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Plumbing. Best practice sealing 3/4 to 1/2 reducer to 1/2 inch nipple


reg

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I have fiited a new 3 bar PRV for my calorifier. In order to fit it I had to use a 3/4 inch to 1/2 reducer and then a 1/2 male/male nipple in order to fit it.

I have done a test assembly to ensure everything fits mechanically, this was OK. I now need to do a final assembly but have read conflicting advice on the best practice for sealing the mating surfaces.

 

So my question is basically what is the best practice for these sort of connections?

.Flat washer? If so what material?

.Ptfe tape? I've read some comments saying no to this.

.Liquid PTFE?

.One of the above or a combination?

 

Again looking for best practice on this.

Appreciate any help, thanks.

 

 

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5 minutes ago, reg said:

I have fiited a new 3 bar PRV for my calorifier. In order to fit it I had to use a 3/4 inch to 1/2 reducer and then a 1/2 male/male nipple in order to fit it.

I have done a test assembly to ensure everything fits mechanically, this was OK. I now need to do a final assembly but have read conflicting advice on the best practice for sealing the mating surfaces.

 

So my question is basically what is the best practice for these sort of connections?

.Flat washer? If so what material?

.Ptfe tape? I've read some comments saying no to this.

.Liquid PTFE?

.One of the above or a combination?

 

Again looking for best practice on this.

Appreciate any help, thanks.

 

 


I’m not totally sure what you mean by a male/male nipple, but in general the point is to look and see how the seal is achieved. If the seal is a flat surface against a washer, don’t use PTFE. If it’s some kind of conical male into a conical female, don’t use PTFE. I would only use PTFE if the seal is made by the threads themselves, eg in a tapered thread.

Edited by nicknorman
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The usual thread on the valves is parallel, so your 3/4 - 1/2 bush is best sealed with PTFE, several turns, unless you are able to bottom the bush out, when a fibre washer will suffice. This would then mean you have a parallel 3/4 thread into which you are inserting  a taper thread nipple. You can obtain a seal using PTFE< but you would be better off with a running nipple and keeping the thread forms consistent. Some pictures would help.

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8 minutes ago, ditchcrawler said:

Why not use a 1/2" prv and save having a reducer ?

Wondered even that would be asked.

Just now, reg said:

Wondered when that would be asked.

Simple answer because I already had it and it has a gauge.

 

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20 minutes ago, Ex Brummie said:

The usual thread on the valves is parallel, so your 3/4 - 1/2 bush is best sealed with PTFE, several turns, unless you are able to bottom the bush out, when a fibre washer will suffice. This would then mean you have a parallel 3/4 thread into which you are inserting  a taper thread nipple. You can obtain a seal using PTFE< but you would be better off with a running nipple and keeping the thread forms consistent. Some pictures would help.

Here are the parts (From Screwfix)

is.jpg.a5a17924078602c7d93f77f3836d8195.jpgis-1.jpg.7959b346ccb18e06b80b1a4d31eff813.jpg

Must admit I wasn't  aware of running brass nipples,  knowledge gained albeit belatedly.

Edited by reg
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I just use a pipe jointing compound for this sort of thing with genersl pipework. :Templars No.1, Boss White  etc. I am still using tins bought over 30  years ago. If the pipe is carrying potable water, use a type that is stated to be suitable for this purpose. Jointing  compounds now seem to be sold under the Fernox brand, avaiable from the likes of Screwfix. 

 

My understanding is that, for general purpose  iron pipes and fittings,  the modern usual  practice is to use tapered male threads and parallel female threads. Tightening the joint firmly causes the taper to slightly expand the parallel female thread to conform with the male  thread, and when jointing compound is used, this produces an efficient seal.  Parallel male threads are normally used on taps and traditional ball cock valves,  which are best connected using a tap connector, which has a captive threaded nut and a sealing washer. General purpose fittings with parallel male threads will generally have  flanges to accommodate a sealing washer that can be compressed the effect a seal when the fitting is screwed fully home. 

 

 

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4 minutes ago, David Mack said:

I suspect these are taper threads, but Screwfix website doesn't say. So ptfe tape where you screw the nipple into the bush. At the other ends, if the fittings you are attaching to have a flat face inside you can use a fibre washer, if not ptfe tape again.

