways.

Ok say a boiler has gas pressure it needs going to it say 18mb but you get more of a drop from the meter say 20mb 2mb drop even if the boiler is getting the 18mb it needs does pipe still need upgraded

also when going the working pressure on boiler does all the other appliances have to be on 2

 

[smillsey]

Cheech if the appliance is existing then no problem with the 2mb drop at all..... Go by what the MI's say, they may only want 16mb!!! 1 mb for a new boiler/cooker/fire everytime!!!

Should all the other appliances be on?

well..... I would say 95% of engineers wont do it with them on, however when designing an installation it is designed to carry the right amount of gas with a 1mb pressure drop at each appliance with ALL appliances running.

I would say YES, however i did my ACS at Logic any they didnt seem to be bothered too much.

Your thinking is bang on Cheech, I put appliances on, as you could get a flame out on cooker when you turn the taps on on a 40kw combi!

As 4m3/hr gas flowing though a poorly G6 meter could cause issues if you got 15mm coming from meter just below the floorboards!

 

[oldplumber]

however when designing an installation it is designed to carry the right amount of gas with a 1mb pressure drop at each appliance with ALL appliances running.

You may want to check your rules again, as I always work on a total drop of 1mb overall so 4 appliances = 1/4 mb drop at each appliance, when designing your system. I may be wrong, but I dont think so, so where are you kirky to give the technical trainig response

 

[smillsey]

oldplumber,

That what i said, If i am calculating a 1mb drop with all appliances on I am effectively allowing a 1/4 mar drop with 4 appliances......

I just said it shorthand, as i would of thought Cheech already knows about effective length thats all...

 

[smillsey]

I just didnt make it very clear haha!

 

[kirkgas]

however when designing an installation it is designed to carry the right amount of gas with a 1mb pressure drop at each appliance with ALL appliances running.

You may want to check your rules again, as I always work on a total drop of 1mb overall so 4 appliances = 1/4 mb drop at each appliance, when designing your system. I may be wrong, but I dont think so, so where are you kirky to give the technical trainig response

i never use the 1/4mb drop for each of 4 appliances, after the 1st tee (which needs the full gas rate) you could have 1m of 15mm copper going to a cooker so if you use 1/4 of the drop here you will be over by the time you get to the last appliance, i dont have time at the minute to write a reply that will explain the full procedure i use, but will try to knock a sensible answer out tomorrow, there are 2 methods in the training notes i use,

 

[tamz]

I always work on a total drop of 1mb overall so 4 appliances = 1/4 mb drop at each appliance, when designing your system. I may be wrong, but I dont think so

There are other ways to calculate this and doing it that way is not always too accurate. They tell you to do it that way to be on the "safe" side. (sometimes by miles)
Nothing wrong in doing that way tho.

 

[tamz]

Beat me to it Kirky

The other method is called the tabulation method.
If anyone is interested PM me and i will send a spreadsheet i made up.

 

[GQuigley67]

gas rate for imperial meters times how long it takes to do a full revolution say 37 seconds and then divide 1098 by 37 = 29.675 KW Gross and for Net divide by 1.11 26.734 Kw