6 replies to this topic

[Brightonnk]

Hi all, I have just been thinking about roughness factors and can't find much information on the internet so was hoping someone could help me here.

 

I have a 3" valve where the body and insert material is carbon steel and the ball and seat metrial is stainlesss steel. I can clearly feel that there is a difference in surface finish but can't find on the internet actual units of finish. I know that a clean commercial steel pipe is about 50 microns... could i take this value for carbon steel and stainless steel too? or is there a table of roughness factors?

 

Thanks

[breizh]

Use the search function in this forum and you will get your answer.

Breizh

[katmar]

What do you want to use this information for?

[Brightonnk]

Hi,

 

Breizh, I have found this information on another post -thanks

 

Katmar, i want to see if there is a difference in absolute roughness and if there is if it effects my pressure drop across the valve.

[katmar]

If you are calculating the pressure drop through a full bore ball valve then there is some justification in wanting to know the surface roughness because the overall pressure drop will be so low that the friction against the wall does become a significant fraction of the overall pressure drop.  But the pressure drop through a full bore ball valve is so low that it is hardly worth worrying about. Full bore ball valves typically have an equivalent length of 3 or 4 diameters, so you could treat the valve as just a piece of pipe the same length.

 

With reduced bore ball valves or globe valves the friction effects are small and are usually neglected.

[Brightonnk]

The valves I am considering are reduced bore valves and did think that the friction factor would be very small -So thank you for clearing that up.

 

Is it correct to assume that the pressure drop across a full bore ball valve would roughly be similar to pressure drop across that of a straight pipe (taking into account the diameter/length/roughness were the same)?

 

Thanks

Neera

[markymaark]

"Is it correct to assume that the pressure drop across a full bore ball valve would roughly be similar to pressure drop across that of a straight pipe (taking into account the diameter/length/roughness were the same)?"

 

Yes, that is what post #5 was saying.  You can relate every, turn, elbow, valve, fitting etc into an "equivalent length".  This makes pressure drop calculations much easier.  For example, a 100 ft length of pipe, with 3 elbows, 2 ball valves, and a check valve may have an equivalent length of 135 ft.

 

Mark