Is this type of cost-effective technology right for your operations?

Each year, thousands of positive displacement compressors suffer serious damage because upstream filters or separators are really not doing their jobs as anticipated by the owner-purchaser. The reputations of machinery engineers are also at risk because they often neglect to understand the full impact of liquid and particulate entrainment in the gas. That said, engineers would do well to study the merits of reverse-filter technology.

Reverse-flow filter-separator technology is a profit generator for best-of-class refineries and petrochemical plants. First applied in the mid 1970s, these flow-optimized, self-cleaning coalescers (SCCs) represent mature, low life-cycle-cost, best-technology solutions for reliability-focused users. A reliability-focused user is far more interested in low life-cycle costs than lowest possible purchase price.

However, since aggressive marketers are known to have clouded the issue with advertising claims, a thorough examination and explanation of facts and underlying principles is in order.

Conventional filter-separators vs. SCCs
To understand how SCCs work, we first must recall how most conventional filter-separators (CFSs) function. In the CFS shown in Fig. 1, the gas enters the first-stage filter elements where its velocity is reduced as it passes through a large filter element area. Initially, the various and sundry contaminants (iron sulfides, etc.) are caught by the filter, but the gas forces gradually sluff it to a particle size that will pass through the filter elements.

The gas and solid particles, as well as the liquids coalesced on the inside of the filter element undergo re-acceleration and are being re-entrained in the collector tube before being led to the next separator section. With wire mesh or vanes in this section typically allowing passage of fine mist droplets and particles—let's call them "globules" of liquid—in the below 3-8 micron size range, a good percentage of liquid and small solids (particulates) remain entrained

In contrast, self-cleaning coalescers or SCCs (Fig. 2) vastly reduce this entrainment and send much cleaner gas to the downstream equipment.

However, SCCs do not accomplish this task by merely making the inlet into an outlet, changing the outlet to the inlet, and calling the "new" device a reverse flow unit. Instead, consideration had to be given to internal configuration, flow pattern and—most importantly—the characteristics of both the liquids and solids to be removed. The designers of this equipment had to adjust their thinking from only pressure-drop concerns to considerations dealing with liquid specific gravities, liquid surface tensions, viscosities and re-entrainment velocities.

In properly designed SCCs, gas first passes through the plenum, then through collection tubes and to the filter elements. The front-end of an SCC represents a slug-free liquid knockout. The de-entrainment section is sized to reduce the gas velocity so as to allow any particulates that might have made it through the filter to either drop out or attach themselves to the coalesced liquid droplets that fall out at this stage. Over three decades of solid experience have proven the effectiveness of this design. Essentially all entrained particulates and mist globules are removed, as are free liquids and large agglomerated materials.

Removal efficiencies examined
Some CFS configurations and models are claiming removal efficiencies with their so-called coalescers that are much better than those actually achieved. These claims are often made for vessels that are much smaller than the well-proven SCCs, and they are virtually impossible to achieve by single-stage CFS models. In addition, these CFS designs are vertically-oriented and their manufacturers or vendors sometimes state—incorrectly—that effective coalescing cannot be achieved in a horizontal vessel.

Upon closer examination, one may find certain CFS configurations to have high pressure drops with "moist" gases, or high velocities, shorter filter elements, virtually never any slug-handling capacity. Moreover, unless a vendor or manufacturer uses the High Efficiency Particular Air (HEPA) filters mandated for use in nuclear facilities and required in hospital operating rooms, filtration effectiveness down to 0.3 micron—considerably less than one hundredth of the width of a human hai—is simply not achievable.

Filter quality examined
Keep in mind that a conventional forward-flow filter separator is considered to be a "coalescer." It incorporates filter elements that operate on the coalescing principle. The filter elements coalesce liquid droplets into 10-and-larger micron size globules to be removed by the downstream impingement vane mist extractor (vanes are guaranteed to remove 8 to 10 micron particles). It is not reasonable to use simple piping insulation as a filter medium and guarantee the removal of droplets in the 0.3 micron size range. Multi-stage configurations are needed and the ultimate filter has to be "HEPA-like," i.e. it has to far exceed the quality of piping insulation.

A good design typically embodies long fiberglass filter elements using certain micro-fiber enhancements that are known to modern textile manufacturers. Low-velocity technology is very helpful and surface area is not as important as the depth of the media through which the gas has to pass.

The thicker the filter element, the longer the gas takes to pass through it, resulting in more and better coalescing of the liquids.

Some SCCs are offered with thin, high-pressure-drop, pleated-paper elements, representing very low contact times and high-exit (re-entrainment) velocities. As dirt builds up, exit velocities rise even higher, resulting in more and more re-entrainment of liquid mists and any associated, shearable solids exiting the cartridges. And the game goes on, as the re-entrained particles get smaller and smaller, thus meeting an artificial guarantee as velocities become higher and higher.

Others offer high-density and -depth media fibers that result in high pressure dropand high exit velocity, and which also re-entrain immediately after passing through the cartridges. Both of these approaches, as well as the downsizing of vessels and internals, contribute to marketing strategies geared to high consumption of elements and, thus, high sales volume and profitability for the vendor.

A competent SCC manufacturer's approach should be just the opposite—to give the user/purchaser maximized reliability, maximized cartridge life and lowest possible maintenance expenses. Years ago, the concept of "self-cleaning" vessels was successfully transferred from oil-bath separator scrubbers. They are still offered for specific applications and incorporate rotating cleanable bundles. This technology evolved...(Read whole article)