Page 15 - Air and gas Drilling Field Guide 3rd Edition

1.2 Engineering Calculations and Units 5

Note that the Earth is not a perfect sphere and, therefore, the acceleration of
gravity will be slightly different depending on whether the free falling body is at a
pole or at the equator. The elliptical form of the Earth dictates that the accelera-
tion of gravity will be slightly greater at the equator than at the poles. For most
engineering applications at or near the Earth’s surface, the average acceleration
of gravity (engineer) terms is used for calculation purposes. Both of these calcula-
tions were made using the exact same calculation method. No special term or
constant was employed to obtain either of the aforementioned results.
The objective of the third edition of this monograph is to allow engineers and
other technologists to carry out the required air and gas drilling calculations in both
USCS and SI units. In particular, the objective is for engineers and technologists
who presently use the USCS units to learn to be comfortable using SI units. Most
examples discussed within this monograph are steady-state flow problems. To facil-
itate this objective, two minor alterations in SI common usage have been made by
the authors to allow for a more unified method of calculation manipulation that are
common to the usage of USCS units. This will allow for transparency in the manip-
ulation of both systems that practitioners working in both systems will recognize.
Nearly all equations in this monograph are derived for use with any consist set
of units. Further, because most equations in the edition are for steady-state flow,
the equations will have few mass (m) or gravity acceleration (g) terms. Therefore,
the first alteration from traditional SI usage will be that any SI data or terms that
contain kg units are to be changed to force units (N) by multiplying appropriately
2
by 9.8 m/sec . An example of this first alteration would be the writing of power in
2
SI units as N-m/sec (watt) instead of the SI purist form of the power unit of kg m /
3
sec (which is also a watt). The second alteration from traditional SI use will be that
2
fluid flow pressure will be given in N/cm instead of the more SI purist recognized
2
pascal (N/m ). Regarding the latter alteration, it is very difficult for either the USCS
or the SI practitioner to visualize the force per unit area magnitude being applied
with pressure to the inside flow area of a 2-in (50.8 mm) nominal diameter pipe
or applied as stress to a small machine part when the area of the pressure or
stress unit is many times greater than the area of the actual flow area or stress area.
Therefore, as is done in the USCS for most applications where a pressure or stress
2 2 2 2
in lb/ft is converted to lb/in (psi), the pascal (N/m ) will be converted to N/cm .
In essence, the authors recommend analyzing the SI system just like the USCS
in that both are treated as an F, L, and T system, instead of treating the SI as an M,
L, and T system. Even though this logic may annoy the SI purist, it will be shown
that employing the SI mass concept (and the USCS force concept) in this manner
is far superior to employing the cobbled-up unit conversions that define force and
mass in the USCS as lb f and lb m , and the use the associated artificial constants
g c and g o [2]. The authors have even seen this conversion concept applied to SI
units in the form of N f and N m with their associated artificial constants g c and g o .
It is suspected that this misguided conversion creation was developed by
chemical and mechanical engineering professors who were motivated by the
desire to have a heat transfer equation that is rationally in consistent force