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Spiral Wound Gasket Compressibility and Pressure Ratings

Spiral Wound Gasket Compressibility and Pressure RatingsIn order to form a reliable seal, spiral wound gaskets do require a certain degree of pressure.  

The applied pressure allows the gasket to be compressed tightly between the mating surfaces preventing any leaks from occurring. 

Factors such as operating temperature and how the flanges are manufactured will affect the pressure that these spiral wound gaskets are placed under. 

Spiral wound gaskets are typically used in ASME B16.5 flat or raised face pipe flanges and pump applications with recess flanges. 

Knowing the maximum pressure, a gasket can withstand is vital to understanding how suitable it will be for your applications and processes. 

Gasket rating systems 

Spiral wound gaskets do come with class and pressure ratings, allowing you to evaluate and choose the right gasket for you. 

The seven pressure classes are as follows: 

  • 150 
  • 300 
  • 400 
  • 600 
  • 900 
  • 1500 
  • 2500 

The higher the pressure rating, the more pressure the flange can handle because it has been designed and manufactured using more metal. 

This means a gasket with a class of 150 will provide a reliable seal under a pressure load of 150lbs matched with a class 150 flange. 

However, for a spiral wound gasket pressure rating, these are typically labelled: 

  • 0-999 psi, 1000-3000 psi (pressure units). 

These ratings are used to identify windings only gaskets. 

Spiral wound gasket density 

Spiral wound gaskets are available in different densities to match various application loads. And it’s important to be aware that different manufacturers will use different filler densities to achieve the same pressure class, as currently, there is no standardisation for spiral wound gaskets in this area. 

Spiral wound gasket density can be defined by describing: 

  • Number of windings per inch 
  • Filler thickness 
  • Assembly pressure 
  • Number of windings without filler 
  • Spot welds, and 
  • Inner and outer rings used. 

These elements can affect the compressibility of a gasket under load and how well they effectively perform. 

The pipe flange standard B16.20 outlines that all inner rings for larger spiral wound gaskets must have specific fillers to prevent buckling. 

So, suitable gasket materials must be chosen, as should the flange design, in order for spiral wound gaskets to perform effectively in varying pressure and temperature applications and ensuring suitability to specific applications. 

Operating conditions affecting compressibility 

Spiral wound gasket compressibility if affected by several factors, including: 

Stress retention and stress relaxation – if a gasket starts to suffer from stress relaxation, its ability to withstand pressure will decrease, and a tight and secure seal will not be provided. Make sure to check the outer ring on spiral gaskets as this is what can prevent the winding from relaxing as pressure is maintained across the perimeter. 

Material thickness – ideally, we’d always advise you to opt for the thinnest gasket material as these are less likely to fail, helping you to avoid flange distortion and misalignment.  Ideally, you should be aiming for comparable compressibility. 

Flange quality – the quality of the finished metalwork can directly affect the sealing of a joint. Surface finish range ideally for spiral wound gaskets should be between 125 Ra to 250 Ra. 

Load sealability – regularly testing for leaks in applications will allow you to test the quality of the gaskets and the load placed on the joint -replacing any gaskets that aren’t performing before more serious issues arise. Use all recommended torque values along with the manufacturer’s specifications.

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Required gasket pressure 

Gaskets require a minimum compression set to provide a strong seal with the flange surfaces, and for additional compression, tightening of the flange bolts can help. 

The chance of leakages decreases as compression loads increase. 

It’s important to know that metal gaskets will require greater stress to compress and seal compared to flexible gaskets.  Whereas non-metallic gaskets rely on friction for their ability to hold internal pressure at the joint. 

Room Temperature Tightness (ROTT) is the universal test that identifies sealing pressure. 

You will find the ratings, class numbers, and pressure numbers (PN) prescribed on all gaskets and flanges.  

For further details on PN ratings and standards, call our team on 01535 274 776. 

Areas to be aware of 

  • For a seal to continue to be effective, consistent pressure must remain on the surface of the gasket.  

Note:  The compression pressure placed on the gasket must be greater than internal fluid pressure from side loading to prevent extrusion of the gasket. 

  • Increasing temperatures in applications can create gasket relaxation. 
  • If too much pressure is placed on flexible gaskets, it can cause extrusion around the flange. 

The team at Specialist Sealing Products can help provide the right technical support for choosing the correct gasket material for your application based on compressibility and pressure. 

Call 01535 274 776 or visit this page to learn more about our products.

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