By Allan R. Budris
Mechanical shaft seals are the number one cause of pump failures, as discussed in my http://www.waterworld.com/articles/print/volume-27/issue-7/departments/pump-tips-techniques/back-to-basics-mechanical-seals-for-water-and-wastewater-pumps.html July 2011 WaterWorld Pump Tips column. Gas-lubricated mechanical seals were not covered, however, because at the time I felt that this seal type was more applicable to chemical pumps and pumps handling toxic liquids, rather than water and wastewater pumps.
However, gas-lubricated mechanical seals offer unique advantages that could be beneficial to water and wastewater pumps. These include higher reliability, reduced system and equipment costs, and low power consumption, in addition to zero emissions. They eliminate the energy requirements of circulating and cooling a liquid barrier, and generate significantly less heat and torque at the seal faces. Further, liquid seal costs associated with gas barrier fluid seal support systems are also simpler and require less maintenance than the buffer or barrier systems required for dual wet seals.
Double Gas Barrier, Pressurized Dual Mechanical Seals
Gas seals (see Fig. 1) run in a clean, controlled fluid environment and are designed to remain non-contacting and non-wearing for the expected operating conditions. Similar to cartridge double seals, this sealing involves an inert gas, like nitrogen, that acts as a surface lubricant and coolant in place of a liquid barrier system or external flush required with conventional or cartridge double seals. Gas seals were first developed for, and applied on, compressors. However, this concept was later adopted for pumps because many barrier fluids commonly used with double seals can no longer be used due to new emission regulations.
The gas barrier seal uses small, well-controlled amounts of a pressurized gas -- such as nitrogen, purified air or other inert gas -- introduced between the seal faces (as a harmless and inexpensive barrier fluid) that helps prevent product emissions to the atmosphere and fully complies with emission regulations. The faces of dry gas seals are carefully etched or contoured with some macro-topographic pattern (such as spiral grooves or a wavy face). These features enable generation of hydrodynamic pressure that prevents contact of the seal faces and allows a minute amount of face separation to take place. Gas seal faces are wider than plain-faced liquid seals and are loaded with lighter spring force.
The use of gas-lubricated mechanical seals in pumps has increased for a variety of reasons. For example, they are non-contacting and do not wear; they produce consistent seal performance, even in a widely varying duty cycle; and they reduce power consumption at the seal faces. The seals generate little heat, and any gas leakage absorbs heat through gas expansion. Further, they eliminate water costs since water isn't needed to flush the seals and there are no water treatment requirements. Also, the cost of the barrier system is significantly less than a typical wet seal support system, and energy consumption is less than 10 percent compared to a wet dual system. Repair costs are lower because, since there is no contact, the seal faces do not wear. It is imperative, however, that the seal faces be separated with a gas barrier film; if there is contact, the seal will not last long running dry at typical pump speeds.
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 specific 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. This can be an issue under transient slow roll conditions as the equipment shuts down, especially for applications where a variable frequency drive with an extended soft start or shutdown is applied. Variable speed drives (VSD) are quite common in water and wastewater applications.
Gas seals can be successfully applied under these conditions, so long as it is 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. 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.
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 can cause two significant problems for gas seal performance. First, they can migrate into the seal face gap and cause 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.
Several design features can, however, be incorporated into pump gas seal systems to mitigate this problem 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.
Conclusion
As discussed, gas-lubricated mechanical seals may help increase pump reliability and lower costs, even for many water and wastewater pump applications, especially if the installation already requires a double seal. The application must, however, be thoroughly reviewed with the pump and/or seal manufacturer, especially if a VSD and/or solids in the process fluid are present. With clear identification of the conditions of service during seal specification, gas seal designs that can handle most application challenges can be applied. This means that with proper upfront engineering and application evaluation, gas seals can provide a low-cost, highly reliable, energy-efficient, zero-emissions sealing solution for many pump applications.
About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits, and litigation support on pumps and pumping systems. He can be contacted via email at [email protected].
References
1. Lai, Tom. "What Are the Basics of Gas-Lubricated Seals? (Part One)," Sealing Sense, 5th Anniversary, Pumps & Systems, November 2009.
