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Correct Selection Of Mechanical Seal Is The Key To Prevent Leakage Of Liquid Pump

Sep 19, 2016

Summary: in modern process more universal, strong corrosive liquid, the right choice is the key to preventing leakage of mechanical seals.

For more than 100 years, people packed a variety of sealing materials, the liquid from the pump shell leakage along the spindle steady decline. Although modern technological process used in the pump, still widely used design-the oldest seal packing boxes, because of its low starting costs and familiar to factory workers.

However, due to environmental issues, using packing methods have gradually been accepted, particularly common in modern process, strong corrosive liquids. Therefore, in practice, to replace the mechanical seal packing more and more.

Basic elements of the seal

Mechanical seal friction is the use of two plane operation principle, achieve the purpose of sealing. Rotating seal face mounted on a liquid pump axis and fixed installation inside the sealing gland on the sealing surface. Due to a sealing surface is in motion, while the other is stationary seal face, so this type of seals called dynamic seals.

Figure 1 rotating and static seal between the face is the most critical factor determines seal performance.

Foundation of mechanical seal as shown in Figure 1, of which 4 leakage pathways need to be sealed:

1. pathway between the sealing surfaces;

2. a rotational surface pathways with the spindle;

3. fixed pathway between the surface of the gland;

4. the pathway between the gland and the stuffing box.

Two leaks after pathway generally uses a static seal, because there is no relative motion between the two sections. This part of the seal is usually referred to as triple sealing, the sealing material for gasket or process compatible liquids of O-rings.

In the design of older seals, located in the secondary seals on rotating surfaces there is a gap, before and after sports in the spindle, so easy to cause premature wear and failure. However in the design of new seal secondary seal in a static state, so you can avoid wear and corrosion problems on the spindle.

In the normal operation of pump, Rotary and stationary surface by liquid in the packing box generated by the pressure to keep it sealed in the State, at the time of starting and stopping, stuffing box pressure generated by the spring pressure to maintain (or even can be replaced by spring pressure).

Most of the mechanical seal design uses softer materials to create a rotating surface, make it harder on the still surface of Rotary friction. For many years, the most common combination is the use of carbon as a surface of revolution, make it run on the ceramic stationary surface. This type of material is still in common use, but still surfaces are made of stainless steel or hardened materials, such as tungsten carbide or silicon carbide.

Regardless of what material, in short between the contact surfaces must be kept a layer of thin liquid films, to play the role of lubrication. However, packing boxes, using a combination of spring-loaded and liquid pressure, you can make the play a good seal between the sealing surface. But sealing pressure is too high, you can affect the formation of liquid film between contact surfaces, leading to increased heat and premature wear. If the seal pressure is too low, and increased clearance between the contact surfaces, likely to cause fluid leakage.

Seal manufacturers are constantly trying to improve the flatness of the contact surface, they use special polishing plates for grinding. Then, the grating plate on its detection using monochromatic light source. From this point of view, these seal contact surface must be handled with care and strictly in accordance with the installation instructions, appropriate protection and ensure that the sealing surfaces are properly seated, this is very important.