Flexible couplings

Nuclear technology

Flexible pipe couplings are uniquely designed for submarine propulsion systems. They are rugged and capable of withstanding high pressures, aggressive environments and large shock movements, while attenuating the noise from rotating machinery.

Flexible couplings comprise a rigid pipe with end flanges that incorporate special noise attenuating features and a slider/swivel arrangement to absorb shock movements. They make a key contribution to submarines’ vital stealth characteristics, and, because they convey seawater at full dive pressure, couplings are designed and tested to meet exacting safety standards.

We are leaders in the design, development and procurement of flexible couplings for submarine propulsion systems. We have supplied six different coupling types for three classes of Royal Navy nuclear submarines, and are developing a new design of sea water flexible pipe coupling for the next generation of nuclear submarine.

Design & methods

Flexible couplings are designed to withstand full dive pressure, attenuate specific noise tones and absorb explosive shocks over the full life of the submarine. The overriding noise attenuation requirement is met by incorporating rubber seals and mounts in a way that does not degrade shock performance. Finite element models deliver a full understanding of performance and result in a balanced design.

Designs are further optimised by thoroughly testing the rubber components and a full-size prototype coupling in laboratories and workshops. They are then successfully proven by sea trials and many years of trouble-free service operation.

The drive to reduce costs prompted a thorough design review, particularly the use of expensive copper-nickel and nickel aluminium bronze alloys. By cladding or plating a relatively cheap steel base with these more exotic alloys we achieved major savings as well as improving resistance to corrosion and shock.

Materials

Forged nickel aluminium bronze is extensively used as it is stronger and more resistant to corrosion from sea water than its cast equivalent.  Copper-nickel has good corrosion resistance and ductility under shock loads, but is being superseded by steel, clad in Monel alloy, for large tubes due to cost considerations.

Components experiencing explosive shock loads utilise high strength steel, nickel plated for general corrosion protection. Fasteners are made of zinc-coated steel except for those regularly immersed, which employ erosion-resistant Monel, also used for parts undergoing high sea-water flow.

Elastomeric materials are crucial to leak-tight integrity and noise performance. The excellent damping and ageing characteristics of natural rubber are ideal for noise attenuating features, while hard natural rubber serves to cushion shock impacts.

A natural rubber seal, reinforced with nylon and protected against saline contamination by a butyl liner, is used for sealing against high pressure sea water. A close monitor on latest rubber developments is maintained through suppliers and research institutions.

Manufacturing engineering

All coupling components are procured to the highest quality assurance standards to ensure the material is free from defects that could lead to a possible sea water leak into the submarine. Rigorous inspection standards are applied and documented throughout the manufacturing process to secure full traceability of parts. This also assures the customer that the assembled coupling will be safe to operate.

Suppliers are selected on the basis of their technical expertise and track record. A full product definition is provided by detail drawings and component specification; contract reviews and progress meetings ensure the design requirements are fully understood throughout the manufacturing process.

Machining tolerances are unusually tight for such large components, with critical dimensions fully checked by our quality inspectors before acceptance. The holistic quality approach continues through assembly and test, with all work undertaken against written procedures by experienced test technicians. This work is supported by designers when required.

Verification

Full design validation is provided by a combination of prototype testing and analysis. 

A number of tests are conducted on prototype couplings these include:

  • A leak and pressure test.
  • An articulation test to prove shock movement.
  • A stiffness test to verify the noise performance of rubber parts.
  • Testing to measure the coupling’s overall noise performance.
  • An endurance/fatigue test check for wear in moving parts, and all couplings receive
  • a full pressure test before despatch to the customer.

Finite element analysis evaluates shock performance, as the couplings are too heavy for practical shock testing. A spring-mass model – tuned to give the same resonant frequencies as those obtained from testing the prototype coupling – generates the final noise curves. The results of this analysis demonstrate the achievement of all key performance parameters and are crucial in the design review and contract acceptance process.

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