Cv conversion to other units
Cv conversion to other units
(OP)
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
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Cv conversion to other units
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RE: Cv conversion to other units
From that description, I'd think you could convert to any units you like (eg, liters per hour per inches of mercury, cubic centimeters per second per atmospheres, etc).
RE: Cv conversion to other units
The units of Cv are gpm/sqrt(psi).
It's unfortunate that the usual statement of Cv's definition does not make that clear.
One complementary common metric equivalent that you might actually run into is
mbar/(m^3/hr)^2
Mike Halloran
Pembroke Pines, FL, USA
RE: Cv conversion to other units
1. "American" Cv - uses US gpm and psi
2. "English" Cv - uses Imperial gpm and psi
3. "European" Kv - uses m3/h and bar
I have included these three in my units conversion program Uconeer which can be downloaded for free from my web site at www.katmarsoftware.com
If there are others that are actually used in industry please let me know and I will add them to Uconeer.
RE: Cv conversion to other units
Katmar, I downloaded your conversion program--thanks for making it available.
Another question: what is the differance between Liters/minute and Normalized liters/minute?--I see that in Europe both seem to be used rather than Cv for pneumatic valves (along with orfice sizes and effective sectional area in the orient)--is there any way to tie all of these together so that it's a bit easier to make comparisons?
RE: Cv conversion to other units
Maybe you should have noted that Kv is not defined in the same way as Cv. Kv is a dimensionless loss coefficient.
Best regards
Morten
RE: Cv conversion to other units
In building engineering services work in the UK, Kv is based on m3/hour and bar. The dimensionless coefficient to which you refer is that which tells us how many velocity pressures we lose in a fitting; this is not the same as Kv. Maybe it's different in the petroleum industry?
Regards,
Brian
RE: Cv conversion to other units
The Kv that I listed, and which is included in Uconeer, is exactly analogous to Cv. The formula is identical, except for the units as noted above. In both cases the liquid density is given as SG relative to water, so that does not change. It is in common use in Germany.
The confusing issue is that the symbol "K" is also used in pressure drop calculations of fittings (including valves) and as you note this is a dimensionless factor that relates the number of velocity heads to the fitting.
For valves of the same type, but different sizes, the K values will be virtually the same. However the Kv (and Cv) of the larger valve will be greater than the Kv of the small valve. It is easy to convert between K and Kv (or Cv) and I'm fairly sure Crane 410 discusses it (unfortunately I do not have my copy with me right now).
regards
katmar
RE: Cv conversion to other units
Because gases expand and contract with changes in pressure and temperature the convention has arisen to refer the volumes back to reference conditions. The reference conditions are called either "standard" or "normal" conditions. Unfortunately there is nothing standard or normal about these conditions and there are literally dozens of conflicting definitions for both terms.
If you do an Advanced Search in Eng-Tips for the terms "standard conditions", "Normal conditions", SCFM etc you will find lots of discussion on this. Another excellent resource on the subject is Milton Beychok's site (Milton is one of the "elder statesmen" on this forum). See http://www.air-dispersion.com/formulas.html
This means that your normalised liters are refered back to some defined state. "Normal" usually refers to 0 deg C and either 1 atm or 1 bar, but you should check with the source of the data.
Uconeer includes a conversion utility for this (Use the fan icon on the toolbar). Note that to convert from one volumetric definition to another you go via the mass based quantity - thanks goodness the mass of a gas is unaffected by temperature and pressure!
regards
katmar
RE: Cv conversion to other units
You are rigt - its K not Kv. Just never use Kv really (only Cv or K).
Best regards
Morten
RE: Cv conversion to other units
I've spent many unpleasent hours trying to convert constants from one set of units to another with inconsistent results.
Where I've ended up is that I convert all input units to what the authors of the empirical equations specify and then convert the result to the units that I want. It is kind of a pain, but I end up not having to try to figure out why MathCad won't let me take the log of psi or losing the fact that I took a fifth root of inches and multiplied it times ft2.5.
Some empirical equations are simple enough that simply changing the constant will work. Others are more subtle. I would suggest using my technique to verify the results you get from messing with the constants before you base an important decision on your arithmetic.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Cv conversion to other units
I agree completely with your sentiments. I have also wasted many hours converting formulas. But there are a few bad ones that I continue to use. Two of my favorites :-
My rule of thumb for liquid piping : Take the flowrate in liters per second, multiply by 1.5 and then take the square root to give the pipe size in inches. I think I originally found it in gpm, and did the conversion to l/s - but that is lost in the mists of time.
Go-anywhere steam tables : Take the pressure in bar absolute and take the fourth root (i.e. press square root twice on your calculator), multiply by 100 to give the steam temperature im degrees C. Example - 100 PSIG = 7.9 bar abs gives 167.7 C. Steam tables give 169.9 C. Good enough for in-plant estimates.
katmar
RE: Cv conversion to other units
Steam is pretty safe to do your empirical equation since you don't often have useable steam at 0C.
My favorite doesn't translate at all. To calculate the volume of a 1000 ft of pipe (in 42 galon barrels) you only have to square the ID (in inches) of the pipe. The result is about 3-4% higher than you get by actually calculating the volume (since I use this for calculating hydrotest volumes the extra works out well for trucks that may not be as full as advertised). This is much more of a coincidince than a physical relationship and it only works for inches, feet, and barrels. If you use meters, mm, and liters the number is some other relationship.
