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Detailed Application Explanation of Spiral Wound Gaskets
2025/02/20
17次
Metal spiral wound gaskets are made by alternately winding a formed metal strip and a soft filler material. When compressed between two flanges, they form a highly effective seal.
The V-shaped protrusion in the middle part of the metal strip functions like a spring, providing excellent rebound to the gasket when operating conditions change. Different materials can be selected for the filler material and the metal strip to accommodate various chemical media.
For those requiring fire safety, flexible graphite can be chosen as the filler material. If there are limitations on the effective compression load of the gasket, modifications to the gasket's structure and dimensions can be made to achieve an effective seal.
A metal spiral wound gasket can include an outer ring, an inner ring, or both. The outer ring aligns the gasket with the flange and serves as a limit for gasket compression, while the inner ring not only provides additional radial strength but also reduces flange erosion and protects the sealing elements.
The excellent rebound and high strength make metal spiral wound gaskets an ideal choice for a variety of operating conditions and environments. They are widely used in oil refineries and chemical plants.
The marking requirements specified by ASME B16.20 are as follows.
Factors Affecting Gasket Sealing Performance
Sealing is achieved by compressing the gasket material to fill the uneven sealing surfaces. This sealing effectively prevents leakage of media. To maintain this condition, sufficient load must be applied to the joint to counteract the hydrostatic end pressure generated by the internal pressure of the system.
The performance of a gasket depends on many factors, including:
Metallic and filler materials, which must withstand the impact of several factors:
a. Temperature: Temperature adversely affects the mechanical and chemical properties of gaskets, and can also influence physical characteristics such as oxidation resistance and rebound elasticity.
b. Pressure: Internal pressure of the media or piping may push the gasket out of the flange surface.
c. Media: The gasket material must be corrosion-resistant to the media.
Joint design: The force between the two flange connections must be sufficient to prevent flange separation caused by the hydrostatic end pressure generated by the internal system pressure.
Appropriate bolt load: If the bolt load is not sufficient, the deformation of the gasket will not meet the requirements, or if the load is too great and crushes the gasket, leakage will occur.
Surface finish: If the surface finish does not meet the gasket requirements, it will not provide a good sealing effect.
各种密封面下法兰、垫片的连接
Gasket Sealing Coefficients "M" and "Y"
The values of "M" and "Y" are used in non-standard flange designs. In practical applications of standard flanges, they are not specified as gasket compression stresses. Our bolt torque tables provide the data that should be used for comparison.
"M" - Gasket Factor
In flange connections, it is the coefficient that provides the required additional preload force to maintain the compression load on the gasket after internal pressure is applied to the connection.
M = (W - A2 P) / A1 P
"Y" - Minimum Design Compression Stress for Gasket
This refers to the minimum compression stress value required per square inch (or in bars) on the gasket contact surface when the gasket provides a seal at an internal pressure of 2 psig (0.14 bar).
Y = W / A1
Where: W = Total tightening force (pounds or Newtons) A2 = Area of the flange bore inside the gasket (square inches or square millimeters) P = Test pressure (psig or N/mm²) A1 = Area of the gasket (square inches or square millimeters)
M and Y for Various Types of Gaskets
Gaskets for Heat Exchangers and Containers
Installation of Gaskets
In flange connections, all components must be correctly assembled to achieve a sealing effect. A common cause of leakage in the connection area is incorrect installation procedures.
Bolt tightening sequence
■ Place the gasket on the flange sealing surface.
■ Bring the other assembled flange into contact with the gasket.
■ Use new bolts or bolts that perform as well as new ones, clean the threads, and lubricate them with a high-quality lubricant, such as an oil and graphite mixture.
■ Place the bolts in the bolt holes.
■ Hand-tighten the nuts.
■ Tighten the bolts according to the bolt tightening sequence shown in the diagram above.
