Reasons and Countermeasures of Electric Submersible Pump Inverted

Abstract: On the basis of a large number of investigation and analysis of field failure accidents, the main reasons for the failure of ESP submersibles are analyzed. The main reason for the failure of ESP submersibles is that there is not enough torque , And then put forward measures to prevent the ESP submersible buckle.

1 down reason analysis

According to a survey of dumping accidents at the scene, it is found that there are three reasons for the undercutting of the electric submersible pump system: First, there is a large back-up torque when the ESP is started and operates normally. This is because the electric submersible pump pipe joints are threaded connection right (M90 × 1.75), and the direction of rotation of the motor clockwise, the motor output torque through the multi-stage centrifugal pump shaft and impeller, guide wheel Passed to the pump cylinder, cylinder pump in the buckle direction of rotation. Especially in the electric submersible pump starts, the starting torque is larger and there is a greater impact, the impact of torque in this role, most likely make the pipe thread connection undercuts. Second, the submersible pump pipe thread preload is not enough (or pre-tightening torque is not enough). The size of the pre-tightening force is an important factor affecting the reliability of the threaded connection. The pre-tightening force can neither be too large nor too small. On the electric submersible pump pipe thread connection, on the one hand, taking into account the submersible pump in the work there is inverted torque, the pre-tightening force of each pipe thread connection should be able to ensure that the axial load of the electric submersible pump system (including the electric submersible pump System self-weight, fluid pressure and preload, etc.) and the horizontal undercut torque under the joint action will not happen undercut phenomenon, even if the pre-tightening torque is greater than a certain amount of undercut torque; on the other hand, the pre-stress can not be too large . If the preload is too large, coupled with the role of submersible pump axial load, it is possible that the thread cross-section of the axial tensile stress exceeds the yield point of the pump cylinder material, plastic deformation occurs at the pump cylinder thread, leading to pre Tight relaxation. At this time, although the two members do not rotate relative to each other, the threaded connection has loosened. If there is reverse buckling torque at this time, this buckling torque all bear by the anti-pouring block; if the anti-pouring block is not strong or insufficient strength, it will cause the anti-pouring block off or be cut off, resulting in electric submersible pump down Buckle well. This shows that the preload should have an appropriate value. However, the actual production of electric submersible pumps used in the design of the pipe thread connection is not pre-tightening accuracy of the calculation, the process does not have the pre-tightening force on the size of the strict control, which is leading to the importance of thread undercut the reason. Third, the anti-down process is not reliable. Due to the existence of submersible ESP electric submersible pump problems, so pipe threaded joints set anti-pouring block. Due to the small thickness of the anti-pouring block, the weld section size is small, carrying capacity is not strong; combined with anti-block and pump head, cylinder are medium carbon steel, welding process is poor, difficult to guarantee the quality of the weld. Therefore, the anti-dumping anti-dumping function is not reliable. This is precisely illustrated by the frequent occurrence of anti-to-block desoldering or even shearing in production.

2 ways to prevent undercuts

2.1 Reasonably determine the pre-tightening torque of each pipe thread

According to the above analysis, the actual pre-tightening torque (Mrb) of each pipe thread connection should firstly ensure that the thread does not loosen under the joint action of the axial load of the submersible pump and the actual back-up torque (Mrp) during the work. For triangular threads, if the effect of axial load is not taken into consideration, the torque Ms needed to tighten the threads and the torque Mp required to loosen the threads should be respectively

Ms = dF b - tan (α + φ v ) / 2 (1)

Mp = dF b -tan (α-φ v ) / 2 (2)

Where, d is the diameter of the pipe thread, mm; F b for the thread preload, N; α for the thread angle, 24; φ v for the triangular thread equivalent friction angle, 9 ° 36.

Into the threaded parameters

M s = 1.1 M p (3)

That is, the torque required to tighten the thread should be greater than the torque required to loosen the thread. For the ESP, the actual maximum back-flip torque M rpmax at work is the starting torque M st . When the electric submersible pump starts with load, the starting torque is 1.1 ~ 2.4 times of the rated torque M R (1.4 ~ 1.8 times for the common 30 ~ 55kW motor), that is

Mrp = M st = (1.4 ~ 1.8) M R (4)

In order to prevent the electric submersible pump in the work due to frequent start-up and undercut, should make the actual pre-tightening torque Mrb meet

Mrb = M s -Mrp / M p = (1.54-1.98) M R (5)

The actual pre-tightening torque should be greater than the rated torque of 1.54 to 1.98 times; on the other hand, the actual pre-tightening torque of the thread can not be too large, that is, the actual pre-tightening force generated by the actual pre-tightening torque can not be excessive. The relationship between the thread preload and the yield point σ s of the thread material and the minimum cross-sectional area A min of the thread is as follows:

F b ≤ (0.5~0.7) σ s -A min (6)

If the pre-tightening torque obtained according to formula (5), the pre-tightening force on the thread cross-section exceeds the range of Eq. (6), the pump barrel material with increased pump wall thickness or higher yield point should be considered .

2.2 Change the electric submersible pump pipe thread rotation

The right-handed change to left-handed, so that you can make the electric submersible pump in the start-up and normal operation of the torque generated will not cause loosening of the screw. Of course, in the submersible pump work process, due to overload, underload and other causes of the motor power failure, but also in an instant undercut torque, but the torque compared with the starting torque is much smaller, will not cause electricity potential In addition, to strengthen the existing anti-dumping measures, strict control of anti-pouring block welding process, to ensure the quality of welding, but also can play a role in preventing undercuts.

The above measures to prevent undercut, can not fundamentally solve the problem of undercutting. Of course, it is impossible to completely eliminate the back-up torque because the torque causing the back-up is not eliminated. But take some measures to control the back-up torque within a certain range. For example, at the junction of the uppermost section of the electric submersible pump and the oil pipe, the movable joint as shown in FIG. 1 can be used to control the undercutting torque within a certain range to avoid the undercutting. The working principle of the joint is that during assembly, the adjusting screw is used to properly control the pressing force between the steel ball and the upper joint so as to ensure that the relative rotation between the upper ring and the lower ring does not occur during normal operation of the electric submersible pump. When the torque exceeds a certain value, the upper joint presses the steel ball into the hole of the upper ring, and the relative rotation between the upper ring and the lower ring causes the torque to be controlled within a certain range, thus preventing the buckle from buckling.