This article addresses common questions and challenges faced in sealing centrifugal pumps.
By Michael Huebner, Principal Engineer, Flowserve Corporation
Question:I was looking at a new seal face and noticed that the surface was covered with small scratches? Is this the same thing as face flatness? Is this a problem?
Answer:Mechanical seal faces must be flat. The flatness of the seal face ensures that there is a uniform fluid film not only around the circumference of the seal face but also radially from the outside diameter (OD) to the inside diameter (ID). This is checked on an optical flat or other inspection equipment as part of the normal quality process. Flatness is often referred to in terms of Helium (He) light bands due to the manner in which the faces are inspected.
There is, however, a difference between the flatness of a seal face and the roughness (or surface finish) on the seal face. In mechanical seals, it is common for flatness to describe the overall (or large scale) deviation from a plane and surface finish to describe small scale deviations. The surface finish of the seal face is equally important but not often considered when discussing mechanical seals.
A contacting mechanical seal operates on a thin fluid film and the leakage rate is a function of the film thickness. In
most contemporary liquid seals, the seal faces operate ina mixed lubrication regime where the surfaces are lightly
contacting on the asperities between the seal faces. This is necessary to reduce the leakage down to an acceptable
level while minimizing face wear and heat generation. It is reasonable to assume that faces with very smooth surfaces would produce thinner effective fluid films and minimize seal leakage. While this is true, it will produce some undesirable effects.
In the past, some seal face suppliers would manufacture seal faces with a “mirror” finish. This term was used because the seal face surface was highly polished and had the reflectivity of a mirror. The surface was free from any visible roughness or scratches. This was commonly seen on Nickel – Tungsten Carbide faces. In the field though, seals with these faces often had poor or unpredictable reliability. The faces were so smooth and flat that it was difficult for a liquid film to form between the faces. This resulted in very high start-up torque and damage to the faces or drive mechanisms.
Seal manufactures recognize that the surface finish of the seal faces is a critical factor in establishing a stable fluid film. The seal faces are manufactured with specific surface finishes which are intended to optimize the formation of the fluid film while minimizing leakage. The most common finishes can have very small scratches crisscrossing the sealing surface. These surface finishes are created during the lapping process. By varying the lapping method, compound material and size, lubrication, and time, the manufacturer can produce specific and reproducible surface finishes.
This ability to control the surface finish also gives the seal manufacturer the ability to produce special surface finishes for challenging applications. In some applications it may be difficult to establish a fluid film due to factors such as a high viscosity fluid. In these cases, the seal manufacturer may specify a higher roughness on the surface to allow for a more stable film and to reduce heat generation in operation. This type of modification is often termed a “matte” lap since the surface finish has a matte appearance.
An end user should rightfully be concerned about using seal faces which have chips, cracks, or other damage. They also must recognize that the seal face is an engineered surface and that some roughness on the faces is intentional and may be a critical part of the design.
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