Pipe Flow Design for Water Distribution
Pipe Flow Design for Water Distribution
(OP)
Hi Everyone,
I am designing a system where there is a tee in the system and two water pipes branch off of the main header. The specs at design flow rates would be..
Q1=100 GPM delta P1=15 psi
Q2=75 GPM delta P2=22 psi
So for this design off the tee, I would need to add a flow orifice or size the piping differently to achieve the design flowrates. If I kept it this way, wouldn't Q1 be higher than 100 GPM and Q2 be lower than 75 gpm since the presure drop would need to be the same to get these rates? I'm assuming pressure in the line before the tee is 22 psi.
Thanks
I am designing a system where there is a tee in the system and two water pipes branch off of the main header. The specs at design flow rates would be..
Q1=100 GPM delta P1=15 psi
Q2=75 GPM delta P2=22 psi
So for this design off the tee, I would need to add a flow orifice or size the piping differently to achieve the design flowrates. If I kept it this way, wouldn't Q1 be higher than 100 GPM and Q2 be lower than 75 gpm since the presure drop would need to be the same to get these rates? I'm assuming pressure in the line before the tee is 22 psi.
Thanks





RE: Pipe Flow Design for Water Distribution
Cv = gpm/sqrt(psid)
[ Yes, it's the same formula used for computing Cv for a valve. The _incredibly_stupid_ convention of defining Cv as "gpm at 1 psid" totally obscures the real units of the measure, because 1 is its own square root. ]
From there, the problem is just like sizing a resistor network to get a desired current ratio, except of course for the pesky square-law nature of the resistors.
Use a spreadsheet, and carry the units too, and you'll be okay.
Mike Halloran
Pembroke Pines, FL, USA
RE: Pipe Flow Design for Water Distribution
Thanks.
RE: Pipe Flow Design for Water Distribution
Which means the flows will balance such that the pressure drops are equal.
You may adjust the pressure drop in either line by changing its size, or the number of elbows, or by inserting a restrictor.
Mike Halloran
Pembroke Pines, FL, USA
RE: Pipe Flow Design for Water Distribution
Thanks.
RE: Pipe Flow Design for Water Distribution
If you actually designing a system to split the flows, you should use control valves. Control valves will allow you to maintain constant flows regardless of changes in pressure in the lines.
RE: Pipe Flow Design for Water Distribution
See ISA S75.01
Paul Ostand
www.ostand.com
RE: Pipe Flow Design for Water Distribution
That does not imply or guarantee that each pipe will achieve the stated flow when they are connected together; it's just a way of stating their relative size.
Mike Halloran
Pembroke Pines, FL, USA
RE: Pipe Flow Design for Water Distribution
You can only have one pressure in the tee where the divergence occurs. This is your 22 psi. You stated that both lines discharge to atmosphere. This means the pressures at the discharge points for the two lines are the same. If the two flows come from the same pressure (22 psi), and discharge to the same pressure (0 psig), then the two lines have the same pressure drop.
If the lines, as constructed, will not give the desired flows with this fixed pressure drop then you have to add in valves or orifices to dissipate the additional pressure drop, or you must accept that the flows will be higher than specified (but this may mean upstream problems and changed pressure drops there).
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pipe Flow Design for Water Distribution
So the line with the less pressure drop (15psi) will get more than the design 100 GPM flow, correct? And the line with the design pressure drop (22 psi) will get the design flow (75 GPM). This is assuming that 22 psi is available right before the tee. This condition would not hurt the system, probably help it.
Thanks.
RE: Pipe Flow Design for Water Distribution
Mike Halloran
Pembroke Pines, FL, USA
RE: Pipe Flow Design for Water Distribution
Thanks.