Want to go a long way toward preventing those excess costs associated with seal failure? Understanding and improving seal system performance starts with a thorough understanding of seal composition, operation, and maintenance.
There is an important, but often misunderstood, element in every pump system.
Seals – particularly radial shaft seals – have a profound effect on the longevity and performance of a pump system. Rubber radial seals provide economical and versatile protection for bearings in pumps, gearboxes, and motors. Their contact with the shaft offers both positive fluid retention and hermetic sealing of the housing. However, they do have practical and technical limits. Careful definition of the application requirements and a holistic approach to seal specification will yield a balance of properties with optimum service life.
Radial shaft seals are used in the power frames of many ANSI and other pump classes that depend on bearings for efficiency and productivity. In fact, up to 27 percent of mechanical failures are due to bearings and 80 percent of those are related to contamination. Bearings in pumps, gearboxes and other mechanical power transmission devices require effective seals to retain media and exclude contamination. The correct seal specification maximizes the system service life and prevents catastrophic failure of moving parts. Following a systematic approach to seal specification facilitates their performance and prevents problems.
The basic operating parameters of radial shaft seals are based upon a basic design (Figure 1). A spring loaded radial lip is usually needed to retain oil or low viscosity fluids. The seal must then be correctly sized for the hardware.
Figure 1. Basic seal design parameters.
Radial seals may have a plain, wave, or helix lip design, each having surface speed limits set by the manufacturer, beyond which fluid control under the lip is lost. Generally, increasing speed reduces other capabilities like the run-out allowance. Most rubber radial seals are limited to 3,500 to 5,000-fpm (17.78 – 25.40-m/sec).
Radial seals are friction devices and, due to oil shear and other factors, increasing speed raises underlip temperature. For example, an increase in shaft speed of 800-fpm (4.06-m/sec) can raise underlip temperature by 25-deg F (about 14-deg C).
Each seal material has an optimum range. Beyond that, thermal stress will harden the compound. Heat aging is a more common cause of failure than wear for common nitrile (NBR) rubber seals. It is important to consider ambient heat combined with the sump or media temperature. With NBR, the increase in underlip temperature of 25-deg F (14-deg C) is enough to reduce seal life by about 50 percent.
Infrared devices or remote sensors measure heat at or near the lip contact zone. Upgrading the seal material to a fluoropolymer or PTFE can extend the seal’s thermal limit to meet many system requirements.
Pressure loading, from system conditions or a fault such as a plugged vent, will mechanically load and distort the lip profile resulting in rapid wear and failure. Standard radial seals are designed for pressures of only about 7-psi (.05-MPa). Special profiles and materials compensate for pressure and in some cases can achieve a PV of 300,000. However, generally speaking, pressure capability and surface speed are inversely proportional.
Radial seals can accept certain ranges of shaft deviation from a true center. These include shaft-to-bore misalignment (Figure 2), or the shaft diameter not rotating in a true circle, known as dynamic run-out (DRO), expressed as Total Indicated Reading (TIR). Outside of the manufacturer’s limit, these conditions could crush the seal lip, leading to rapid wear, or open a gap causing leakage.
Directionality on the shaft and surface roughness are prime causes of leakage failure. A surface is actually a series of microscopic peaks and valleys (Figure 3). A surface that is too smooth has difficulty supporting an oil film leading to a high underlip temperature. Too rough a surface and peaks can project through the lubricating film and abrade the lip.
Seal manufacturer handbooks often provide roughness and texture specifications based on industry standards, usually RMA or DIN. Even if roughness is correct, a shaft can have directional lead (a spiral or screw pattern) from the initial turning or grinding method. While an inward lead might be beneficial, an outward pattern can auger more oil under the lip than its pumping action can manage. A good target is a roughness value of 8 – 17-μin Ra (.20 – .43-μm).
Electronic tracing instruments accurately assess surface finishes. The industry standard recommends holding lead to less than ± 0.05-deg. Housing bore roughness is less critical at 100-μin Ra (2.5-μm), and here lead is not consequential.
Both shaft and bore should have chamfers of 15-deg to 30-deg, or a smooth radius. Square corners guarantee a rolled lip or bent seal case. All contact surfaces should be free of burrs and nicks that could cut lip elements or score seal cases.
Installation and Assembly
Installation is another prime area for seal failure. The seal should be lubricated with the same fluid used in the application and must be square in the bore. Care should always be taken to direct installation forces to the outer diameter of the seal case. This can be done with an arbor press and suitable tools. A hand tool can also be used, along with a simple block of wood as a buffer. Hammer strikes on the seal case can also cause distortion of the seal profile (Figure 4).
Nitrile rubber works well in a wide range of mineral based oils. However, when used with polar solvents like acetone, it can cause catastrophic swells observed as a softening, often accompanied by physical destruction (Figure 5). Its weather aging characteristics are not good and surface cracking could occur from extended ozone or ultra-violet light exposure.
Similarly, a compound like ethylene-propylene resists all those conditions but swells rapidly from contact with aromatic hydrocarbons and mineral oils. Some oil additives are beneficial to gears by sacrificial transfer but can build deposits at the seal lip. Some lubricants are based on synthetics that resist oxidation and shear but can attack rubber compounds.
Particularly in slurry pump applications, chemical compatibility is very important to ensure proper operation and function. Consultation with both the lubricant and seal supplier can avoid problems.
Additional Seal Options
If pumps are located outdoors or in places with high external contamination, an auxiliary V-ring seal can often significantly reduce wear to the radial seal lip (Figure 6). These can be installed to run against metal seal cases and turn with the shaft, providing a slinger action. They add little power loss and can run dry or with grease prepack. However, many ANSI and especially API pumps are migrating to labyrinth or bearing isolators at the thrust and line ends. These sealing devices have non-contacting stator and rotor elements, or lightly loaded axial faces, and thus have extremely low internal wear and can match the bearing L10 life. They are usually machined from metal or plastic and have excellent chemical inertness, a wide temperature range, and good abrasion resistance. They are principally effective in excluding wash down fluid sprays and particulate contamination from the power frame. Some labyrinth products have a vapor blocking ring to control humidity ingress during shutdowns, while face types do so by contact elements.