Five performance factors affect the life of a mechanical seal and guide the determination of an optimum replacement cycle for that seal. They also lead to improved seal life and mean time between repair (MTBR), thereby reducing the Life Cycle Cost (LCC) of the pump population.
Last month we reviewed the first two of these factors. Here are the other three.
Seal Application Limits
The third factor is published mechanical seal application limits, typically defined by pressure, speed and temperature, are prone to misinterpretation.
Most seal manufacturers say that operation of the seal at one of these limits will result in a minimum life span of two years; three years with some seal types such as those specified in Standard API 682. The published limits, while reliable for steady state operation, may be overstated or unreliable for cyclic operation. An exception may be API 682 seals, which specifically address that shortcoming by including a series of cyclic tests to qualify a specific seal type for a range of services.
Generally, no fixed rules exist on how much particular transient-pressure, speed, temperature, gas or solid entrapment, or vibration a seal can tolerate. Expert advice is needed in this area.
Seal Face Deflection
The amount a seal face deflects during a transient depends on several factors, including the magnitude and rate of the transient, and the robustness or strength of the seal face. The bottom line is that seal face deflections occur as operating conditions change and may either improve or starve lubrication of the seal faces. A reliable seal is one having a low degree of sensitivity to the expected or unexpected transients. The sensitivity of a seal can be defined by its ability to maintain more or less parallel seal faces during all possible transients.
Sensitivity can also be controlled by properly selecting the materials and the environmental control system. For example, seals with hard/hard face combinations, such as tungsten carbide (WC) and silicon carbide (SiC), which are preferred for dirty fluids or in some cases for high pressures, are very vulnerable to complete or partial loss of the fluid film. Any type of damage to these materials, when run against each other, is typically unrecoverable and only gets worse as time progresses.
New face materials and treatment technologies, such as diamond coatings, promise significant improvement in this area. On the other hand, hard/soft combinations with carbon-graphite as the wearing material offer the distinctive advantage that they can carry much higher loads, sustain longer periods of inadequate lubrication and, most importantly, more readily recover after damage to the carbon-graphite face.
Silicon carbide against a hard carbon-graphite is generally accepted to have one of the highest load capability limits of the readily available materials, best tolerates dry running and, therefore, should be the preferred material combination for many applications.
Limiting Application Parameters
Without question, an application becomes more difficult and risky when using a larger shaft diameter, faster speed, and higher temperature, pressure and solids content. It is imperative that the robustness of the faces and their ability to dissipate heat efficiently are optimized accordingly.
In many tough applications, it may be wise to consider custom engineered mechanical seals that are designed specifically to deal with unusual events or predefined operating modes of the equipment. Different face designs and lubrication technologies can be selected to increase the life of the faces.
Seal life well in excess of three years is possible for the large majority of applications. The seal face shown in Figure 1 is an example of a robust design that is used in high pressure or speed applications with the ability to handle severe transients and periods of inadequate lubrication. The carbon-graphite face is shrunk-fit into a metal housing and reinforced at the inside diameter with a hardened steel ring. The API 682 and Hydraulic Institute publication Mechanical Seals for Pumps: Application Guidelines offers good rules for specifying an engineered seal.
The fourth factor is all face seals must leak a minute amount to sustain an adequate lubricating film during all operating modes.
Because seals operate in these favorable lubrication regimes, energy consumption is minimized, which makes seal operation more economical. The amount of leakage is typically a few milliliters per day or less and not visible. This may be significantly higher for special applications involving high pressures, speeds or large shaft diameters. Consult your seal vendor for the calculation of expected leak rates.
Mechanical seals meet today's emission limitations in the vast majority of applications. Predicting the leakage and friction behavior for any given application is possible with a good degree of accuracy, which helps set operator guidelines for normal, questionable, and failure behaviors of the seal.
The leak rate increases or decreases when conditions such as pressure, temperature, or speed are changed. This means the leak rate varies depending on how the pump is operated and how responsive the seal faces are to any transient operating conditions. The key to a low and consistent leakage pattern is to maintain face flatness. The Key Performance Indicators (KPIs) of the seal may differ depending on the face technology and materials used.