3 replies to this topic

[proinwv]

IF you are familiar with Crane Technical Paper 410, it states on p 1-7 the limits for using the Darcy equation as >10% and >40% DP based on P where P=inlet pressure, psig.

I would have thought that the condition would be based on % of P', psia.

For example, having an inlet of 0 psig and an outlet of -1 psig. If we base the allowable DP on P1 psig, then there is none, where as if we use the abs pressure, the allowable DP is 1.47 psi for the 10% limitation.

Any thoughts?

Thanks!
PAUL

[Art Montemayor]

Paul:

There are no restrictions on using the Darcy-Weisbach equation in the pressure region below local atmospheric pressure. As Crane’s Tech Paper 410 points out:

Δ P = (f) (L/D) (v²/2 g) (ρ/144)

where,
Δ P = Pressure Drop, in psi;
f = the Darcy Friction Factor;
L = the pipe length, in feet;
D = the pipe internal diameter, in feet;
v = the fluid’s mean velocity, ft/sec; and,
g = the gravitational constant, 32.2 ft/sec².

I have often, during the past 45 years, found mechanical engineers to be trained in (and persistent) using the term “psi” indiscriminately – without noting that they mean <u>gauge</u> or <u>absolute</u>. Of course, to a chemical engineer, this is lunacy and an invitation to calculation and decision errors. All the mechanical engineers I’ve dealt with (except the ones that worked for me) always treated this subject in a cavalier manner: they knew what they meant (either gauge or absolute), so there was no concern on their part. I’ve often been a hard disciplinarian about using proper, defined units in all engineering products, and never depending on the other person(s) to “know” what it is that you are trying to communicate. I have gone into great detail in the past about such terms as “Standard Conditions” and have not made many friends because of the stand that I always take. Your example points out a similar case.

If you are dealing with compressible fluids, you should be employing absolute values. This is a classical Ch E “golden rule”. It is my belief that Crane Tech Paper is expressing generalities and forgetting that they have published a Nomenclature page at the beginning of the Paper that they are bound to. In that Nomenclature, as you adroitly have noted, they define

P = pressure in psig; and,
P’ = pressure in psia.

They have failed to take into consideration that to deal in psig forces the reader to work in units that define some pressures in the negative range values and worse still, in 0 psig (which of course is a positive 14.696 psia). Clearly, Paper 410 should have noted the use of absolute pressure units for the inlet pressure (the units for Δ P make no difference). There are other errata or omissions in Paper 410 that I’ve found through the years and it has been depressing to witness that these keep being repeated, uncorrected, through each printing. In the past, I’ve turned in my marked-up copy of Tech Paper 410 to a representative of Crane - on their request - and the comments and critiques that I noted have been seen by them. However, I doubt if there will be any corrections made to the Paper. The reason, of course, is that Crane is no longer interested in sinking money into Paper 410; the piping, valve, and fittings business is no longer that profitable where this can be justified. It’s sad, but true. Crane went into making their product a veritable “Bible” for those in Fluid Mechanics on a voluntary basis. I still have my original 1957 printing version. It’s sad that no one else has contributed as much – nor is any other volunteer seen for the foreseeable future. Fluid Mechanics and fluid flow study is the loser and consequently less advancement in this area continues.

Let’s hope that someone else "steps up to the plate" in the near future and helps out in the field of fluid flow engineering – especially in the areas of 2-phase, supercritical, and vacuum flow. A lot still needs to be done.

(Thanks Milt Beychok, for your timely and excellent tips on how to employ BB code. May your tribe increase.)

[proinwv]

Art, thank you for your reply! It confirms my thoughts and your explanation is excellent. As a ME, I take no offense, as I learned early on ALWAYS define your units. (Us ME's can get in our own trouble with inches v. feet etc.).

I recently called the publisher (vendor?) of TP410 as I have a 20th printing, 1981, but it does not list any edition!

She told me that there had not been any substantive changes, and no need to purchase the latest "printing". This reinforces your experience.

This is unfortunate because it is a great summary of flow and it is widely used.

PAUL