(A guide to properly and improperly cementing & joining PVC PIPE)
by Freeman Newton <notwen@telus.net>
Having a leak on the Job? Ah, naughty! Consultants (who are perfect, of course) get aggravated. Supervisors have been known to lose their hair and customers to get fits ... and just who is at fault? YOU?... well, maybe not.
A contractor with a flawless record for cementing PVC pipe might not be so successful the next time. Why? Because of changes in the work force.
Why do leaks occur? How can we prevent this? From whom do we get information to prevent this from happening?
It's SIMPLE ! Go ask you friendly supplier of cement for instructional data. And if he cannot help you, CHANGE your supplier !
You want to prevent leaks from happening? Here's a list of things to AVOID.
1. No primer was used. 2. Primer was allowed to dry out. 3. The pipe was wet and damp. 4. Incorrect cement type was used. 5. Insufficient cement was used. 6. The pipe is moved just after cementing. 7. People didn't wait long enough before doing pressure testing (.... just like concrete needs 'time' to achieve max. strength) 8. The cement was old (cements have a definite shelf life, not so apparently for primers). 9. The wrong primer was used (yes, even Varsol was once used by the City of Toronto ... good lubricant, maybe, but totally useless as a primer. Every single cemented joint became unglued.! A fiasco!)
This is about PVC ... although here are other cementable plastics such as CPVC, ABS and even Plexiglass ..... all of which are to some degree or another inter-cementable and some are interweldable like PVC & acrylics.
PREPARATION is the key to good results. Cut the pipe square. Bevel the ends. Deburr. Clean off moisture & dirt. Make sure that you have all the materials available before starting ... cement ... primer ... rags ... proper applicator (one for the primer & another for the cement) and people on hand to help out.
One presumes, however, that the pipe will fit the socket! It is rare when it doesn't. The pipe should go into the (dry) socket for about 1/3. The so-called interference fit of a properly made tapered socket to a well made piece of pipe. Sometimes the pipe will bottom out on the socket. The so-called net fit. But if it hardly goes into the socket or if it wobbles around in a net fit socket, something is wrong. REPLACE!
You are now ready to start cementing. There are many different types of PVC cements available as well as multiple purpose cements.
Tetrahydrofuran is the solvent for PVC (or CPVC), THF for short. Primers may contain some acetone or MEK. The cements will contain some solids and additives to improve performance in varying field conditions ... gap filling ... humidity ... clarity ... low temperature usage
Primers have a dual purpose. To remove any traces of oil (from hands, etc.) and parting agents (from moulded fittings) and to soften up the PVC. This is particularly important in cold weather conditions. Never, EVER let the primer dry out. While the primer is still wet, apply the cement.
Primers, being essentially pure THF, tend to dry out quickly. This is a problem when you are up a ladder trying to join large fittings on a hot day. Use a good sized applicator! The rule of them is to get a cheap (unvarnished) paint brush that is HALF the size of the pipe diameter!
In other words the brush length will be the same as or slightly more than half of the depth of the socket ... which means that multiple passes and overlaps are not necessary, which are not only time consuming but cause a time delay that might allow the primer and the cement to prematurely dry out before joining especially during hot weather !
The normal rule particularly for larger sizes and predominantly Sch. 80 is to prime the socket first, then the end of the pipe and then re-prime the socket. While the primer is still wet, apply the cement to the pipe end first, then into the socket (careful! don't leave a puddle) and then put another coat on the pipe.
Push the pipe into the socket ... for small fittings, twist the pipe, too ... hold for a short time. Wipe off excess cement with a rag. Leak failures are commonly due to insufficient cement being used. At the joint one should always notice some excess cement oozing out which, as mentioned above, has to be wiped off.
Do not move that freshly cemented joint for a while. Do not apply line pressure until the joint has a chance to dry. Your friendly supplier should be able to give you all this data.
THREADED FITTINGS To prevent leaks, the rule of thumb is to use tfe tape with threaded fittings. One wrap is good enough for the small pipe sizes but it's a good idea to use two wraps for larger fittings such as 3" dia and up.
The key to success is to count the number of turns that are needed to tightly dry fit into the socket. Then apply the tape, turn to the number of dry turns you did and then add on two more complete turns. With small, say, 1/2" sized pipe it is possible to keep on turning and turning and turning due to the lubricity of the tape ... This will stress the female thread and you can break it! I've done this ....but you shouldn't !
As you (should) know, in Canada and in the USA, moulded fittings are threaded to iron pipe standards. So don't imagine you can fit a normal machined thread to a moulded socket ...you'd have to make the thread to IPS standards. Another point ... when you put on the tape, DON'T put it on to follow the thread but AGAINST the thread, otherwise it will tend to get scraped off and get bunched up.
PROBLEM SOLVING ! We have a leak. What shall we do? But what shall we NOT do !
What we must NOT do is to add on any cement to the outside of the fitting. It will do absolutely nothing for you. Also, do not add anything else in the fond hope that you can cure the problems by using silicon RTV caulking compound or fiberglass. We have 3 options.
1. Cut out the leaky connection and replace with a new one.
2. Use a repair coupling, if permitted.
3. Backweld.
BACKWELDING is an art. It is done with a hot air welding gun and a compatible weld rod. First of all, all traces of cement have to be scraped off where you want to weld. Secondly, drain the the line, if you can and thoroughly dry off the suspect area. Thirdly, wait for 24 hours after cementing before attempting to backweld. Why? Because the (welding gun) heat will stress the connection by forcing some of the solvent in the cement into a gaseous state. However, sometimes backwelding is very necessary even for a perfectly cemented job such as as pipe line handling commercial 93% sulfuric....after waiting for 24 hours !
But if the cementing instructions had been followed, you should never, ever need to backweld. Get a copy of these instructions. Ensure that that they are complied with.
And please do not rely on 'Joe' just because he tells you that he had done (some) cementing before. If all the (apparently many 'Joes') were given instructions and had followed them, this article would be totally unnecessary ... and contractors would not be knocking on your door "to get a fix", either ... and you yourself should not even be bothering to read this article, should YOU !
The above article is a rewrite of the authors l984 HAVING A LEAK COSTS TIME ON THE JOB which appeared in a construction newspaper ... and was prompted by, of course, by the poor cementing practices of 'others'. Maybe something should have been said about the IPS Company. Their Weld-On 705 is recommended for Pools and Spas. Their Weld-On 7ll, 717 & 719 cements are designed for Sch. 80 as well as for large diameter pipe. Ask them for advice.
On a personal basis, I like the Laramy welding guns. WHY? Mainly because they have such a useful little booklet on welding.
Freeman Newton, White Rock, B.C. Canada Nov. lst 2002 Revision (threaded connections) Nov. 2004
Back to finishing.com Home Page