Gas Pipe sizing - have i got it???

[Simonbeasley1]

Im doing my gas course. we are working out of the corgi essential gas safety. i am stuck on understanding pipe sizing. My tutor does it by trial and error i think the same way as copperboard does it. The book & Viper gas does it another wat and tells you the actual pipesize to use.

Does anyone know how to gas size using corgi's & vipers way. you have a few appliances, several sections of pipework from meter to those appliances. It requires you to multiply the length (including addition for fittings) by the number of sections to achieve a common drop across those sections but not exceeding 1mb. Where i am getting confused do you multiply by the total amount of section in the overall installation? what happens when you come across only 2 section from meter and an appliance in a multiappliance install. is it multiplyed by 2?

would really like some help.

thanks

Simon

 

[kirkgas]

i do know how to do it, but not sure if i can explain it well enough by writing it here without a corgi book in front of me, as it is easier for me to talk someone through an example
from memory(which can be fraught with danger) you must only have 1mb drop across installation, if you have different pipe legs you split the 1mb drop equally between all pipes, easy, however the table in your book give flow rates per 3,6,9,12m etc lengths of pipe allowing for the 1mb drop in their calculations, so if you have say 4 pipes you are now looking at a pressure drop of 0.25mb per leg, if you simply use the tables in the book your pipe sizes will be too big ( as you end up allowing 4x1mb drops) so to work out pro rata pressure drop you need:
gas flow rate req'd as per your example (check chart to see what pipe size you need) if flow rate equals 16m of 22mm pipe and your example needs 6m of pipe you do 6/16 to give the pro rata drop for your example, you then refer back to chart to get actual size of pipe you need, told you this wasnt going to be easy to write down cause now i think i'm going off the rails a bit, will try to remember to look tomorrow and attempt to write down as if i was answering it myself, i will use actual example in corgi book as that should hopefully make it easier for you to follow

 

[bengasman]

The multiplication method is an approximation that may work out getting larger sizes than you need.
Trial and error might not get you through the exam, as you will be required to get the minimum usable size.
The most effective way is to use partial drop per length on combined use i.e. 2 appliances use the same pipe for a quarter of the length, you allow 1/4 mbar drop over that section.
In real life you don't actually calculate, but use an educated guess and err on the size of the larger choice ass it leaves room for future extensions/upgrades and too large is fine, but too small means you have to redo it. At least that is how I do it, and most guys I know.

As all that counts for you at the moment is to pass the assessment, you should follow the way your book uses. Just draw up a dozen different scenarios with 3 or 4 appliances each, and calculate them all to get the hang of the system.

A first step if you find it difficult, is to work out a dozen or so single appliance exercises with different size appliances and different lengths to give you the beginning of a gut-feeling about what size you need for a certain load over a certain length.
Once you got that, do a dozen with 2 different appliances fed by a "main" pipe over part of the distance, and then a split to the 2 separate appliances with different loads; this will give you an advanced "feel" of what "sounds" about right.
I bet you can work out the next step.

 

[gas4you]

If you have a 5 metre run of pipe for one appliance, then find out the gas rate of this appliance and look up the gas carrying capacity for this length of pipe from the standard charts, adding 0.5 m per elbow/T and 0.3m per swept bend.

If you had two appliances you can either add their gas rates together, or double the actual length of the supply pipe for your calcuation, and treble it for 3 etc.

 

[Paul Barker]

Actually it's one of those things they focus on in training which has never been used by 99% of installers of yesteryear because all pipes are undersized.

Because manufacturers are clamping down installers of today who give a fig are now pipesizing better than those of the past but they still aren't doing it by proper calculation they are just using their experience and when they have everything running make sure they have <1mb drop across the peipework in practice.

So in the real world what you are worried about isn't applied. With experience most people know you need 22mm for a 24kw upwards for any length at all and pretty soon need some 28mm in there, and for 40kw stuff take a load of 28. Cookers and fires fire it most of the way in 15mm switch down fires to 8mm at the end. This is because 15mm is cheap and easy to work with.

 

[bengasman]

Right as you may be Paul, he won't get his ACS if he can't pipesize.

 

[kirkgas]

i dont think there are any complex calculations in ACS, might be some multi choice questions but nothing too deep, its nice to get your head round so that you are installing properly though

 

[Simonbeasley1]

Thanks for all your replies - I can pipesize using the Uk Copper board way which is to estimate the pipesize say a 37KW combi (will likely to need a 28mm) and depending on the length and additional fittings you divide that length by the total length that run of pipe can be using the m3/hr, pipe size & length table. i apply this to all sections add up the pressure drop and hopefully it will be below 1mb otherwise i need to increase up to the next pipesize. Just would have been nice to be able to calc the pipe diameter rather than this trial and error. I do agree experiance plays a part in this.

Will carry on using that technique, will knock up a couple of tables that i can input my figures into.

Got a headache today with working out the overall gas volume in pipework & meter to determine if i can work on that system and then working out the purge amount for the pipework & meter to identify whether you can purge to atmosphere or use a burner on a hob say.

Got their in the end - all good fun. Really enjoying it which is quite sad.

Flues,, Ventilation & controls next week.

Cheers

Simon

 

[bengasman]

i dont think there are any complex calculations in ACS, might be some multi choice questions but nothing too deep, its nice to get your head round so that you are installing properly though

I can't be asked to calculate. Just ask the customer if oversizing for 20 or 30 quid sounds like a plan, to allow for future additions or upgrades. They always say yes.
At the end of the day, nobody knows if they would like a nice big range cooker in 5 years time; if they do, the 30 quid would have been the best investment they ever made. If they don't, 30 quid on the price of a quality installation is not going to make a lot of difference over 10 - 20 years.

 

[kirkgas]

simon keep plugging away and you will get there, there is a lot in the flues and ventilation so you will have a sore head next week as well!! the class im taking at the moment have just done testing and purging, have already done ventilation, they are doing flues and flue testing next week so perhaps my head will be nipping as well