How much water can flow down a 2" pipe?

Discussion in 'Shower & Bathtub Forum & Blog' started by johnfrwhipple, Mar 28, 2010.

  1. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,406
    Location:
    North Vancouver, BC
    I would like to know the maximum flow rate of a 2" line. I have 3/4" supply lines at 70 PSI. The water has to travel through a 2" P Trap and 2" drain line 36" and drop into a 4"x4"x2" wye. Why is so hard to find this information out?

    Is there any resource online where I can look up the formula for converting pipe size and pressure into maximum flow rate in Gallons per minute or Litres per second.

    I have a luxury en-suite renovations I'm working on and want to do some precise calculations to insure the system works as planned. If I switch to a 3" drain line - what do they handle?

    I can work out the math, I just need the variables.

    Can it be worked out simply by taking the smallest fitting and using the water pressure?

    So many questions, I hope there's answers. Maybe I'll rig something up in my basement - my poor wife will love that!
  2. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    According to the UPC a 2" trap and drain can handle a 30 GPM intermitant flow.

    Do you have an open 3/4 pipe discharching water or is it passing through a fixture such as a shower valve?

    If you are using a valve then the paperwork should give you the flow rate in gpm.

    If you are discharching through an open pipe then turn it on and see how many 5 gallon buckets you can fill in 1 min. A practical test would most likely give you better results in that situation anyway.

    A 3" trap and drain can handle up to 50 gpm intermitant flow, according to the UPC
  3. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,406
    Location:
    North Vancouver, BC
    Thank you for your information. I tested the flow rate myself last night at my home.

    I have the Quick Drain USA under flood test as I type. I built a temporary box for it and ran a water flow rate test and then flooded the drain with a plug in the line to see if it leaks.

    I was there for an hour last night and the water level did not move off the mark. I suspect it's bullet proof like I first thought.

    I managed to run 1.15 Litres of water per second down the drain - both with and without the drain cover installed. I could not get more volume than that and my neighbor was not home to borrow his hose.


    Water Flow Rate Results

    Location: North Vancouver
    City pressure (3/4 Supply, poly B to copper): 104 PSI
    Fittings on line: From city connection we have 2-3/4 1/4 ball valves, 1 coupling, 1 tee - all 3/4" the 3/4" 1/4 turn ball valves look like the smallest fitting.
    Volume of water from this line: 1.15 litres per second
    According to the UPC a 2" trap and drain can handle a 30 GPM intermitant flow (30 gallon [US, liquid] per minute = 113.562 353 52 litres per minute or 1.8927 Litres per second
    Drain type: Quick Drain USA
    P Trap size: 2" solid
    Waste line size: 2"

    In order to push the envelope with my testing I will need to run more water into my lab! The drain was handling the 1.15 Litres per second ease. Will try again with more water...
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,825
    Location:
    Cave Creek, Arizona
    Your incoming water volume will be limited by the valve and any devices connected to it, regardless of how much water is available from the piping. There are too many variables to calculate the drain capacity, because the drain seldom is full of water. Usually there is a substantial amount of air aspirated into the drain water and that is what fills the pipe and limits the capacity. Downspouts have the same problem and "anti cavitation" roof drains restrict the amount of air so the pipes can handle more water. But there are no "anti cavitation" shower drains because they would create a tripping hazard.
  5. Basement_Lurker

    Basement_Lurker One who lurks

    Messages:
    668
    Location:
    Victoria, BC
    Have you consulted pages 597 and onward of the BC plumbing code?
  6. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Your drain pipe would not be pressurized by a broken pipe running water in the shower or whatever. It is just a gravity situation
  7. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,406
    Location:
    North Vancouver, BC
    Jimbo - how does a drain pipe have pressure? If it is all vented I thought there was no pressure in a pipe.

    Sanjeev I'll look up your reference, I'll have to borrow my plumbers code book.

    Thanks men.
  8. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    So my question is this: After algore recovers from the stroke he is going to have when he sees this shower thing you are building, are you willing to stand in ankle deep water in the shower with the 2" drain that gets a little hair buildup over the grid, and a little scum buildup in the trap? We have two basic facts: most of our building codes allow a 2" drain for a shower, and you are building a "monster" shower from the sounds of it, so you get to decide how you want to drain it.
  9. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,406
    Location:
    North Vancouver, BC
    "The Monster Shower"

    Every day this shower is a simple design, one handheld and one rain head.

    Options are the key. This shower will have a steamer and two benches as well. The choice of body jets or water fall! Both?

    My client is from across the pond and Swiss Shower's are a common place luxury that we are able to re create in his home thanks to the luxury of having 2 supply lines of both hot and cold.

    To debate with Al Gore over this point would take some time because it's hard to just call out waste when the client can walk to the store for groceries, commute from his door step and lives in a multi family hi rise with a shared and far more efficient hot water supply.

    All of the fixtures have water restrictors and not all will be on at once. I'm planning for worst case scenerio (House Guest) and will insure the system can't over flow. Off course with increased volume the risk of a massive overflow is greater with a larger system. The Quick Drain USA drain is easy to clean and routinely snaking the line for hair is a wise idea and a service schedule will be drawn up and recommended by me.

    If the drain is incapable of handling the flow we will add restrictors or replace the drain.

    As for standing in ankle deep water I don't think this will happen. In my flow test the drain did filled with a few inches of water but that water was directed into a 30" box 5.5" wide. With the channel drain I'm calculating that the shower will drain cleaning and no standing water should be present. That said this is our first shower with a trench drain and this number of spray heads. I think ankle deep water would only be present if the drain was getting restricted by hair and debris or if all fixtures where on (which won't happen).
  10. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    You hadn't mentioned the channel drain....and that is much preferred over a basic round drain in an application like yours. Still, why not just do 3" and never worry ??
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