As water process system operators know, there is a tangible relationship between water filtration complexity and value. String wound filters are a perfect example in general water sediment filtration applications, given that string wound filters are high-load workhorses available at competitive costs. However, when misapplied, these same virtues can quickly become detrimental, primarily fiber shedding. Below we’ll explain the concept of fiber shedding, its risks, and prevention suggestions so that process system operators can achieve the well-balanced performance and cost benefits they require.
To start, let’s explain what string wound filters are:
- String wound filter cartridges are consumable filtration elements that are placed into fixed filter housings, and used to extract visible suspended solids from fluid streams including dirt, piping scale, and organic sediment.
- String wound filters are most common in water applications for front-end sediment removal and general protection of sensitive downstream process steps. They do not filter any color, aroma, flavor, or chemical properties.
- These filters work best with ambient temperature water that is pure or contains mild chemicals stemming from upstream treatment (such as low-grade alkali and acid pH balancing).
- String wound filters are specified by their filtration size (typically ranging from 1/2 microns to 200 microns), length, and diameter. In general, longer lengths provide higher flow rates, and larger diameters provide higher soil holding capacity.
- These filters are available in polypropylene, glass fiber, cotton, nylon, and polyester materials, with polypropylene being the most common.
- String wound filters are constructed by coiling string lengths around a plastic or stainless-steel core tube, building up overlapping layers of strings to create a dense filter media layer that captures soils from fluid streams passing through the filter.
Because of the way that string wound filter elements are constructed, they present an inherent risk known as fiber shedding. In short, fiber shedding occurs when pieces of the filter element detach and get carried into passing fluids. At best, fibers may shed from the outside of the element’s surface due to normal wear but are recaptured by the intact deeper layers of the element, maintaining full filtration efficacy. At worst, fibers are shed deep into the element body and create large flow paths through the element, rendering the filter ineffective and immediately introducing a risk to overall system integrity. For this reason, users must actively defend against fiber shedding as a matter of total process safety.
Water Quality & Performance Concerns
Now that we understand the general concept of fiber shedding, let’s review the most common risk factors that fiber shedding presents:
Downstream Media Contamination – shed fiber materials are considered contaminants that adulterate the fluids they are carried in. These free-floating fibers can remain suspended in the process fluid all the way to final use or packaging, fouling subsequent process steps and final products. In addition, fiber particles can deposit downstream within piping components and cause issues such as plugged instrument ports, cracked pump seals, and scarred valve seats.
Filter Overloading – fiber shedding is often the result of a filter being used too long to the point that the filter becomes fully saturated beyond its capacity. Once overloaded with soils, the filter will substantially drop in its flow capacity, causing incoming process fluid to cut flow paths through the element and loosen the filter’s string windings. Once the string pack is loosened, filter strings have a much greater chance of being broken off.
Soils Release – as string wound filter elements loosen and fibers shed away, the soils that were retained within those fibers will also be released into the process fluid stream. Should the filter element loosen enough that flow paths through the filter develop, the filter is then considered fully compromised as it will allow soils to be released from the element and to flow downstream.
Hydraulic Impacts – when fiber shedding occurs, system hydraulics suffer overall. In systems with progressive filtration elements, free-floating fibers shed from coarse elements can plug up downstream fine elements and cause overall flow restrictions. In systems with only coarse filtration, compromised string wound filters can reduce pressure drop as larger flow paths are developed, which can cause supply pumps to ramp up and overrun flow setpoints.
Regulatory Compliance Issues – in regulated process environments, shed fibers are treated as non-compliant foreign contaminants that trigger all sorts of regulatory scrutiny. In water treatment applications, fiber particulates can bring treatment parameters out of specification, impede necessary chemical reactions, subvert legal labeling requirements, and even pose health and safety risks (specifically in the case of municipal and bottled water processes).
Knockdown Systemic Impacts – loose fiber materials can wreak havoc on sensitive process systems, especially with potable water for human consumption. Not only can fiber contaminants foul downstream sensors and cause automated process control calibration to drift, but they can also pose direct safety risks such as plugging relief valves and drain orifices. Fiber shedding occurs slowly over time, and may not be caught before it causes detrimental knockdown effects on other parts of the system.
Combating String Wound Filter Concerns through Proper Operation and Care
While the above list of fiber shedding concerns may seem daunting, there are straight-forward methods that operators can employ to defend against these issues occurring, such as:
- Select Appropriate Filter Media – string wound filters are preferred for general sediment removal from ambient temperature, high flow, water-based fluids. Purchasers should specifically check flow rates, pressure drops, sediment load volumes, chemical properties, and temperatures for compatibility. Any single parameter that falls outside of a string wound filter’s compatible range must be treated as a risk that can lead to fiber shedding. In those cases, operators should consider more suitable filter types such as pleated filters, bag filters, and melt-blown filters.
- Inspect and Maintain Elements – operators must routinely inspect their string wound filter elements and their housings for any signs of the concerns listed above, especially physical and hydraulic stresses that precede fiber shedding such as indentations, discoloration, chaffing, and loosening. String wound filters generally cannot be cleaned and reused, so operators must monitor the filter element’s loading and then replace the filter before it becomes fully impacted.
- Maintain Consistent, Mild Operating Temperatures – String found filter elements are sensitive to temperature cycling, and normal thermal expansion and contraction can loosen spiral windings, leading to premature shedding. For this reason, string wound filters work best at consistent, normal temperatures. In addition, different fiber materials have varying minimum and maximum temperature ratings, so purchasers must be sure to select a filter material that falls well within their application’s temperature range.
- Reduce Flowrate & Abrasion – many specifying engineers select string wound filter elements near the top of their flow and pressure ratings to keep cost down (that is, getting the smallest filter size that works for the application). In practice, stepping up to the next size filter provides more headroom over the system’s expected flows and pressures, which in turn reduces the velocity and force being exerted onto the string windings.
Seeking Ultimate Performance and Protection with String Wound Filters
In modern process system design, we often recommend additional protection solutions to be considered along with single filter modules to ensure overall system reliability. Such additional protection solutions may include automated differential pressure instruments and alarms, multi-plex filter housings for redundancy, balancing orifices or valves, inline sight gauges, and downstream turbidity sensors. With or without these additional protection options, string wound filters provide practical, effective sediment removal so long as they’re properly selected for the application at hand and actively maintained over time.