Evaluation of Jet-Bit Pressure Losses

An experimental and theoretical study was conducted to resolve discrepancies between observed and calculated pressure losses for fluids flowing through the nozzles of jet bits. The use of a nozzle discharge coefficient of 0.95, which is commonly used for roller-cone bits, results in the calculated pressure drop being up to 30% more than actually occurs, with the average error being 14.7% for 770 data points measured with nine different fluids. A relationship is presented to calculate the discharge coefficient as a function of borehole pressure, nozzle size, and mud density, which reduces the mean absolute error to 3% for these data points. Simply increasing the discharge coefficient to a constant value of 1.02 results in reducing the error to 4.1% without using the more complicated relationship.