ABOUT FLANGES

Revised: 12/28/2007

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Ring Gasket Selection /
Relation to Connection Material,
Well Fluids, and Application.

Gasket and Groove Specification

 API Spec 6A specifies ring gaskets in the following materials:

All of the above ring gasket materials meet the requirements of NACE MR0175 for H2S service. For extreme critical service ¹ (high H2S with high CO2) some users may specify corrosion resistant alloys such as Inconel 625. The selection of ring gaskets should follow the specification of the connectors sealed by the gaskets. 

API Spec 6A specifies connections in 45K, 60K, and 75K yield strength material. Commonly, manufacturers use the following commercial steels to make flanged or hubbed connections.

API Spec 16A for Drill Through Equipment specifies the same yield strength material as API Spec 6A, and Spec 16A manufacturers frequently use all the same low alloy steels, but rarely if ever do they use 410 Stainless. NACE specifies all of these materials as satisfactory for Hydrogen Sulfide Service (H2S) per MR0175 provided the hardness does not exceed 22 Rockwell C (22 RC), equivalent to 237 Brinell Hardness (237 HBW) per ASTM E 140.

Both API Spec 6A, and 16A, allow corrosion resistant alloy weld inlaid ring grooves, although no particular advice appears in either API document as to when or why to specify such inlay. Ring groove inlay typically provides no benefit on surface equipment except for preventing rust damage between made-up periods on blowout preventers and manifolds.  API Spec 17D requires corrosion resistant material inlay of ring grooves for all sub sea applications. Manufacturers commonly use 316 or 309 stainless steel and Inconel 625 for ring groove inlay welding.  316 or 309 stainless steel inlay typically has a lower hardness than the base metal of low alloy integral flanges and therefore has a greater tendency to deform along with the coining of the ring gasket during make-up.  If connection make-up must occur underwater (separate connectors assembled together while submerged) operators must use SRX or SBX ring gaskets.

For extremely corrosive well environments, API Spec 6A permits the use of low alloy steels with all well wetted surfaces overlaid with corrosion resistant alloy, provided it does not weaken the design. Manufacturers and users often specify a complete overlay of well wetted surfaces using Inconel 625, as this material in its stress-relieved condition has similar mechanical properties to the base metal. Inconel 625 exhibits excellent corrosion resistance to almost any well fluid.  

Gasket Material to Connector Application

For connectors made of carbon or low alloy steel without inlay, use carbon steel ring gaskets for any service as no benefit derives from stainless steel  gaskets .

Use carbon steel ring gaskets with connectors that have 300 series stainless steel weld inlaid ring grooves that will experience frequent make-up.  This will minimize deformation of the connector ring groove.

For connectors made of 410 martinsitic stainless steel, use 304 or 316 stainless steel ring gaskets for any service.

For connectors made of low alloy steel with Inconel 625 ring groove inlay, use any API Spec 6A ring gasket desired.

For connectors made of low alloy steel having all well wetted surfaces overlaid with corrosion resistant alloy (e.g. Inconel 625), use 316 stainless steel or corrosion resistant alloy ring gaskets selected for the  service condition ¹.

Gasket Configuration Application

After a material selection some connections and service applications also require a decision between available ring gasket configurations.

For surface applications, 6BX flanges and 16BX hubs join together with BX ring gaskets, 16B hubs join together with RX ring gaskets.

API Spec 17D connectors, or any connectors used for sub sea applications, require a specific type of gasket should an underwater make-up occur, or should an operator anticipate one. Only SBX and SRX ring gaskets will reliably seal when made-up under water. SBX gaskets fit in connectors with grooves dimensioned for BX gaskets and SRX gaskets fit only 5000 psi segmented flanges. These SBX and SRX gaskets have vent holes that allow any water trapped below the gasket during make-up to vent to the inside of the connection.

For what to do when you need an SBX or SRX gasket and you don't have one, see: Test Rack Tips.

Connection of 6B flanges can utilize three ring gasket profiles, resulting in two different stand-off measurements, and at least theoretically, two different expected performance situations. Type R ring gaskets come in two profiles, oval and octagonal. Type R ring gaskets seal along small bands of contact on their O.D. and I.D. where the gasket has coined against the groove sides on a flange. The gasket supports both weight and bolt tension. Vibration ¹ and bending ¹ that may occur during drilling operations may require periodic tightening of bolts when operators use R ring gaskets in 6B flanged blowout preventer stack assemblies

Type R oval ring gaskets perform well ¹ in almost all ¹ static flanged well head assemblies.

Type R oval ring gaskets perform well ¹ in oil or gas well work-over situations with low flanged blowout preventer stack weights ¹ and "short made-up connection periods."

Type RX ring gaskets have a pressure-energized design feature that makes them desirable for 6B flanged blowout preventer stacks and manifold assemblies. They seal only on their O.D. and have a larger load-bearing surface on their I.D. to support weight ¹ and resist  vibration ¹.

Type RX ring gaskets provide an extra measure of performance   potential ¹ when used in flanged blowout preventer stacks of  considerable weight ¹ and where operators expect to encounter   pressure ¹ during extended ¹  time on the hole.

See caution about alignment problems that may occur when interchanging R and RX gaskets in assembled equipment groups.

To identify which numbered ring gaskets seal specific flanges see Flange Slide Rule Program.  To identify which numbered ring gaskets seal specific hubs see Hub Slide Rule Program.

To find out what flanges a numbered ring gasket fits, see What Flange Does This Ring Gasket Fit?

For more information about ring gaskets, see Ring Gasket Handling and Inspection Before Assembly.

For more information about ring gasket sealing, see Field Appraisal of Ring Grooves.

For more information about flange connection make-up, see Flange Make-up, Stand-off and Drift.

For more information about hub connection assembly, see Instructions for Installation and Care in the Catalog for Clamps for Clamp Hub Connections.

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