Seal in long-term savings

Figure 1. The largest cost component is unscheduled downtime.

The cost of an individual seal can range from as little as 50 cents to several hundred dollars. Production costs incurred by seal failure are much higher. The results of interviews we did in Europe in 2005 and 2006 with seal manufacturers, distributors and end users put the downtime cost of standard seal failure at around $3,800 per hour per pump without an in-line spare. (There is no downtime cost with an in-line spare pump.) This takes into account a process shutting down, two hours to replace a seal, lining out the process and start-up. The cost certainly could be higher for a complex process. Pump failure also can lead to other costs, e.g., environmental, safety and quality ones.

What does this mean? When manufacturers opt for low-cost elastomeric seal replacement, the short-term savings may not always translate into long-term value. Conversely, a premium-priced part that outperforms its cheaper rival may deliver significant savings. The difference is that the value can be measured during use rather than at the outset. Extending the mean time between repairs (MTBR) has a strong impact on reducing cost and improving production parameters such as rate and quality.

Successful seal performance requires careful consideration of process conditions, elastomer properties and seal design. A deficiency in any of these areas will result in a reduction of seal service life and the possibility of sudden seal failure. Chemical attack, high temperatures and mechanical stresses are common culprits.

Perfluoroelastomer pluses

Kalrez perfluoroelastomer parts resist more than 1,800 aggressive industrial chemicals and can handle temperatures up to 327ºC. The combination of long-term resistance to chemical damage, swelling and elevated temperatures enables users to minimize expensive seal failures and improve MTBR, thereby reducing maintenance and operating costs and increasing productivity.

For instance, C.D.R. Pompe, Senago, Italy, has dramatically extended the lifetime of O-ring seals operating in aggressive process media by switching to Kalrez Spectrum 6375 perfluoroelastomer parts. “The previous O-rings of fluoroelastomer, mounted on our type FC 35/50 mechanical seal and operating with impure sulfuric acid for dust laying service, failed each time after a maximum service life of only two months. By comparison, the Kalrez Spectrum 6375 O-rings are still operating after two years in an identical critical environment without problems,” says Roberto Michelucci of C.D.R. Pompe.

Dow AgroSciences, Drusenheim, France, has seen the life of dynamic and static seals rise from one day to an average of two months by switching from PTFE to Kalrez Spectrum 6375. “Each time we performed a clean-in-place procedure at 80°C or made a product change on the filler line, the PTFE seals became mechanically damaged. This meant we had to change the PTFE O-rings on a daily basis,” explains Michel Blondel, mechanical maintenance leader at the plant (Figure 2).

Figure 2. Michel Blondel (left) credits change in material with boosting filling-machine seal life more than fifty-fold.

“The use of Kalrez Spectrum 6375 allows us to increase the operating uptime of our filling machines and to improve overall reliability of our packing line. We have also significantly reduced the time spent in O-ring replacement,” he says. Blondel also reports cost savings as a result of major improvements in safety and reliability, and reduced maintenance.

These high-performance perfluoroelastomer seals can help to improve safety by controlling fugitive emissions and preventing contamination of process streams. As chemical makers know, it’s costly not to strive to decrease environmental risk. Regulatory demands will continue to drive manufacturers towards cost-efficient environmental solutions. As an example, the European Union’s Integrated Pollution Prevention and Control directive (IPPC, 96/61/EU ) will take effect this October, compelling process plants to reduce the loss of volatile organic compounds through unexpected leaks, evaporation, flaring or spills.

Seal replacement scenarios

Seal replacement is a reality. Whether needed once, twice or repeatedly — due to failure or a fixed schedule of planned system checks for safety, service, quality or economy — such replacements involve unit and downtime costs. We have identified four seal-replacement scenarios to gauge the impact that the choice of sealing solution can have on total system cost:

1. Seals that last for the life of process equipment;
2. Replacement once or twice due to failure;
3. Repeated replacement; and 
4. Maintenance by replacement on schedule.

In the first scenario the seals don’t have to be replaced as they last for the life of the pump or other process equipment (Figure 3). Strictly on a cost basis, the use of perfluoroelastomer parts isn’t justified. Seal selection is driven by cost alone.

Figure 3. An expensive material offers no advantage if a seal lasts for the life of the equipment.

Price isn’t the controlling factor for the second scenario (Figure 5). Seals are replaced due to failure or for maintenance reasons once or twice during the life of the equipment. Failure incurs the expense of unplanned downtime. Using perfluoroelastomer seals could be the most-cost-efficient solution.

Figure 4. Using a more expensive perfluoroelastomer material pays off if it avoids unscheduled downtime.

In the third scenario, the seals need to be replaced several times. Perfluoroelastomers should offer substantial savings because in process environments involving elevated temperatures and aggressive chemicals they should last longer than conventional sealing materials.

When seal replacement is regularly scheduled for reasons of safety, service, quality or economy, the downtime cost incurred in replacing existing seals is usually constant (Figure 5). The added expense of switching to perfluoroelastomer seals should be offset by doubling, tripling or even quadrupling the periods between overhauls.

Figure 5. Longer time between overhauls should offset the added cost of a perfluoroelastomer seal.

Take the long view

Each plant has a unique seal replacement rate and downtime cost. In addition, a site must consider its chemical environment and risks when evaluating the potential savings of integrating perfluoroelastomer seals in assembly. However, concentrating mainly on purchase prices can do your plant a costly disservice. Consider the total system cost, and factor in the price of unproductive time and problems such as leakages. This may well show that a premium sealing solution can lead to significant savings. At the very least, devoting time and thought to longer-term factors will be invaluable from a strategic standpoint.

Additional information is available at http://www.dupontelastomers.com/Products/Kalrez/optimize.asp.

Russell Schnell is an applications engineer for DuPont Performance Elastomers, Wilmington, Del. E-mail him at [email protected].