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Purging procedure question...?

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Hello all, I'm currently studying for my initial ASC assessment and was wondering if any of you fellas can help me with a niggling issue I have re purging. I know all the claculation methods for volume etc, but on the actual process my tutor taught that you open the furthest point on the pipework, open the ECV and alow the calculated volume to pass. I asked him that where you might for example have a 5 mtr long 15mm branch to a cooker point, would the gas not possibly 'compress' or at least 'hold' the air in that branch as it passed throught the main pipe to the furthest point, effectively not doing a complete purge. He advised that you open the ECV slowly and the slow rate will draw the air from the branches and fully purge the pipework, he advised that you didn't need to open each branch. Now reading the CITB G2 book, that advises a procedure of opening the furthest branch, allowing an amount to pass, then repeat at each branch until the calculated volume has passed. That to me would make complete sense as it did in the classroom, as you ensure complete purging......? in addition to this conflicting advice, the G2 advises that if the volume is greater than 0.02m, the purge gas must be ignited at the purge point....but does that then mean that even though your purging a percentage of the volume at each point, it must be ignited at each point...? ( I know the total volume of gas and its build up would constitute a risk and hence the igniting, but if the main pipe purge was ignited do the smaller perentage small branch purge volumes have to be ?....and also are the purge volume calculations to be taken as a 'minimum' volume, or the correct volume ?

Sorry if that all seems to be a little too intense, I really should be in the bloomin pub right now :(..... but I can't move on until the niggling doubt has gone away and I'm a stickler for wanting to get everything bang on....

cheers fellas.
 
re read what the purpose of purging is for and you can conclude the answer.
also after purging...re-establish other appliances eg relight.
 
I think your tutor is confusing you , the idea of a purge is to get rid of the air in the pipework so you can light the appliances
 
gasmarc said:
i think your tutor is confusing you , the idea of a purge is to get rid of the air in the pipework so you can light the appliances
Click to expand...
really?
THOUGHT IT WAS TO ELIMINATE EXPLOSIVE GAS / AIR MIX!
 
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OP you only need to worry about high level issues such as pockets of air/compressibility and purge velocities when dealing with much higher volume purges such as might be found on very large industrial installations or indeed ,the purging of large diameter mains. Domestic purging is a much simpler affair.
 
clarify your procedure (or your explanation of it) you purge the calculated volume from the furthest appliance then purge each other appliance till you can smell/detect gas you dont purge the volume to each appliance, or split the purge volume between appliances, it is straightforward, purge volume calculation to furthest appliance then smell/detect at all other appliances
 
OP -This is worrying!

Quote - "my tutor taught that you open the furthest point on the pipework, open the ECV and alow the calculated volume to pass. "

Purging is undertaken after a let by and tightness test. You have the ECV open, with the pipework pressurisd, and then open the furthest tap/valve with you standing with the open tap! So that you can control the escape in a safe manner.

Standing at the ECV with an open end at the other end of the house is asking for trouble.

When you have confirmed the presence of gas by smell, switch of the tap/valve you are purging, and then go and read your meter to see how much has been purged.

Do this for each branch/appliance on the pipe work. It does not matter how many times you switch off and return to read the meter, it is not a problem as you should have already completed a successful tightness test, so you are absolutely sure there are no leaks.

Choosing the purge point can be confusing. Usually it will be the Gas Hob that is furthest away. Simply open the taps and stand with it until you can confirm the presence of gas by smell, switch off, and then return to the meter and read the dial.

The oddball ones are appliances without taps/valves to atmosphere, for example, a boiler. In this case it is quite acceptable to disconnect the inlet/isolation valve from the main gas valve, and open the isolation cock so that gas passes to atmosphere in a controled manner (with you standing with it!) and when you confirm the presence of gas, reconnect and test with LDF before returning to the meter to check the quantity passed.

Be in control of your chosen purge points. There is potential for a nasty accident if you leave an open tap unattended
 
Thanks a lot fellas, really appreciate the advice from all of you, JayDee, thanks for the pointers too, every bit helps. Got to be honest too, our tutor covered the subject far more comprehensively today and conceded that his 'real world' advice last thing Friday evening was misleading.....I'll shall avail of you lot to keep him on his toes in future...thanks again.
 
Well put JayDee. You don't leave an open tap/ended pipe after breaking seal to allow gas to escape while sat at the meter watching a dial.

I was always taught to allow gas to escape till smelt (noting time to do so), closing your purge point, then check meter for volume passed, and you can then generally have a rule of thumb for how much longer to purge for at the same point. i.e. if half of volume required has been passed in 1 min, then approx 1 min further required. I was told for ACS assessments, that if I got this question, the answer is to let out some, seal it, go to meter, check volume passed, go back, let a bit more, seal it, go to meter, check volume passed, go back... and so on, as many times as it takes, but you NEVER leave the purge point open and unattended.
 
Bang on DJ

I think the OP may have misunderstood his tutor.............................Or his tutor has misunderstood the fundermentals of purging ;-)...............Education!..........Don't start me on that one!..............
 
I have never looked at the meter ever when purging , i just crack the furthest joint from the meter until you smell gas and then nip up and do Tightness test sorted
 
gasmarc said:
I have never looked at the meter ever when purging , i just crack the furthest joint from the meter until you smell gas and then nip up and do Tightness test sorted
Click to expand...

why dont you do it properly? doing it properly is probably quicker than your way, as the regs say you do TT before you purge, your way must then include 2 x TT, i do one, then purge properly then reconnect fitting and test with LDF, you say you dont go near the meter when purging, but you purge till you smell then go back to the meter and do another TT
 
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kirkgas said:
why dont you do it properly? doing it properly is probably quicker than your way, as the regs say you do TT before you purge, your way must then include 2 x TT, i do one, then purge properly then reconnect fitting and test with LDF, you say you dont go near the meter when purging, but you purge till you smell then go back to the meter and do another TT
Click to expand...

if you are saying do a tt before purging then surely your way must involve 2 tt's?? otherwise how can you be 100 percent sure the connection has sealed after 'nipping up'?
i think gasmarc is saying that after the initial tt before an installation, pipework is obviously sound so just purge till he can smell then tt at end
 
Kirkgas your saying i do one, then purge properly then reconnect fitting and test with LDF,
So if i know a system is sound after doing a tightness test
Your saying using LDF to check the joint is fine and the system is sound ?
 
Strewth................it was all oh so clear about 6 posts ago...!"!!!
 
gasmarc said:
Kirkgas your saying i do one, then purge properly then reconnect fitting and test with LDF,
So if i know a system is sound after doing a tightness test
Your saying using LDF to check the joint is fine and the system is sound ?
Click to expand...


that is the procedure, you MUST do a TT when you turn the gas on, if you confirm it is ok you MUST purge as per the procedure, then you protect/cover/insulate/paint the pipes, on completion of the proper purge (if you have disconnected an appliance at an inlet valve it is perfectly acceptable to LDF the connection, as it is perfectly acceptable to do so when refitting boiler parts after a service, there would be no need to do a second TT unless............and i hope we all know why/when/where we MUST do a TT, it may have been the way a read your procedure, but it seemed to say you purge till you smell then you do a TT to confirm all is OK, if you havent doen an initial TT (on new install) then your procedure isnt correct
 
Kirkgas I am sure you are correct in your method on a new installation, I personally always do a tightness test after joining two pipes regardless of using LDF on a joint.
 
gasmarc said:
Kirkgas I am sure you are correct in your method on a new installation, I personally always do a tightness test after joining two pipes regardless of using LDF on a joint.
Click to expand...

do you mean you do one before and one after? so you do TT then disconnect a fitting to purge then a second TT to confirm you have tightened the fitting properly? if so that is up to you but surely unnecassary? as the procedure is noted that when you disconnect gas fittings you only need to test with LDF to confirm it is gas tight, and if you only do one TT after you reconnect how would you know that any drop recorded was not caused by you
 
Yes two tests . regardless what regs say I always do a test afterwards as i like to sleep 100% sound LOL
 
The main point of a purge is to fill the gas meter with greater than 90% gas.... normally 100%, this is why its always 5 times the badge capacity. If this is done and you still have a gas / air mix in the pipework a flash back in the pipe work will extinguish when it hits the greater than 90% gas.

pipe work of normal domestic diameters has the strength to hold a flash back but volume in the meter with a 5 to 15% mix will explode, explosions in confined areas are bombs..... possibly taking out the gable end of the house.

The only time the volume has to change is on bigger pipes, in that situations above 0.02m3 by the time the gas air mix has reached the point that you are purging with your ignition source the meter MUST be full of gas.... so cant explode.
 
gasmarc said:
Yes two tests . regardless what regs say I always do a test afterwards as i like to sleep 100% sound LOL
Click to expand...

as you say you do what you want above the regs so you KNOW you are doing things properly
 
everyday I try and learn something new ,being doing it for years now still know jack lol
 
gasmarc said:
Yes two tests . regardless what regs say I always do a test afterwards as i like to sleep 100% sound LOL
Click to expand...

am the same before and after and use my sniffer to test when theres gas there when purging
 
Using a sniffer is not a good idea when purging. It will pick up small amounts of gas and definitely won't tell you when you have 90% gas unless you are spending around a grand for a gasco.
Do it the way it says in the book.
 
never been a problem for me, i just wait till it goes off the scale have ye seen how much gas comes out when doing 5 times the on the meter so to speak i no what it says in the book but that scares me the amount of gas into someone house and i no lots of people and not met anyone who does it that way all say the same, scary
 
It is not in the least scary if you think about it. Unless you were purging into a box of 0.2m3 (less than 2 ft x 2ft x 2ft) then you will never get anywhere near lfl's.
 
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