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Specific Gravity
is a dimensionless quantity indicating how many times
a certain volume of a material is heavier than an equal
volume of water. Specific gravity values associated
with bit hydraulics pertain to the mixture of water
and polymers or drill muds that are used as circulating
fluid. Pure water at 62°F has a specific gravity
of 1.00 and a density of 62.4 pounds/foot3.
Typically, water and drill mud mixtures will have a
specific gravity in the range of 1.00 to 1.30 resulting
in density values of 62.4 pounds/foot3
to 81.1 pounds/foot3.
The use of circulating fluids with a specific gravity
greater than 1.00 will result in higher fluid pressures
at the bit face.
In some cases, the density of a drill
mud mixture may be given as mass per fluid volume. For
example, where a circulating fluid is said to have a
density of “10.2 pounds/US gallon”, the specific
gravity of the mixture is determined by first converting
the given density ( 
) to “pounds/foot3”:
The specific gravity ( 
) therefore is a ratio of the circulation fluid’s
density and the density of pure water:
TFA or “Total
Flow Area” is defined as the combined cross-sectional
area of all of the fluid input ports: either the sum
of the areas of the internal fluid passage canals or
the sum of the areas of the face discharge holes or
slots moulded into the bit crown.
The actual fluid pressure (P)
developed at the face of any bit may be calculated by
using the following formula:
Where:
“ Q ” represents
the circulation pump rate given in “inch3/sec”
“ A ” represents
the TFA for the bit given in “inch2”
“
” represents the specific gravity of the circulating
fluid
“ ”
represents the density of water: a constant of 0.0361
pounds/inch3
“ g ” represents the acceleration due to gravity:
a constant of 384 inches/sec2
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