Introduction To Drilling Hydraulics Calculations
Drill Bit Pressure Drop Calculation
1) the changes in pressure resulting from a change in elevation are negligible.
2) the upstream velocity (V.) is negligible compared to downstream (as below figure).
Laboratory studies show the pressure drop through the nozzle does not correspond precisely according to Eqs. above. As a result, a discharge coefficient factor, Cd is applied to make laboratory results correspond to theoretical pressure drops. The discharge coefficient will vary, depending on nozzle type and size, A value of 0.95 is representative, however, for most field situations.
Applying Cd above equation becomes:
EQ.1 
The velocity in the nozzles is equal to the flow volume divided by the area, or V = Q/A
Converting to common field units, the VN becomes as follows:
EQ.2 
Where:
AT = total nozzle area, in^2
Q = flow rate, gal/min
VN = nozzle velocity, ft/sec
Using Eq. 1&2 And solving for PB:
EQ.3 
Drill Bit Hydraulic Horsepower & Impact Force Calculation
Hydraulic Horsepower at the drill bit
A measure of the energy per unit of time that is being expended across the bit nozzles. It is commonly calculated with the equation HHP=P*Q/1714, where P stands for pressure drop in pounds per square in., Q stands for flow rate in gallons per minute, and 1714 is a conversion factor necessary to yield HHP in terms of horsepower. Bit manufacturers often recommend that fluid hydraulics energy across the bit nozzles be in a particular HHP range, for example 2.0 to 7.0 HHP, to ensure adequate bit tooth and bottomofhole cleaning (the minimum HHP) and to avoid premature erosion of the bit itself (the maximum HHP).
EQ.4

Impact Force Calculations at the drill bit
Impact pressure which affect impact force is the momentum change of the drilling fluid distributed over the impacted area at the hole bottom. This pressure, acting downward and to the sides, attempts to overcome chip holddown pressure and remove cutting from beneath the bit. pressure and remove cutting from beneath the bit.
In drilling Hydraulics, Impact Force can be calculated through this equation.
EQ.5 
Example On How To Calculate bit pressure drop, Hydraulic Horse Power And Impact Force
3. Calculate the hydraulic horsepower expended at the bit from Eq.4