3 replies to this topic

[h2no]

Hi everyone,

 

I have a DN50 process water line with a hand valve controlling the amount of flow (usually set to about 2.5m³/h).

I've checked the supplier of the hand valve's data, it has a kv value of 91.7 when fully opened.

This would mean (if I understand correctly) that a flow of 213m³/h is possible when this valve is fully opened.

The pump supplying the water can give up to 600m³/h if necessary at 5.4 bar(g).

 

To protect connected vessels we need to limit the amount of water that could flow through this line to a maximum of 40m³/h.

 

A restriction orifice would be an easy fix but the manufacturer told us the resulting pressure drop over the orifice is large enough to cause cavitation.

he suggested installing multiple orifices to create a step-wise pressure drop.

 

This made me wonder why we haven't noticed any cavitation when using the hand valve to limit the flow...

 

Any input on all of the above would be very welcome

 

[MTumack]

What Temperature is the fluid?

 

Unless this is a steam application, I find it very unlikely that a room temperature water line would see any flashing through the valve until you start to see quite excessive temperatures @ 5.4 barg. You'd be looking at temperatures of around the 150°C / 300°F in order to see any flashing across the valve, and you wont see any cavitation without any partial flashing which would cause bubbles of steam to form which would implode and cause high impact stresses on the outlet side of the Control Valve.

 

In a standard utility line service I don't think you'd have an issue, my guess is since you have a hand valve there you are looking at line temperatures from 10 to 30 °C.

 

Check the reference attached.

 

Attached Files

•  fluid flow Cashco_Fluid - CV SIZING.pdf   763.73KB   24 downloads

[h2no]

Hi MTumack,

 

Thanks for the reply.

The temperature of the water is around 25-30°C.

According to the manufacturer a flow of 40m³/h is too high for DN50, minimum line size would be DN80.

Am I overestimating the amount of water that can flow through this valve/pipe?

213m³/h does seem rather high (water velocity of 30m/s...) but I didn't expect 40m³/h to be an issue.

Edited by h2no, 29 September 2015 - 02:14 AM.

[MTumack]

h2no,

 

You can have that much flow through the pipe, the question is more how long you want the pipe to last.

 

I'll assume you have Sch 80 pipe for mechanical strength (and cost, Sch 80 is likely cheaper than Sch 40), therefore with ~40 cu.m/h you are probably looking at a velocity around 20 ft/s (guesstimating here based on experience), depending on how you want to calculate erosion velocity, that likely fails this check. Although, depending on how progressive you are, what material choices you've made, and how clean your process is one could make an argument to use a C value of 200 to 250 or so, which would have you pass this check.

 

I will guess the pipe will probably be quite loud, but again that is user specific... some guys want to be able to have their kids sleep next to the line undisturbed, some guys don't care if it literally howls.

 

I'd say my general experience is that specifications call for around 8 to 12 ft/s for these type of water lines. (Which puts you in that 3" NPS your manufacturer recommends.) Some guys don't care though, and I have seen some specify up to 30 ft/s line velocities.

 

So I guess I ask you, do you want to flow 40 cu.m/hr through the line? Why / Why not? What is the maintenance cost of leaving as is? What is the cost of upsizing?

Edited by MTumack, 29 September 2015 - 05:20 PM.