Mechanical seals are used in machines like pumps and compressors as a method to control the liquid flow. The basic mechanism of a mechanical seal is a rotating shaft passing through a fixed housing. This spinning shaft and the stationary part have a static seal provided by an O-ring. Similarly, another part is attached with an O-ring that provides a static seal to the stationary housing. These seal faces are then pressed together with a predefined force that actually results in the sealing. While there are different types of mechanical seals, some of the common advantages of these are:
-Consumes less power
The critical part, however, is to select the most appropriate one from the different kinds of mechanical seals that are available today. Let’s look at these types of mechanical seals. There are 6 most commonly used mechanical seals types – pusher, non-pusher, balanced, unbalanced, balanced, conventional, and cartridge.
Pusher seals have a spring that is axially mounted to the shaft. This moves along the rotating shaft and keeps contact with the faces of the seal. Pusher seals are simple to use, easily available and are inexpensive. The downside of using pusher seals could be the chances of fretting of the shaft.
The main difference between the pusher and non-pusher seal is that these use sealed bellows instead of springs, which means they don’t move axially to keep contact with the seal faces. Just like pusher seals, they are easy to use and inexpensive. They also work well under high pressure and low-temperature applications. The bellows, however, need regular upgrades for better functionality in harsh environments.
Balanced mechanical seals
Balanced mechanical seals are extremely able to withstand high pressures, meaning they are suitable to handle liquids with low lubrication qualities.
Unbalanced mechanical seals
Unbalanced mechanical seals are used in harsher conditions like misalignment of shaft and vibrations. They cannot withstand high pressures and can be used only in the low-pressure range. The seal may fail if the force of the fluid is increased. They, however, are inexpensive, allow slight amounts of leakage and are highly stable.
Conventional mechanical seals
Conventional mechanical seals are simple seals that are widely used for applications that originally have lip seals instead of mechanical seals. These seals are used during the upgrade of such applications. Hence these seals need to be properly aligned to the shaft of the pumps.
Cartridge mechanical seals
Of all the different types of mechanical seals, cartridge seals are the modern one and many of the newer products launched by the pump builders have these types. They are easier to maintain, easy to replace, and also very simple install that neatly fits inside the housing. They also provide the required static sealing to both the rotating shaft and the fixed housing.
Now that you are aware of the different types of mechanical seals, the next important thing to understand is how to select the right seal for your pump. There are five crucial steps to select the pump mechanical seal you need.
Step 1: Figure out the right type
Any type of seal you chose comes in possibly two variants – Single or double seal. A single seal is easy to use and inexpensive. However, they are not really recommended in the hazardous environment because of their susceptibility to leakage. Conversely, double seals are suitable for use when liquid is unstable, toxic or hazardous. There are two seals in the double seal type that are oriented in opposite directions and has a high-pressure barrier liquid between the two seals. In case a seal fails, the barrier liquid flows into the atmosphere instead of the process fluid reducing the environmental risks.
Step 2: Liquid Type
Identifying the liquid type is another important step in choosing the right seal. Your selection of the seal should factor on the type of fluid that is going to be processed, chemical compatibility of the fluid and the vapor pressure.
Step 3: Temperature
When we were looking at different types of mechanical seals earlier, we have seen that few are suitable for higher temperature pressure while few are not. The seal should be selected based on liquid temperature and the seal's ability to withstand those temperatures.
Step 4: Material
Not just the characteristics of the seals, but the outer material of the seal should also be taken into consideration while choosing the right one.
Step 5: Safety
Safety of the environment also people work around is critical when choosing the appropriate seal type. A wrong seal may be hazardous to both human and environment alike. Double seals are hence not only reliable but recommended too to use.
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