5 replies to this topic

[SR23]

I've recently conducted an experiment investigating losses through pipes and fittings with water as the fluid and I'm a little unsure how to proceed calculating the theoretical pressure drop through the globe and gate valves for my lab report.

 

I know that for bends you need to essentially sum the frictional losses along the pipe and the head loss from the bend using Darcy-Weisbach and H_l = k*u^2/2g.  

 

When calculating pressure loss across a valve is it the same procedure or do you not need to take into account frictional losses from the pipe? I think I'm getting a little confused by this as well from my experimental data as my results show the differential pressure across the valves increasing as the flowrate decreases - surely this is wrong? I thought that the differential pressure is proportional to flowrate and so shouldn't it be decreasing as flowrate decreases?

 

Any help would be much appreciated. 

Edited by kkg14, 13 December 2016 - 02:05 PM.

[shan]

It is possible lower water flow with larger differential pressure across a valve because of flow regime transferring from turbulent flow to laminar flow.

[SR23]

It is possible lower water flow with larger differential pressure across a valve because of flow regime transferring from turbulent flow to laminar flow.

 

Would you mind explaining why that would happen?

 

Also, would that still be possible with Reynolds numbers ranging between 8000 - 20000? These are all pretty much in the turbulent regime and my results show that as the flowrate decreases, the pressure drop increases steadily from 0.1 bar-0.4 bar in increments of 0.1. Not sure if my results are correct or not.  

[Pilesar]

The details of your experimental setup matter. If the flow is regulated by the valve, then reducing the opening of the valve to reduce flow would result in a higher pressure drop across the valve. The sum of the individual pressure drops in the system must equal the overall pressure drop. 

[SR23]

Yes, the flow was indeed regulated by the valves. Here's a picture of the experimental setup to clarify things as well. 

 

 water_rig.jpeg   79.71KB   1 downloads

 

Ah, so what you're saying is that the differential pressure measured from the valves is the total pressure drop experienced by the system from all the pipes and fittings?

[Pilesar]

Valve pressure drop is: (total pressure drop) minus (pipe and fitting pressure drop). You can often get good values for the source point (beginning) and sink point (ending) of your system. The pressure drops through the fixed diameter pipe and fittings can be calculated pretty reliably by standard equations. You can assign to the control valve whatever pressure drop is left over in your system calculations. In the real world, you change the flow rate by changing the opening of the valve. In your calculations, it is often easier to measure the flow rate and then back-calculate the pressure drop across the valve because you know that the total pressure drop must be made up by the sum of the individual pressure drops.