Fluid Pressure in Pipe 1

[ThermalMan]

I have 1,075 gpm of water flowing through an 8" pipe at a velocity of 7.05 ft/s and a pressure of 75 psi. I need to branch off that 8" line to divert enough flow to reduce the pressure from 75 psi to 65 psi. How do I calculate the amount of flow to divert to reduce the pressure in the main line? Any help would be appreciated.

 

[imok2]

tpmartin185I, believe this is the formula you need:


New flow rate= measured pressure/rated pressure x 0.5 x rated flow. divert 65psi/75psi x 0.5 x 1075 GPM = 465 GPM

Please check with others to follow

 

[TD2K]

You need to look at the entire system this 8" line is installed in to answer the question, there's no way to give you an answer with the information you have provided. There are just too many variations:

Let's say you had a centrifugal pump feeding this line with 1075 gpm and 75 psig being its operating point. If you wanted to drop the pressure to 65 psi by 'bleeding' off water, you'd need to look at the pump curve to see what flow you needed to put through the pump to bring the discharge pressure down to 65 psi from 75 psi. The difference between that flow and 1075 gpm would be the flow through your new side line. If the curve is steep, the additional flow might be small, if the curve is quite flat, you could be talking a lot more flow.

Let's say on the other hand this 8" water line comes off a 24" distribution header. Practically speaking, you might not be able to draw off each additional water to drop the pressure to 65 psi in this system.

Let's say the 8" line is actually a long pipeline with the delivery pressure being 75 psi when flowing 1075 gpm. Depending on the length of line and supply pressure device (pump, etc), you might not need to flow very much more water to drop the pressure simply due to additional friction losses.

We need a more complete description of your system to give you an idea how to calculate the necessary flow.

 

[Latexman]

Put in an orifice plate that'll give you 10 psi permanent pressure drop at the flow conditions given. My handy dandy orifice plate spreadsheet says an orifice diameter of 3.983" is needed for 10 psi pressure drop for 1075 gpm of water in a 8" sch. 40 line. Now you have all 1075 gpm at 65 psi! No need to divert any.

Good luck,
Latexman

 

[ThermalMan]

Thanks for the help! Here is some additional information. This is an 8" pipe, closed loop system with a centrifugal pump. At the end of the loop, there is a piece of process equipment that can not see more than 75 psi or it will shut down. With minor and friction losses, the equipment normally will see 55 psi. Theoretically, the equipment will never see 75 psi, but just in case, I want to put in a bypass with a valve that will open and divert flow at a set pressure (65 psi), thus relieving pressure in the main 8" line. In order to size the valve properly and economically, I need to know the minimum amount of flow to bypass in order to relive the pressure 10 psi.

Thanks.

 

[vinquest]

tpmartin185,
If you already had the intention for a bypass, why not consider a pressure regulating valve or an adjustable choke valve for the by-pass line, if your centrifugal pump cannot be replaced.
This will allow you a great extend of flexibility to regulate the line pressure in relation to your downstream process.
Otherwise, you can also trim down on the pump impeller to the required pumping rate/dp. Check with the pump specialist.

 

[Latexman]

What's the dead head (no flow) pressure at the piece of equipment given your fluid and system? If that's under 75 psi, I wouldn't do anything. If it is close enough to 75 psi to worry you, or higher, trimming the impeller until it isn't an issue is a great idea if it'll still provide the needed flow. If that's out for some reason, you could put a variable frequency drive (VFD) on the pump motor and cut back on the pump speed (and flow and head and . . . ) based on the pressure in the equipment, in the line, or at the discharge of the pump. It depends on what you have, what you want, and what's close enough. I'd try to set it up so it does modulate during the normal variations of the process, just to keep all the parts moving and not have a static failsafe system which is more problematic when it is needed, unless it is inspected and tested frequently. Diverting the flow will work. It's not the energy conservation-wise way to do it. I'd design it to handle the total flow with a control valve in it with a pressure control loop. Again, set it up so it modulates during the normal variations of the process.


Good luck,
Latexman