David
RE: Cv conversion to other units
I find it ever so sad that we have "favorite equations".
David
RE: Cv conversion to other units
We should not feel sad about this. I have accepted that I am a techno-junkie nerd and that I get my kicks from things that "normal" people would find boring. Years ago I was working my way up the corporate ladder and had got to a purely management position with virtually no technical work. I was bitterly unhappy - to the point of feeling physically sick on my way to work every morning. I realized that I had to get back into the technical realm. Perhaps I do no earn as much as I would have if I had remained on the corporate ladder, but at least I look forward to my work every day now.
yours in engineering
katmar
RE: Cv conversion to other units
I have found the reference in Crane 410 for converting between Cv (valve coefficient) and K (dimensionless resistance coefficient). In my 1988 Metric edition (410M) it is equation 3-16 on page 3-4. For those who don't have Crane 410 handy the conversion is
Cv = 29.9 * d^2 / sqrt(K)
Cv is USgpm and psi
d is pipe inside diameter in inches
K is dimensionless resistance coefficient
This allows you to express the flow characteristic of any fitting (eg an elbow) as a Cv value, but Cv's are usually only used for valve sizing. It is probably more useful in reverse - i.e. to express a valve with a known Cv as a K value to allow you to sum all the K's to get an overall pressure drop for a pipeline.
katmar
RE: Cv conversion to other units
Please excuse this belated post. As you know, I was bedridden for 6 weeks and am now getting back to half-normal speed. I couldn’t read this thread and let the opportunity go by to pop yet another engineering balloon – this time, the error in one of my favorite Crane Tech Paper #410 equations: the conversion of Cv to K.
As you have correctly cited, Crane gives us the equation for Cv:
Cv = 29.9 * d^2 / sqrt(K)
However, they screwed things up by not leaving well enough alone. They proceeded to solve for K with the following:
K = (891) * d^4 / (Cv)^2 (which is not correct)
The correct equation for K is, of course,:
K = (894.01) * d^4 / (Cv)^2
There are some more errata in Crane’s which I have noted in my copies (I still have my original 1957 printing) and this raises the subject that I would like to bother you with: you recently wrote an interesting post on your disenchantment with the “K’s” as used in Crane and although I read the thread, I have not been able to find it lately. Could you help me by remembering which thread it was that you expounded some very interesting and experienced finding on the information found in Crane’s regarding the use of “K’s” in calculating head loss?
As I recall, your findings dove-tailed with my past experience in using Crane’s data and equations and I want to relate this information to another engineer I am helping out. I know this changes the theme of this thread, but I thought it would get your attention and I always enjoy discussing fluid mechanics with you anyway.
Best Regards,
Art Montemayor
RE: Cv conversion to other units
Second question, what is "d" when looking at a valve. Let's take for example a valve with the following characteristics: Orifice Dia = .25", pipe size NPT = .38",and Cv = 1.00. Would d be the .25 or the .38?
Cooperjer
Cooperjer
Mechanical Engineer
RE: Cv conversion to other units
3.28 capacity: the rate of flow through a valve usually stated in terms of CV or KV.
John
RE: Cv conversion to other units
I am pleased you are back and active here again. I think the thread to which you are referring is thread798-135792 It was in the Chemical process engineering forum, which is probably why it did not result in much discussion.
My objection to the Crane 410 "K" values was that they are not applicable to laminar flow calculations, but Crane example 4-7 explicitly shows them being used for laminar flow. I recently got involved with some calculations for a liquid fertiliser plant where CMS (condensed molasses solids) was being pumped. The very high viscosities involved make this a definite laminar problem, and I found huge discrepancies between the various methods.
What really surprised me was that the "equivalent length" method, which tends to be looked down upon as an inferior short-cut method, was far better then using turbulent flow "K" values.
I put together a few examples to illustrate this, using a Sched 40 standard radius 90 degree welded elbow. The two fluids are water at 2 m/s (Reynolds = 205000) and molasses (visc = 4000 cP, Reynolds = 10.1).
For water I got:
Constant K = 0.3, press drop = 0.600 kPa
Equivalent length = 20, press drop = 0.749 kPa
Hooper 2-K method K = 0.316, press drop = 0.632 kPa
Darby 3-K method K = 0.335, press drop = 0.670 kPa
For molasses I got:
Constant K = 0.3, press drop = 0.019 kPa
Equivalent length = 20, press drop = 8.193 kPa
Hooper 2-K method K = 79.5, press drop = 5.136 kPa
Darby 3-K method K = 79.5, press drop = 5.136 kPa
In both cases the equivalent length method gives results that are a bit high, but in the ball-park. For laminar flow the Constant K method is out by a factor of 265. This is way beyond acceptable.
Back to the Cv to K conversion. I think the discrepancy you point out between the two Crane formulas is just rounding error. In my notes, where I worked to 4 significant digits I got a value of 29.83 in place of Crane's 29.9, and a value of 889.8 in place of their 891.0. In view of the other likely uncertainties I don't think it is too critical.
regards
Harvey
RE: Cv conversion to other units
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.
RE: Cv conversion to other units
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
RE: Cv conversion to other units
Kv ~ 5 * A[cm2] * ALPHA
ALPHA = flow factor according to DIN 1952
Kind Regards,
Hahor