High pressure swivels can be divided into two general categories:
- Coaxial swivels, such as coring swivels and washpipe assemblies, and
- Side port swivels (also referred to as side entry swivels), such as cementing heads and rock drilling machines.
In a coaxial swivel, the fluid is conducted axially between axially aligned, relatively rotatable conduits. Typically, one of the conduits is guided for rotation by bearings within a bearing housing, and the other conduit is fixed to a support frame that is mounted on the bearing housing.
In a side port swivel, a bearing guided mandrel rotates inside a bearing housing, and the fluid is conducted radially from a radially oriented housing port to a radially oriented mandrel port that connects with an axially oriented bore.
Within these two basic types of swivels, the rotary seal challenges vary depending on whether the fluid being conducted has abrasive or lubricating qualities. For example, oilfield washpipe assemblies and cement heads conduct harshly abrasive fluids, while a high pressure hydraulic swivel conducts a clean fluid with lubricating qualities. The rotary seal challenges also vary depending on the speed of rotation. For example, a hose swivel occasionally rotate by a few degrees or a few revolutions, while other types of swivels rotate continuously.
What are the key challenges in high pressure swivel sealing?
One challenge common to most high pressure swivels is component pressure breathing. The high pressure causes dimensional changes to key components, which affects extrusion gap clearance and may affect bearing mounting clearance. Kalsi Engineering has developed floating backup ring arrangements that make the extrusion gap clearance immune to the pressure breathing of the pressure housing. The floating backup rings are easily adaptable to hydraulic swivels, side port swivels, and coaxial swivels. Our rotary seal handbook teaches how to implement floating backup rings using a simple stacked housing assembly. In hydraulic swivels and side port swivels, the stacked housing arrangement isolates the bearings from the effects of pressure breathing, which improves the pressure capacity of the rotary seals by minimizing runout of the mandrel.
Another swivel seal challenge is seal generated heat. As the differential pressure across a rotary seal increases, the interfacial contact pressure between the seal and the rotating mandrel increases, which causes increasing friction, heat, and seal wear. Higher rotary speeds cause increased temperature and accelerated wear. Kalsi Seals address these challenges by using rotation to hydrodynamically pump a thin film of lubricant into the dynamic sealing interface between the seal and the mandrel. The lubricating film reduces friction, heat, and wear, and the cooler operating conditions promote high pressure seal performance.
In typical hydraulic swivels that use cap seals, most of the seals can be exposed to differential pressure acting from either side of the seal. Deformation and seal shuttling caused by the reversing pressures introduces lubricant between the O-ring and the plastic cap, and between the O-ring and the groove, and the result is undesirable seal slippage. The preferred way to use Kalsi Seals in hydraulic swivels is an arrangement that only puts differential pressure across the seals in a single direction, eliminating the adverse deformation and seal shuttling associated with reversing pressures. Unlike cap seals, the plastic liner of our highest pressure capacity seals is bonded to the elastomer, so there is no possibility of slippage between the liner and the elastomer. The difference between the breakout friction of the plastic and elastomer portions assures that relative rotation occurs at the shaft to seal interface.
In swivels that conduct high pressure abrasive fluids, third body wear of the seal can also be a significant issue, because differential pressure can drive the abrasives into the dynamic sealing interface. High pressure packing rings that are typically used in coaxial washpipe assemblies, are particularly susceptible to this due to the texture of the packing rings, and due to a lip load that eventually becomes very light in the absence of differential pressure. The typically recommended practice with Kalsi Seals is to partition the abrasive fluid from the seal lubricant with a seal that is exposed to little or no differential pressure. This requires a lubricant pressure that is equal to or slightly above the pressure of the abrasive fluid being conducted by the swivel. One rotary seal partitions the abrasive fluid from the seal lubricant, and the other rotary seal retains the lubricant pressure. This division of function between two seals prevents the high pressure from forcing abrasive fluid into the dynamic sealing interface.
What is the recommended implementation for high pressure swivel seals?
In hydraulic swivels, a pair of Kalsi Seals are used to define each hydraulic circuit, as shown schematically by our hydraulic swivel seal brochure and Chapter E2 of our seal handbook. Chapter E2 will be updated in the near future to show how to use floating backup rings in hydraulic swivels using the general stacking housing arrangement that is shown in Chapter E5. Whether or not floating backup rings are used, the bolt-on bearing housing arrangement that is shown in Chapter E5isolates the bearing housings from pressure breathing, and helps to assure minimal runout of the mandrel. For hydraulic swivels that have high pressure while rotating, Kalsi Seals with the enhanced lubrication wave pattern are the preferred choice because of their ability to lubricate with low viscosity lubricants.
Recommended design practices for high pressure washpipe assemblies that conduct abrasive fluid are provided in Chapter E4 of our rotary seal handbook. Recommended design practices for side port swivels that conduct abrasive fluid are provided in Chapter E5 of the handbook. Obvious simplifications are possible when the fluid being conducted can be used as a seal lubricant. Plastic Lined Kalsi Seals have the highest pressure capacity, followed by Dual Durometer Kalsi Seals. In applications that are sensitive to startup torque, plastic lined seals or dual durometer seals with a low friction treatment are recommended.