Positive displacement motor horsepower

If the positive displacement motor is lifted off the bottom of the borehole and circulation continues, the motor will simply continue to rotate at 408 rpm. The differential pressure, however, will drop to the value necessary to overcome internal friction and rotate, about 100 psi. In this situation the motor produces no drilling torque or horsepower. 

As the positive displacement motor is lowered and weight is placed on the motor and thus the bit, the motor speed continues but the differential pressure increases, resulting in an increase in torque and horsepower. As more weight is added to the positive displacement motor and bit, the torque and horsepower will continue to increase with increasing differentiated pressure (i.e., standpipe pressure). The amount of torque and power can be determined by the pressure change at the standpipe at the surface between the unloaded condition and the loaded condition. If too much weight is placed on the motor, the differential pressure limit for the motor will be reached and there will be leakage or a mechanical failure in the motor. 

A 61-in. outside diameter positive displacement motor of a 1 2 lobe profile design (where performance data are given in Table 4-114) has rotor eccentricity of 0.60 in., a reference diameter (rotor shaft diameter) of 2.48 in. and a rotor pitch of 38.0 in. If the pressure drop across the motor is determined to be 500 psi at a circulation flowrate of 350 gal/min with 12.0 Ib/gal, find the torque, rotational speed and the horsepower of the motor. 

Total Pressure Loss. Since bit life is not an issue in a short deviation control motor run operation, it is desirable to operate the positive displacement motor at as high a power level as possible during the run. The motor has a maximum pressure loss with which it can operate. This is 580 psi (see Table 4-114). It will be assumed that the motor will be operated at the 580 psi pressure loss in order to maximize the torque output of the motor. To obtain the highest horsepower for the motor, the highest circulation flowrate possible while operating within the constraints of the surface mud pump should be obtained. To obtain this highest possible, or optimal, circulation flowrate, the total pressure losses for the circulation system must be obtained for various circulation flowrates. These total pressure losses tabulated in the lower row of Table 4-117 represent the surface standpipe pressure when operating at the various circulation flowrates. 

Variations in pressure and volume demands have a serious impact on blower reliability. Since blowers are positive-displacement devices, they generate a constant volume and a variable pressure that is dependent on the downstream system s back-pressure. If demand decreases, the blower s discharge pressure continues to increase until (1) a downstream component fails and reduces the back-pressure, or (2) the brake horsepower required to drive the blower is greater than the motor s locked rotor rating. Either of these results in failure of the blower system. The former may result in a reportable release, while the latter will cause the motor to trip or burn out. 

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