To eliminate leakage, mechanical seals are the method of choice in sealing most pumps in ethanol plants. However, eliminating process leakage in rotating equipment does present many unique challenges to achieving long-term reliability. A primary consideration for reliability is identifying the appropriate seal design and materials of construction, and a critical part of the process is determining and installing the correct environmental controls for each application.
Seal Design versus Fluids Pumped
Process conditions in ethanol plants range from systems that operate under a vacuum, which tends to create negative pressure in pump stuffing boxes, to booster pumps that operate with high suction heads that create above average stuffing box pressures. To further complicate matters, many of these systems change erratically due to the nature of the process and the human error factor. Clean-in-place practices introduce a completely different set of operating controls to consider when selecting the correct seal system for ethanol applications.
To achieve maximum reliability, all characteristics of the various products pumped in the ethanol manufacturing process have to be considered since they directly affect the seal system. These include fluids that are extremely abrasive, viscous and sticky slurries as well as ethanol, which must be maintained at a vapor pressure high enough to prevent it from flashing to a gas. These difficult conditions are further complicated by process temperatures that range from 90-deg F to 220-deg F. In addition, incorrect selection of stuffing box design, which is now available from pump manufacturers, can also create adverse conditions in and around the seal area that can accelerate erosion and wear and cause premature failures.
Fluids pumped in ethanol plants fall into two general categories: 1. Clean fluids like ethanol and water and 2. Abrasives such as mash, non-fermented compounds and syrups. Mash is cooked ground corn mixed with water and additives and then fermented. After fermentation and distillation, the slurry containing non-fermentable solid compounds is evaporated to thick syrup. The syrup is further processed into dried distiller's grain.
A multiple mechanical seal is required to seal these slurries (see Figure 1 above). These seals must be flushed with a clean liquid, operated in a double mode and be capable of pressure variations, temperature changes, clean-in-place solutions and deadheading. Seal facings, metal parts and elastomers must be compatible with this range of operating conditions to ensure reliability.
Environmental Control Plans
Environmental control plans must provide proper lubrication and cooling throughout the range of normal operating conditions as well as upsets or abnormal operation. To be compatible with the ethanol process, water is the desired flush fluid. To conserve water, the flushes are maintained in closed-loop systems that can cause concerns for reliability. Closed-loop systems require steps to circulate and cool the water to maintain an effective flush through the seal (see Figure 2 at left). Otherwise, undesirable heat generated by the seal will result in early failure.
Pumping clean fluids-ethanol and water-requires sealing solutions to enhance lubrication and cooling. These applications are handled with single seals designed with materials that allow for cooler operations. To further enhance cooling, environmental control plans should be employed to remove heat and ensure proper lubrication.
Using an evaluation and selection process, plant operators have been able to increase their mean time between repairs (MTBR) from less than one year to more than three years.
Alternative mechanical seals for slurry applications are currently being pursued. These designs use single seals in place of double seals and eliminate water as the barrier fluid. For example, one design incorporates specially formulated grease as the barrier fluid. An automatic grease attachment maintains the barrier grease to the seal. The greaser attachment also provides a simple visual indicator check for refilling.
Pumps & Systems, April 2008