Seal Flush Filtration: Comparing Resin Bonded to Melt Blown Filter Cartridges
Filters are widely used in heavy industrial applications such as Pulp and Paper mills to protect the seals of rotating equipment such as pumps and refiners.
The gland box usually consists of a hard sleeve mounted on a shaft, with winds of packing compressed in a housing to form a seal on the sleeve. To lubricate and cool the seal, a fluid must be applied to the packing – this is often surface water from a lake or river.
Filtration is employed to ensure that particulate in the water, such as sand or silt, is removed. Otherwise this particulate can become embedded in the packing, cause wearing of the packing sleeve, and lead to pre-mature failure. Such a failure can cause significant upset to the operations of the plant, including downtime, reduced output, thousands of dollars in lost revenue, and a costly repair. A typical filtration system consists of a number of pre-filtration stages, followed by a final cartridge filter close to the seal being flushed.
Historically resin bonded filters have been used for industrial and commercial seal flush applications. That product however is intended mainly for highly viscous fluids or high temperatures, not for ambient temperature water as seen in this application. In fact, better results can be achieved by utilizing newer, more advanced filter products such as the Graver Stratum C series, multi-zone melt blown filters, as discussed below:
Graded Pore Structure A graded pore structure is beneficial in applications where there is a broad particle size distribution, such as in surface waters. A graded pore structure means that there is a distinct difference in the size of the pores within the filter from the outside to the inside – with more open voids or pores at the surface of the filter to capture larger particles, and smaller voids towards the centre of the filter to capture the finest particles. The result is complete utilization of the entire volume or depth of the filter, the capture and retention of a wide range of particle sizes, and extremely high ’dirt holding capacity’.
All depth type filters are manufactured from fibers that are formed in such a way to create a deep matrix of pores which capture particles from the fluid passing through the filter. Filters using long fibers are unlikely to have shedding issues, as the fibers are intertwined and held in place within the matrix. Short fibers, conversely are more likely to come loose, and be carried out of the filter in the liquid flow.
Filter Life Typically filters are changed based on differential pressure across the filter. This “terminal” pressure drop is typically no more than 35psid, but can vary depending on the application. However, some users opt to change filters based on other criteria such as a reduction of flow, on a scheduled planned maintenance basis, or based on visual inspection of a filter in a plastic housing.
Filters that provide the longest life utilize the entire depth of the filter to capture contaminant. Depth filters that capture particulate mostly on the surface of the filter tend to be changed more frequently, especially if change is based on a visual inspection.
Under no circumstances should two or more filters be stacked on top of each other in a filter housing. The correct length of filter should always be used. If filters are stacked, there is a high risk of fluid by-pass through the area where the two filters are butted together.
Filter Manufacturing Process
Nominally rated filters can vary significantly from manufacturer to manufacturer, as there are no industry standards for testing and rating filters. Some nominally rated filters are as low as 50% efficient at their rated micron retention, while others are designed to meet a more stringent 90% efficiency. To ensure consistent filtration results from batch to batch, the manufacturing process has to be strictly controlled.
Graver Stratum C Series advanced melt blown filters offer a superior alternative to resin bonded filters for protecting the seals in rotating industrial equipment. Users can expect more precise filtration, less fiber migration, longer on-stream life, and ultimately better seal protection. The same conclusions presented here can be applied to mechanical seals as well, where clean water is required to cool the seal and flush contaminant away from the seal faces.