1. Rod Breakage/Disconnection: Caused by excessive load, wear, or operational errors, resulting in rod fracture, uncoupling, or stripped threads. Manifestations include: a sharp drop in pump efficiency, a rapid rise in the dynamic fluid level, and a decrease in current (approaching the no-load level) within a short period. While torque and axial force traces remain steady lines, the absence of downhole pump operating torque causes the polished rod torque to fall well below the normal operating range; similarly, the axial force drops below normal because the axial component generated by fluid pressure on the rotor is missing. The depth of the break can be calculated based on the axial force resulting from the weight of the remaining rod string.
2. Pump Leakage: Caused by wear on the G-type screw pump's stator rubber, which reduces the interference fit between the stator and rotor and diminishes the pump's pressure-bearing capacity, leading to increased leakage. Manifestations include: a gradual decline in pump efficiency, a gradual rise in the dynamic fluid level, and a gradual decrease in current. Increased leakage reduces the pump's active torque, causing it to fall below the normal range. The axial force also drops below normal—due to the reduction in fluid-pressure-induced axial force—yet remains higher than the weight of the rod string submerged in the produced fluid.
3. Stator Swelling: Caused by the action of the produced fluid, the stator rubber swells, increasing the interference fit between the stator and rotor. While pump efficiency and dynamic fluid levels remain normal, the current increases. The heightened interference raises the initial torque between the stator and rotor, causing the polished rod torque to exceed the normal range; in severe cases, the stator and rotor may seize, though the axial force remains normal.
4. Stator Debonding: Caused by insufficient bonding strength of the stator. Manifestations include a sharp drop in pump efficiency, a rise in the dynamic fluid level, and a fluctuating decrease in current. Upon debonding, the pump's active torque drops to zero, and the polished rod torque decreases significantly. Additionally, irregular friction between the stator and rotor causes irregular fluctuations in the polished rod torque. As the axial force generated by fluid pressure drops to zero, the polished rod axial force falls below the normal range but remains higher than the weight of the rod string in the produced fluid. 5. Tubing Leakage: Tubing leakage is caused by cracks in the tubing wall. Symptoms include reduced pump efficiency, a rise in the dynamic fluid level, and decreased electric current. Because the operating pressure differential of the single-screw pump decreases, the pump's active torque drops, causing the polished rod torque to fall below the normal range; the axial force generated by fluid pressure decreases, resulting in a polished rod axial force that is slightly below normal but still exceeds the weight of the rod string submerged in the fluid.
Maintenance Procedures for G-Series Screw Pumps
(1) Check the piping and connections of the G-series screw pump for any looseness. Rotate the pump by hand to verify that it turns freely.
(2) Add bearing lubricating oil to the bearing housing, ensuring the level aligns with the center line of the oil gauge; replenish or replace the lubricant in a timely manner.
(3) Unscrew the priming plug on the pump body and fill the pump with priming fluid (water or slurry).
(4) Close the gate valve on the discharge line, as well as the valves for the discharge pressure gauge and suction vacuum gauge.
(5) Jog the motor briefly to check for the correct direction of rotation.
(6) Start the motor. Once the pump is running normally, open the valves for the discharge pressure gauge and suction vacuum gauge; after observing appropriate pressure readings, gradually open the gate valve while monitoring the motor load.
(7) Try to keep the flow rate and head of the G-type screw pump within the ranges specified on the nameplate to ensure operation at peak efficiency and achieve the intended energy-saving results.
(8) During operation, the bearing temperature should not exceed the ambient temperature by more than 35°C, and the absolute maximum temperature must not exceed 80°C.
(9) If any abnormal noise is detected, stop the pump immediately to investigate the cause.
(10) When shutting down the G-type screw pump, first close the gate valve and the pressure gauge valve, then stop the motor.
(11) During the first month of operation, change the lubricating oil after 100 hours of use; thereafter, change the oil every 500 hours.
(12) Regularly adjust the packing gland to ensure proper leakage from the stuffing box (a steady drip is ideal).
(13) Periodically check the shaft sleeve for wear and replace it promptly if significant wear occurs.
(14) When using the G-type screw pump in cold winter weather, remove the drain plug at the bottom of the pump casing after shutdown to completely drain the medium and prevent freeze damage.
(15) If the G-type screw pump is to be out of service for an extended period, disassemble the pump completely, wipe away any moisture, apply grease to rotating parts and mating surfaces, reassemble, and store properly.
Common faults and maintenance of G-type screw pumps
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