■ During the initial tightening, the tightening force of any bolt should not exceed 30% of the recommended value, as doing so may cause the flange to warp and crush the gasket.
■ When the recommended bolt torque is reached, check the bolt torque individually in a clockwise direction to ensure that the bolts have been evenly tightened.
■ Due to creep and stress relaxation, it is necessary to prestress the bolts during equipment operation to ensure adequate stress loading during operation.
Prestress Applied to Bolts for Thermal Expansion
Compensation for thermal expansion, relaxation, creep, hydrostatic end pressure, and residual effective stress of the gasket is achieved through bolt preloading.
Due to the different materials of the flange and bolts, their coefficients of thermal expansion may differ, which can affect the bolt load. In cases of significant thermal expansion, it is necessary to provide a minimum stress on the bolts and allow for pipeline expansion to achieve gasket compression.
The outer ring of the gasket serves as a limit for gasket compression. For gaskets without an outer ring, measures must be taken to prevent thermal expansion from damaging the gasket by exceeding its elastic limit.
Troubleshooting for Leak Points
The general method to identify the cause of leakage is to carefully inspect the gasket at the leakage point.
Observe the results
Effective corrective measures
Severe corrosion of the gasket
• Replace with better corrosion-resistant materials
• Replace with materials that have better cold flow resistance characteristics
Excessive extrusion of the gasket
• Replace with materials that can withstand higher loads - i.e., denser materials.
• Replace with materials that can support higher loads
Gasket crushing
• Provide a stop ring to prevent gasket crushing or redesign the flange.
• Check the gasket dimensions to ensure it is the appropriate size and specification.
Due to extension beyond the raised face flange or towards the flange hole
Mechanical damage caused by internal protrusions
• Ensure proper alignment during gasket installation
• Choose a softer gasket material
Without obvious compression
• Choose a thicker gasket material
• Reduce the gasket area to provide a higher compression load per unit area.
• Altering the gasket size and shifting the stressed portion of the gasket closer to the bolt area can effectively reduce [the issue/problem].
Flange bending
Caused by excessive deflection and bending of the flange.
• Rely on auxiliary rings to provide stiffness to the flange.
The thickness of the gasket at the outer diameter is thinner than that at the inner diameter.
• Choose a gasket made of soft material to reduce the required sealing stress.
• Reduce the gasket area to decrease the sealing stress.
• Tighten the bolts in sequence according to the specified tightening procedure.
The gasket is not uniformly compressed around its perimeter.
• Ensure accurate flange alignment.
• Add a reinforcing ring to the flange to better distribute the bolt load.
•Choose a gasket material with lower stress.
Periodic variation in thickness around the perimeter of the gasket
• If possible, increase the number of bolts to achieve a more uniform distribution of bolt loads.
• If the flange is warped, it can be reprocessed or a gasket made of softer material can be used.
Selection of gaskets
Requirements for Temperature and Chemical Resistance
Determine that the gasket you order can withstand the operating temperature and resist the effects of the medium in use. Check the chemical compatibility of the metal and filler with the sealing medium. As a general practice, the metal used to make spiral wound gaskets and metal jacketed gaskets should be similar to the flange material.
The compressibility of flexible graphite makes it an excellent filler for making metal gaskets. Although flexible graphite cannot be used with strong oxidants like nitric acid and sulfuric acid, it can operate at temperatures up to 450°C. PTFE filler material provides excellent chemical resistance below 260°C. According to ASME B16.20, an inner ring should be used when PTFE is used as the filler material to prevent radial instability of the wound portion.
Working Pressure
Working pressure has a direct impact on the design and selection of metal gaskets. Higher pressures may cause the gasket to blow out, while low-pressure applications require a gasket design that can seal with low bolt loads.
Garlock gaskets suitable for high-pressure applications include:
■ Kammprofile gaskets
■ Spiral wound gaskets with an inner ring
■ Solid metal gaskets
Gaskets suitable for low-pressure applications include:
