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Abengoa Bioenergy, formerly High Plains Corporation, owns and operates five bioethenol facilities throughout the United States and Europe, with a total yearly production capacity of 195 million gallons. Given the harsh nature of its working environment, signs of wear on the company’s many tanks, pumps, and seals is to be expected. But when demand is high, production must run at the pinnacle of efficiency, and seal reliability can have an enormous impact on the company’s bottom line.
Most process pump seals at Abengoa’s York, Nebraska plant had a mean time between failures (MTBF) of 60 to 120 days, however one seal lasted merely one day before leaking. The situation was further complicated by the nature of the product being created at the York facility – a viscous mixture containing between 30 and 35 percent solids. The standard solution for this application is a double seal, although the time and expense required to install the support equipment needed by pressurized seals – as well as the possibility of leaking barrier water diluting the process fluid – created its own concerns.
Mechanical Seals in Pumping Applications
A. Elements of mechanical seals
An end-face mechanical seal, often called simply a mechanical seal, is a type of seal employed in rotating equipment, such as pumps and compressors. The seal uses both rigid and flexible components to maintain a sealing interface with the rotating shaft, and are both mechanically and hydraulically loaded to maintain contact with the rotating shaft. Mechanical seals are comprised of four parts:
1. Primary sealing surface
2. Secondary sealing surface
3. Actuation mechanism
4. Drive mechanism
The primary sealing surfaces are the key to the seal, and are typically comprised of a combination of very hard materials – such as silicon carbide or tungsten carbide – embedded in the casing; softer materials are used in the rotating seal assembly. Other materials may be used as needed, depending on specific requirements of the application, such as temperature and the chemical properties of the fluid. Secondary sealing surfaces are those parts of the seal that require a fluid barrier, but are not rotating. The actuation mechanism shown in Fig. 1 is a mechanical spring, although the pressure of the sealed fluid will oftentimes aid the spring in actuating the seal. The drive mechanism of the seal ensure that only the primary sealing surfaces are allowed to rotate – although these must rotate in conjunction with the rotary shaft, the primary and secondary sealing surfaces cannot be allowed to rotate relative to each other.
All mechanical seals must contain these four components, although these components may interact with one another in a wide variety of ways. Mechanical seals are usually categorized as “pusher” or “non-pusher,” depending upon the nature of the secondary sealing surface relative to the rotating shaft. Pusher seals feature a dynamic secondary seal which is allowed to move axially with the shaft; non-pusher seals do not. A “cartridge seal” is a prepackaged variety that is commonly found in more complex applications, or in situations where the design of the equipment made the use of a typical component seal impractical, such as horizontally split or vertical pumps.
Because all mechanical seals are designed to use the process fluid for sealing actuation and lubrication of the rotary shaft, the seals are designed to leak. Normal leakage rates will vary by seal and application, but might range from a barely perceptible drip to a readily apparent stream. Some seals might not leak to a measurable degree, some seals might leak erratically, and others may leak in a constant or progressive fashion. However, it should be noted that some form of mechanical contact is necessary for very low or imperceptible leak rates; hydrostatic or hydrodynamic non-contacting (“full lift off”) seals tend to have the most visible leak rates.
B. Tandem and double seals
Due to the “mandatory leak” nature of mechanical seals, great care must be taken when employing seals in hazardous processes such as chemical and petroleum processing. In these instances, a secondary (or “containment”) seal is placed in line with the primary seal along the rotary shaft. This containment seal is filled with a neutral liquid or gas called a barrier fluid (if the containment cavity is unpressurized, this medium is called a “buffer fluid”). In a tandem seal, the primary seal will leak process fluid into the unpressurized containment cavity; once the pressure in this cavity has risen by a certain amount, the process operator will know that the primary seal has failed; if the operator inspects the containment cavity and finds it drained of fluid, they will know that the secondary seal has failed. This capacity of tandem seals to allow the process fluid to leak into the containment cavity make these types of seals useful in applications where the process fluid would create a dangerous situation if allowed to leak into the atmosphere, or when the process fluid would change state (i.e. sublimate or combust) if allowed to contact with air.
Double seals are commonly employed in gas applications, or in processes which contain a process fluid that is toxic, abrasive, corrosive, viscous, or contain an appreciable percentage of solids per unit of fluid. The barrier cavity of double seals is pressurized, and requires the use of secondary support equipment to keep the barrier cavity pressurized and the barrier fluid re-circulated as needed.
Reducing Costs with the John Crane Type 5870
The John Crane Type 5870 flushless seal features many advantages over the typical seal types used in ethanol applications. Chief among these is the fact that the 5870 is able to operate without the support equipment normally required for double seals. The unit comes preassembled from the factory, and – as was the case with Abengoa – can be installed trouble-free in just a few hours’ time. The 5870 features open-profile, abrasive-resistant sealing faces that allow the seal to be placed near impellers, allowing for maximum cool running and clog-free performance. The 5870 also features a single-coil spring allows for a large range of shaft motion due to cavitation and pulsation.
The greatest advantage of the 5870 is its reduction in operating costs. This is primarily due to the absence of a required support system, as would be found on other double seals. However, “down the line” process costs are also avoided by the 5870’s design. When a seal employs a support system, there is always the concern that barrier fluid will mix with process fluid and require separation later in the process. For Abengoa, this would have necessitated another separation process down the line, in order to remove the barrier fluid from the product for re-circulation. This feature also reduced the plant’s water dependency, which would have dramatically increased operating costs during drought conditions when water prices skyrocketed. And because seals primarily fail due to poor flush conditions, eliminating this liability increased the robustness of the seal significantly.
From a cost standpoint, Konwinski said the seal quickly paid for itself. “The cost of a double seal, as well as the cost and manpower associated with a flush system, quickly adds up [. . .] We are pleased with John Crane’s solution and are happy to report that everything advertised about the product is true.
http://www.johncrane.com/iPortal/upload/banner//S_5870.pdf
http://www.abengoabioenergy.com
http://www.sealsentinel.com/interphex/Day1-Story2.html
http://www.pspglobal.com/mechanical-seals/fluid-leakage.html