It is not always easy to know which sealing system is best for your application. Operating parameters such as pressure, temperature, and speed may not be the only things to think about. Explosion protection (ATEX), environmental protection and operational safety can be equally important. If you are in the food or pharmaceutical industry, then GMP, FDA-compliant materials and hygienic design will be major issues. It’s best to seek professional advice and get the seal manufacturer on board as early as possible. This article compares different sealing systems for agitators and special equipment.

Lip seals and mechanical seals are the most popular choice for equipment with rotating shafts. In principle, both are suitable for simple applications at pressures up to 6 bar, temperatures up to 200°C and speeds up to 2 m/s. Lip seals are not suitable for vacuum applications, and dry running or liquid lubricated seals are normally the solution of choice.

When the going gets tough (pressure > 6 bar and speed > 2 m/s), the only solution we would recommended is a liquid-lubricated mechanical seal. Besides the operating limits, you need to consider other aspects as well such as service life, seal monitoring capability, air emissions (TA Luft) and explosion protection (ATEX). Table 1 shows an example of key selection criteria.

Seal Life

Seal life is a crucial consideration when you choose your seal. Long service life increases the availability of your system and cuts down your repair and spare parts costs. You can expect a lip seal or dry running mechanical seal to last for 2 years on average. Liquid lubricated mechanical seals will last much longer (7 – 10 years), and they have a proven track record going back many years as shaft seals in agitators and other special machinery.

Leakage can be monitored, for example, at a drain port on all of the cartridge seals discussed in this article. Not only that, you can measure the level of wear on a Burgmann SeccoMix (dry-running seal) when the machine is stopped. To get reliable leakage monitoring results on a lip cartridge seal, the sealing function of the outboard lip seal must be intact, which is not always the case. If verification of the sealing function is even possible (and it may not be), it would take an awful lot of outlay.

To comply with stringent environmental regulations, e.g. TA Luft, it is important to choose a sealing system which keeps emissions to a minimum.

Agitators and special machines are not specifically mentioned in TA Luft, but you still have an obligation to minimize emissions.

The gas phase normally needs to be sealed on top drive machines with rotating shafts. On this type of machinery, multiple seal systems with provision to extract leakage to a gas collection system, similar to what is required for compressors and buffered sealing systems, ensure compliance with air quality (TA Luft) guidelines.

Process fluids come into direct contact with the seal on side or bottom drive machines, so the pump guidelines are probably most appropriate. What this means is that either the drives have to be hermetically sealed or you need multiple mechanical seals with unpressurized or pressurized buffer fluid.

This means that all of the systems discussed in this article are suitable for top drive machines if the seals are buffered and leakage is extracted to a gas collection system. On the other hand you should use multiple mechanical seals and a suitable buffer system on machines with side or bottom drive.

ATEX Risk Assessment

The user is responsible for conducting a risk assessment in ATEX applications [1,2]. Top-drive agitators are particularly critical where applications in an explosive environment are concerned, because a potentially explosive atmosphere can build up inside the machine.

The dry-running SeccoMix agitator seal is approved for all ATEX zones. The suitability of the seal has been demonstrated in type testing. The temperature of the potential ignition source can be monitoring by directly measuring the seal face temperature with a Pt 100.

If you are using lip seals in ATEX Zone 0, monitoring of the potential ignition source is mandatory. However that is virtually impossible to do, because a temperature sensor cannot be mounted directly under the lip. It is not good enough to measure the temperature of adjacent components, because the measurement is not accurate enough.  This violates the requirements of ATEX 95, because there is no reliable way of preventing the temperature from rising above the allowed limit and it is not possible to initiate corrective action in time. You can implement primary explosion protection (inertization) with the appropriate monitoring system instead.

Radial Shaft Deflection

Another aspect to consider when you are selecting your seal is radial shaft deflection. Maximum radial shaft deflection for agitators is defined in DIN 28161. However, these thresholds are often exceeded in practical application.

The Burgmann SeccoMix is the ideal choice for these applications. It can handle ±1.5 mm of deflection in the seal area without the seal’s functionality being impaired.

Lip seals are not suitable for large radial deflection. Even minor shaft deflection causes loss of sealing function and increased wear.

Complex designs with auxiliary bearings do exist, which prevent radial deflection at the seal between the lips and the shaft. So-called compensating elements such as O rings installed between the seal housing and the mounting flange can also be used to compensate for radial shaft deflection. However, in such cases you can probably never be sure that the sealing function will remain intact if you have a significant level of shaft deflection. This can be a big problem if you are handling critical media that pose an emissions risk. It is also extremely difficult or impossible to verify that the sealing function of the compensating elements remains intact.

Even with these elaborate designs, manufacturer’s specifications only allow a maximum of ±2 mm radial deflection at the impeller, which is in line with DIN 28161 requirements. Liquid-lubricated mechanical seals such as the Burgmann M481 K(L)-D comply with the shaft deflection requirements of the DIN standard.

Agitators and special equipment are frequently used in the pharmaceutical and food industries. FDA-compliant materials are mandatory in most cases, and sometimes you need sterile versions which meet hygienic design standards. FDA-compliant materials are available for all of the seal types which are mentioned in this article. Abraded particles from the carbon seal face can sometimes cause problems. The SeccoMix, for example, has what is called a leakage cup which traps the particles, preventing them from contaminating the product. Trials are currently underway on new materials made of plastic or SiC with a special coating, etc. which can add an extra margin of safety.

If you really need a sterile seal, then your best choice is a special mechanical seal which meets hygienic design standards. Due to the nature of their design, lip seals are simply not suitable. 

For more information on agitator seals, contact Eric Vanhie, Director of Engineering/Product Management at ebnow@eagleburgmannseals.com.

References
[1] Himstedt, M.; Beyer, M.. Explosionsgeschützte Rührwerke – Beispiele für eine Zündgefahrenbewertung. TÜ 45 (2004) No. 5, pp.36-41

[2] Beyer, M.: Systematische Zündgefahrenbewertung an explosionsgeschützten mechanischen Geräten. TÜ 45 (2004) No. 3, pp.27-31