Compatibility with Fluid
The gasket should obviously not be affected by fluid being sealed over the whole range of operating conditions. If any doubt exists, then the gasket manufacturer should be consulted.
The gasket selected should have reasonable life expectancy at the maximum temperature encountered (or the minimum temperature if for a low temperature application). A broad indication of the temperature pressure ratings of the common gasket materials is shown in the figure below.
Gasket materials are designed to compress under load to achieve the initial seal. However, to retain that seal, the gasket should be able to resist flow (or creep) to prevent loss of surface stress by bolt reduction. This property is very important and is the one that most readily separates high quality from low quality gaskets.
Under ambient temperature conditions, most gasket materials do not creep significantly, but as the temperature rises beyond 100°C, creep becomes a serious consideration.
For all applications but particularly for low temperature applications, the following points should be observed:
- The gasket should be completely dry when installed (gaskets for such applications should be stored in a dry atmosphere).
- The required flange loading should be applied at ambient temperature.
- The above information is intended as a guide to the maximum possible ratings of each class of jointing. It does not imply that all the gaskets within each generic type are suitable for the temperatures and pressures shown.
- Even if the material chosen is theoretically suitable for the temperature and pressure, other factors should be considered such as available bolting, flange facing type, shock loadings, etc.
- Consultation with gasket experts should take place at the design stage to ensure that the gasket selected is suitable for all conditions of the application.
The gasket has to be suitable for the maximum internal pressure experienced; this is often the test pressure, which can be greater than 2 times the flange rating at ambient temperature. Vacuum conditions need special considerations but as a guide:
- For coarse vacuum (760 torr to 1 torr): flat rubber or compressed asbestos fibre gaskets.
- For high vacuum (1 torr to 1x10-7 torr): rubber ‘O’ rings or moulded rectangular seals.
- For very high vacuum (below 1x10-7 torr): specialised seals required.
There are many factors apart from those already considered that affect the selection of the correct gasket material and type.
- Cycling conditions.
- If the service conditions include frequent thermal or pressure cycles, then the gasket has to be resilient enough to allow for the flange movements and strong enough to resist the mechanical loading.
- Vibration: If the pipeline is subjected to undue vibration, then the gasket has to withstand the mechanical effects involved.
- Erosive Media: Certain media (e.g. solids suspended in liquids) can slowly erode gaskets leading to a much shorter life than expected. In such cases, choice of gasket material and selection of gasket dimensions are critical.
- Risk of Contaminating the Fluid: Sometimes the effect of contaminating the fluid by leaching chemicals from the gasket should be considered. Typical examples are in the sealing of potable water, blood plasma, pharmaceutical chemicals, food, beer, etc.
- Corrosion of Flanges: Some flange metals are prone to stress corrosion cracking (e.g. austenitic stainless steel). When using these, care should be taken to ensure that the gasket material does not contain an unacceptable level of leachable impurities which may induce corrosion. Such impurities include chloride ions.
- Integrity: When integrity of a gasket is of prime importance (e.g. when sealing a highly toxic chemical), the choice of gasket may be influenced by the requirement for a larger safety margin. As an example, a spirally wound gasket with an outer retaining ring may be selected in place of a compressed asbestos fiber gasket.
- Economy: Although a gasket is a relatively low priced item, the consequential expense of leakage or failure should be considered when deciding on quality, type and material of the gaskets.
The following guidance is offered where pre-selection has not been carried out.
RTJ (Ring Type Joint) Gaskets
RTJ gaskets are forged rings that fit into the machined groove of an RTJ flange. RTJ gaskets are generally used for high pressure applications. Sealing is by metal-to-metal contact between gasket and flange. Solid metal joint rings have excellent tightness and tolerance to temperature and pressure changes once correctly bolted up. Very close attention must be given to their bolting up. Rings and groove faces must be free of imperfections.
There are four different types of ring commonly available: Types R, RX, BX and AX. The most commonly used is Type R.
These are either oval or octagonal in cross-section. The oval RTJ is the original design. The octagonal RTJ is a modification to the oval design and provides better sealing. R type rings may be specified for Class 150 to 2500 flanges though are typically found on Class 1500 flanges and often Class 900. The piping specification will state whether an octagonal or an oval joint is to be used. R type rings may be used on either flat face or raised face RTJ flanges.
RX gaskets fit and seal into the same groove sizes as do R type gaskets. Note that the RX gasket is wider than the R type gasket and the flange face-to-face separation will therefore be greater.
RX gaskets are normally specified up to Class 5000 API 6A Type B flanges. They are used when a more effective seal is required which is resistant to vibrations, shock loadings, etc. (e.g. on wellheads and Christmas trees).
The asymmetric cross-section makes the gasket self-energizing. The outside bevel of the ring makes the initial contact with the grooves of the flange and thus preloads the gasket against the groove outer surface.
These are only used on API 6A Type BX flanges and are rated from Class 5000 to 15000.
The pitch diameter of the ring is slightly greater than the pitch diameter of the flange groove. This preloads the gasket and creates a pressure energized area.
Type BX gaskets are NOT inter-changeable with R or RX gaskets. The groove on a flange which accommodates a BX gasket is dimensionally different to that for R and RX gaskets.
When correctly fitted, the flange face-to-face separation using a BX gasket is zero.
Note: It is particularly important to check the flange face-to-face separation which must be uniform around the entire circumference of the flange. RTJ flanged joints are particularly susceptible to uneven bolt tensioning and misalignment of the ring within the groove.
AX Ring Joint Gasket is pressure energized; the higher the pressure, the better the seal performance. AX gasket is usually used for hydraulic connector, instead of heavy API interface flange with time-consuming installation. AX gaskets are primarily used in the oil, gas, petrochemical and offshore industries. They are also commonly used on valves, pipe-work assemblies and vessel joints and are used to seal flanged connections subject to high pressures and temperatures.
RTJ Gasket Identification and Specification
- Type: Whether R, RX or BX. If R, state whether octagonal or oval. The type of ring to be used will be specified in the piping specification.
- Ring Number: For example R46 will fit a 6 inch NB Class 1500 RTJ flange.
- Material: A variety of materials is available. Again check with the piping specification for the correct material. The material grade will have an identifying code. For example: Soft Iron: D; Stainless Steel 316 : S316
- Standard: Either ANSI B16.20 or API 6A; as specified in the piping specification (these two standards are equivalent and interchangeable).
- Identification: The type, ring number and material will always be marked on the side of the ring.
Spiral Wound (SW) Gaskets
The standard of SW gaskets can vary considerably between manufacturers, and they should be obtained only from reputable suppliers.
Most Spiral Wound Gaskets now being used are Spiral Wound 316 stainless/stainless Windings and Graphite Filler. These gaskets have a 316 stainless/stainless inner ring and coated carbon steel outer guide ring, but on some occasions the outer ring could be stainless steel to provide corrosion resistance to the external environment.