What's special about working with PVC pipe of 6" or 8", and cementing large joints?

I’ve cemented ordinary white PVC pipe of smaller sizes for years. Occasionally I’ve gone as big as 3" or, maybe, 4", but for low pressure applications, and that was quite a few years ago. I want to make something with at least 6" for potable water in my home system, which is driven by a pump and should cycle to 45 psi regularly, higher if the pressure switch malfunctions (quite some time ago an adorable little mouse got in there with sunflower seed shells and partly jammed it, and I watched it cycle upward past 140 psi and peg the gage).

It looks like 6" schedule 40 should be fine, maybe even 8", though in 8" I might want to go to schedule 80 – and 8" schedule 80 plumbing gets pricey fast.

In particular I wonder about cementing joints. I’ve seen people make joints in PVC up to around 18", but there’s a crew of guys doing it and they’re using equipment to slide the pipe home quickly. These are situations where you could be making the last joint in a very expensive assembly, and the pieces slide partway together and then freeze up. I think one is supposed to choose the viscosity of the cement appropriately but don’t know how to.

What I’m doing is making a settling chamber for suspended solids. I have a lot of mica or similar grit in my water and it rapidly fills filter housings before the filter even clogs. We are talking particles of up to a couple millimeters, and they settle rapidly. They sink like rocks. Well, actually, they sink AS rocks. You can settle a lot of them out of kitchen sink tapwater just by running it through a drinking glass. My water’s not turbid, it isn’t really a filter I need, and I’ve bought a centrifugal sand trap but it’s so tiny it would fill up too frequently to maintain easily. My idea is basically to make something like a sink drain trap, with a small emptying port in the bottom. I want it to be able to trap a couple liters of grit before it needs to be emptied.

Thanks for any advice!!

Maybe you can get a small hot water tank and use that as a settling chamber. They come in all sizes and all the plumbing fittings are installed. Including a drain tap for removing sediment. Probably lots of used ones for sale.

If you use a hot water tank. I would install some extra valves on the in and out lines.
So they can isolate the tank. and also vent to the air. When you want to drain the sediment. Close the out put and in put lines. Then open the output one to the air. This will allow a fast drain when the sediment valve is opened. But not drain down the rest of the system. Close the sediment valve. Open the inlet valve. Wait till water comes out the top outlet to air valve. Then shut that one.
Tank is flushed, filled and the rest of the system will have just a bit of air in it. Three way valve would be great for this.

Duh.
Just shutoff valves on the in and out. Then replace the pressure safety valve with a more normal type. Seeing as you are not using it as a heater. Done.

Napier,

If I may suggest, look into using a hydrocyclone. Here is one for example - https://www.irrigationking.com/3-plastic-hydrocyclone-sand-separator-220-gpm.html

They are small but effective (will take out smaller particle sizes) and are convenient to flush out the collected solids. You can setup a sprinkler valve arrangement to flush out the solids every day.

Good luck with whatever you pick.

Just be sure and use the cement intended for larger pipe sizes. The cans are marked. Plus the right primer. If you want to look at some cool fittings to make your tank really nice, here you go:

https://flexpvc.com/
Here is a neat 4" Tee with a 3/4" side outlet to bring the supply in without a bunch of reducers:

https://flexpvc.com/cart/agora.cgi?p=PVC-Fittings-Tees-Reducing&p_id=401-416&xm=on&ppinc=detail

Dennis

My back of the envelope math says that if the pipes have a mating surface of 1 inch, then an 8 inch pipe will make have about 8 times the mating surface as a 2 inch pipe.
PVC glue doesn’t so much ‘glue’ the joints together as it does melt the PVC and then resolidify a few seconds later. Anyone that’s assembled PVC knows that you have about 2 or 3 seconds after you slide the two parts together to get them fully seated and straight. After that very small window there’s no turning back. If something is wrong (ie you cut one of the pipes at an angle and it went together crooked), you’re tossing it starting over.
Further, because of all this, it can take a bit of force to fully seat a pipe into a coupler.

My WAG as to why you see a multiple people and/or equipment to assemble these larger pipes is a combination of them simply being difficult to slide together as well as pipes that large are likely long as well. It may take multiple people/machines simply to wrangle them so they go together properly.

For a home project, I don’t think you need to run around hiring extra hands. Just have everything ready to go…and do it. If you’re concerned about the expense of buying a new pipe if you screw up, I’d suggest getting one a foot or so longer than you need, that way if there’s a problem, you can cut the end off.

Also, if you have that much grit in your water, my suggestion would be to call the municipality and ask them.
But as far as filters, perhaps you could install, say, another two. The first one being very porous (maybe even some kind of mesh instead of an actual filter), just to get the really big stuff out, the next one less porous and by the time the water reaches the currently installed filter, there should be considerably less sentiment in it.

Tanks would be appealing except that they have small inlets, so water enters them at high velocity (which for their intended purpose is fine, but not for mine). Small inlets stir the tank. I need to get the water velocity down in the settling chamber.

Thanks! The “centrifugal sand trap” I referred to is a hydrocyclone. For some dumb reason they integrated a small screen with it, so it can clog, and its outlet design looks fragile and comes with all sorts of printed warnings to treat it gently, whereas I think I’ll have to be emptying it very frequently. I don’t trust it.

