This paper presents techniques which will allow Engineers to interface a digitizer to IBM PCs (or compatibles) through an asynchronous communications adapter port (RS232). Also presented is a sample application program which uses this presented is a sample application program which uses this interface to perform pumping unit torsional analyses. The same interface could be adapted to other engineering programs, e.g. well log analysis, mapping, etc.
After designing and installing pumping unit equipment, the equipment should be checked to verify that it is operating within design limits. One of the important factors which affects the life of a unit is the net torque on the gearbox. API STD 11 E outlines a procedure to calculate the net torque on a gearbox using information supplied by the manufacturer and the actual loads on the unit in service.
In the API procedure the net torque is stated to be the difference between the torque generated by the rod load and the torque generated by the counterbalance. The equation used by API to calculate the net torque is:
The torque generated by the counterbalance is a product of the counterbalance moment and the sine of the crank angle.
The torque generated by the well load is the product of the torque factor and the polished rod load, with the polished rod load being corrected for the structural imbalance of the unit.
The torque exerted by the counterbalance load is in the direction opposite the torque due to the well load, thus the difference between the two torques is the net torque.
The Torque Factor is a geometric value, based on crank angle, which converts well load to crank shaft torque. Torque factors for a unit can be obtained from the unit manufacturer or may be calculated using unit structural dimensions. Torque Factor charts are usually presented in the form of torque factor versus polished rod position and crank position. Polished rod position is presented in nondimensional form with zero position is presented in nondimensional form with zero representing the bottom of the stroke, and unity (1.0) denoting the top of the stroke. Torque factors and polished rod position are tabulated in 15 degree increments from 0 to 345 degrees in reference to the crank arm.
Actual loads can be obtained from a calibrated surface dynamometer card, The dynamometer card is a recording of polished rod load versus polished rod position. To use the polished rod load versus polished rod position. To use the torque factor method, the loads from the dynamometer card must be calculated from the card at polished rod positions which correspond to the nondimensional positions on the charts. Typically, the loads are determined by "digitizing" the card by hand in 15 degree increments. The corresponding counterbalance torque is calculated by multiplying the counterbalance moment by the angle which corresponds to the same increment on the charts.
Calculating torques on a finer grid than the 15 degree increments used in the API method grid would increase the chances of calculating the actual peak torque. The API STD 11E outlines equations which allow calculation of torque factors at any rod position, when the basic dimensions of the pumping unit are known. When using a finer grid, hand "digitization" of the dynamometer card becomes very time consuming and increases the chances of human error.
To reduce the amount of time spent on calculating torsional analyses, we created a torsional analysis program which could evaluate the required equations for any grid size desired. To reduce effort spent on digitizing the dynamometer card, digitizing tables were investigated as a source of inputting the recorded data to the program. Larger digitizing tables were already interfaced to our mainframe computers: however, remote offices did not have direct access to this equipment and had to send their dynamometer cards to a central processing area for digitizing. This was inconvenient and resulted in time delays of days in obtaining torsional analyses - an impractical situation for an effective maintenance program.