ABS PCs

[JLspacemarine]

I just can't find any large pressure rated PVC locally so would it be okay to use ABS pipe and fittings as a pressure chamber on a APH? Even if it's DWV?

The non-pressure rated ABS pipe and fittings are still pretty thick when they are Schedule 40 (6-7mm), so it seems like they can take some pressure. Right?

[Silence]

I did some research and learned a few things. Apparently the use of ABS is a rather contentious question, and there's not too much information on it. Several pages linked here, though.

If your pipe is cellular core, then it'll be useless for containing pressure. If it's solid core but not pressure rated, then that's where the controversy begins...my guess is it may be okay at low pressures, because it's well-documented that ABS tends to tear apart, not shatter. This account seems pretty scary, but it was a hybrid potato cannon (where you ignite pressurized gases in stoichiometric ratios) and the escaping air was searing hot. I doubt high temperatures are an issue for a water gun pumped by hand, but Ben could probably say for sure.

I'm not sure what pressure that spud gun reached, but it was probably much higher than what you'll attain in a water gun.

Your pipe may not specify whether it is cellular core or solid core, but if it's the latter, then cutting it will leave those parallel saw marks, maybe up to one centimeter in width.

One thing I learned was that ABS seems to weaken with use. You would only be using it at low pressures, but having the pipe continually under pressure could be a problem...hard to say. However, since some ABS pipe is pressure-rated, I can only assume that those types, at least, do not degrade over time.

Incidentally, I also learned that PVC degrades rather quickly when exposed to sunlight. There's also the fact that it loses a good amount of strength in mild heat, too, but not as much as in the cold; ABS maintains its strength from -40 degrees F/C to 180 degrees F. Interesting.

[JLspacemarine]

I guess the pipe I can buy locally is solid core, since it is Schedule 40 and there is no foam or anything not ABS inside.

So, is it worth the try for a good water gun? Or too dangerous? IMO ABS pipe and fittings seems to be pretty strong, but I don't want anything to burst in my face if I eventually try to use this for containing pressure.

[captianfear]

I made a PR water gun with ABS last year. It hasn't shattered yet. I just asked the plumbing guy if it would hold a bike pumps air pressure and he said it would. It work. It handled 80 to 120 psi average.

http://www.isoaker.net/isnet/topic3727.html

[JLspacemarine]

Sounds good. Thats a very good example. But have you verified if your ABS piping was pressure rated or DWV?

[captianfear]

I believe it was pressure rated but I do not remember the rating. I read the words on the side and it was schedule 40 I think. That number rings a bell. It is basic pipe you get at home hardware.

[Ben]

Schedule 40 refers to dimensions, not pressure rating. Most DWV pipe is sch. 40. Unless the pipe has a pressure rating on it I would assume that it's not rated.

I've never come across pressure rated ABS, though I have read such a pipe exists.

The tee you used in your water gun captainfear is most definitely not pressure rated. Curved tees generally aren't pressure rated. Remember that the pipe and fittings both must be pressure rated for safety. Will anything bad happen in your case? Most likely not, but it's good to know for the future and reaching for the straight tee if it's there should solve the problem.

As for temperature issues, the air undoubtedly will be hot if you increase the pressure enough, however, the plastic won't be due to the difference in specific heats. It takes much more energy to heat plastic basically.

[C-A_99]

This brings us to another possible issue: the reducers commonly used in any projects that require 3" or larger pipe, such as WBL's, SuperCannons, etc. I haven't seen pressure ratings on the ones with "Charlotte" printed on them. Judging by the thickness and composition of the plastic, they appear to be safe, and we've been using them for years.

Edit: "Charlotte", not "Charlotte Pipe", I was talking about fittings.

[Silence]

C-A 99: Funnel-shaped reducers generally aren't pressure-rated, and the ones stocked in hardware stores probably never are. Go online or find a real plumbing store like Noland's and make sure you get pressure-rated ones. For 3" PVC, you should be able to find reducing bushings, which are pressure-rated.

Ben: How hot? PV=nRT suggests the increase in n will account for most of the increase in P. And wouldn't the plastic absorb the heat as the system equilibrates?

JLspacemarine: I suppose I never really gave my recommendation, so here it is: go for it. The plastic *may* fail somewhere down the road, but it'd still be safe (since it wouldn't shatter) (and also since it wouldn't be spewing fiery hot gas).

[C-A_99]

I don't recall seeing 3" reducing brushings at Lowe's (which is where I usually go for nearly everything except O-rings, straps, and other miscellaneous material), they seem to carry up to 2". The funnel reducers I use (which I have on my douchenator, and have been there for 2 years, I also have an endcap from the same manufacturer) are very common in just about homemades I've seen here. The SuperCannon II also uses some kind of funnel shaped reducer. I haven't seen any brushings that size anyway. (the brushing + coupler combination that's nearly always used in homemades using 2" pipe)

Edit: Actually, most fittings, including brushings, elbows, endcaps, etc. don't have ratings written. The only fitting I remember seeing ratings on is the ball valve, though they may exist in check valves too. Possibly, fittings are generally designed to stand higher pressure than the pipe they go on, so they're not rated or a rating isn't shown.

[Silence]

Yeah, I'm not sure what to do here...I keep getting the impression that you're not supposed to use large-diameter PVC for containing pressure. This page also recommends reducing bushings, but doesn't say where you can get it in large sizes. Interestingly, McMaster-Carr sells neither pressure-rated reducing bushings nor pressure-rated reducing couplings.

Also – they're called "bushings," not "brushings."

[Ben]

McMaster-Carr is likely intentionally being convoluted on the pressure rated part because they want to prevent people from using them with compressed gas. They generally won't say fittings are pressure rated. However, if you go down to the "Specifications Met" part and choose "National Sanitation Foundation (NSF)" and then "NSF 61" you'll end up with pressure rated PVC pipe and fittings. You can see from here that most of the reducing bushings in sch. 40 are pressure rated and any pressure rated reducing couplings are sch. 80 and expensive.

@Silence: You can quantify how hot a perfect gas is with isentropic process relationships. Isentropic means constant entropy, which also means reversible adiabatic.

Looking at the difference in specific heats I noticed that they actually aren't too different for PVC so the PVC could get rather hot. Of course, the heat transfer won't be immediate and the peak PVC temperature would of course be lower than the peak air temperature simply because the energy is more spread out, but this is something to investigate further.

[adronl]

Originally Posted by C-A_99

This brings us to another possible issue: the reducers commonly used in any projects that require 3" or larger pipe, such as WBL's, SuperCannons, etc. I haven't seen pressure ratings on the ones with "Charlotte Pipe" printed on them. Judging by the thickness and composition of the plastic, they appear to be safe, and we've been using them for years.

Lowes carries Charlotte but most all of their products above 2" are all drainage and not pressure rated excluding pipe that states pressure rated. For smaller though they have a section specifically rated sch 40 pressure pipe but the section tends to not be very extensive or organized well. Generally the same goes for Home Depot although they don't advertised a section for sch40

You can get larger sch 40 Charlotte but you gotta go to a commercial plumbing supplier.

[Ben]

To clear something up that I missed earlier, most fittings don't list pressure ratings. If the fitting is PVC and has NSF-pw or NSF-61 stamped on it, it's pressure rated. If the fitting has NSF-dwv stamped on it, it likely isn't pressure rated but sometimes is and typically that only occurs when the fitting is stamped with both pw and dwv.