Clamping down on joint failures with formed-in-place gaskets

Formed-in-place gaskets cna replace pre-cut versions in almost any application and perform as well or better. This article describes how formed-in-place gaskets form strong, leak-free permanent joints; how special compounds eliminate maintenance tasks such as gasket replacement and fastener retorquing; and how anaerobic and silicone gasketing materials serve a nearly unlimited range of applications.

07/06/2001



Plant Engineering - December 2000
MAINTENANCE
Feature: Clamping down on joint failures with formed-in-place gaskets
Robert Sevigny, Application Engineer, Loctite Corp., Rocky Hill, CT

Key concepts

  • Formed-in-place gaskets create strong, leak-free, permanent joints that withstand the stresses of temperature, pressure, and vibration.

  • Special compounds eliminate maintenance tasks such as gasket replacement and fastener retorquing.

  • Anaerobic and silicone gasketing materials serve a nearly unlimited range of applications.



    When servicing equipment, gasketed joints are a blessing. When running equipment, they are often a headache.

    Equipment vibrates. Fluids get hot. Gaskets erode, fatigue, and break. Joints leak. A gasket failure can shut down a machine or a whole process worth thousands of dollars per minute. These reasons are why gaskets—those low-cost shapes of rubber, cork, cellulose, or composites—rank high on every plant maintenance checklist.

    Plant personnel regularly inspect joints and retorque bolts to restore gasket compression. They replace worn gaskets during maintenance shutdowns, and sometimes on-the-spot in emergencies. They buy and catalog countless shapes and sizes of gaskets and store them carefully to prevent embrittlement.

    While gasketing is necessary, many of these tasks are not. Formed-in-place gaskets ( Fig. 1 ) can replace precut versions in almost any application and perform as well or better.

    A dynamic system
    A gasketed joint is not a static object. It must maintain its seal against temperature fluctuations or extremes, chemical action, internal pressures and surges, and machinery shock and vibration. A successful seal depends on proper interaction between the flange, fasteners, and gasket.

    The gasket is the most important element in the system, creating a physical barrier that prevents the escape of fluid. When properly applied, formed-in-place gaskets create a longer-lasting seal against a full range of operating stresses.

    Traditional precut gaskets, while seemingly convenient and inexpensive, have deficiencies that affect joint maintenance and performance. Contact pressure between a precut gasket and flange must exceed a minimum sealing stress, which depends on the viscosity of the fluid passing through the joint, type of gasket material, gasket shape, flange stiffness, and surface finish.

    Above the minimum sealing stress, pores and gaps in the gasket material are squeezed shut and the gasket conforms to irregularities in the flange surface. Sealing stress is at the heart of at least four common gasket problems.

    Compression set. A gasket inevitably loses its resiliency. Pressure surges on flanges have a pounding effect on gaskets, gradually reducing their thickness and opening the way for leaks. Compression set is the primary reason maintenance personnel must periodically check and retorque gasketed joints.

    Breakage. Overtightening fasteners can crack, tear, or rupture the gasket directly under the fastener heads.

    Extrusion. Internal and external pressures can squeeze the gasket out from between flanges. Gaskets are most likely to extrude at the midpoint between fasteners, where sealing stress is the lowest. Gaskets can also extrude where flanges are "cocked" because of imprecise machining or tightening of bolts in an improper sequence. Leakage starts to appear when pressure in the area of low gasket compression falls below the minimum sealing stress.

    Abrasive wear. Because they are compressible, precut gaskets allow some side-to-side flange movement, which can abrade the gasket material. This movement also can loosen fasteners, leading to a loss of sealing stress.

    Another drawback of precut gaskets is obsolescence. Finding replacement gaskets for older equipment can be nearly impossible, especially if the manufacturer has changed the design on newer models. In addition, precut gaskets can be damaged during transport and handling, and can become brittle when stored for long periods.

    Formed-in-place gaskets
    Liquid gasketing compounds (introduced in 1974) have steadily evolved to meet the demands of modern industry. Formed-in-place gaskets have proven to be equal to, or better than, precut products in a wide range of maintenance and repair applications.

    There are two basic kinds of gasketing compounds. Anaerobic compounds, most often used on rigid metal flanges, cure to a tough, thermoset plastic in the absence of oxygen. These compounds endure high resistance to solvents and hot oils, withstand high operating pressures, and withstand temperatures up to 400 F (204 C).

    Silicone gasketing materials cure in the presence of atmospheric moisture and provide effective seals for components made of pressed or rolled sheet metal. They withstand temperatures as high as 600 F (315 C). Their elongation properties make them a good choice in joints that show extreme differential thermal expansion.

