bar 2

[McCormick93]

A firm in the US is designing systems for a process plant in China. Over here, when we say "100 psi steam" it is understood that it means psig. Over there, they use "bar" for pressure. A 100 psig system would be 7 bar (gauge) or 8 bar (absolute). In places where "bar" is used, is it assumed gauge or absolute?

 

[25362]

For years I was used to the following conversion factors:

1 atm = 14.696 psi, rounded off to 14.7 psi
1 bar = 14.504 psi, rounded off to 14.5 psi
1 kgf/cm² = 14.223 psi, rounded off to 14.2 psi

 

[lewtam]

kgf/cm2 is a bastardization of the metric system that appears to have been a well meaning but ignorant attempt to offer a suitable metric equivalent to psi.

Let us never speak of it again.

bar, psi, kPa, "hg, mmH20 and atm all make sense when used in their proper context, but kgf/cm2 has no place in our industry.

LewTam Inc.
Petrophysicist, Leading Hand, Natural Horseman, Prickle Farmer, Crack Shot, Venerable Yogi.

 

[JohnGP]

lewtam, here, here!

Let us not forget also the mighty MPa, where Newtons and mm can come together in mutual harmony in stress calculations, even if you have to use the odd power of 10 occasionally for forces and bending moments.
John

 

[sailoday28]

It has been stated on this thread
"If you zero a gauge calibrated in bar(g) in Farmington, NM, USA then zero bar(a) would be -2.5 psia which is an impossible concept.

Normally liquids and gases in equilibrium are in compression. This by our convention- rusults in positive pressures. It is possible to stretch liquids and gases and the resulting stresses result in negative absolute pressures. The Vanderwals EOS, particularly for the liquid end when entering what is thethe two phase region along an isotherm will reveal negative absolute pressures.

Regards

 

[stanier]

The SI Unit for pressure is kPa. I have seen thatexpressed as KPaA or kPag for clarity. kPa ia assumed to be gauge unless otherwise defined.

 

[katmar]

lewtam, I see no difference between kgf/cm2 and any other way of expressing force per unit area. It is widely used in Europe. It may not be your unit of choice, but there is no reason to regard it as inferior to any other. It certainly makes a lot more sense than "Hg or mmH2O where you are using a column height to represent a force per unit area.

sailoday28, just because a simplified model predicts negative absolute pressures does not mean that it is true in reality. A model, no matter how elaborate, is only an estimate of the truth.

stanier, in the process engineering world kPa is assumed to be absolute unless otherwise defined. This is the danger when data sheets are passed from mechanical engineers to process engineers (or vice versa). Rather specify what you really mean and assume nothing.



Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zdas04]

Katmar,
The definitions of kg are all very self importantly limited to mass tems. There is no allowance for a kgf in any definition I can find. kgf/cm2 is exactly the kind of bastardization that SI purests complain about in the imperial system. The Newton is a perfectly reasonable force term, but apparently there is some problem with it. kPa is pretty small at about a hudred per bar so rather than come up with a human-size force/unit area unit (maybe N/cm^2) they cobbled up kgf/cm2.

Any arbitraty unit can work, I just find it humorous that the same people who say 5,280 ft/mile is stupid use kgf/cm2 with a straight face.

David

 

[katmar]

David, you are correct - any unit with which we are familiar seems to be rational - even 5280 ft per mile. lewtam was (and you are) correct to call kgf/cm2 the Metric equivalent of the psi. It is important to remember that Europe used the Metric system long before the SI system was formulated. Although some units are common to Metric and SI, they are not the same systems. Also, the "Metric" system was never as rigorously defined and codified as SI is. When I was school (OK, a long time ago!) we learnt about the cgs and mks systems, which were both "Metric" systems, and each country in Europe had its own flavor of Metric.

Europeans carry the baggage of their Metric systems in much the same way as Americans carry the baggage of Customary units. So, for a German engineer it feels as "right" to talk of kgf/cm2 as it does for an American engineer to talk of psi. For both engineers it is simply a case of what they are used to.

My disagreement with lewtam's post was simply that I could not understand why he finds the psi to be acceptable, and the kgf/cm2 not. Both use non-SI units for force per unit area. What makes kgf/cm2 inferior?

regards
Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Jankog]

The Barg is usually used in situations where it involves stress calculations. Pipelines are designed on Barg. The Bara is more process related.

 

[Ashereng]

In our market, pipelines are designed in kPa(g).

 

[sailoday28]

katmar (Chemical)
From a Google search.

Lyman James Briggs, May 7, 1874—March 25, 1963 | By Peter Briggs ...At room temperature, the maximum attainable tension in water was over 250 bar, ... and is still cited regularly in the literature on metastable water. ...

http://www.nap.edu/html/biomems/lbriggs.html

- 35k - Cached - Similar pages

To me tha above states absolute negative pressures (tension in fluids) are possible. A true EOS should be able to support real life measurements, etc.

Regards