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Plants seal their fate

July 16, 2008
Contending with fugitive emissions becomes more of a priority

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Fugitive emissions from U.S. chemical plants top 300,000 tons every year, according to estimates from the Association Francaise de Normalisation (AFNOR), La Plaine Saint-Denis, France, an international standardization organization. This represents roughly a third of the total organic emissions from the sites. It’s exactly the same story in Europe, too.

“There’s no doubt that in the future there will be more regulation and inspection to decrease fugitive emissions,” says Dana Mathes, corporate director, environmental, health and safety operations for Dow Chemical, Midland, Mich.

However, sealing technology hasn’t received the attention it deserves, says Brian Ellis, director general of the European Sealing Association (ESA), Tregarth, U.K. “The EU (European Union) didn’t realize that it’s very hard to reduce emissions if seals aren’t doing their jobs properly. This has been the major focus for all of our members for over the last ten years or so.” He adds: “Sealing should be seen as much more of a support industry these days.”

“Market forces are pushing the development of ever-tighter sealing materials, but those forces depend on where you are. In the U.S., the driver is mainly regulation and compliance,” notes Jim Drago, manager, business development, for Garlock Sealing Technologies, Palmyra, N.Y.

However, the diversity of chemicals makes sealing a challenge, he adds. “The chemical industry is processing so many different oxidizing materials that we can’t use the carbon and graphite which are ideal for refinery hydrocarbon applications,” says Drago. The move away from large plants manufacturing one or two products to smaller facilities making a large range of products via batch processes adds to the challenge, he notes.

Another complication comes from the increasing popularity of piping made from polymers. “The challenge here is to generate enough pressure to create a proper seal without cracking the flange.”

Key resource

The crucial importance of sealing is highlighted in “Sealing Technology BAT Guidance Notes.” This 71-page document gives advice on the best available techniques (BAT) for industrial installations covered by the EU’s integrated pollution prevention and control directive. It’s published by the ESA, following close consultation with the Fluid Sealing Association (FSA), Wayne, Pa., and a number of seal suppliers.

Available on the ESA’s website, it provides an introduction to all aspects of fugitive emissions. Red bullet points highlight where the authors believe specific challenges might arise while green bullet points indicate their recommended approaches to achieving BAT.

According to the document, the scale of fugitive emissions depends on nine factors:

  1. Equipment design
  2. Equipment age and quality
  3. Standard of installation
  4. Vapor pressure of the process fluid
  5. Process temperature and pressure
  6. Number and type of sources
  7. Method of leak determination
  8. Inspection and maintenance routine
  9. Rate of production

Disregarding unsealed sources such as storage tanks, vents, flares and spills, many losses stem from leaks in the sealing elements of particular types of equipment, including agitators, compressors, flanges, pumps, tank lids and valves. Overall, valves typically account for 60%, relief valves 15%, tanks and pumps 10% each and flanges 5% of fugitive emissions, notes the document.

According to an estimate cited in the Guidance Notes, for every pump in an average plant there will be 32 valves, 135 flanges, one safety valve and 1.5 open-ended lines. Hence, with so many potential sources, leaking losses can be hard to determine. The age of equipment and how well an installation is maintained can significantly influence the extent of emissions, but generally the important causes of leaks include equipment failure, pollution of the sealing element, incorrect process conditions and ill-fitting internal or external sealing elements.

A changing relationship

Ellis offers a number of observations on how the drive to tackle fugitive emissions is dramatically changing the relationship between sealing suppliers and the chemical industry.
“Traditionally the sealing suppliers went through a large number of distributors, creating a huge gap between themselves and the industrial users. So the person who actually carried out the final installation was not necessarily qualified at all.”

In the days of asbestos, he says, this wasn’t really an issue because asbestos had such a wide variety of uses but, with its phasing out, new sealing materials are much more application specific. “For example, you might only get three or four applications per material now, where there would have been 15 or 20 for asbestos. Of course, the new ones way outperform asbestos but they must be used by the user in the right application and be installed properly. This is a real challenge that faces the chemical industry — installation is absolutely crucial now, in a way that it never used to be. There is also the challenge of overcoming the conservatism of engineers on chemical plants, particularly in some European countries where there is a very great reluctance to adopt new solutions.”

A success story

One company not resistant to change is Dow Chemical. Indeed, Mathes says the company aims to outpace regulations. “So our own internal targets are designed to anticipate these changes and stay ahead of them.”

In fact, Dow has been setting corporate emissions targets for 15 years now. Many of these are focused on volatile organic compounds (VOC) and the fugitives that form a substantial part of them. Currently more than two million points of emissions are monitored at over 20 sites around the world. Since 1994, the result of these efforts has been a 62% cut in VOC emissions even though the company has increased production by 38% over that period.

Although bulk manufacturing still is an important part of Dow’s portfolio, its move to lower volume manufacturing of a larger number of products, especially performance chemicals, has brought new challenges in terms of fugitive emissions.

So, wherever possible, the company’s engineers try to design out the opportunities for fugitive emissions to arise — for example, by reducing the need for pipe connectors and their associated gaskets in new plants.

“By putting sections of pipe rack together and welding them, we’ve learned how to engineer out flanges. We also opt for seal-less or tandem seal pumps now, rather than single-seal pumps. These, of course, have implications for process safety and maintenance: fewer opportunities for spillages and simpler maintenance has important implications in terms of long-term cost of ownership,” notes Mathes.
At existing plants, Dow uses strategies such as live loading of gate valve packings to improve their performance. Such approaches are particularly important because 80% of the company’s fugitive emissions come from these valve packings and pump seals.

