Combining Water Streams

[Docseven]

Hello, I am getting tripped up on this application.

An outlet of a water filtration system is currently flowing with a pressure gauge on the outlet reading 20 PSIG, at 190 GPM, through additional piping (I have geometry and can work out friction factor impact as applicable) and empties at the top of a 38' (feet) higher vertical standpipe inside a storage tank.

If another stream is added to this by connecting a second filtration in parallel, but this one with output @ municipal supply pressure and 110 GPM, how would I calculate the combined pressure which would be read on that same local gauge?


I believe that because it will still be emptying at the same outlet inside the tank, just need to calculate the friction losses for the additional velocity through the pipe to the tank with the additional flow. In what circumstance could the additional stream cause the initial stream flow rate to decrease, stop, or worst case reverse?

 

[MikeHalloran]

The output flow from one system can affect the output flow from a connected system if they are driven by gravity, or by a non-positive displacement pump. ... i.e., probably.

If you want the discharges to not interact with each other, give each its own standpipe.

I'm impressed by the confidence you show in the respective output flows. Are the systems (with which I am not familiar) driven by positive displacement pumps at a fixed flow rate, or do you have flowmeters installed, or, as I suspect, you have been provided one flow and one pressure for each, and the actual operating point is dependent on the downstream system's load curve?



Mike Halloran
Pembroke Pines, FL, USA

 

[Docseven]

The current flow of 190 gpm @ 20 psig before going to the standpipe is measured with a gauge and flow meter for the treatment plant.

The flow through the additional filtration system is coming into that system at 75 PSIG off a header maintained at that pressure. There is a flow meter on the additional filtration system, and the inlet valve will be throttled open until it is reading 110 GPM. Pressure will not be at supply pressure at that time like I originally said (currently, with only part of the additional filtration system in place and discharging to a standpipe in another tank, pressure @ outlet is 12 PSIG downstream of all valves).

 

[LittleInch]

I think your original belief is correct.

To calculate the increased pressure at the junction between your two supplies you need to add the static head (38 feet) to your now increased frictional head losses 9 probably at least double what you know have. A good initial estimate is that pressured drop in the same pipe with the same fluid is proportional to the square of the velocity / flow. So if you current pressure drop is say 10psi at 190gpm , then this would increase to around 25 psi at a new combined flow of 300 gpm.

Then you need to work out the flowrate from your initial system to see if it can still do 190gpm @ 25psi instead of 10 psi.

If it can then great, but likely to be less. So now you need to iterate and take some middle figure.

If the new flowrate through the old machine is only say 150 gpm, the work out what a combined flow of 280 gives you. Now you should get more through your old system and gradually narrow the gap between combined flow. This works because your new system clearly has lots of pressure available so can maintain 110 gpm at a wide range of output pressures due to your control valve.

Does that make sense? Put your own data in my explanation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

The inlet pipe into the standpipe should be oversized so your proposal may work.

Add the flows together (total of 300 GPM) and determine the headloss from the junction of the combined streams to the inlet pipe exit into the standpipe. This headloss would have to be less than 3.5 psi for you system to work.

38 feet of water in the tank is equivalent to 16.5 psi. So you only have a small margin of head to work with.

The pressure measured just after the pipe junction will be the backpressure at the tank (38 feet of water head) plus the headloss of the combined flow from the pipe junction of the combined streams to the inlet pipe exit into the standpipe.

http://www.cheresources.com/invision/topic/13399-mixing-of-two-pressure-streams/