Confused about working pressure

[ant101]

Hope somebody can help me with this question. Im training to be a gas engineer, please be patient with me.
Am I correct to say that to test your working pressure at the meter you attach your gauge and for example turn a minimum of three rings on cooker and expect a reading of 21mb + or - 2mb. If this is so and the reading was 21mb and then you were to turn the boiler and fire on should the reading still be 21mb or is it ok as long as it is in the range of 19 - 23 mb.
Thanks for any advice

 

[gas4you]

Yes you are correct

In reality though testing with 3 rings on a hob will not give the same reading as say a 31Kw combi on full for HW.

I only use the 3 rings as a last resort.

Also personally anything below 20mBar I still call in as low pressure and get it set for 22mBar.

Things will all become more apparent 'out there working' as compared to theory in the classroom.

Good luck

 

[ant101]

Yes you are correct

In reality though testing with 3 rings on a hob will not give the same reading as say a 31Kw combi on full for HW.

I only use the 3 rings as a last resort.

Also personally anything below 20mBar I still call in as low pressure and get it set for 22mBar.

Things will all become more apparent 'out there working' as compared to theory in the classroom.

Good luck

Thanks dave, It was 22 mb with 3 rings on but with boiler on HW and fire full on and then 4 rings and both oven and grill full on it dropped to 19mb. Is this normal?
sorry to be so deep.

 

[bengasman]

A hob could take 10 kW which means you are now sucking 40 kW out of the pipe.
IF the test is in winter, AND during peak hours, AND the incoming pipe is a bit small due to several splits to supply various test set ups in a training centre, AND governor is a bit old, then yes, to be expected. Don’t forget that a water gauge is only accurate to 0.5 mbar so what you had was possibly 19.5. nothing to worry about I would say

 

[gas4you]

What you describe is often caused by undersizing of the gas carcass.

It could also be that the govenor is not responding properly or even the supply from the street is not up to scratch.

If I have ANY doubts I always call in TRANSCO (NG now) to inspect and hopefully correct.

Always start as you mean to go on

 

[baxpoti]

Working pressure, the gas supply pressure with the appliance running on maximum, as opposed to burner pressure, should also be checked at the appliance.
For a standard efficiency boiler a 1mb drop from the meter to appliance is the maximum permissible.
High efficiency boilers with zero gas governors (with or without offsets) can operate safely with a working pressure as low as 14mb at the appliance 21±2mb at the meter.

 

[chrishutt]

High efficiency boilers with zero gas governors (with or without offsets) can operate safely with a working pressure as low as 14mb at the appliance 21±2mb at the meter.

But you are still required to limit the pressure drop to 1 millibar.

 

[bengasman]

But you are still required to limit the pressure drop to 1 millibar.

i thought it was 2 these days, have i lost the plot?

 

[corgiman]

But you are still required to limit the pressure drop to 1 millibar.

i thought it was 2 these days, have i lost the plot?

never heard it being 2 mate, its always been 1 as far as I know

 

[Gasguru]

........High efficiency boilers with zero gas governors (with or without offsets) can operate safely with a working pressure as low as 14mb at the appliance 21±2mb at the meter.

Have a word with the manufacturers - it's dependent on the gas valve employed and the burner. Some have had problems with melting burners as the fuel/air mix goes a little lean.

Apart from the fact that it contravenes GSI & U regs and other appliances tee'd off the line may not be safe.

21±2mb W.P at meter, 1 mbar max drop to appliance. Can't believe this still causes confusion.

 

[bengasman]

21±2mb W.P at meter, 1 mbar max drop to appliance. Can't believe this still causes confusion.

2mb difference at meter, 1mb drop, easy to mix 'em up

 

[ChrisR]

Plenty of room for confusion.
There's a pressure drop across the regulator and the meter, and the regulator isn't "perfect" in that the output pressure from it does vary with the amount of flow going through.
The BS on gas meters allows for a 2mbar drop at full flow.

Ant101 your training notes will tell you that the working pressure at the meter is allowed to drop by 4mbar at full flow. That means that you can have 19mbar at very little flow, dropping to 15mbar at full load. Then a 1mbar drop in the pipework leaving you with 14mbar.

Appliances are required to be safe at 14mbar, but that doesn't mean they have to work. Some boilers (GW Hxi, eg) won't light reliably at 14mbar.

If the pressure goes any lower, unregulated burners such as on hobs can go out. That can lead to a dangerous situation if a hob is turned right down with a boiler off. When the boiler comes on, the gas flame can go out, meaning unburnt gas is escaping, if there's no flame failure device on the hob. When the boiler goes off again, much more gas escapes of course.

 

[bengasman]

May have missed them of course, but I am pretty sure the 4 mbar drop allowance is not something that was in my training. Take your word for it though.
The one thing that strikes me as odd, when tolerances are added, it is okay to measure 15 mbar in stead of 21 and call it perfectly safe. 1.5 mbar drop due to flowresistance in the downstream pipe is unsafe. It seems that the standards for the installer or householder are a lot more stringent than for the network operator.
Is there a guidance note somewhere that explains why that is, or am I asking too much?
One thing I have found that makes our live easy in this respect; Transco are pretty quick and easy when it comes to rectifying. We may blame them for being TOFO-boys, but when I reported a low pressure at a commissioning at 6 pm Saturday nite, they were there within 2 hours, and left my client with a working boiler. That is not bad for a free at point of use service.

 

[ChrisR]

Ben the 1.5 isn't unsafe until it's applied on top of the other losses.

The 4mbar isn't lost by the network operator, it's inside the meter/governor/regulator.

The 4mbar has come into the training relatively recently but I don't know when. You'll find it in the current Viper book.
I teach/assess ACS part time, by the way.

 

[bengasman]

Ben the 1.5 isn't unsafe until it's applied on top of the other losses.

The 4mbar isn't lost by the network operator, it's inside the meter/governor/regulator.

I meant safe as in ‘legal’. Though I doubt a lot of installers will rip up the pipe they have just stuck in when they find 1.5 drop, unless I am mistaken, it is what they should do. Or maybe I should say: must do.


As the meter is owned by Transco, I include that in the network operator’s part.

It is not something I am overly concerned about to be honest. I am too lazy to calculate pipe sizes, so if I think I might come anywhere near the limit I take a size bigger. Added cost is marginal on a whole job and the customer always agrees to it when you point out that it will safe them a lot of money if they decide on a bigger cooker. I do find it interesting from a technical point of view though.

As I am always looking for more info and knowledge, can you recommend any books?
Got the standard corgi book, and excellerate or whatever it is called and the blue viper book. Not the latest one apparently.