Mechanics of an Electric Submersible Pump Failure Mode
- Maston L. Powers (Consultant)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- February 2002
- Document Type
- Journal Paper
- 62 - 67
- 2002. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 3.1.2 Electric Submersible Pumps, 4.1.2 Separation and Treating, 5.6.4 Drillstem/Well Testing
- 2 in the last 30 days
- 489 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 5.00|
|SPE Non-Member Price:||USD 35.00|
The occurrence of electric submersible pump (ESP) failures caused by spinning diffusers is tolerably frequent in moderateoperating- cost circumstances. However, extremely high wellservicing expenses are associated with many ESP applications. Examples include deep wells, arctic locations, offshore wells that must be killed with high-density fluids, and seafloor completions. In these or other high-cost circumstances, early pump failures of any mode cannot be tolerated.
Longitudinal compressive force is imposed on the diffuser stack of ESPs during assembly to prevent diffuser rotation. If this is done improperly, the diffusers can spin because of torque transferred from the impellers, resulting in early pump failure. This paper analyzes the mechanics of the spinning-diffuser failure mode and demonstrates why some pumps with inadequate compressive force can pass common pump tests but fail in this mode. Equations are developed to calculate the restraining force as it changes under varying conditions and the minimum value required to prevent diffuser spinning. Testing procedures are proposed to emulate the effects of well conditions conducive to diffuser spinning, thereby detecting the defective pumps currently being missed. Practical examples are included that illustrate the utility of the concepts presented herein.
Longitudinal compression is imposed on the diffuser stack of ESPs during assembly to prevent diffuser rotation. If this is done improperly, the diffusers can spin because of transferred impeller torque, resulting in an impeded flow condition and early pump failure. Examination of pumps that have failed in this mode may disclose evidence of high temperature and diffusers with circumferential wear and/or shortening because of wear caused by relative movement at the contact with adjacent diffusers. This mode of failure begins at or near the pump top and progresses downward, as wear further loosens the diffuser stack and the head, developed by upper stages, deteriorates.
Some pumps that pass API performance specifications during common testing procedures subsequently fail because of diffuser spinning. A possible explanation for a portion of these occurrences is that an otherwise strong pump might meet test standards in spite of an incipient diffuser-spin condition. It should be pointed out that a spinning diffuser is audible when horizontal testing is employed and would not go unnoticed. Most cases of tested pumps failing in the subject mode are probably the result of service conditions.
The assumption of floating impeller pumps is made throughout this paper. However, the effects described here are equally applicable to pumps with a fixed impeller design.
Restraining the Diffuser Stack
Impellers transfer torque to diffusers by three means: the mechanical drag of thrust-washer contact, which increases with increasing impeller thrust; disk friction and other hydraulic drag; and the dynamic portion of the head generated by the impeller. Torque of a single impeller is defined in Eq. 1.
At shutoff, it is known that generated head is 1/2 dynamic and 1/2 static.1 A reasonable estimate of maximum transferable torque would be one-half that calculated with Eq. 1, with PI at maximum value. This assumption is made herein, although experimental data would be more desirable.
Diffuser rotation is prevented by frictional contact between stages and with the pump head and base. The contribution of lateral contact with the housing is neglected here. The static torque capacity of a diffuser/diffuser contact (or a diffuser/head or diffuser/ base contact) is expressed in Eq. 2. The minus sign in this equation reflects the fact that FN is a compressive force and, therefore, negative.
Once spinning has begun, it will continue unless FN doubles, or possibly triples, because the value of KK is 1/2 to 1/3 of KS. Values of KS (and KK) should vary somewhat, depending on the fluid wetting the diffuser surfaces; thus, the value of KS when a pump is in service may be different from that during testing. This would affect the minimum value of FN required to prevent diffuser spinning and could account for the subsequent failure of some tested pumps.
The initiation of diffuser spinning can be illustrated by envisioning the diffuser stack as a torqued shaft restrained at the ends with couples distributed throughout its length. The interstage contacts would be equally spaced planes of weakness. Fig. 1 illustrates the couples (tD) imposed by each diffuser, the interstage torque transfers (ti), and the end reactive torques for a six-stage pump. Also shown is a plane view of a longitudinal line along the diffuser stack, demonstrating (in great exaggeration) the distortion caused by the distributed torque. Note that ti is zero at midpump and increases toward the ends, where it assumes values of ntD/2. The equation for torque capacity (tC) of the interstage contacts was derived from Eq. 2 by substituting the appropriate value of FN. At the pump top, FN =FDR. This is augmented by an additional force, at subsequently lower contacts, equal to the sum of the pressure-based force and the buoyed weight of all stages listed previously. Eq. 3 (in which 0 n' n) was, thus, derived.
Eq. 3 shows that the contact between the top diffuser and the pump head (n'=0) has the least torque capacity, with tC increasing at each successively lower interstage contact. Therefore, diffuser spinning should begin at the top stage, where ti is maximum and tC is minimum. The previous analysis assumes that KS has the same value at all interstage, diffuser/head, and diffuser/base contacts. Because some variation in KS occurs, spinning should begin near the top of the diffuser stack but not necessarily with the top stage.
Fig. 2 graphically illustrates ti and tC for a pump with adequate FDR, and tC for a pump for which FDR = 0. Diffuser spinning may occur at any contact for which tC falls below the ti curve. As the head developed by upper stages declines, tC of the lower stages is diminished, and diffuser spinning progresses downward.
|File Size||402 KB||Number of Pages||6|