Connecting 15 mm supplies to kitchen sink deck mixer

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I have just changed our kitchen sink deck mixer.

The new one has the same hole centres for the supply tails and the same thread on these tails: 3/4" BSP, I think.

The copper supply pipes terminate in adaptors with captive pipe nuts. These adaptors are solder-ring extensions of the pipes, and appear to convert 1/2" bore pipes (15 mm external diameter) to 3/4" bore extensions. I had assumed that these extensions retained the nuts in the same way as 15mm to 1/2" tap connectors for basin taps/cistern ball valves - that is, trapped by a flange close to the connection end of the extension on which a fibre washer sits to make the seal.

One of the connectors leaked, but only slightly, and all efforts to stop this by tightening it failed. So I assumed that the fibre washer had failed. This worried me because it now seems impossible to obtain the original size fibre washers for 1/2" tap connectors, so I was concerned that I might neither have, nor be able to buy, correct size ones for these 3/4" connectors.

The pipe dropped enough after unscrewing the connector from the tap tail to allow me to pull the nut down a small way in order to expose the presumed flange and fibre washer. I couldn't at first see, so felt with my finger to remove what I assumed would be the remains of an old fibre washer.

But I could not feel any washer! The leak had been very slight, so there had to be something to make the seal!

So I got a powerful lead lamp to replace the flashlamp I had been using, and a small hand mirror (originally from a motorbike handlebar!).

I was very surprised to see (a) no fibre washer at all, and no visible flange. Instead, close to the top of the pipe extension (which slides a small distance into the tap supply tail) was an OLIVE.

After ensuring that everything was clean, and well lubed with Vaseline, I reassembled the connection and, with the disadvantage of having to use a basin tap wrench (plus extension to its tommy bar to improve my leverage), did the nut up as tight as I could - probably about as tight as one would on a normal compression fitting.

This cured the leak, but the job raises questions, which I hope some members may be able to answer:-

1. Should this type of connection be made with an olive? The tap tail is chamfered internally at the bottom, which certainly suits an olive much better than a flat flexible washer. However, surely the pipe nut on the extension/tap connector adaptor, is not chamfered internally so as to compress an olive?

2. Couldn't the amount of torque required to compress (re-compress, in this case) an olive easily be too great for the thin stainless steel deck of the sink? I noticed some flexing as I tightened the nut right up.

3. I could not see if the 3/4" olive had been pushed along the spigot of the adaptor as far as the flange which I had expected to find (with a worn-out fibre washer sitting on it). If this type of 3/4" tap connector extension/adaptor for 15 mm pipe does incorporate a washer flange, then this is taking the strain of trying to compress the olive, with a flat bottomed pipe nut behind it. The shape of the olive as viewed in my mirror suggested that this might be so - it looked more flattened at the back end, where the nut or flange bears on it, than at the back, where it is compressed by the chamfered bore of the end of the tap supply tail.

What do members think/advise about all this?
 
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If you could pop a picture of the fitting(s) up then we may be able to understand what's being used. You can use a compression connector w/ olive for connecting to taps. It's not the normal approach and only issue is they are usually metric rather than imperial. Your pipe will be 15mm rather than 1/2". If there's any leaks, a little PTFE tape around the olive will sort that out.

Vaseline isn't idea on potable water BTW as it's a petroleum product, use silicone grease where possible.
 
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1. Should this type of connection be made with an olive? The tap tail is chamfered internally at the bottom, which certainly suits an olive much better than a flat flexible washer. However, surely the pipe nut on the extension/tap connector adaptor, is not chamfered internally so as to compress an olive?

It's no used very often but it's an acceptable way of connecting 15 mm pipe to 1/2" tails - the backnut will probably be from a compression fitting and so is designed to do the job you describes.

2. Couldn't the amount of torque required to compress (re-compress, in this case) an olive easily be too great for the thin stainless steel deck of the sink? I noticed some flexing as I tightened the nut right up.

Not a chance

3. I could not see if the 3/4" olive had been pushed along the spigot of the adaptor as far as the flange which I had expected to find (with a worn-out fibre washer sitting on it). If this type of 3/4" tap connector extension/adaptor for 15 mm pipe does incorporate a washer flange, then this is taking the strain of trying to compress the olive, with a flat bottomed pipe nut behind it. The shape of the olive as viewed in my mirror suggested that this might be so - it looked more flattened at the back end, where the nut or flange bears on it, than at the back, where it is compressed by the chamfered bore of the end of the tap supply tail.

There won't be a flange or a washer - tube into tail, olive onto tube, backnut over olive and tighten.

What do members think/advise about all this?

Keep an eye on the tail and if it's weeps then either tighten or PTFE over the olive
 
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Thanks!

The supply pipes are indeed modern 15 mm, but the internal diameter of this pipe is 12.7 mm (1/2") and the old 1/2" (classed by bore diameter) and the modern 15 mm (external diameter) are identically sized. This situation is unlike that applying to the larger ones such as 3/4"/22 mm 1"/32 mm, where the new version of the pipe is slightly larger in ext diam than the old one. In the latter cases the fittings are larger and adaptors/reducers are needed when adding a modern pipe to an old system.

There is much confusion over pipe and fitting sizes. The main problem is that, while the imperial size is easilly recognised as the bore of the pipe, the fittings and their threads carry the same designations as the pipes, despite being physically much larger than the bore of the pipe for which they are intended. Since the introduction of our half-hearted metrication of pipe sizes, the situation for 15mm/1/2" pipe is unaltered from pre-metrication days. The fitting are still classed as 1/2" and have 1/2" BSP thread. The potential for misunderstanding has increased because of the retention of 3/4" BSP threads for large tap tails. In the case of low pressure supplies to a bath, where 22 mm pipes will have been used since 3/4" became obsolete, you can't attach the pipe direct to the bath tap with a normal olive, but have to use either a reducer/adaptor or a special olive on the pipe tail.

I should have mentioned that the supply side which leaked is the hot (low pressure) side. The tap has separate waterways. You are right about Vaseline and potable water, but we are talking about tiny quantities of the grease which leach slowly into cold water. I do have silicone grease, but silicones are devils for spreading and making the floor slippery (even after supposedly washing it!) if I accdentally smear some onto the floor surface - slip sideways and steady myself with the hand which has just been applying the grease - you know!

Very interesting that you confirm that the seal I have found (by olive, not fibre washer) is a recognized method for connecting to a tap with a 3/4" tail, and that the same may sometimes be found on taps with 1/2" tails.
 
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Petroleum products shouldn't be used primarily because of the potable water aspect but also due to the adverse reaction it can have with some synthetic materials used in potable water appliances and fixtures, sealing washers, tap washers, internals of hoses etc.
 
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