Clamp/Hub Connectors: Piping Connecting System

1. Introduction to Clamp/Hub Connectors

Clamp/Hub Connector is a type of connection system which consists of three components, Hub, seal ring, clamp along with bolting. Applicable piping classes, sizes are generally considered as defined by the CLIENT specifications. Fig.1 shows the components of a clamp connector.

Clamp Connectors Components

Fig. 1 Typical components of Clamp Connectors

The purpose of this post is to provide guidelines for design, material and inspection of clamp connectors. This will be particularly helpful to Trainee and Piping Engineers to understand the minimum requirements to be considered for procuring clamp connectors.

This topic is split into three sections as listed below, to capture each aspect in detail

  • Design
  • Materials
  • Inspection

2. References used for Clamp/Hub Connectors

ASME B16.5 Pipe Flanges and Flanged Fittings
ASME B31.3 Process Piping

ASME B36.10

Welded and Seamless Wrought Steel Pipe

ASME B36.19

Stainless Steel Pipe


Rules For Construction of Pressure Vessels & Alternative Rules


Material Properties

NACE MR 0175/ISO 15156

Petroleum and natural gas industries — Materials for use in H2S-containing environments in oil and gas


3. Description of Activities

  • This document gives the stepwise approach for minimum requirements to be captured while procuring clamp connectors.
  • 1.1 Design

  • Hubs shall be designed as per ASME Sec VIII Div. 1 – Appendix 24 for internal pressure and clamps shall be designed as per ASME Sec VIII Div. 2 – Part 4 for external loads. Alternatively hubs can be designed based on vendors proven calculations wetted by reputed third party


  • The complete clamp connector assembly shall be designed as per ASME Sec VIII Div. 2 – Part 5 “Design by Analysis”.

    The design of the clamp connector depends on the below information. If any of the below parameters are changed, it is necessary to re-check if the selected clamp connector is suitable for the new parameters. As a minimum following to be specified.

  • Size of the connecting pipe.
  • Schedule/thickness of the adjoining pipe to which the hub shall be welded and valve end bore
  • Pipe ID/valve bore.
  • Design pressure and design temperature to which the clamp connectors shall be designed. (It shall be noted that special attention shall be given if the clamp connectors shall be designed for line design pressure and temperature or fully rated class condition. Especially, if the clamp connectors are used as mating connection with hub ended valves, then the design pressure shall be the fully rated class condition as valve are designed to fully rated class condition. The hub shall be designed to suit the fully rated class pressure-temperature conditions).
  • The material of the hub, seal ring, clamps and bolting.

Once the above information is available the following process shall be followed in selecting the clamp connectors. Manufacturer’s hub selection table/s to be referred.




Determine nominal pipe size, schedule (or pipe OD / ID), Pipe material and operating pressure / temperature
8” Sch. XXS, API 5L X60 @ 3705 psi – 100 deg. F 18” Thk 27mm, API 5L X65 @ 6250psi 100 deg. F


Select suitable hub/clamp size – normally this would match the nominal pipe size
8in for 8” Sch. XXS 18in for 18” Thk 27mm


Select suitable seal ring to match the pipe bore – normally this would be the smallest seal ring without restricting the pipe bore – see also Note 1.
Bore = 174.64

Select size 67 to suit bore of 174.64

8in67 8” Sch. XXS


Select size 160 to suit bore of 403

18”160 18” Thk 27mm


Select hub material grade compatible with pipe material
Select A694 F60

8in67 8” Sch. XXS A694 F60

Select A694 F65

18in160 18” Thk 27mm A694 F65


Preliminary sizing complete – check pressure capacity – Note 2
8in67 A694 F60

Pmax @ 100 is 9949 (acceptable)

18in160 A694 F65

Pmax @ 100 F is 5860 (not acceptable)


Check bending capacity – Note 3


If acceptable – sizing complete – Note 4
Final Connector size: 8in67 8” XXS A694 F60 Choose next size of clamp or lower size of the seal ring.

Final Connector Size: 20in160 20” Thk 27mm A694 F65


If pressure / bending is not acceptable then consider resizing connector using a smaller seal ring, or larger clamp (or both).

Note 1: Particular care shall be taken while selecting the seal ring for piggable lines as a wrong selection of seal ring shall result in restricting the pipe bore. This would create hindrances while carrying out pigging operations in these pipes.

Note 2: The selection of the seal ring completes the preliminary selection of the clamp connectors. Once all the components are selected, it shall be checked if the selected clamp connector maximum allowable pressure is more than the design pressure and also the bending capacity of the clamp connector is sufficient. If both, design pressure and bending capacity of the clamp connector are within acceptable limits then, the selection of the clamp connectors is complete.

