Cv conversion to other units 1

[foxtrot]

I'm looking for converion factors or approximations that will allow me to convert between Cv, Normalized liters/min and any other units used to specify flow through a valve

 

[katmar]

Cooperjer,

You would use the ID of the pipe. The reason for wanting to convert from a Cv to a K value is that you can then multiply the K value by the velocity head to get the pressure drop. So the calculated K value and the velocity must apply to the same diameter.

 

[Montemayor]

Harvey:

Thank you for the referenced Thread #. You are always a great help. Our mutual experience with Crane’s Tech Paper runs parallel. I felt indebted to this resource from Crane as I was developing my career from 1960 – onwards. However, with the slow demise of the fittings business, very little development or improvements were done to this great product during the past 20 -25 years. Recently, it has largely been left to individuals like Hooper, Darby, and others to continue the much-needed refinements and development work in fluid mechanics – especially in the measurement and calculation of fluid flow. There is still a lot to be done in this area and the improvements seem to be coming in at a dribble.

Your experience with the velocity head factor (a.k.a. “K”) doesn’t surprise me. When I was working Dupont projects back in the late 1980’s, we knew the K factors for gradual contractions were overestimated by up to 250%. We also knew that the general feeling that the K value should be virtually the same for different sizes of valves of the same type did not result as exactly true in the field. So, as a result we resorted to continue using Equivalent lengths (L/D) – especially in evaluating relief valve piping, since these gave consistent, conservative results.

Although I’ve had more experience and time to employ the pondering of how the engineering of fluid flow problems is done today and how we got here, I often get depressed when I consider that very little (in my opinion) has been done in original, developmental work in this very important area. Ever since Julius Weisbach’s proposed equation in 1845, Darcy’s improvements in 1854, Nikuradse’s work in 1933, the formulation Colebrook-White in 1939, and the data compilation of Moody (actually, Hunter Rouse’s) in 1944, it wasn’t until the 1950’s that the present use of the Darcy-Weisbach equation came into popular employment by engineers. Up to that time, engineering of fluid flow problems was largely done with empirical equations other than the Darcy-Weisbach model – like the Hazen-Williams equation and others. In the last 50 years, the only significant improvements that I’ve seen are:

1) The development of explicit formulas or - versions of the Colebrook Equation – to better resolve the friction factor in an analytical manner;
2) The recognition and continuing study of 2-phase flow phenomena and the rigorous calculation of pressure drops in this medium;
3) The development of Hooper’s 2-K system, followed by Darby’s 3-K method.

And even the Colebrook-White equation has inherent errors – some claim 15 -25% (Egad!). But yet, to this day I still detect a failure (or reluctance) on the part of authors and professors to attack the subject of inaccuracies in the products produced by the various equations and methods employed in fluid flow calculations. It’s as if the subject were a taboo or akin to mentioning “sex” during a church service. It just isn’t done. And this is very unfortunate because it leaves students (and us old engineers) with a bad taste in their mouths and wondering: just what, exactly, do those computer print-outs really represent? I sincerely hope this situation changes fast in the near future – our energy supplies continue to diminish and our need for energy efficiency (vis-à-vis low pressure drops) also becomes acute.

Regarding the Cv to K conversion: The square of Crane’s 29.9 is 894.01. And since at my age I can relate to their earlier printings, I know the number 891 was generated with what we had available then – a slip stick (“slide rule”) – and it was accurate enough then. The point I failed to make was that this is, to me, a clear reminder of the level of fluid flow improvements made by Crane in the past decades. We mustn’t rely on Crane to deliver us better flow calculation products. I’m afraid we have to come up with other new, or pioneering, resources to improve the present level of fluid mechanics and fluid flow information and tools for engineering calculations. And in my “golden” years, I don’t know where to expect this to come from.

Yours in Engineering,
Art Montemayor

 

[hahor]

German Kv [m3/h] = 0.8566 * cv [gpm]

Kv ~ 5 * A[cm2] * ALPHA

ALPHA = flow factor according to DIN 1952


Kind Regards,
Hahor