The issue of installation is extremely important at Dow, too. The company invests a lot of time and effort to make sure it has the best engineering standards. However, if field data from a Dow plant anywhere in the world suggest a change would be beneficial, it happens straightaway and is immediately implemented globally, says Mathes.

Dow also focuses on comprehensive and company-wide training strategies.

“We did have concerns about how flanges were being put together, so we implemented a flange-fitting training strategy. When combined with our engineering standards, such strategies have led to a tremendous increase in productivity. The more we do this, the fewer emissions and leaks we have, the fewer spare parts are needed, etc: it’s an increased cost/productivity benefit,” Mathes notes.

Drago also stresses the importance of correct installation. “If you don’t adhere to the rules, you won’t get the performance. We can sell the best gasket or packing set in the world, but there’s no point if it isn’t installed properly.”

New options

To address sealing challenges and surmount the inherent conservatism of many operating companies, members of ESA and FSA are developing better technologies.

Figure 1. Suitable for top-mounted drives, this seal features a central rotating seal made of solid silicon carbide and wider sliding faces. Source: Burgmann.

For example, the new AGSZ agitator gas seal from Burgmann Industries, Wolfratshausen, Germany, targets low RPM machines. Designed for use on top drives in tanks with standardized connections, the central rotating seal (Figure 1) is made of solid silicon carbide and has wider sliding faces, which produce a gas film of much greater stability, says the company.

The result is safe, non-contact operation even in rugged agitator applications. Buffer gas consumption has been reduced by 75% compared to the previous AGS model while leakage measurement variations fall within a much narrower range. The stationary spring-backing of the seal faces makes the AGSZ less sensitive to radial movement such as shaft wobble or shaft skew. Such a stationary design has a proven track record on pump seals, notes the company.

A specially designed clamping ring for torque transmission allows up to ± 2-mm. axial movement of the shaft. The floating bearing on the shaft is integrated into the seal. Acting as the fixed bearing between the rotating shaft bushing and the stationary housing, it reportedly provides effective protection against the negative effects of movement at the sealing gap.

The high strength coating on the seal faces guarantees very good emergency running characteristics in start-up and slow-down phases or during failure of the buffer gas supply, according to Burgmann. Being double-balanced, the seal remains closed even when there’s no buffer gas. In addition, a reduction in the number of wearing parts helps to lower maintenance costs and simplifies installation.

Meanwhile, DuPont Performance Elastomers, Wilmington, Del., has added to its Kalrez Spectrum family of perfluoroelastomer parts that are specifically designed for touch services such as those involving amines. Hot primary amines are notoriously aggressive toward elastomers, causing their premature degradation.

In lab tests, Kalrez Spectrum 6380 parts exhibited what the company describes as excellent overall chemical resistance and mechanical properties for static and dynamic sealing applications in harsh processes at temperatures up to 225°C. In particular, they showed very low swell after 672 hours of exposure to ethylene diamine, ethanolamine and ammonium hydroxide, compared with alternative perfluoroelastomer parts commonly specified for such environments.

“Before introducing Kalrez Spectrum 6380, we subjected pilot seal samples to rigorous field testing in an aggressive primary amine process at 210°C, operated by a major chemical company located in the Texas Gulf Coast area,” notes Tony Dorta, global marketing manager. “Several perfluoroelastomer parts from various manufacturers were used, but none offered service in this process environment beyond four months. By substituting Kalrez Spectrum 6380, the company succeeded in tripling seal service life to one year. As a result of this field test and our own laboratory trials, we are confident in recommending the latest Kalrez product for processes involving hot amines, strong acids and oxidizing media.”

W. L. Gore & Associates, Newark, Del., has introduced One-Up pump diaphragms suitable for use with most chemicals and at elevated temperatures and pressures.

Thanks to a proprietary polytetrafluoroethylene (PTFE) on the wetted side, they last significantly longer than other conventional PTFE diaphragms, claims Gore. “These diaphragms provide a dramatic improvement in service life and are considerably stronger, with greater flex life.” In general service pumps, the diaphragms’ long life and versatility will reduce both maintenance and operating costs — or your money back, says the company.

Figure 2. Proprietary bonding process between the PTFE and the elastomer overcomes traditional problems with both leaching and delamination. Source: Garlock Sealing Technologies.

Garlock has launched the Gylon Style 3545 seal. Made of pure PTFE, it suits media such as strong caustics, strong acids, hydrocarbons, chlorine, and cryogenic fluids as well as glass-lined equipment, plastic piping and low bolt-load applications. Its highly compressible outer layers seal under low bolt load, making the seals appropriate for many non-metallic flanges. Compressible layers conform to surface irregularities, especially on warped, pitted or scratched flanges, while a rigid PTFE core reduces the cold flow and creep normally associated with conventional PTFE gaskets. The soft PTFE can be cut easily from larger sheets, which reduces inventory costs and expensive downtime, while the rigid PTFE core facilitates installation, especially on large diameter flanges and in hard-to-reach areas.

Garlock also offers the Stress Saver 370 for use, e.g., with non-metallic flanges It features raised molded-in sealing rings that seal with 75% less surface area. The pure PTFE sealing surface resists a host of chemicals; a proprietary process is used to bond the PTFE to the elastomer, so that it won’t leach or delaminate (Figure 2).

For the chemical industry, the challenge clearly is to use new technologies in a way that satisfies the demands of both the regulators and the shareholders.

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