Last month's Sealing Sense provided an overview of pump gas seal benefits and general design issues. This article will discuss some application conditions that must be considered when applying gas seals to pumps.
As noted in Part One, gas seal technology for pump applications offers a number of benefits, including zero emissions, high reliability and low power consumption. Gas seal support systems are also simpler and require less maintenance than the buffer or barrier systems required for dual wet seals.
However, some application conditions require special consideration when choosing gas seals for rotating equipment. Considerations include selecting the optimal topographical seal face features to create face separation and finding the seal types and arrangements best suited for the process fluids being pumped.
Gas seal faces are designed to generate hydrodynamic forces that separate the seal faces during operation of the rotating equipment. These forces are proportional to the rotational speed of the shaft. Seal face topographies are designed for optimal performance within a specified range of rotational speeds. The gas film stiffness can be significantly reduced if the seal is operated at rotational speeds below the lower limits of the design.
Normal operating speeds of the equipment may be within the gas seal's design range. However, transient slow roll conditions can occur as the equipment shuts down, especially in applications where a variable frequency drive with an extended soft start or shutdown is applied. High temperature applications may also implement slow roll conditions to avoid shaft droop during periods of non-operation, and pumps with steam turbine drivers are typically slow rolled for long periods.
Gas seals can be successfully applied under these conditions so long as they are recognized at the time the gas seal is specified. The face topography can be modified to rely more on hydrostatic forces and less on hydrodynamic forces to create face separation. This can be effective even under slow rotational speeds, but the possibility of increased barrier gas consumption must then be considered.
Another option is to apply a coating to the hard face (typically silicon carbide) that will minimize the potential for scuffing of the seal faces if contact occurs during the slow roll condition. Diamond-like coatings (DLC) and true diamond coatings can be applied for this purpose.
Many topographical seal face patterns are designed to create hydrodynamic load support and seal face separation when the seal faces rotate in a specific direction, but act to draw the seal faces into contact when the direction of rotation is reversed. These seal face patterns are called unidirectional.
For most pump applications, a unidirectional seal face does not present problems because the pump shaft only rotates in one direction. However, this may not be the case for some pump system designs with a significant length of vertical discharge and a piping configuration that allows the fluid in the discharge line to drain back through the pump when the pump is shut down. The draining of the discharge line can cause the impeller to rotate in a reverse direction, resulting in reverse rotation of the pump shaft and seal faces.
Most gas seals readily tolerate short duration reverse rotation without damage to the seal. However, if the pumping system design allows reverse flow through the idle pump for long periods, it becomes imperative to recognize that this condition can result in reverse rotation of the seal. Make sure to advise the seal manufacturer of this pumping system design situation during the selection process.
As with rotational speeds, the face topography for unidirectional seal faces can be modified to rely more on hydrostatic and less on hydrodynamic forces to provide face separation for successful performance. In this case as well, some application conditions and seal designs may lead to increased seal face separation with a corresponding increase in barrier gas consumption.
Bidirectional seal face topography can also be employed. While bidirectional topographies are able to generate hydrodynamic separating forces regardless of the rotational direction, they typically generate lower gas film stiffness. This can potentially allow seal face contact when axial perturbations of the seal rings occur.
Solids in Process Fluid
Solids are a common cause of mechanical seal failure in pump applications, but can be particularly problematic in some gas seal designs. Solids may be dispersed throughout the process stream or may be dissolved and crystallize out of solution between the inboard and outboard seals. In either situation, the solids cause two significant problems for gas seal performance.
First, gas seal faces are designed to operate with a gap between the mating faces on the order of 0.0001 in (2.54 µm). The solids can migrate into the seal face gap as shown in Figure 1 and cause third body erosion, degrading the seal face topography and diminishing its effect. For gas seal configurations where the process fluid is at the I.D. of the inboard seal faces, centrifugal forces will act to aid the migration of solids into the gap between the seal faces.