Section 4 Design and construction

4.1.1 In the design and arrangement of turbine machinery, adequate provision is to be made for the relative thermal expansion of the various turbine parts, and special attention is to be given to minimising casing and rotor distortion under all operating conditions.

4.1.2 Turbine bearings are to be so disposed and supported that lubrication is not adversely affected by heat flow from adjacent hot parts of the turbine. Effective means are to be provided for intercepting oil leakage and preventing oil from reaching high temperature glands and casings and steam pipes. Drainage openings and drain pipes from oil baffle pockets are to be sufficiently large to prevent excessive accumulation and leakage of oil.

4.2.1 Turbine rotors, cylinders and associated components fabricated by means of welding will be considered for acceptance if constructed by firms whose works are properly equipped to undertake welding to equivalent standards, for rotors and cylinders respectively, to those required by the Rules for Class 1 and Class 2/1 welded pressure vessels, see Vol 2, Pt 1, Ch 4 Requirements for Fusion Welding of Pressure Vessels and Piping.

4.2.2 Welding is to be carried out in accordance with the requirements of Ch 13, 4 Specific requirements for fusion welded pressure vessels of the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials) using welding procedures and welders that have been qualified in accordance with Ch 12 Welding Qualifications of the Rules for Materials.

4.2.3 Before work is commenced, manufacturers are to submit for consideration details of proposed welding procedures and their proposals for routine examination of joints by non-destructive means.

4.2.4 Materials used in the construction of turbine rotors, cylinders, diaphragms, condensers, etc. are to be of welding quality.

4.2.5 Where it is proposed to construct rotors from two or more forged components joined by welding, full details of the chemical composition, mechanical properties and heat treatment of the materials, together with particulars of the welding consumables, an outline of the welding procedure, method of fabrication and heat treatment, are to be submitted for consideration.

4.2.6 Joints in rotors and major joints in cylinders are to be designed as full-strength welds and for complete fusion of the joint.

4.2.7 Adequate preheating is to be employed for mild steel cylinders and components and where the metal thickness exceeds 44 mm, and for all low alloy steel cylinders and components and for any part where necessitated by joint restraint.

4.2.8 Stress relief heat treatment is to be applied to all cylinders and associated components on completion of the welding of all joints and attached structures. For details of stress relief procedure, temperature and duration.

4.2.9 For all welded components, weld procedure tests are to be in accordance with Ch 12, 2.7 Destructive tests for steel butt welds of the Rules for Materials.

4.2.10 Production weld tests are to be performed according to the requirements of Ch 13, 4.5 General requirements for routine weld production tests of the Rules for Materials.

4.3.1 Smooth fillets are to be provided at abrupt changes of section of rotors, spindles, discs, blade roots and tenons. The rivet holes in blade shrouds are to be rounded and radiused on top and bottom surfaces, and tenons are to be radiused at their junction with blade tips. Balancing holes in discs are to be well rounded and polished.

4.3.2 Surveyors are to be satisfied as to the workmanship and riveting of blades to shroud bands, and that the blade tenons are free from cracks, particularly with high tensile blade material. Test samples are to be sectioned and examined, and pull-off tests made if considered necessary by the Surveyors.

4.4.1 Main turbine rotor discs fitted by shrinking are to be secured with keys, dowels or other approved means.

4.5.1 Care is to be taken in the design and manufacture of turbine rotors, rotor discs and blades to ensure freedom from undue vibration within the operating speed range. Consideration of blade vibration should include the effect of centrifugal force, blade root fixing, metal temperature and disc flexibility, where appropriate.

4.5.2 For the vibration and alignment of main propulsion systems formed by the turbines geared to the line shafting, see Vol 2, Pt 5, Ch 4 Shaft Alignment.

4.6.1 Pipes and ducts connected to turbine casings are to be so designed that no excessive thrust loads or moments are applied by them to the turbines. Gratings and any fittings in way of sliding feet or flexible-plate supports are to be so arranged that casing expansion is not restricted. Where main turbine seatings incorporate a tank structure, consideration is to be given to the temperature variation of the tank in service to ensure that turbine alignment will not be adversely affected.

4.7.1 In the arrangement of the gland sealing system, the pipes are to be made self-draining and every precaution is to be taken against the possibility of condensed steam entering the glands and turbines. The steam supply to the gland sealing system is to be fitted with an effective drain trap. In the air ejector re-circulating water system, the connection to the condenser is to be so located that water cannot impinge on the L.P. rotor or casing.

4.8.1 Turning gear is to be provided for all turbines to facilitate operating and maintenance regimes as required by the manufacturer. Manuals are to be provided and are to comply with the requirements of Vol 2, Pt 1, Ch 1, 4.6 Through life operation principles 4.6.7.

4.8.2 The turning gear for all propulsion turbines is to be power-driven and if electric, is to be continuously rated.

4.8.3 The turning gear for all turbines is to be fitted with safety interlocks which prevent steam valve actuation for turbine operation when engaged, see Vol 2, Pt 1, Ch 3, 4.14 Machinery interlocks. Indication of engaged/disengaged is to be provided at all start positions. In the case of hand operated turning gear turbine operation may be prevented by manual means including the provision of warning devices or notices.

4.8.4 The remote control of power driven turning gear is to be designed so that power is removed if the operating switch is released.

4.8.5 Means are to be provided to secure the turning gear when disengaged.

4.8.6 Overload protection arrangements are to be provided to prevent damage to the electric motor and the turning gear train.