- adds pressure at the base of the accessible water column in terms of short duration pulses.
It's a bit like a hammer drill.
It is not like a shotgun cartridge. Or pumping up a car tyre
Experience suggests that it is highly effective at dislodging congealed fat and represents no danger to pipework and only a small danger to stupid people who lean over to see whats going on without glasses on.
But then, I have tried it many times and you obviously have not.
And a mate of mine, a plumber, used to be keen on pouring a Winchester of conc nitric acid into blocked drains and did find that worked very well. That as in the days before plastic drain pipes too.
Most certainly, PVC pipes can withstand boiling water. It's not easy to find a specific figure, but the melting point of PVC in the form it is used in pipes is probably around 200 deg C. What temperature it might start to soften at, and lose some physical strength is another matter, but it should still be fine for use as a waste pipe.
With polyethylene, it's a different matter. Sources give the melting point as between 120 and 141 deg C, but, of course, it softens at a much lower temperature as anyone who has poured boiling water into a PE container will have found out. So anyone who tries to use PE pipe for waste could be in for a nasty surprise if they pour boiling water down it.
How much of a surprise if caustic soda is used can be calculated - to some extent. A bit of Googling will show that when sodium hydroxide is dissolved in water it releases 44kJ of energy per mole - in other words about 250 calories per gram. One calorie will raise the temperature of one gram of water by 1 deg C. So, for example, adding one gram of sodium hydroxide to half a teaspoon (2.5 ml) of water at freezing point will raise its temperature to boiling point.
And adding more will raise its temperature *above* boiling point. Well above boiling point, in fact, and not "...approximately the same temperature". If you look at Graph 1 on page 31 here:
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you will see that a 50% solution of sodium hydroxide boils at about 142 deg C at atmospheric pressure (and so even higher if under pressure in a blocked pipe). In other words, the temperature of the solution will be above the melting point of polyethylene.
Regardless of its effect on the pipe itself, I'd imagine a bigger concern might be its effect on any joints. As basically the last thing anyone needs are leaking joints in inaccessible places.
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