gas hob pressure

[dme1]

I have just installed a replacement hob which will be checked by a
corgi man.
But I have a problem. I have 20mb at burner jet but this drops
drmatically when I turn another burner on. With the gas lit you can see
the flames reduce in size when more than 1 burner is on at a time. This
happened with our old hob but I put it down to the age of the thing.

I did an extension some years ago and then about 5 years ago moved my
boiler to the garage.This was checked by a corgi man, he actulaly made
the final gas connections. I think the gas hob problem coincided with
boiler move but as I say, i put it down to the age of hob.


Here is a diagram of the pipes (approx including allowance for elbows)


2mtrs @ 22mm 10mtrs @ 22mm
Meter------------------TEE---------------------------BOILER 1.6m3/hr
|
| 12mtrs @ 22mm 14mtrs @ 15mm
|---------------------REDUCER------------HO­B 1m3/hr


I used the following link and calculations for pipe sizes all look ok?
Any ideas what the problem may be? I'd like to know whether Corgi man
will need to lay new pipes etc.
http://www.cda.org.uk/megab2/build/pub124/sec3.htm#4.3 Copper Tube Sizing

Reply »

 

[ollski]

undersized pipe or blocked pipe.

 

[chrishutt]

Not necessarily. The only pressure we are given is the standing pressure at the hob, which doesn't tell us much. We need to know the working pressure at the meter outlet and at the hob to determine what the pressure drop across the pipework is.

 

[ChrisR]

We all I think hesitate to advise on DIY gas, because the number of things which have to be done correctly, for work to be safe, is too long a list to explain.
But the problem is either the regulator on the meter, or a restriction in the pipe. Have to suggest you leave it to your gas man to diagnose.

Youcan get an idea by watching the hob (one small burner) when the boiler comes on. If the pressure collapses, call transco to check the regulator/governor on the meter.

 

[dme1]

How do i test working pressure at the meter?
I have a manometer (up to 30mbar) but when i did the standing test at the meter to test for leaks the gas blew the water out of the manometer (I opened valve too quickly- and resolved this by following instructions and opening it slowly) If I connect it again and turn valve on slowly or not I guess I'll have water over my face again

 

[dme1]

We all I think hesitate to advise on DIY gas, because the number of things which have to be done correctly, for work to be safe, is too long a list to explain.
But the problem is either the regulator on the meter, or a restriction in the pipe. Have to suggest you leave it to your gas man to diagnose.

Youcan get an idea by watching the hob (one small burner) when the boiler comes on. If the pressure collapses, call transco to check the regulator/governor on the meter.

I agree but i'm still interested in the cause of this.
I am just going to turn boiler on as you suggest, but does the pipe size seem too long ..

i.e. 14mtrs 22mm then reduced to 15mm for another 14mtrs?

 

[dme1]

the small burner was not affected when boiler came on...
I wonder how llong to wait for a corgi man ?

 

[ollski]

What is the heat input (kw) of the appliances?, presume that you have calculated elbows in the lengths already....sounds ok though.

 

[dme1]

What is the heat input (kw) of the appliances?, presume that you have calculated elbows in the lengths already....sounds ok though.

Yep, I included an allowance for elbows (.5m per elbow) as per the pipe sizing guide.

1 rapid @ 3kw
2 semis @ 1.75kw each
1 auxilary @ 1kw

 

[doitall]

Well for starters the first 2m is well undersized.

For seconds the 15mm is also undersized.

 

[ChrisR]

Agreed but I wouldn't have expected to see flames drop appreciably going from one small burner to 2. Would check the working pressure at hob to observe. Is the restrictor open?
If you tell transco you've been advised the governor's faulty they'll come and check it free within 4 hours. If it's got a black knob on top they'll probably change it anyway!

 

[dme1]

Corgi man has just been. I installed it all ok. The problem was restiction in the pipe. It was actually flux in the original isolating valve, £45 for piece of mind and a working hob was money well spent, although he said my instalation was done OK. If I'd replaced the valve myself I would have resolved the problem.


Thanks for all your feedback.

 

[doitall]

Corgi man has just been. I installed it all ok. The problem was restiction in the pipe. It was actually flux in the original isolating valve, £45 for piece of mind and a working hob was money well spent, although he said my instalation was done OK. If I'd replaced the valve myself I would have resolved the problem.


Thanks for all your feedback.

And the undersized pipe, what has he done about that, or didn't he try it with everything running.

According to your figures, the 2m x 22mm will give you 2m³/hr, and not 2.9m³/hr.

Piece of mind I don't think so. The first 2m length needs to be 28mm.

The 15mm needs to be reduced in length by approximately 2m.

 

[dme1]

Corgi man has just been. I installed it all ok. The problem was restiction in the pipe. It was actually flux in the original isolating valve, £45 for piece of mind and a working hob was money well spent, although he said my instalation was done OK. If I'd replaced the valve myself I would have resolved the problem.


Thanks for all your feedback.

And the undersized pipe, what has he done about that, or didn't he try it with everything running.

According to your figures, the 2m x 22mm will give you 2m³/hr, and not 2.9m³/hr.




Table 3 Gas discharge, m3/h, in straight horizontal copper tube with 1.0 mbar differential pressure between the ends, for gas of relative density 0.6.

Length of pipe run, m
Size of tube 3 m 6 m 9 m 12 m 15 m #20 m# 25 m 30 m
10 0.86 0.57 0.50 0.37 0.30 0.22 0.18 0.15
12 1.5 1.0 0.85 0.82 0.69 0.52 0.41 0.34
15 2.9 1.9 1.5 1.3 1.1 0.95 0.92 0.88
#22# 8.7 5.8 4.6 3.9 3.4 #2.9# 2.5 2.3
28 18 12 9.4 8.0 7.0 5.9 5.2 4.7

This table (sorry about format) shows that 2.9m3/hr will be supported by a 22mm pipe up to 20mtr with a 1mbar drop?
Are you saying that every house with a boiler and hob needs 28mm pipe?


Piece of mind I don't think so. The first 2m length needs to be 28mm.

The 15mm needs to be reduced in length by approximately 2m.

 

[doitall]

No, I'm saying you have a total pipe length from the meter of 38m.

you have a load on the first 2m of 2.9 m³/hr.

The discharge for 38m of 22mm pipe is 2m³/hr.

The 15mm has a load of 1.3 m³/hr.

14m of 15mm will discharge 1.1 m³/hr, and is just on the mark, if the length too the Tee was 28mm, but could do with the 22mm extending by 2-3m.

Pressure drops.

Boiler 0.24.m/bar
Hob 1.17m/bar.
Both together 2.2 m/bar