Quick check please!

[JH2015]

Hi

I've got some measurements that don't add up according to my calcs. Please would someone be kind enough to check and see if I'm doing something obviously wrong, or there's a problem with my measurements?

All the info is in the attached sketch. I've measured 1.2-1.4 barg at the T, and am getting 1.2 l/s out of the pipe. According to my calcs I should be getting more like 2 l/s.

1.2 l/s would be produced by 0.5-0.6 bar, not 1.2-1.4.

Pipe is blue plastic water pipe, 25 mm (20.1 mm ID). It's curved, which I've modeled as 2 x 45 deg bends.

Many thanks for the help.

 

[LittleInch]

Can we confirm units here. Is the distance on your sketch mm or m?

Need to look at the internal diameter / arrangement of that "tap" - there could be a lot of losses there are also the exit losses from the end discharge.

Even at 1.2l/sec that's nearly 4m/sec which is moving.

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

 

[JH2015]

Hi. Thanks for the reply.

Sorry, all lengths/distances are mm.

Since posting someone else has also mentioned the tap. I had assumed it would open to full bore and have therefore modeled it in friction loss terms as an open gate valve. Maybe that's the issue though?

The tap looks like this one:

https://www.fanevalleystores.com/product/250264/plasson-3407-plastic-25mm-stop-tap

Many thanks again

 

[bimr]

There is significant headloss across the tap. Here is a cross-section of the typical stop tap valve:

Attached is an example of the head loss. It shows 1 meter of headloss at 0.42 l/s (1.25 m/s).

 

[LittleInch]

Is the valve a quarter turn valve or a screw action much like a normal sink tap or stop cock?

Even a quarter turn ball valve will often have a reduced bore for a plastic valve.

At those sorts of pressures and flows I suspect that is where a lot of your extra unaccounted losses are coming from.

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

 

[bimr]

No, it is not a quarter turn.

http://www.plasson.fr/sites/default/files/fichiers/Plasson_Specifications_VANNE PP_2014.pdf

http://www.plasson.fr/sites/default/files/fiches/reference/robinet-a-soupape-droite-340725.pdf

 

[JH2015]

I feel stupid now - I could have looked up that cross section myself couldn't I? Thank you for pointing out the obvious. It is indeed one of these types rather than a quarter turn.

Looking at the section it is clearly nowhere near full bore when open.

I can use my measured results to calculate the loss coefficient for it.

Thank you very much for the replies.

 

[LittleInch]

Maybe you could.

The key thing to remember its ok making assumptions, but when your results don't work, go back and see if the assumptions were correct.

Some of the metal stop cocks are gate valves, some are like this. If you want more flow swap out the valve for one with a smoother flow path.

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

 

[HPost]

It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.

Richard P Feynman