Section 3 Requirements for Piping Systems Fittings

3.1.1 Care must be exercised when designing piping components for the diving system to ensure that gas or liquid flow is in the proper direction through the component. Most components have a designed direction for flow and this should be observed. Where components permit bi-directional flow, they shall be installed to take best advantage of the design. For example, valves serving as both inlet and outlet on high-pressure vessel shall be installed so that when they are closed the vessel pressure acts from below the seat and not on the valve stem packing.

3.1.2 Piping components shall always be installed in accordance with the manufacturers' recommendations, unless a deviation is approved by LR.

3.1.3 All manually operated piping system components shall be readily accessible and easily operable under normal and emergency conditions.

3.1.4 All piping system components shall be selected to permit adequate flow for the most demanding mission conditions expected for the diving system. These conditions shall be specified when justifying the selection of a component.

3.1.5 Adequate stop valve and/or check valve protection shall be provided to prevent loss of control of the system, e.g. isolation valves shall be provided for all gauges and regulating valves and double valve protection shall be provided on fill and drain lines for all gas pressure vessel. Double valve protection shall consist of one isolation valve for each pressure vessel bank and isolation valves for each pressure vessel in the bank. Pressure vessel drain lines shall be separate for each individual flask. Flask drain lines shall not be manifolded. Hull and back-up valves are required on PVHO’s to prevent uncontrolled depressurization or flooding.

3.1.6 Brazed joints are permitted in copper and copper-alloy piping systems. Special care must be taken to ensure surface cleanliness and full penetration of the weld joints in order to prevent entrapment of particulate and eliminate uncleanable surfaces.

3.1.7 Mechanically attached fittings such as elastic strain preload (ESP), swaged (Category I UIPI) and shape memory alloy (SMA) types are not permitted in breathing gas piping systems.

3.1.8 Pipe threads are typically not allowed in life support systems or systems subjected to external pressure without specific LR approval. Experience has shown that pipe thread connections are susceptible to corrosion, shock and vibration damage, and leakage. Consideration must be given to pressure limitations due to a reduction in wall thickness of the pipe at the tapered threads. Should a component only be procurable with pipe threaded end fittings, a means must be provided upstream and downstream of the component to permit its removal without disturbing the threaded joints. Any compound (e.g. anti-seize thread tape) or lubricant used in threaded joints shall be suitable for the service conditions.

3.1.9 Flared pipe fittings and their joints shall conform to the range of wall thickness and method of assembly recommended by the manufacturer. Care should be taken with cutting and flaring tools so as to not induce work hardening of the tube end, which can make the material more susceptible to brittle fracture. Flared fittings are to be of LR type approved.

3.1.10 Flareless, mechanical friction or bite-type connections shall not be used on piping components where failure could cause uncontrolled depressurization or flooding of pressure vessels, life support systems, electrical assemblies or other life-critical components. The use of such fittings in control and monitoring systems may be permitted only if the component can be quickly isolated from the rest of the system in case of failure and redundant means of providing the control and monitoring functions is available; or the fittings are on LR approved equipment.

3.1.11 Fittings where direct impingement of entrained particles may occur, such as short radius elbows and tees, shall be made from monel. In areas where carbon steel or stainless steel construction is permitted, fittings with the same composition may be used if the velocity limit is reduced by half. Piping downstream of control valves shall be made from monel, for a distance of 10 times the pipe diameter. For pipe sizes below DN 50, carbon steel shall not be used; stainless steel shall be used, subject to the velocity limitations given in the Rules and Regulations for the Classification of Ships, July 2022 Pt 5, Ch 12 Piping Design Requirements.

3.1.12 In carbon steel and stainless steel piping systems, branches shall be made with equal tees, reducing tees or swept outlets. In monel piping systems, branches should also be made with equal tees, reducing tees or swept outlets, but branch outlets (weldolets) and branch fittings (nipples) may be used.

3.1.13 Oxygen piping systems shall be kept as simple as possible, with the smallest possible number of valves, fittings, branches and nozzles. Consideration should be given to the combination of several instrument connections in one fitting or nozzle.

3.1.14 Oxygen piping shall be located as far as possible from other piping. Pipes containing flammable fluid or steam shall not be laid within 1 m distance of oxygen piping. The free space may be utilised for lines carrying water, nitrogen or other nonflammable fluids at ambient temperature. At points where the above requirements cannot be fulfilled, the oxygen system shall be fire protected.

3.1.15 Running of oxygen piping through enclosed spaces should be avoided. If unavoidable, flanged connections or valves shall not be used and the enclosed space shall not contain lines carrying flammable fluids. Oxygen piping shall not support other pipes.

3.1.16 Expansion bellows shall not be used.

3.1.18 Changes in diameter shall be minimised.

3.1.19 Dead ends in piping shall be avoided.

3.1.20 The oxygen flow in piping shall be in one direction only. If flow in two directions cannot be avoided, arrangements are to be analysed for potential risks.

3.1.21 To assist removal of the degreasing agent after cleaning, low points in equipment and piping, shall have drain connections without a valve but closed with a blind flange or other provisions to remove the cleaning agent, see also Pt 5, Ch 4, 6 Testing and Cleaning.

3.1.22 Connections for purging and venting shall be situated such that purging and venting of the system can only be done in the normal direction of the oxygen flow.

3.1.23 Nitrogen systems used for automatic purging shall be equipped with a filter and two pressure indicators. They shall be designed to prevent the possibility of oxygen flowing back into the nitrogen system.