    The table compares the performance characteristics of anaerobic and silicone formed-in-place gasket materials.

    Gasket compounds eliminate the need to purchase and store numerous sizes, shapes, and types for the full range of plant equipment. A few tubes or cartridges of material can replace an entire inventory of cut gaskets. The compounds assume any flange shape, so gaskets are always available, even for the oldest machinery.

    Anaerobic gasket compounds conform totally to the flange surface, covering all irregularities and filling all gaps, thus eliminating leak paths ( Fig. 2 ). The finished joint provides metal-to-metal contact ( Fig. 3 ), making the assembly structurally stronger and restricting side-to-side motion. Bolts, flange, and gasket act as a unit, sharing operational stresses.

    Joint integrity does not depend on a minimum sealing stress. There is no gasket compression set and no need to retorque flange bolts. The thermoset material has no pores or gaps to permit seepage or weeping, and is highly impermeable to gases and fluids, including strong solvents.

    Silicone gasket compounds meet the challenge of sealing joints formed with less-rigid stamped sheet metal parts. In manufacturing stamped sheet metal components, it is difficult to maintain a consistent, flat joint face. Consequently, gaskets for such applications usually consist of thick, highly compressible materials, such as a cork composite. These materials are expensive and have the same shortcomings as most other precut gaskets.

    Silicones offer high oil resistance and high elasticity to allow for joint movement while maintaining a seal. The materials adhere strongly to clean metal surfaces to prevent leaks, while low shrinkage ensures effective gap filling.

    Using formed-in-place gaskets
    When properly chosen for the application, formed-in-place gaskets help create effective, durable joints requiring little or no maintenance. Despite their virtues, however, gasket compounds should not be viewed simply as replacements for every type of precut product. A joint is a system, and all its components—flanges, fasteners, and gaskets—must work together.

    Before using any formed-in-place gasket, it is essential to understand how the joint was designed, and why. In large part, this information is necessary because gaskets have functions beyond providing a seal.

    For example, the original gasket material may have been specifically chosen to dampen vibration or insulate against heat transfer. Metallic gaskets are often selected to help accommodate differences in thermal expansion between two dissimilar flange materials.

    Some gaskets fulfill dimensional requirements. In some instances, a gasket acts as a shim in an assembly to adjust the relative position of components such as gears, or to control the preload on bearings. Whenever these gaskets are eliminated, it is essential to insert a substitute shim.

    Unlike precut versions, anaerobic formed-in-place gaskets do not "cushion" high spots on the flange surface. It is important to chamfer bolt or dowel pinholes to eliminate raised metal. This action ensures an effective seal and eliminates stress risers in the joint. It is also a good practice to apply a continuous pattern of gasketing material inside bolt or dowel pinholes to eliminate secondary leak paths.

    Although anaerobic gasket compounds have a nearly indefinite "open time" once applied, it is advisable to complete the assembly within 1 hr to reduce the potential for particle contamination. Immediately after assembly, all fasteners should be run down and torqued to specification. Subassembled components may require slave fasteners to ensure consistent clamp load during the curing process.

    Figure 4 provides some suggestions for using silicone gaskets.

    —Edited by Ron Holzhauer, Managing Editor,
    630-320-7139, rholzhauer@cahners.com


    More info
    Mr. Sevigny is available to answer questions about formed-in-place gaskets. He can be reached at 860-571-5100.


    Comparing formed-in-place gaskets
    CharacteristicsAnaerobicRTV silicone

    Cure mechanism

    Absence of air and presence of metal

    Atmospheric moisture

    Open time

    Indefinite

    3%%MDASSML%%5-min maximum @50% relative humidity/20 C

    Cure through volume

    0%%MDASSML%%1.25 mm

    0%%MDASSML%%6.25 mm

    Tensile elongation

    5%%MDASSML%%64%

    250%%MDASSML%%600%

    Shear resistance friction coefficient

    1

    0.16

    Operating temperature range

    -54%%MDASSML%%204 C

    -70%%MDASSML%%315 C

    Operating pressure range

    All pressures; 34 MPa max

    Low pressures; 1.75 MPa max

    Solvent resistance

    Excellent; all automotive fluids

    Fair; no fuel or aromatics

    Hot oil resistance

    Excellent

    Fair/good

    Surface oil resistance

    Good

    Poor/fair

    Substrate compatibility

    All metals, thermosets, and crystalline thermoplastics

    All metals and engineering plastics

    Flange requirements

    Close-fitting machined castings and structural stampings only

    Stampings and nonstructural components; 0.5%%MDASSML%%125-mm standoff desirable

    Application methods/options

    Tracing methods, silkscreen stencil, transfer, and roller

    Tracing methods only



    Plant Engineering





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