Mechanical seals prevent leaks of the pumped fluid and the loss of expensive pumped products. They keep or seal the pump so that the fluid stays inside it. Since about the 1950s, mechanical seals have almost completely eliminated the inefficient and expensive stuffing box.
They are installed where the pump shaft enters or leaves the pump housing. Seals of various types, sizes, and configurations are available. However, they all use the basic principle of a combination of fixed and rotating surfaces.
1. How does the pump’s Mechanical Seal work?
The mechanical seal works by using two very flat overlapping surfaces, which makes leakage difficult to occur. One surface does not rotate with the axis (stationary), while the second surface rotates with the axis (rotates).
When the two faces rub together, the fluid film between the two faces migrates for cooling and lubrication purposes. Ideally, the pumped fluid will flow out between the two faces, enter as a liquid, and evaporate when it reaches the atmosphere.
It must be pointed out at this point that all mechanical seals leak some very small amounts of steam, so even the best mechanical seal design will lose a small part of the pumped fluid.
2. Pump mechanical seal parts
All mechanical seals usually consist of three basic components:
Generally, there are various types of mechanical seals. According to design and layout, mechanical seals are generally divided into the following categories:
3. Type of mechanical seal
Mechanical seal can be divided into the internal seal, external seal, push-in seal, Non-push seal and Balanced seal. Here are the details of each type of mechanical seal.
The design of the internal seal is that the rotating part of the mechanical seal is located in the pump seal cavity. For internal seals, fluid and pressure are applied to the outer diameter (outer diameter) of the seal. Generally, internal seals are used for higher pressure applications compared to external seals.
The design of the external seal is that the rotating part of the mechanical seal is located outside the pump seal cavity. Normally, external seals are used for chemical applications of non-metal parts: the pumped fluid does not come into contact with the sealed metal parts, so expensive and/or special materials are usually not required.
Push-in seal is a design that pushes a dynamic secondary seal (o-ring, wedge or other type of equipment) through the shaft as a means of compensating for surface wear and/or shaft movement.
This type of seal is designed to not use dynamic secondary seals. Generally, non-push-in seals are metal bellows or elastic bellows seals.
In a balanced seal, the hydraulic pressure used to close the sealing surface is significantly reduced. The advantage of a balanced seal is that less heat is generated because the pressure is reduced to force the surfaces together. Therefore, they can withstand higher pressures than unbalanced seals.
Mechanical seals act without any reduction on the full hydraulic pressure present in the seal chamber in unbalanced seals. In low-pressure applications, unbalanced seals are the best choice.
Double seal: In this structure, two (2) mechanical seals are used face to face, back to back or in series (facing the same direction), thereby allowing buffer fluid or gas to be introduced between the two sets of sealing surfaces. Double seals are mainly used to seal volatile organic compounds, dirty, non-lubricated or very viscous products. In addition, they are also used for products that are cured or changed state.
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