Selecting Modern Reverse-Flow Filter-Separator Technology

Kathy | October 1, 2005

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 the gas stream leaving the CFS.

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 to filter vessels with a rotating cleaning mechanism and to the present state-of-art, i.e. the back-flushing of individual elements while remaining on-stream.

Further, competent manufacturers still offer maximized performance from even conventional vessels by utilizing tried and true designs with maximized internals. They will not advocate the use of downsized versions that violate certain velocity and pressure-drop criteria, thereby incurring high maintenance and non-sustainable, or non-optimized performance.

This takes us back to HEPA filters. Designed and developed for air filtration, HEPA filters recycle the air many times within a closed system and periodically add fresh makeup air to achieve the desired air quality. In the hydrocarbon processing industry, there is usually only a single-pass opportunity to achieve clean gas. It is rarely feasible to recycle process gases several times to obtain the desired gas purity. Since absolute, beta-rated filter elements are simply not able to achieve these results, many inferior designs call for one or more “conditioning” filters, or vessels, to be placed upstream of their “coalescer.”

Also, be on the lookout for offers that allude to the advisability (or just the merits) of installing downstream vessels to clean up certain liquid streams to which the gas has been exposed. A relevant question to ask is “Why does the liquid have to be cleaned up if the upstream vessel(s) has done its job of, say, protecting the treating tower?” Without fail, the answer will point to liquids, or mists or corrosion products in the form of small solids particles that were not adequately removed upstream of the tower. Hence, foaming and treating agent contamination were not eliminated. This means tower upsets, additional filtration for liquids and even the possible need for carbon beds or filters to remove trace liquid aerosol contaminants.

SCCs have been successfully implemented to protect such process streams and to eliminate or prevent contamination-related upsets. Time and again, bottom-line results show that self-cleaning coalescers protect equipment and safeguard reliability.

How to specify and select the best equipment
Superior self-cleaning coalescers can remove iron sulfides, viscous fluids and slugs because of their inherent low pressure drops (4″ to 6″, or 100 to 150 mm H2O). Moreover, low velocities and other important considerations conducive to good separation and low life cycle costs must be taken into account here.

With input from the user or destination plant, a competent vendor can assist in drawing up a good inquiry specification. Within the specification there are many options to consider. The choice, quite clearly, depends on process conditions and related parameters, some of which are as follows:

  • Dry filter: for gas with associated solids
  • Dry filter, self-cleaning: for gas associated with solids
  • Line separator: for gas containing entrained liquid mist
  • Vertical or horizontal separator: for gas with entrained liquid globules (mist, aerosol)
  • Vertical or horizontal separator: gas with entrained liquid particles (mist) and free liquid (slug) removal
  • Vertical or horizontal filter-separator: gas with entrained liquid globules (mist, aerosol) and stable solids
  • Reverse-flow mist coalescer: gas with entrained liquid globules (mist, aerosol). Removal to sub-micron particle size and extremely high efficiency
  • Reverse-flow mist coalescer with slug chamber: gas with entrained liquid globules (mist, aerosol), slugs and (stable or unstable) solids. Removal to sub-micron or better, at high efficiency (can be furnished in self-cleaning configuration while in full service)
  • Oil-bath separator-scrubber: gas with liquid globules (mist) and solids (stable or unstable). Removal to 3 microns at 97% efficiency by weight
  • Tricon 3-stage separator: gas with entrained liquid globules (mist), slugs and solids (stable or unstable). Removal to 3 microns at 97% efficiency Evaluating the proposed configurations

Once the various bidders submit their offers, they must be evaluated using life-cycle costing and suitability criteria. An objective evaluation must keep in mind the following:

1. Velocity: Once the gas stream enters the vessel, there should be no internal configuration that would accelerate the gas back to pipeline velocity. Causing the motion of gas to increase in velocity will only cause the liquid to shear into smaller and smaller globules.

2. Pressure Drop: In no instance should a piece of separation equipment be designed with more than a 2 psi pressure drop from flange to flange when the vessel operating pressure exceeds 500 psig. At less than 500 psig, the flange-to-flange pressure drop should be limited to one psi or lower. Pressure drop consumes energy, and energy costs money.

In no design of separation equipment should the pressure drop across an element arrangement be allowed to exceed 0.5 psi. As filter elements become wetted and 50 percent plugged, the pressure drop increases four-fold.

If, for example, the initial pressure drop is 0.5 psi, and the elements become half- plugged, the pressure will increase to 2 psi. Once the elements become three-quarters plugged, the pressure will increase to 8 psi. This is 16 times the initial pressure drop and a change of elements is now unavoidable. Keep the initial filter element pressure as far below 0.5 psi as possible to avoid frequent element change-out. Remember, the filter elements have to be disposed of and this disposal can become expensive.

3. Filter Element Cost: Always ascertain the cost of replacement elements. Some vendors will practically give away vessels in order to generate spare parts sales. Find out the inside diameter, the outside diameter and the length of the proposed elements and how many of these make up the vessel internals. Using this information, calculate the surface area on the inside of the elements and the velocity of the gas entering the elements.

Additionally, from this information, determine the exit velocity leaving the elements. Note that this velocity should not exceed the re-entrainment velocity of the liquid. Some of the reverse-flow coalescer offers you might receive will turn out to be “egg beaters” that take whatever liquid enters the vessel and shear it into orders-of-magnitude amounts of smaller globules that are then re-entrained in the gas stream. Liquid globules can be sheared so small that they cannot fall out again until they re-coalesce downstream. But, all the same, the liquid is there to do its damage to downstream equipment.

