Mechanical Seal Systems How to Select the Right Seal for Mixing Applications
The basic sealing task in mixing applications is to seal the rotating shaft as it passes through the vessel wall. Depending on the operating conditions — pressure, temperature, agitator speed, etc. — different types of seals are used. As this article shows, mechanical seals offer many advantages over other types of seals. Particularly if hazardous or explosive substances are being mixed, a mechanical seal is mandatory.
Mechanical seals are generally regarded as mechanical engineering components, and are used e.g. in pumps as a sealing component. The situation is somewhat different in processes and applications of mixing technology. In these cases, the mechanical seal is generally implemented as a unit that can be exchanged. These are referred to as “cartridges”. Whereas single-acting mechanical seals still have a component character, double- and triple-acting mechanical seals are regarded as mechanical seal systems owing to their complexity.
A mechanical seal system consists of the mechanical seal cartridge, the hydraulic components (e.g. pressure compensator) and the “installation”, which comprises the piping, instrumentation and mountings. In some cases, this also includes a seal liquid refilling system, as shown in Fig. 2 (see picture gallery).
As a consequence, in most mixing systems, only the complete mechanical sealing system can provide reliable sealing of the vessel. The sealing function of the vessel can be guaranteed and maintained if the mechanical sealing system has been correctly selected.
Supply Systems — What are the Tasks?
Supply systems ensure safe and reliable operation of the mechanical seal. A seal is regarded as being technically sealed when the pressure in the seal liquid chamber is always higher than the vessel pressure. The supply of seal liquid is thus of primary importance to safety.
Supply systems must be able to fulfill the following tasks in order to guarantee a reliable mechanical seal operation: See next page ...
Pressure maintenance: Alternatives for pressure maintenance are continuous flow systems or pressure compensator arrangements.
Cooling: The physical processes taking place on the seal faces are very sensitive to high temperatures. If critical values are exceeded, this may lead to localized areas of drying, resulting in “hot spots” and greater shear stresses on the surface of the seal rings. The sealing function is compromised as soon as the surface structure has been destroyed (blistering). Therefore, the heat conducted to the seal from the vessel and that caused by friction in the seal interface must be continuously removed. Continuously operating cooling systems (Fig. 3) are extremely important for reliable operation.
Flushing: In many processes, corrosive or erosive substances contaminate the surfaces of the seal rings. To protect the rings against these substances, they are flushed with a liquid that is compatible with the process taking place in the vessel and the product-wetted mechanical seal components.
Seal liquid with automatic refilling system: An outstanding characteristic of mechanical seals is their very small leakage rate, even at elevated vessel pressures. A leakage rate of only 20-50 ml/day can be expected during normal agitator operation at vessel pressures up to 70 bar. Nonetheless, it is still recommended to monitor the leakage and refill automatically when necessary, in particular under conditions of continuous operation. An automatic refilling system can be seen in Fig. 2 that is controlled by the level indicator of a pressure compensator.
Seal liquid — emergency supply: In the event of an unexpected increase in the leakage rate due to damaged seal rings, the amount of escaping seal liquid may no longer be sufficiently replenished. To maintain the positive pressure difference between the mechanical seal and the vessel, and thus maintain the lubricating function, the seal liquid (often water) is circulated through the mechanical seal with a higher flow rate. This allows the reactor to continue operating for a certain time after a leakage has occurred. Fig. 4 shows an overview of the application criteria for modules of seal liquid supply systems.
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.
Pressure Compensator Systems
1. Mode of operation of the pressure compensator
The pressure compensator is a hydraulic cylinder with a piston rod (Fig. 8). The entire circular area AB of the lower side of the piston is subjected to the vessel pressure PB. The seal liquid pressure PS applied to the upper side of the piston can only be generated with the smaller annular area AS because only the atmospheric pressure is exerted on the piston rod. The forces are balanced in the static state: PB x AB = PS x AS
The surface area ratio AB/AS is always greater than 1. This ensures that the pressure in the seal liquid is always higher than vessel pressure. Differential pressures of 2-10 % are typical.
2. Pressure compensator for a single agitator
As shown in Fig. 2 and Fig. 8, the lower chamber of the pressure compensator is connected to the head-space of the vessel via the seal flange (yellow). The upper chamber is connected to the seal liquid chamber (red). This arrangement ensures that the pressure in the sealed chamber (without auxiliary power!) automatically follows the vessel pressure. The seal chamber pressure is usually constant in continuous flow systems (Fig. 9).
The inboard pair of seal rings is directly exposed to corrosive products and/or high temperatures. They are therefore regarded as particularly critical. Especially in batch operations or under very fluctuating operating conditions during continuous operation, these rings are completely unloaded by the pressure compensator.
3. Pressure compensator system for multiple agitators
Pressure compensators are generally equipped with a refilling hand pump. However, an automatic refill system is recommended if there is more than one agitator to exclude possible errors by the operating personnel (Fig. 10). The refilling pump is switched on when an LAL signal is sent from the pressure compensator (in addition compare Fig. 2). The pump is stopped with LAH. This configuration means that the pump operates for a short time and thus has a long service life.
Position monitoring of the pressure compensator provides very sensitive monitoring of the leakage behavior of each individual seal. This enables countermeasures to be initiated in good time if premature failure of the seal is imminent.
4. Pressure compensator system with independent cooling
Many plants don’t have a reliable central cooling water supply. An independent cooling system for several mechanical seals can be seen in Fig. 11. Provided that the pumps and the heat exchanger are connected to an emergency power generator, the seals can be kept at their normal operating temperature, even if there is a complete failure of the external power supply.
Integration into Processing Plants
Reliable operation of the mechanical seal depends on monitoring and control of the seal liquid supply systems. Its integration into the overall process control system is facilitated if the programmer has been provided with clear functional descriptions, logic diagrams and any necessary cause-and-effect diagrams.
The survival probability of the seal increases if the controlling parameters are clearly defined and interlocks have been properly implemented.
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