Clamp Connectors

It was my luck to participate in Northwest Redwater Refinery project. As introduced in previous post, LC Finer unit deals with heavy residue from Vacuum and Distillation Unit.  The unique process conditions and commodity properties, made us facing challenges daily. Clamps connectors were one of them.

Because of the high temperature and high pressure process, a lot piping systems were fall into #1500 and #2500 rating. Meanwhile, as H2S and hydrogen exposure conditions, alloyed and stainless steels had to be applied. In this case, conventional connections such as flanges, could be extremely heavy, bulky and expensive. After a careful study, Grayloc connectors were selected over flanges in classes of 1500 and 2500. It was a brilliant and brave decision, which demonstrated the engineering capabilities of Fluor.

Clamp Connector Components

The clamp connector consists of three types of components, the seal ring, the two hubs and the clamp assembly, normally consisting of two clamp halves and four bolts. 

 

Advantages of Clamp Connectors

Clamp connectors offer considerable advantages opposed to conventional connections such as flanges, especially in high pressure piping systems.

Pressure Energized, Metal-to-Metal Sealing

Clamp connectors do not rely on the conventional flange gaskets and correctly tensioned bolts to maintain the integrity of the seal. As the hubs are drawn together by the clamp assembly, the seal ring lips deflect against the inner sealing surfaces of the hubs. This deflection elastically loads the lips of the seal ring against the inner sealing surface of the hub, forming a self-energized seal.

Size & Weight

Up to 75% lighter and significantly smaller than a comparable ANSI or API flange. 

360° Orientation

Radial bolting on clamp connector allows 360° orientation around the pipe.

Less Bolting, No Alignment

Only 4 bolts to tighten rather than up to 24 on a conventional flange, with no bolt holes to align.

Minimize Maintenance Downtime

Much quicker make-up and release time

Leak free: reduce emissions

A proven track record of gas tight leak free operation when correctly installed. The pressure energized bore seal withstands temperature and load variations, which ultimately can cause conventional flanges to leak.

Lower bolt torque

Typically 50% less than a ring joint flange. Sealing integrity is as well unaffected by over-tensioning bolts.

Reusable seal rings 

Unlike spiral-wound gaskets, reusable metal seal rings give greater flexibility for hydro-testing and commissioning prior to service.

Disadvantages of Clamp Connectors

Flanges are commonly used. ASME 16.5 for flanges and flanged fittings, ASME 16.47 for large diameter flanges regulate all aspects of flange manufacture and uses. On the other hand, clamp connectors are available at several manufacturers and these manufacturers will have different types even in the same rating class. Although designed to meet ANSI B 16.5, ASME VIII DIV 2, API 6A/Q1 and other design codes, clamp connectors are not standardized dimensionally under ASME code, therefore introduce a risk for incompatibility during piping installation, if administered improperly during the design.

1.               During NWR project, Grayloc was client’s favorite. All hub ended valves and fittings were tagged and logged as specialty items. It put massive work load on material management.

2.               SPEC breaks:

There are many concerns at SPEC breaks, such as pipe wall thickness and materials. Since the wall thicknesses and thus internal diameter for each nominal pipe size could be different per application, the internal diameter of the seal rings may vary. Therefore, different hubs and seal rings may be required for identical Nominal Pipe Sizes.

In our practice, for the SPEC break connectors, each component was selected, described and tagged.  Material engineers worked out a long list on all SPEC break Grayloc connectors. Then CAD support team put them into SmartPlant 3D database. When modeling, designers had to pick the right connector from the drop down list by tag number, to ensure bill of material is correct.

3.               Nozzles on vessels:

Since hub faced nozzles are not standard, a set of clamp connector had to be ordered. One piece of hub was sent to the vessel vendor for the nozzle and the rest parts were shipped to fabrication shop for the pipe spool.

4.               Break spool require extra pair of hubs:

Unlike flange gaskets, seal rings are “T” shaped and much wider. When clamps are disassembled, the hub faced item can’t be removed easily as flanged. In most cases, an extra set of clamp connector has to be added at 90 degrees direction as a removable spool. Showing as below:

5.               Some items require special design due to the uniqueness of clamp connectors:

Blind Hubs - for blinding of a Pipeline / Equipment

Blind or Spectacle Seal rings - for isolating sections of pipe or in-line blind service

Restriction Seal rings - to reduce Pressure / Flow

Insulating Connectors - to prevent galvanic corrosion or to provide electrical isolation

Orifice Seal rings - for Flow measurement (with integral or replaceable Orifice Plates)

Orifice Unions - With Pressure taps for Flow measurement with Orifice Seal rings

Butt weld Hubs for Orifice Unions shall be of streamline bore design and are to be provided with Orifice tap holes of which the centerline on the inside is located 25.4 mm (1 inch) from the Orifice Plate plane.

Manufacturers provided different solutions. But the construction and operation of them are much more difficult than using flanges.

6.               Stress calculation

Most Manufacturers include pressure / temperature rating tables for the materials applicable in their catalogue. These pressure ratings may exceed the ASME 16.5 table 2 rating values for flanges, but do not include allowances for External Loads (Forces / Bending Moments) acting on the Connection due to Pipe Stresses. Although it is explained that the clamped connection can withstand severe external loading without leaking, it may seem upon calculation, that the selected components are not able to withstand the combined load of internal pressure and external Loads.

As our stress lead Junaid Khan mentioned: The biggest drawback is low Vendor allowable external forces and moments. On NWR these hub clamps were used on high temperature piping, complying with recommended loads necessitated the use of spring supports and obtaining required flexibility was a pain. The purchase documents should specify minimum acceptable external loads.  ASME Section VIII, Div-1, Appendix 24 “Design Rules for Clamp Connectors” and API publication” Bending Moment Calculations for special API Hub/Clamp combinations” provide a basis for establishing reference values. On NWR this was not done and the piping team was pretty much left at the mercy of Grayloc, the Client’s preferred Vendor. 

Above is the observation from a designer’s point of view. There might be other concerns on project management, procurement, delivery and installment sides. Clamp connectors usage is spreading, the disadvantages can be avoid with more experience and regulations on them.