4. Vessel Life: Under ordinary circumstances, separation equipment should have a useful life of 20-25 years. Needless to say, corrosion problems, internal explosions, vibration or pulsation, overloads, hydrate formation, lack of routine maintenance, incorrect or faulty maintenance practices, misapplication or use of equipment under unsuitable operating conditions, re-placing elements with unsuitable or poor-quality substitutes and various other forms of mistreatment can adversely affect vessel life.

5. Reliability of Vendor: If a piece of separation equipment is bought and put into service under conditions that deviate from the design intent, it may not live up to expectations. Such underperformance will usually manifest itself rather quickly. These unpleasant surprises can be avoided by selecting a reliable vendor as the source of supply. The individual or team engaged in the selection and evaluation task should ask:

  • if the vendor has the facilities to manufacture the equipment, or “farms it out” to sub-vendors
  • who builds the essential parts such as the filter elements, the mist extractors, other internals, and the vessel itself
  • who does the x-raying, hardness testing, ultrasonic examination, magnaflux examination (both wet and dry, if required), stress relieving, hydrostatic testing, grit-blasting and painting, and final preparation for shipping

6. Value: How important is proper performance of the separation equipment to the protection of downstream equipment? Certainly, monetary value has to be placed on repair and maintenance of the downstream installation.

To what extent would rotating equipment such as turbines, turbo-expanders, centrifugal or reciprocating compressors, internal combustion engines, dehydration, amine or molecular sieve units, refinery or petrochemical processes, meter runs, power plants, fired heaters, plant fuel, municipal fuel and/or, perhaps, gas coming in from producing wells be affected by potential performance deficiencies of the separation equipment?

What are prudent downtime risks and what would be the cost of rectifying problems with downstream equipment caused by defective filtration equipment?

A reliability-focused organization demands answers to these questions!

7. Follow-up: Who will ultimately make the determination if the goods specified and purchased are, in fact, the goods received? Will the responsibility change hands from selection to purchasing to operation with a relaxed regard for what was intended to happen and what is actually happening? In that case, only the very best and most conservatively-designed piece of separation equipment should be purchased.

Contrary to “conventional wisdom,” there have been no “super breakthroughs” in the design of separation equipment in the past 30 years. On the other hand, considerable changes have been made in presentation and marketing methods over the past two or three decades. Some marketing claims as to how far the state-of-the-art has advanced during the past several years (or even in recent months) are truly stretching the imagination. Beware, since they may simply be designed to sell spare parts and/or just stay alive in a highly competitive environment.

Life cycle cost calculations
Life cycle cost (LCC) calculations also must be used to determine the wisest equipment choice. Life-cycle-based filter equipment cost is the total lifetime cost to purchase, install, operate and maintain (including associated downtime), plus the downstream cost due to contamination from inadequately-processed fluids or even the risk of damaging downstream equipment,and (finally) the cost of ultimately disposing of a piece of equipment.

A simplified mathematical expression could be:

LCC = Cic + Cin + Ce + Co + Cm + Cdt + Cde + Cenv + Cd

LCC = Life Cycle Cost
Cic = Initial cost, purchase price (system, pipe, auxiliary services)
Cin = Installation and commissioning cost
Ce = Energy consumed by incremental (i.e., higher) pressure drop across the equipment offered
Co = Operation costs, if applicable
Cm = Maintenance and repair costs
Cdt = Downtime costs
Cde = Incremental repair cost, downstream equipment
Cenv = Environmental costs
Cd = Decommissioning and/or disposal costs


Energy, maintenance and downtime costs depend on the selection and design of the filtration equipment, the system design and integration with the downstream equipment, the design of the installation and the way the system is operated. Carefully matching the equipment with the process unit’s or production facility’s requirements can ensure the lowest energy and maintenance costs and yield maximum equipment life.

When used as a comparison tool between possible design or overhaul alternatives, the life-cycle-cost process will show the most cost-effective solution, within limits of the available data.

Concluding thoughts
Initial investment costs go well beyond the initial purchase price for your equipment. Investment costs also include engineering, bid process (“bid conditioning”), purchase order administration, testing, inspection, spare parts inventory, training and auxiliary equipment. The purchase price of filtration equipment is typically less than 15 percent of the total ownership cost. Installation and commissioning costs include the foundations, grouting, connecting of process piping, connecting of electrical or instrument wiring and (if provided) connecting of auxiliary systems.

But, suppose now that a team of engineers goes through the planning, the bidding, the procurement, the installation and evaluation stages of the separation equipment and finds that it matches the requirements exactly. Then comes the spare-parts purchasing stage and, at that point, cheap, incompatible sets of fiberglass pipe insulation elements are bought. Suppose further that these are to be installed, when dictated, by the best operating practice assigned to the installation.

Chances are the element manufacturer will have made all kinds of promises and a few dollars will have been saved. However, what happens when these substitutes are installed? There is noquestion about it—the separation equipment can no longer live up to the job specifications and bad things start to happen at that point.

So then, to the reliability-focused and risk-averse user, life cycle costs are of immense importance. In contrast, repair-focused users are primarily interested in the initial purchase price. There is consensus among best-in-class industrial and process plants that only the truly reliability-focused facilities will be profitable a few years from now, and only they will survive.

A frequent contributor to Maintenance Technology, Heinz Bloch is the author of 14 comprehensive textbooks and more than 300 other publications on machinery reliability and lubrication. He can be contacted at hpbloch@mchsi.com; Internet www.machineryreliability.com




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