Page 16 - Air and gas Drilling Field Guide 3rd Edition

6 CHAPTER 1 Introduction and Units

Table 1-2. Units that Define Mass [1]

Unit System Mass Unit Dimensions
2
2
USCS Slug F T /L ¼ lb sec /ft
2
2
SI Kilogram F T /L ¼ N sec /m
(weight) units to correct to mass based equation [2]. Since specific heat c p in the
o
SI is in units of kcal/kg-K and in the USCS is in units of BTU/lb- R, a heat transfer
equation for SI calculations would have to be in mass rate of flow _ m and a heat
transfer equation for the USCS would have to be in weight rate of flow _ w.Itis
our contention that for those of us who were not brought up using the SI as
our primary system of units, it is easier for engineers to carry out calculations
in both systems if both unit systems are manipulated in exactly the same manner.
This requires use of the kilogram definition given in Table 1-2. It is likely that this
kilogram definition will “ruffle the feathers” of some SI purists.
There is one other important change to be made in the application of the SI to
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the drilling calculations made in this monograph. N/m or the pascal (Pa)asa
pressure unit will not be used. Most engineers have trouble correlating these
values to small cross-sectional areas such as a 2-in nominal pipe ( i.d. of 2.0 in,
2
or 50.8 mm). In its place we will use a pressure term unit of N/cm . It is only nec-
4
essary to multiply this pressure unit by 10 to obtain pressure in pascals or by
0.6897 to obtain psi. The convenience of using this alternate SI pressure term
is illustrated by Figure 1-1, where both scales can be placed easily on the same
gauge face for easy reading (and reference to one another).

200 300
N/cm 2
300 400
psi
200 500
100 400
100 0 0 700 600 500

2
FIGURE 1-1. Pressure gauge with both psi and N/cm units.