Flowrate in a vertical straight up-and-down pipe 2

[Tatyana851]

Hello. Does anybody know how to determine the direction of the flow in a straight pipe with 1 globe valve knowing its diameter, length, and pressure (at each end is the same)?

Thank you.

 

[bimr]

Flow moves from high pressure to low pressure in accordance with Bernoulli law.

 

[Tatyana851]

bimr, that is right that the flow behaves according to the Bernoulli's Law. However, in my case the pressure gauge is the same at each end of the pipe (30ft long pipe, no pressure drop), and the whole system is not described (whether there is a shallow tank on top and a free discharge on the bottom or something more complicated). The length, diameter , and pressure information are given.

I have been trying to apply different assumption to understand why the pressure drop is equal to 0? Might it be a pump?

 

[danw2]

30 feet of water column has a head pressure of 13 psig.

At no flow, the lower gauge will read 13 psig greater than the top gauge.

During flow from top to bottom when the total pressure drop equals 13 psid across the 30 feet the top gauge will read the same value as the lower gauge.

 

[Tatyana851]

What if I place a glove valve mid way, and read the same pressure gauges. How will the pressure drop change?

Thank you.

 

[bimr]

When you have a pressure pipe situation, the pressure will read the same in all sections of pipe, as you are reading dynamic head. Dynamic head is the total equivalent height that a fluid is to be pumped. This particularly true for small systems with low flow. Note that the differential head loss across items such as valves starts at zero with low flow and increases with flow velocity.

This is not true for gravity pipes such as danw2 is describing, where one end is open to the atmosphere. There, you are measuring static head, not dynamic head. Normally, there should be some head loss across a globe valve. This can be measured by installing pressure gauges on each side of the valve.

For a situation that you are describing (probably a low flow scenario), the only method to determine flow is with a flow meter.

 

[Tatyana851]

Thank you guys

 

[Kuhuh]

I am confused, Wasn't your original question to determine the direction of flow? I would just start to close the globe valve. If its pressurized system the pressure on the upstream side will increase. If it gravity feed then upstream side could remain the same but down stream side will equalize to atmospheric.