Note 3: Bending stress as derived from piping stress analysis shall not exceed allowable bending stress of clamp connector.

Note 4: If the selected clamp connector maximum allowable pressure is less than the design pressure, then the maximum allowable pressure of the clamp connector can be increased by either choosing a smaller seal ring or larger clamp or both. Again, check if the new selected clamp connector maximum allowable pressure is more than design pressure.

Other points to be considered while design the clamp connectors are

  • For stud bolt diameter in sizes above 2.5” in ASTM A193 B7, mechanical properties of the bolt material considered in the design calculations shall be as per the mechanical properties of the finished stud bolts.
  • If the bolt size is 1.5” and above then bolt tensioner shall be used for tightening the bolts unless otherwise mentioned in the CLIENT specification. Conical washers shall be used while operating the bolt tensioner as this will provide required surface to mount the tensioning tool. Conical washers shall be supplied by hub vendor.
  • Clamps shall be provided with lifting lugs if the weight is above 25kgs unless otherwise mentioned in the CLIENT specification.
  • Valve Vendor Support

If the clamp connectors are used as mating hubs with hub ended valves then the hub manufacturer shall supply hub fabrication drawings along with all details to valve vendors for fabrication of hubs. Valve ends with hub and pipe hub supplied by hub vendor when assembled shall form a leak proof joint. It is recommended to invite hub supplier during kick-off meeting with valve vendor to agree the seal ring size.

1.2 Materials

  • The material of hub, seal ring, clamps and bolting shall be as per the piping class.
  • All carbon steel hubs shall be supplied in normalized or quenched and tempered condition or as mentioned in CLIENT specification.
  • Carbon content and carbon equivalent in the hubs shall be as per ASTM standard or as mentioned in the CLIENT specification.
  • All stainless steel material shall be supplied in solution annealed condition and nickel alloy UNS N06625 shall be supplied in annealed condition unless otherwise mentioned in the CLIENT specification.
  • Seal ring shall be always of seamless construction.
  • Clamps shall be of forged construction, cast material is not acceptable.
  • If the clamp connectors are being used is sour service, then all the materials shall comply with latest version of NACE MR-0175/ISO 151516.
  • Clamps shall be painted as the piping class.
  • Painting of hub face beyond seal ring contact area to be in accordance with project coating specification.
  • For clamp connectors used in API 6A pressure classes (1.e API 2000, API 3000, API 5000, API 10000, API 15000 and API 20000) the mechanical properties of the material used in the design calculation for hubs and clamps shall be as per API 6A Table 6. The same information shall also be passed on to vendor, so that they shall use the same values in their design calculations.
  • When weld overlay is required, extent of overlay in the seal ring pocket shall be extended till the contact surface of the seal ring on hub face.
  • Clamp connectors shall be inspected as per applicable ASTM standards.
  • Manufacturer shall demonstrate material weldability by supplying testing rings. The number and the size of the test rings shall be as advised by the Welding Engineer or the CLIENT specification.
  • As per the construction code, if PWHT is required after welding of hubs with pipe/fitting, then it shall be informed to the Manufacturers that they shall perform PWHT simulation as per the PWHT requirements of the Project.
  • NDE examination of the clamp connectors shall be in accordance with applicable manufacturing code.
  • Clamp connectors which are used in API piping classes, the testing requirement of the clamp connectors shall be as per the PSL requirements mentioned in API 6A.

1.3 Inspection

Appendix I

Typical material used for hubs, seal rings, clamps and bolting

Hub Material Seal Ring Material Clamp material Bolting Material
Carbon Steel A105N/A694 F60/F65/F70/F75 AISI 4130 / 4140 + PTFE coated Forged AISI 4130/4140 A193 B7/A194 2H for non-sour service.

A193 B7M/A194 2HM for sour service.

Low Temp Carbon Steel A 350 LF2 AISI 4130 / 4140 + PTFE coated Forged AISI 41430/4140 A320 L7/A194 7 for non-sour service.

A320 L7M/A194 7M for sour service.

Stainless Steel A182 F316/F316L A 182 F316 + PTFE coated SS316/SS316L A193 B8M/A194 8M
Duplex Stainless Steel A182 F51/ A182 F53/A182 F55 A182 F51/ A182 F53/A182 F55 + PTFE coated SS316/SS316L A 276 UNS S32760
Nickel Alloy UNS N06625 B564 UNS N06625 or CS with Inconel625 weld overlay Alloy 718 + PTFE coated Forged AISI 4130/4140 A193 B7/A194 2H for non-sour service.

A193 B7M/A194 2HM for sour service.


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