Connecting Into An Existing Water Main

There are some references regarding economical fluid speed inside pipes, I'm pretty sure you could locate some by browsing around. But, actually, if this is not a critical application (like if it was a domestic water supply or to replenish a reservoir for some reason), and with a water flow rate of only 30 l/min, you will be fine with even less... Its the flow rate and associated drag in fluid that matters, specially in long pipes.

Of course, for the above statement, I am assuming that its a straight pipe with no relevant difference in heigh. If there are curves, elevation changes and any constraint in the outlet (I mean, if you need a minimum pressure of 4.999 bar in the use point), please inform, in this case, people here will need to check the pressure drop along the pipe taking all this information in account. Each curve of section change really matters and make a huge difference in the pressure drop.

If you want to dig further, the following link gives you the manual indicating some equations from a software that is comonly used and that you could try.

http://www.eng-software.com/products/methodology/pipe_flo.pdf

I also found a on-line calculator that gave me the value of 0.82 bar of pressure drop in the mentioned conditions, which is for my perception an acceptable value.

http://www.pressure-drop.com/Online-Calculator/index.html

BUT, DO NOT FORGET: if there are curves, bends, and changes in elevation, please inform! All this will do make your available drop at use point vary a lot.

Look at this link, at 50mm dia pipe, for 4200m long, your drop will be only 0.88 bar

http://www.pressure-drop.com/

or as per this too it is about 0.65 bar for 4.2 Km length and dia 50 (2")

Q = A X V where Q is the discharge in m3/sec, A = area in m2 and V is the velocity in m/sec is simple equation used in conjunction with the pressure loss chart provided in any Mechanical Engineers handbook. Hope this will be useful to solve your problem.