Calculating Pressure/Flow in after Tee fitting

[RohitGogna]

Hi,

I am very rusty on my fluid mechanics and need to implement the design shown in the image.

Our pump is pumping Warm water (50-60 Celsius) through a 4" pipe which then transitions to a 6" diameter Tee fitting. From this Tee fitting it splits to a 4" diameter re-circulation line and 8" line which provides water to some "shower heads". The pump will be on a timer switch so this system will only be running for about 30 minutes to an hour as required. Also note that all pipes are Stainless Steel, and that the GPM values in the picture shown were arbitrarily picked just for this example.

I would like the shower head to output 120 PSI and when the shower heads are not in use there will still be some flow running through the re-circulation line to keep the pump happy.

This used to be an existing system that recently failed and we are now making some modifications to provide the cheapest solution to fix it. (adding Tee, globe valve and timer switch)

So I have a few questions:

1) How do I determine what the Pressure would be in the 4" re-circulation pipe and the 8" pipe for the shower heads if I knew the flow in pipes?

The method that I have been using is assuming there would 130 PSI going to both the 8" diameter pipe and the 4" re-circulation pipe, determining the pressure loss/100 ft and subtracting that from 130 PSI. But this has resulted in me getting close to 0 Pressure at the end of the 8" piping and that I would need the globe valve to restrict close to 70 PSI of water, which I believe is very inaccurate.

2) Which length would i use to determine the pressure the loss? The total length of all the piping or just the length to the furthest location?

I'm fairly confident that we would only use the longest length but if someone could please confirm

3) Is there a industrial standard/rule of thumb for determining what the minimum safe flow for a centrifugal pump would be? (i.e. 25% of design flow?)

Thank you for your help in advance

Please let me know if you require any other information in order to help me.

 

[bimr]

1. The 6-Inch pipe with 700 gpm flow has a headloss of approximately 1.7 psi/100 feet. For 8-Inch pipe, the headloss would be 0.4 psi/100 feet.

2. Use the length of pipe to the farthest fitting.

3. Without knowing any additional details, 25& is appropriate for a rule of thumb.

You probably will not be able to obtain 900 gpm through a 4-Inch pipe.

 

[RohitGogna]

Thank you so much for your reply.

I just wanted to confirm though, is it correct to assume that there would be 130 PSI of water going through the recirculation line and the shower head line then?

 

[bimr]

With the information submitted, it is likely.

 

[LittleInch]

At any tee or junction the pressure is the same at the start of each branch.

I don't understand how you got to nearly 0 pressure at the end of the 8"pipe when it was so short.

When analysing a network it can get complex depending on how many flow paths are active at the same time, so the worst case might not be the lowest run.

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