Mechanical Seal Designs for Wastewater Pumps & Guidelines for Slurry Pump Wear Resistance

Q. What mechanical seal designs are recommended for wastewater application pumps? A.In wastewater, two seal arrangements are commonly used: single seal and dual-pressurized (double) seal. Single seal designs, shown in Figure C.2, are normally either a stationary pusher seal with the springs isolated from the process fluid or a rotating elastomeric bellows seal with a large single coil spring wetted by the product.

Single seals, also known as pusher seals, can be further broken down into split, shown in Figure C.4, and nonsplit designs. Nonsplit pusher seals for wastewater applications are almost exclusively a cartridge design that fits into the pump stuffing box or seal chamber. However, because of the pump type and available space, some pusher seals may be noncartridge designs. Split pusher seals are mounted outside the pump stuffing box or seal chamber to facilitate the assembly of the seal halves. Advantages of single and split pusher seals are outlined below:

• They are hydraulically balanced and provide maximum interchangeability in pumps and other equipment without equipment modification.
• They eliminate sleeve wear and minimize power consumption compared with packing.
• They have a wide operating window, from vacuum to positive pressure.
• They can be installed without sleeve or shaft replacement, even if the pump was previously packed.
• The springs are typically a nonclogging design or are located outside the process fluid, which prevents them from clogging.
• They eliminate excessive leakages that lead to bearing failures and corrosion issues.
• Large pumps benefit from split seals by saving equipment teardown for initial installation or seal replacement.
• They reduce maintenance and operating costs for large pumps through reduced flush requirements and repair costs.
• While the initial cost of split seals can be slightly higher than nonsplit component or cartridge seals, the overall life-cycle costs (LCC) can be less. The savings associated with split seals are achieved by not having to remove the pump or realign the equipment for seal repair.

Pusher seals by definition have a secondary sealing element that is pushed along the shaft/sleeve to compensate for wear, vibration and shaft movement in the form of axial end play or runout. With stationary pusher seals, the wetted areas exposed to the process fluid are the seal faces, primary and mating rings, and minimal adaptive hardware sections of the sleeve and gland plate. With these designs, the profiles are relatively smooth and offer good to excellent resistance to abrasion and erosion when used with the appropriate materials. Elastomeric bellows seals, shown in Figure C.3, are available as either shaft-mounted component seals or cartridge seals, based on the pump design. Specialty nonsplit and split designs also are available for specific applications involving highly abrasive services and/or large rotating equipment. Elastomeric bellows seals, also known as nonpusher seals, are designed to prevent contact between the shaft and the portion of the bellows touching the primary seal face. Because of this feature, elastomeric bellows seals can withstand more angular misalignment than pusher seals. Elastomeric bellows seals have a large single coil spring that is not subject to clogging like small multiple spring designs that are exposed to the process fluid in other pusher seal designs. Elastomeric bellows seals offer fair to good resistance to abrasion but are typically limited in metallurgy to various grades of stainless steel and nickel-copper alloys. Abrasion and erosion may not be a factor in seal selection, depending on the piping plan. For more information about wastewater pumps, reference HI’s guidebook Wastewater Treatment Plant Pumps: Guidelines for Selection, Application and Operation. For more information about mechanical seals, reference HI’s guidebook Mechanical Seals for Pumps: Application Guidelines. Q. What guidelines should be followed regarding selection of slurry pumps based on good wear resistance? A.Pump wear depends on the pump design, abrasiveness of the slurry, the specifics of the application or duty conditions, the way the pump is applied or selected for the duty, and the conditions of service. Wear inside the pump varies depending on the velocity, concentration and impact angle of the particles. Wear is normally most severe in the impeller seal face area of the suction liner, followed by the vane inlet and exit. The casing wear amount and location also vary with the shape of the collector and as a percentage of the actual operating conditions compared with the best efficiency point flow. Many slurry pump wear parts may last for years with only routine maintenance. Services, such as transportation of high concentrations and abrasive or large solids, can reduce a part’s life to several months. Larger pumps with thicker sections, more wear material and slower operating speeds can improve life in all applications, though the increase in product cost may not be warranted in each particular case. Analytical and numerical models are available for making qualitative predictions of wear. Their limitations and the variability of slurry service mean that wetted component life prediction is still only good for estimation and should not be used for guarantees. These estimates are normally based on the specified operating condition of the pump and may vary if the pump operates under different conditions. An LCC evaluation of the capital, power, wear and other expenses associated with pump operation can be used to estimate the best balance between different pump designs. Such analysis is largely theoretical because wear can be unpredictable in actual service. Ranking the slurry into light (class 1), medium (class 2), heavy (class 3) and very heavy (class 4) services, as shown in Figure 12.3.4.2a, provides a practical tool for pump selection. This ranking is based on aqueous slurries of silica-based solids pumping (Ss = 2.65). It also provides guidance for mineral slurries if an equivalent specific gravity for the mineral slurry is used to determine the service class. The boundary lines between the service class areas in the chart approximate the limits of constant wear modified for practical considerations and experience. Capital and operating cost considerations are such that different (higher specific speed) designs may be employed for the lighter service classes. More information on slurry pumps can be found in ANSI/HI 12.1-12.6 Rotodynamic (Centrifugal) Slurry Pumps for Nomenclature, Definitions, Applications, and Operation.