What Determines Seal Leakage? - Pump Systems Problems, Pump Repair, Pump Installation, Pump Help & Resources

What Determines Seal Leakage?

Written by Fluid Sealing Association

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Measuring Leakage

Leakage limits may be given as a volumetric rate, such as milliliters per hour (ml/hr), or as a mass leakage rate, such as grams per hour (g/hr). For volatile organic compounds (VOCs), limits are sometimes expressed as a concentration, i.e., 200 parts per million (ppm), of the VOC. This is usually referred to as an allowable emission rate; the measurement is taken according to EPA Method 21. An emission rate of 1,000-ppm is equal to a mass leakage rate of 5.6-g/hr. Another rough rule of thumb is that a milliliter of liquid contains about 20 drops; therefore, a volumetric leakage rate of 3-ml/hr would be about 60 drops per hour, a drop per minute.

Some states, notably California, have a "no visible leakage" requirement; in practice, this means that visible leakage of three drops per minute is considered a major leak and the pump must be shut down for repair.

Other Factors Affecting Leakage

Factors that tend to increase leakage, other than those that cause seal face damage are:

High fluid viscosity (typically above 30-cP or 32-cST)
Low seal balance (60 to 65 percent range)
Low face pressure due to unloading forces
Very narrow faces (< 0.100-in)-Edge chipping, surface damage and wear have a more dramatic effect on leakage than wider face widths
Face treatments or special lapping techniques to increase sealing gap
Composite face materials (due to rougher surface finish)
Excessive converging sealing gap due to thermal distortions
Excessive divergent sealing gap due to pressure distortion
Wiping action of seal face over mating ring
Distorted seal faces (high and low spots from some mechanical condition)

Summary

Mechanical seals are designed to produce an acceptably low level of leakage to function effectively. Many individual internal and external factors, as well as interactions between them, affect the rate of seal leakage. Some design features or treatments enhance reliability but contribute to some marginal increase in leakage. The source of most leakage usually is the interface between the two seal faces, but can also come from secondary seals such as O-rings.

A balance must be established between steady state and transient operating conditions to control the negative factors that lead to increased leakage. Contact your mechanical seal supplier to determine the design that best establishes this balance for a specific set of operating conditions.

Next Month: How do I determine bolt torque for flange connections?

We invite your questions on sealing issues and will provide best efforts answers based on FSA publications.

Fluid Sealing Association

Sealing Sense is produced by the Fluid Sealing Association as part of our commitment to industry consensus technical education for pump users, contractors, distributors, OEMs and reps. This month's Sealing Sense was prepared by FSA Members Gordon Buck and Tom Lai. As a source of technical information on sealing systems and devices, and in cooperation with the European Sealing Association, the FSA also supports development of harmonized standards in all areas of fluid sealing technology. The education is provided in the public interest to enable a balanced assessment of the most effective solutions to pump technology issues on rational Total Life Cycle Cost (LCC) principles.

The Mechanical Seal Division of the FSA is one of five with a specific product technology focus. As part of their educational mission they develop publications such as the Mechanical Seal Handbook, a primer intended to complement the more detailed manufacturer's documents produced by the member companies. This handbook served as the basis for joint development of the more comprehensive Hydraulic Institute publication: Mechanical Seals for Pumps: Application Guidelines. Joint FSA/ESA publications such as the Seal Forum, a series of case studies in pump performance, are another example as is the Life Cycle Cost Estimator, a web-based software tool for determination of pump seal total Life Cycle Costs. The Sealing Systems Matter initiative also was launched to support the case for choosing mechanical seals that optimize life cycle cost, safety and environmental compliance.

The following members of the Mechanical Seal Division sponsor this Sealing Sense series: