Mechanical Shaft Seals (MSSs) are critical sub-components in Electric Submersible Pump (ESP) Seal Chamber Sections (SCSs), as they are the primary barriers used to isolate wellbore fluids from the motor oil. However, several ESP operators have observed relatively high MSS failure rates in certain applications, resulting in significant impacts on overall ESP system reliability and operational costs. In some cases, during teardowns, wellbore fluids were found in the lower chambers of the SCS, without any signs of damage to the MSSs or the bags/bellows in the SCS. Thus, to improve the run-life of ESP systems in their applications, operators identified the need to better understand the main influential factors affecting leakage rates through the MSS, including factors associated with the MSS characteristics and its operational conditions.
To accomplish this, a test apparatus was built to allow the testing of MSSs in a wide range of operational conditions. This paper summarizes the technical challenges and key learnings that arose from building and commissioning such a test apparatus, as well as conducting several tests on multiple MSSs of a single design.
Through commissioning of the test apparatus and subsequent testing, several limitations of the apparatus were identified and corrected through upgrades. Many of these upgrades were related to ensuring reasonably accurate measurement of the leakage rate through the MSSs, in both directions. Unexpected behaviours of the MSS were also observed during some tests. One example is the interesting phenomenon of reverse-pumping where, under certain conditions, leakage occurs in the opposite direction of the applied differential pressure. Finally, questions arose about the importance of certain aspects of quality control for MSSs and/or SCS assembly procedures.
The above effort to build and commission a MSS testing apparatus has resulted in unique insight into shaft seal behaviour and continues to shine light on gaps in the industry understanding of factors affecting leakage through MSSs. It certainly provides evidence that there are many ‘unknown unknowns’ about MSS performance under challenging downhole operating conditions, and that the effort is worth continuing to support better MSS design/selection, and SCS assembly, as part of improving ESP system reliability.