Downhole High-Pressure Equalizing Safety Valves: A Solution-Variable Labyrinth Seals (1987)

ABSTRACT Surface Controlled Subsurface Safety Valves (SCSSV) with equalizing devices are historically short lived. The critical seal area of the equalizing valve being the primary failure device. The deficiency of equalizing devices are somewhat acceptable in wireline retrievable SCSSV, but less acceptable in high capacity wells using tubing retrievable safety valves where down time is critical and pulling or workover expense is high. To eliminate this problem and to improve well security, several conventional designs were evaluated (Figure 1). The deficiencies were identified and a new design was developed and tested. The principal weakness of the equalizing safety valves was their inability to protect the critical sealing surfaces. The design solution (Figure 2) involved an equalizing valve section and labyrinth seals with all critical sections, such as chamber areas and orifice diameters calculated to allow careful control of fluid velocity to minimize erosion of critical seal areas and to optimize equalizing time. This paper will demonstrate the advantages of using labyrinth seals to protect the equalizing section of tubing retrievable safety valves. Although the conventional types of equalizing tubing retrievable safety valves that operate in shallow, low pressure clean gas environment have a short life relative to non-equalizing safety valves, it is the purpose of this design to withstand virtually any downhole environment available. INTRODUCTION The primary design concern of the equalizing section was to minimize and/or eliminate damage at the critical sealing areas of the safety valve due to harsh downhole conditions. Conventional equalizing designs suffered from a non-existance of critical seal area protection. With the use of labyrinthseals to reduce the pressure drop across the equalizing device the service life of this component can be dramatically increased. This is due to the basic design properties of the labyrinth seal. Through a series of labyrinth seals the upstream pressure is decreased by dissipation of kinetic energy at each cavity. Figure 2 shows the basic flow path through the equalizing section. The percent reduction in pressure across a given length of labyrinth seals is described by an exponential curve, as seen in Figure 3. There are many types of labyrinth seal designs. Of these designs the "straightthrough" type was best suited for this application due to its simplicity, and it required the least amount of radial wall thickness. This is advantageous when packaging the labyrinth seals into the outer wall of the safety valve. DESIGN CONSIDERATION The labyrinth seal design was developed using the theory presented in a paper by Childs and Scharrer [ 1 ]. This paper was based on Iwatsbos solution for predicting leakage rates for rotating and non-rotating labyrinth seal designs. For the purposes of this paper the zero-order solution was used to predict the leakage rate for a non-rotating labyrinth seal configuration using methane gas as the operating fluid. The following data was used in all labyrinth seal calculations: (Mathematical equation available in full paper)