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.
Rotational Speeds
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.
Rotational Direction
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.
Figure 1. Solids migration into seal face gap
Spring Loads
Second, the spring loads that provide the closing force of the seal faces in pusher type seals are typically about 50 percent lower for pump gas seals than for liquid lubricated seals. These lower spring loads are employed to avoid overcoming the gas film stiffness created by the hydrodynamic opening forces (Fo) and minimize the risk of seal face contact.
Figure 2. Forces on gas seal faces
Due to these low spring loads, pusher type gas seals can be susceptible to secondary seal hang-up, which occurs when the dynamic secondary seal cannot move axially to allow the flexibly mounted seal ring to track the mating ring. Hang-up can occur when the seal ring is displaced axially from the mating ring, resulting in excessive barrier gas consumption, or when the seal ring is displaced axially toward the mating ring, resulting in excessive closing forces and possible seal face contact.
Design Solutions
Several design features can be incorporated into pump gas seal systems when solids are known to exist in the process fluid. One approach is to prevent the solids from entering the seal chamber area. This can be achieved using solids exclusion devices that are mounted at the impeller end of the seal chamber. These devices may include volute-like geometries that convert the rotational velocity of the process fluid in the seal chamber into axial velocity, directing the fluid and its solids out of the seal chamber.
Seal configurations with process fluid at the O.D. of the inboard faces also help to reduce the migration of solids into the seal face gap.
Other designs may use a restriction at the seal chamber throat such as a labyrinth seal (see Figure 3). API Plan 32 can also be used to prevent the ingress of process fluid and solids into the seal chamber with a close clearance bushing and injection of a clean fluid into the seal chamber.
Special dynamic secondary seal designs can be employed to minimize the chance of hang-up. These designs typically minimize the amount of drag that the secondary seal creates when it moves axially. They may also prevent solids from collecting at the process side and restrict axial movement of the secondary seal. The designs often control the secondary seal squeeze in the radial direction by using a canted coil or leaf spring to energize the secondary sealing surfaces against the axial sealing surfaces.
Gas seal designs incorporating metal bellows seals are also available. The use of a metal bellows seal instead of a pusher type seal completely eliminates O-ring secondary seal hang-up.
Conclusion
The conditions described in this article are some of the most common causes of failure when gas seals are applied to pumps. However, with clear identification of these conditions during seal specification, gas seal designs can be applied that mitigate these application challenges. With proper engineering and application, gas seals provide a highly reliable, energy efficient method for providing zero emissions sealing solutions for pump applications. Contacting a seal manufacturer with complete application details first can ensure optimum results.
Pumps & Systems, December 2009
Next Month: How do I prevent gasket blow out?
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