Mechanical Seal Systems
How to Select the Right Seal for Mixing Applications
Continuous Flow Systems
1. Thermosiphon effect
Water cooling and circulation pumps are not very popular because the pipework and pumps increase the capital cost. During operation, they lead to additional costs for the cooling water and electricity supplies as well as extra maintenance. For simple sealing tasks, however, these additional elements are not necessary if the thermosiphon effect in the seal fluid and natural convection into the surrounding area is used (Fig. 5). Hot liquid (red) has a lower density than cold liquid and thus rises upwards and causes the seal liquid to circulate. Cooling of the seal liquid in the storage vessel can be intensified by fitting it with a cooling water jacket.
Constant pressure of the seal liquid in the seal chamber can be achieved by using a pressure overlay of nitrogen.
If the thermosiphon effect is insufficient to remove the generated heat quickly enough, then the seal liquid must be circulated with a pump. Owing to the large amounts of heat, natural convective cooling with air has to be replaced or supplemented by forced cooling, e.g. using cooling coils in the storage vessel (Fig. 6). This is particularly necessary if parts of the mechanical seal in contact with the product protrude into the liquid product, e.g. for side-entry or bottom-entry agitators. The forced circulation cooling system can only be operated reliably if it is equipped with suitable monitoring instruments, e.g. flow meters and temperature sensors.
2. Continuous flow system for multiple agitators
In practice, continuous flow systems as shown in Fig. 7 are required for reliable operation that uses several mechanical seals. The pressure control valve is the most important component for sealing the agitator. It controls the pressure in the seal liquid circuit to such a level that is approximately 10 % higher than the maximum vessel pressure. The pressure is generally kept, even if the vessel pressure changes. The pressure accumulator fulfills a very special safety function. Should the pumps or the pressure control valve fail, e.g. during a power failure, the high pressure in the seals is maintained by valves. During this time, the pressure accumulator ensures that the pressure in the seal liquid circuit is higher than in the vessel and also supplies more seal liquid to replenish leakages.