Installing a high-performance liquid filtration system can be a sound investment for your process or lubrication streams but you can easily undermine the entire system’s performance by installing an inaccurately sized or improperly specified filter.
Because it must serve as a barrier to contaminants, the filter acts as a restriction or obstacle to an otherwise free-flowing stream. The filter specification should lead to a filter that keeps the process or lubrication stream flowing at the desired capacity while achieving the required fluid purity standards.
It’s common to strive for short-term cost control by specifying a filter to support the minimal performance threshold. However, specifying without a thorough understanding of guidelines can result in an undersized filter. An undersized filter creates an unwanted restriction, a pressure drop on the line, and thus a strain on the entire system. In addition, the filter itself will suffer strain and, as such, yield a brief service life.
Conversely, a filter that’s properly fitted — or even conservatively oversized — can produce long-term savings. A larger filter can support a system with a less expensive pump and motor. In addition, its superior cleaning capabilities will result in less frequent fluid and filter change-outs.
Accurate filter specification requires a review of 12 critical parameters. Several of these are interrelated. Let’s look at each of them.
- Flow rate and regularity of flow. A filtration system obviously must be able to accommodate the stream’s flow rate. Indeed, flow rate is the most critical parameter in determining the appropriate filter size; so, flow rate data will be among the supplier’s first requests. Flow rate analysis also will consider the regularity of the stream’s flow — whether it’s steady, intermittent, variable, or potentially subject to sudden increases.
If flow rate information isn’t readily available, check the system’s original mechanical drawing for details. If you still lack data, the supplier will request additional information to estimate flow rate and will probably recommend a slightly upsized filter, to err on the side of caution.
- Reservoir size and usage rate. These data can be used to support calculations of overall system flow rate. In addition, they’re particularly important in applications that recirculate oil through a side-stream (kidney loop) filtration system because the filter must be able to adequately clean the fluid within the required reservoir turnover rate and time.
If the filtration system is undersized, the contaminant either will enter the system more quickly than the filter can process it or will linger and settle in the reservoir to create a lasting contamination issue. By specifying a filter that cleans the contents of the reservoir more quickly and more frequently, you can avoid system damage from residual or acute contamination. This attribute is particularly important for systems with smaller, modern reservoirs with higher usage rates.
The traditional rule-of-thumb for reservoir turnover is to remove and clean 10% to 20% of the fluid per hour. Today, filtration systems can be specified for an even higher turnover rate. To accurately assess the performance of a filtration system fitted to a re-circulating system, test the purity of the fluid as it enters the reservoir and exits the filtration system.
- Viscosity. Analysis of the process or lubrication stream’s viscosity also is essential to proper filter fitting because viscosity can affect flow rate. While water and other low-viscosity fluids experience little variation in viscosity, the viscosity of other fluids such as oils can change significantly with the operating temperature (or temperature variations throughout the system). So, a filtration system that processes higher viscosity fluids (Figure 1) requires special attention for a sound specification fit. Viscosity can be an important factor, for instance, in turbine and gear oil applications.
Figure 1. This unit provides particulate removal to 3 micron and water removal to less than 100 ppm while handling flows from 50 to 800 gal/min.
Ideally, the filter size specification, as well as pump and motor specifications, will be based on the measured or predicted viscosity of the liquid at the time it flows through the filtration system. Equipped with this information, it’s easier to produce a reliable prediction of the fluid’s flow rate and the performance demands on the system.
- Material compatibility. The filter vessel and element obviously must suit the fluid. Otherwise, filtration system parts could deteriorate and enter the fluid stream — meaning that today’s O-ring could become tomorrow’s contaminant. This parameter is particularly important in applications using synthetic and ester-based lubricants, although compatibility also should be considered with systems using mineral-based fluids.
Consider all aspects of the filter, including the vessel, adhesives (if present), seals, filter media, and instrumentation (gauges, flow sights). A compatibility check can be done quickly and easily by comparing filtration system materials with those currently used in the process. If the system has been successfully operating, then a filter fabricated of the same materials should seamlessly integrate with the existing system. The fluid’s supplier, of course, can provide information on compatibility and hazards.
- Pressure and differential pressure. The system’s operating pressure (pump pressure) determines filter vessel sizing. It’s a good practice to size the filter vessel for a pressure at least 33% higher than the maximum operating pressure to ensure that the vessel is well within the limitations of its pressure rating. Filter element sizing is based on the differential pressure across the filter.