Screwfix were out of stock so I bought one from City Plumbing. about £8. It doesn't specify the thread but looking at it now it looks parallel but as its half eleven I am not going to get a fitting to screw into it tonight. on it it says DN15 CW617N 1/2" CE0038

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9 minutes ago, Ronaldo47 said:

I just use a pipe jointing compound for this sort of thing with genersl pipework. :Templars No.1, Boss White  etc. I.......

Thanks for that write up, very useful.

I have heard others suggest pipe compound but maybe you could tell me Is it difficult to undo the joint afterwards?

That's my main concern with using it as I'm already working blind and at full stretch fitting it together so have a preference for ease of disassembly. Mind you a gaurernteed leak proof joint would override that preference methinks.

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25 minutes ago, ditchcrawler said:

Screwfix were out of stock so I bought one from City Plumbing. about £8. It doesn't specify the thread but looking at it now it looks parallel but as its half eleven I am not going to get a fitting to screw into it tonight. on it it says DN15 CW617N 1/2" CE0038

Ouch that's a painfull difference in price.

Screwfix was £1.29 and £1.34 Inc for Nipple and Bush respectively 

 

However I appreciate that when it comes to plumbing it sometimes a question of  needs must.

 

 

Edited by reg
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Pipe jointing compounds (other than some specialised types specifically intended for use on damaged threads [Templars  No 2?] that dad, a gasworks pipe fitter,  used to use in the 1960's) that use meths as a solvent, are generally non-setting and should be  removable after a couple of decades without difficulty.   

 

I have tried using PTFE tape (dad did have some in the late 1960's), but my experience was that  there is a knack to applying it to make sure that it doesn't get pushed out to the exposed part of the thread and leave an unreliable seal. With a smear of compound applied to the inside of the female thread and a smear applied to the end of the male thread , you can be sure that compound will be present over the entire surfaces of the mated threads of the two parts.  Useful where access is difficult as you don't have to worry about the tape unwinding itself before the parts are screwed together. 

 

 

Edited by Ronaldo47
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14 minutes ago, ditchcrawler said:

I thought you paid £1-29

No the prices I quoted were for the previously pictured  Bush and nipple.

 

Just checked back on post, thought you were replying to those quoted prices for nipple and Bush. I take it now that you were referring to the cost of the PRV @ £8 in which case that seems quite reasonable.

Edited by reg
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For threaded connections where a washer is not the appropriate solution, I find PTFE tape unreliable (and rather second rate) in comparison to a modern thread sealant such as those from Loctite or the generic equivalent such as FLOMASTA THREAD SEAL from Screwfix (currently £5.99). Once I started using these products I have never gone back to PTFE tape as I find the liquid sealants to be very reliable and effective.

 

If a washer is required, e.g. on parallel plugs into parallel threaded holes, bonded rubber seals are best (Dowty washers)

 

For really difficult situations with iron pipes, larger gaps etc, Boss White and hemp can still be a good solution, but its messy.

 

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1 minute ago, jonesthenuke said:

For threaded connections where a washer is not the appropriate solution, I find PTFE tape unreliable (and rather second rate) in comparison to a modern thread sealant such as those from Loctite or the generic equivalent such as FLOMASTA THREAD SEAL from Screwfix (currently £5.99). Once I started using these products I have never gone back to PTFE tape as I find the liquid sealants to be very reliable and effective.

 

If a washer is required, e.g. on parallel plugs into parallel threaded holes, bonded rubber seals are best (Dowty washers)

 

For really difficult situations with iron pipes, larger gaps etc, Boss White and hemp can still be a good solution, but its messy.

 

Still have a part hank of hemp in the garage

 

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I still have my dad's ball of hemp string. The last time I used it was on a running coupler around four decades ago, about the only time you encounter parallel male thread on a piece of ordinary iron pipe these days. 

 

I have a pre-war home encyclopaedia whose D.I.Y. section includes gas fitting. It recommends using soap on iron pipe joints!  Coal gas was supplied at a lower pressure downstream of the meter than natural gas is today, so I guess it might have been satisfactory.

 

Mind you, its electrical section shows how to install a 2A or 5A socket as an extension from a 15A socket rather than providing it with its own 5A fuse, something that as far as I know has never been allowed under the electricity regulations. 

Edited by Ronaldo47
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