# pressure loss when combining water lines **URL:** \ **Category:** Factual Questions\ **Created:** [May 11, 2016, 10:08pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577 "2016-05-11T22:08:18Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![sotexreb](https://avatars.discourse-cdn.com/v4/letter/s/bc8723/32.png) [@sotexreb](https://boards.straightdope.com/u/sotexreb)\ **Post date:** [May 11, 2016, 10:08pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/1 "2016-05-11T22:08:18Z")
I am trying to figure out how to calculate the pressure difference in a water pipe if: i have 2 1" lines that will be combined into a 2" water line to serve a 4 story hotel. I know the flow rates are different but i need mathematical proof to show the client the pressure loss in their system would force them into a break tank and booster pump situation. Anyone? I know there are formulas for pressure loss but how does one combine them like this? ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [May 11, 2016, 10:36pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/2 "2016-05-11T22:36:04Z")
Ordinary geometry [area of a circle = π \* (radius)²] should be enough. The cross-section of a 1" pipe is .785 square inches and for a 2" pipe, it’s 3.14 square inches. You’d need four 1" pipes to equal the cross-section area of a 2" pipe. ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [May 11, 2016, 10:53pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/3 "2016-05-11T22:53:59Z")
Pressure loss from friction is going to be dependent on the flow rate and the line size. Static pressure loss is only dependent on vertical elevation. Combining pipes will not generally cause a loss in pressure as long as the combined pipe has the same cross sectional area as the two pipes that meet. In fact, it usually results in less pressure loss. For example, in my Cameron Hydraulic Data handbook, a flow of 10 gpm through one-inch steel schedule 40 pipe will result in a frictional head loss of 6.81 feet per 100 feet of pipe length. If two one-inch pipes each carrying a 10 gpm flow combine in a two inch pipe the combined 20 gpm flow in a two-inch pipe will have a frictional head loss of only 0.878 feet per hundred feet. A two-inch pipe has more than twice the cross sectional area as a one-inch pipe and correspondingly lower velocity. Friction loss is proportional to the square of the velocity. If you want a mathematical proof you could trot out the Darcy–Weisbach equation, but honestly most engineers just whip out their Cameron’s and look it up. Of course, it’s not just straight pipe, you need isometric drawings. Every fitting or elbow has a friction loss. They’ve all been calculated for you in terms of equivalent pipe length. Isn’t that thoughtful? For example, in a one-inch pipe a standard 90 degree elbow has the same frictional loss as 2.62 feet of pipe, while a long radius elbow is equivalent to only 1.40 feet. I guess what I’m trying to say is you need a lot more information that that you’re combining two one-inch lines into a two-inch line to say you need a booster pump. Based on the data you’ve given you haven’t demonstrated that. ---
**Author:** ![MikeF](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MikeF](https://boards.straightdope.com/u/MikeF)\ **Post date:** [May 12, 2016, 6:18pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/4 "2016-05-12T18:18:54Z")
I believe this is covered in Volume 14 of the Kinsley Manual 😉 ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [May 12, 2016, 11:04pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/5 "2016-05-12T23:04:27Z")
> [@gotpasswords](#): > > Ordinary geometry [area of a circle = π \* (radius)²] should be enough. > > The cross-section of a 1" pipe is .785 square inches and for a 2" pipe, it’s 3.14 square inches. You’d need four 1" pipes to equal the cross-section area of a 2" pipe. Pipes are labeled in one dimension (1", 2", etc.) but exist in 2-dimensions (height x width). That’s why you need to figure the area, like gotpasswords said. ---
**Author:** ![Hairy\_Bob](https://avatars.discourse-cdn.com/v4/letter/h/ecc23a/32.png) [@Hairy\_Bob](https://boards.straightdope.com/u/Hairy_Bob)\ **Post date:** [May 13, 2016, 8:26pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/6 "2016-05-13T20:26:10Z")
> [@t-bonham@scc.net](#): > > Pipes are labeled in one dimension (1", 2", etc.) but exist in 2-dimensions (height x width). That’s why you need to figure the area, like gotpasswords said. Not to pick nits, but pipes have a length as well, which is also significant when calculating system requirements. Basically, what Bill Door said. ---
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [May 14, 2016, 12:23am UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/7 "2016-05-14T00:23:13Z")
> [@sotexreb](#): > > I have 2 1" lines that will be combined into a 2" water line to serve a 4 story hotel. What does this mean exactly? Do you have two 1" lines from the street and you want to use 2" near/in the building? Where is the transition? Depending on pipe length, your maximum flow may already be choked by the 1" lines although a larger pipe downstream would slightly reduce the flow loss from friction. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [May 14, 2016, 12:44am UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/8 "2016-05-14T00:44:34Z")
So how many 1" pipes will he need for the 6" fire riser? ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [May 14, 2016, 12:53am UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/9 "2016-05-14T00:53:37Z")
> [@Nefario](#): > > What does this mean exactly? Since there is no way that this is a real situation, it means it’s a homework question. ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [May 14, 2016, 5:06pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/10 "2016-05-14T17:06:45Z")
> [@Dag\_Otto](#): > > Since there is no way that this is a real situation, it means it’s a homework question. If it is, it’s a terrible homework problem. There’s no way enough information here to do anything with it. No flows, no line lengths, no fittings, no static head, no initial pressure. It’s like asking “How deep is a hole?”. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [May 14, 2016, 9:28pm UTC](https://boards.straightdope.com/t/pressure-loss-when-combining-water-lines/754577/11 "2016-05-14T21:28:47Z")
If he is switching 2 1" pipes for a single 2" pipe he won’t have any loss, he should gain a little from less friction over the length.