3" Main for Car Wash?

Molo · Jan 10, 2009

An 80 year old lifetime plumber told me he installed 3" line rather than 4" because the 3" line would not allow the sand and particles to accumulate along the bottom of the pipe.
Is this a sound theory?

By the way, I'm not taking a test on drain lines, just wondering a bit.

Probedude · Jan 10, 2009

molo said:
An 80 year old lifetime plumber told me he installed 3" line rather than 4" because the 3" line would not allow the sand and particles to accumulate along the bottom of the pipe.
Is this a sound theory?

By the way, I'm not taking a test on drain lines, just wondering a bit.

Yes, too large of a pipe and the velocity of the waste moving through it slows down (so I've read)

Sponsor · 2022-01-29T12:20:58-0800

Redwood · Jan 10, 2009

Sand shouldn't get past the separator.

http://www.watts.com/pdf/ES-WD-SA-SERIES.PDF

Molo · Jan 10, 2009

Redwood said:
Sand shouldn't get past the separator.

http://www.watts.com/pdf/ES-WD-SA-SERIES.PDF

A separator, sounds like the clay traps that they have in the sinks at the ceramics school.

Is is accepted theory that using a smaller diameter allows for less settling on the bottom of the pipe?

Redwood · Jan 10, 2009

molo said:
A separator, sounds like the clay traps that they have in the sinks at the ceramics school.

Is is accepted theory that using a smaller diameter allows for less settling on the bottom of the pipe?

Yes it is, and it is a requirement on car washes garages etc. here.

The smaller dia will have more scouring but...
Somewhere it will settle and that is why sediment separators are required.

SewerRatz · Jan 10, 2009

Here they want 4" cast underground. Even though 3" could handle it since most car washes have a large sump pit that holds the waste water to be reused to wash your car with, only fresh water is the final rinse. So this sump does have an overflow in it which leads to separator then drains out.

hj · Jan 11, 2009

drain line

The approved plans should have shown the pipe sizes AND the required sand trap/separator. The 3" line will have more velocity, UNTIL it is full of water, then the friction loss will probably negate any supposed benefits. IT will also be more difficult to clear when it does fill with sediment. I have never seen, or done, one with anything less than 4" pipe.

SewerRatz · Jan 11, 2009

Only once in my almost 20 years doing this work I seen 3" under ground. Was at a Venture store. I had to replace the under-sized grease trap. When we dug it there was 3" cast iron everywhere. Way they did it was ran 3" pipe for all the indirect drains shallow and the outlet of the old trap was 3" that tied into a 4" line that was 5 foot deep. The inspector told me I can use 3" pipe if the new proper sized trap had 3" inlets and outlets. Otherwise it would of all had to been ripped out and replaced with 4" piping.

3" Main for Car Wash?

Molo

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Redwood

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Molo

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hj

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