Sanitary Mechanical Pump Seal
In the first of a multi-part series, I will discuss the design and features of sanitary mechanical pump seals. In subsequent posts, we will discuss more advanced topics, such as seal face materials, configurations, selection, and application.
Where there is a rotating shaft, a shaft seal is involved. The shaft seal prevents leakage of product into the atmosphere. The pump seal serves as a barrier between the pump casing and the atmosphere. In the sanitary world especially, we are concerned with minimizing leakage.
The two basic seal designs we see in the sanitary world are the lip or O Ring seal and the mechanical seal.
The lip or O-ring seal is relatively straightforward in design. It utilizes a rubber ring sliding against the shaft. It is usually positioned using a slot in the shaft and a shaft sleeve. This seal design is good for applications with low-pressure differential and low speed applications. We generally only see this seal type applied in low pressure, slow running positive displacement pump applications. We do not see this seal design in sanitary centrifugal applications where shaft speeds between 1750 and 3600 RPM. While it is not the most robust of seal designs, it is effective, low cost, and easy to maintain and repair.
The second seal type, which will be the focus of the rest of this post, is the mechanical shaft seal. A mechanical seal consists of two main components: a rotating part, the seal ring, and a stationary part, the seal seat. The stationary seal seat is usually fixed to the pump housing. The rotating ring is axially pressed against the stationary seat, usually through the use of a spring. The tiny space between the seal faces is called the seal gap. When the pump shaft is not moving, the seal faces are pressed against one another.
When the pump shaft starts rotating, the pressure differential created between the pump chamber and atmosphere causes the seal faces to separate. The pressure differential draws product between the stationary and rotating seal faces to create a lubricating film that is 0.00025 mm thick. This fluid film serves to lubricate and cool the seal faces, preventing wear. If properly designed and installed, the low pressure and high temperature in the seal gap will cause seal fluid evaporation and prevent leakage.
If poorly designed or if seal faces wear, the pump will leak. The rate of leakage is determined by a variety of factors including, but not limited to: surface roughness of seal faces, flatness of seal faces, vibration and stability of the pump, speed of rotation, temperature, viscosity, and type of pumped media, pump pressure, and seal design and assembly.
In future posts, we will discuss specific applications regarding sanitary pumps that cause seal face wear and media leakage and what Holland can do to help you minimize sanitary seal failure.
Holland Applied Technologies