Moreover, though this has been hard to track down, hydrocyclone efficiency is very dependent on flow rate and drops to zero as flow rate drops to zero. See for examples:
https://www.tandfonline.com/doi/pdf/10.1080/02786828508959066
http://www.ajer.org/papers/v3(6)/O036120126.pdf
The trouble with this for home use is that my flow rate will vary quite a lot, unless I place it between my pump entrance and my pressure tank, which location would be difficult to add a component to.
I have hunted for a suitable one anyway, and am not finding much. The possibility remains, but it’s not great. The Irrigation King example you linked comes set up for 2" lines at the smallest, and flow rates way higher than my home. I have found some others for lower rates, so something may come of it…

My water comes from a private well. I don’t think the municipality would have anything to say.
The brute force method of loads of filters is a possibility, I guess. I already have three filters in parallel that each hold a large 24" cartridge, but there’s just so much of this stuff!

Heavens! OK, my entire mindset about what is or is not available in PVC has just been recalibrated! I may try these people first when trying to acquire fittings, but in any case know to try them before concluding something is unavailable. Thank you for this excellent resource!

Well, yeah, that’s kind of the thing. There’s a limited time to get things where you want them. Also, when I assemble smaller PVC I like to not only slide them together, but also twist, so there can’t be a dry path in the joint that runs all the way from the interior to the exterior. In my house’s drain plumbing there are several joints that leak from time to time because of this, and I’ve done various awful things like painting the outside of the joint with the heaviest cement I can find, and hose clamping rubber sleeves over the joint, as these leaks turn up. The right fix might be to have my drain plumbing completely replaced, including tearing up drywall to get to some of it, but that is not very appealing! So, twisting would have been nice when they built my house, but I’m not at all sure it can be done, especially by one person handling 6" pipe.
And I fear, fate being what it is, the joint I can’t have a do-over on because of where it is in the assembly, thats the one that will screw up. That’s why I’m trying to do some homework now.

You know, if I can grouse a bit more about my house’s original construction, I think the plumber must have been awful. For the supply pipe he used the thinner copper that is only rated for hot water heating, not for supply pressure. For the drain pipe, not only did he leave multiple leaky joints, he also cut almost completely through a support joist, which let an upstairs wall shift downward, cracking the drywall and making the toilet and major appliances lean backwards, until the toilet actually hit the wall behind it. And one fitting in the drain plumbing actually didn’t get cemented at all, on one side. I had to cut out a portion of the line and splice in a section, and use unions because the pipe on each side of this location couldn’t be moved to slide fittings into place. And the longest run of drain plumbing travels 26 feet with only 4" of drop, so it’s practically horizontal and solids settle in it. When I had to unscrew my bathtub faucet to replace it (which blocks access to the pipe inside it so I can’t put a backing wrench on it), the copper pipe just twisted. I dread replacing my shower head because I think the pipe it’s on will break or something. Working on plumbing in my house is like doing surgery on a hemophiliac.

Have you checked into commercial and industrial supplies? What is your price range? Sounds like you are trying to go on the cheap. Personally I’d find a non-leaking 40-50 gallon water heater and pipe that in with a 1" line. Lay the tank on a horizontal and the water in and out are through the drain and pressure fittings if they are on the same side and mounted on top. Draining through the inlet or outlet may not be the most effective though so it may be a one usage device. The sediment drain can go off some type of electric timer to drain into your sump pit as often as needed. Your pump and household demand doesn’t exceed 10-15 gpm very often does it?

I think the issue with the 6" & 8" pipe sizes are the time you have to get it put together after you started applying the cement. The first area is setting by the time the last area is put on. And this is before you even put the pieces together. How do you expect to diminish the turbidity in an 8" pipe that can’t be accomplished in a 15-20" diameter water heater tank?

I think a “sand separator” is what you are looking for. No need to do the engineering on your own; there are some made for home use. I’ll link to one example:

SandMaster Sand Separators

Note I was talking about both turbidity (cloudiness) and turbulence (vigorous random flow that prevents settling) at different points. It’s turbulence, not turbidity, that I want to reduce. I will have to put something like a low-percentage stainless steel perforated plate across the beginning of the large diameter. I figured I would cut a disk of this to closely fit the ID of the pipe socket and place it in there before sliding the pipe in.

Another possibility might be using a big filter housing that has a lid that comes entirely off, so I can get in there with a suitable baffle.

Yes, “sand separator” is another term for many of the same products. The delicate one I already have was sold as a “sand separator” on Amazon. That said, I think I was unwise so far to focus so much on Amazon as a vendor, and will check out your link and others. That may well be my answer. Thank you!

SandMaster is interesting! Looks like it does exactly what I need!

It does have to be placed between the check valve and the storage tank. I think like other cyclones it requires a minimum flow rate to function, which isn’t gonna work downstream of the tank when there’s nothing but a toilet slowly refilling. In my system the check and tank are coupled directly, so I think I’d need to add another check valve upstream and then the separator between the two check valves. Also, I don’t have enough room there or even very close to there, so I’d be extending by maybe ten feet the incoming pump line, which is currently only a couple feet long. However that should be fine. The line already runs about 200’ vertically and 30’ horizontally before it reaches the house.

The right size is about $500, which I can handle. I already have $500 invested in the filter cartridges and housings. I am not crazy about their steel construction, as I fear rust, but I guess it must be OK, right?

SandMaster is just one vendor – I am investigating others…