Abstract
A failure analysis was performed on a leaking slab gate valve used on an oil well Christmas tree assembly. The valve was removed from service after approximately 1 year of service, when it was observed that oil continued to pass the valve upon completely closing it. As a result of the leaks, slabs of these valves were being replaced frequently and at very high cost. However, that did not solve the problem. Valve failure from internal corrosion was suspected. The factors that were considered in the analysis included materials of construction, valve design, scaling, corrosion, valve lubricant, and operational conditions such as increased water cut. The objective of the failure analysis was to determine the most probable cause of this valve leak.
Based on the results from the failure analysis, it was determined that the most probable cause of the leaking valve was ceased seat motion resulting from scaling on the mating surfaces of the floating seats and the valve body. An increased water cut likely caused the scaling deposits when the petroleum-based lubricant was dissolved away by the crude oil. It was concluded that all construction materials held up in service, and that the valve design did not cause the leak. The absence of corrosion and presence of scale was unexpected to the operators.
This paper describes the failure analysis, discusses the various failure scenarios that were considered, and provides recommendations for operation of similar valves.
