Sealing Systems Comparing Life-Cycle Costs of Different Sealing Systems for Special Rotating Equipment like Agitators and Dryers

Choosing the right sealing system for a specific application is not always easy: often there are too many possibilities, and sometimes the gaps in one’s know-how about advantages and disadvantages of different sealing systems are too big. This article will help you to evaluate different sealing systems for agitators and special rotating equipment, including operating limits and arising costs.

Related Vendors

EagleBurgmann Germany GmbH & Co. KG

Italvacuum, Srl

Angst+Pfister AG

There are several ways to seal machines with rotating shafts, but the best-established methods are lip seals and mechanical seals. These in turn are available in different versions: single or multiple seal, component or cartridge seal, dry-running or lubricated, with or without flush, to mention just a few. Table 1 gives an overview of the basic operating limits of different sealing systems.

In general all of the mentioned sealing systems can be used for undemanding applications with low pressures, temperatures and sliding velocities. However, lip seals are not suitable for vacuum applications. In this case mechanical seals, either dry-running or liquid-lubricated, are the standard sealing device. And as process conditions and shaft speeds increase, only liquid-lubricated mechanical seals can be recommended without reservations.

As plant costs continue to increase, however, the costs of the sealing systems are becoming a more and more important factor in decisions on plant design. A reasonable way to evaluate investment is to calculate life cycle costs (LCCs), which include not only investment and installation costs but also maintenance and operating costs for a defined period of time. A full life cycle analysis should also include the costs of production losses due to downtime; for simplicity, these costs are ignored here. The LCC is defined as: LCC (4 years) = Investment costs + Installation costs + Maintenance costs + Operating costs.

Three typical cases of operation

We have chosen three typical cases as application examples.

• Case 1: Top-entry agitator in a steel vessel with a non-critical process medium (operating temperature 150 °C; operating pressure 3 bara; sliding velocity 2 m/s). The sealing products compared are a cartridge-type lip seal, and a dry-running mechanical seal (Burgmann SeccoMix 481 L). Neither of these seals requires a supply system, and both have an expected life of two years.
• Case 2: Top-entry agitator in a steel vessel with a critical process medium where emissions to the atmosphere must be prevented (operating temperature 100 °C; operating pressure 6 bara; sliding velocity 2 m/s). The comparison here is between a cartridge-type lip seal pressurized with nitrogen, and a liquid-lubricated dual mechanical seal (Burgmann M 481 KL-D) with a barrier system. The life of the lip seal is expected to be two years, compared to seven years for the mechanical seal.
• Case 3: A steel dryer with an overhung side-entry shaft (operating temperature 200 °C; operating pressure 1 bara; sliding velocity 1 m/s). Here the options we chose are a cartridge-type lip seal pressurized with nitrogen, and a liquid-lubricated dual mechanical seal of robust design that is particularly suitable for media with a high solids content (Burgmann MR…L-D). The life of the lip seal is expected to be two years, compared to five years for the mechanical seal.

In case 2 + 3 it is assumed that the nitrogen supply for the lip seal is already available on site. As liquid barrier medium for the mechanical seal water is assumed.

This article concentrates on cartridge seals with integrated bearings, because these have become the standard for agitators and special rotating equipment. Cartridge seals are quick and easy to install, and the fact that the floating bearing for the shaft is normally integrated into the seal minimizes shaft movements in the area of the seal, thus increasing seal life.

Comparison of LCC

Figures 1, 2 and 3 show the breakdown of the LCC for these three cases. Costs were calculated over a period of four years, assuming 6000 operating hours per year and a shaft diameter of 80 mm.

The investment costs for the seal and if necessary for the supply system are shown in yellow. Maintenance costs for repairs and spare parts are in red, while the installation costs are in green. The operating costs for power and barrier media are shown in blue.

It is obvious that in every case, investment costs make up the greatest part of the LCC. The longer the period over which the LCC is calculated, however, the higher will be the influence of the operating and maintenance costs. This means that sealing systems with long operating lives are always preferable. The purchase cost of the cartridge lip seal is strikingly high. This is due to the intricate design and the additional bearings to absorb radial movements of the shaft.

Many plant operators are still nervous of the costs of installing and operating a supply system. From Figures 2 and 3, however, it is clear that these costs are not as most of the plant operators would have assumed. In particular, the operating costs of the liquid-lubricated seal system are lower than those of the nitrogen-pressurized lip seal, because of the latter’s relatively high nitrogen consumption and the high costs of nitrogen.

Maintenance costs are another big part of the LCC, especially for seals with short operating lives, so it is clear that longer-lasting seals save money. This is especially true if you add in the costs of losses production during maintenance, which we have not included here because it varies so much from one plant to another.

In summary, we can say that mechanical seals, either dry-running or liquid-lubricated, have significant cost advantages compared with lip seals. In addition, mechanical seals have been well established for many years as reliable sealing solution for agitators and special rotating equipment items such as dryers.

The authors are with Burgmann Industries GmbH & Co. KG

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