Design, specification and material selection of mechanical systems and components
Design questions test whether you can set a design basis, select materials for the actual environment, and write a specification that a contractor can price and a checker can verify. Hong Kong conditions - high humidity, marine chloride, dense urban sites and long plant life - drive many of the correct answers.
Core rules and concepts
- Design basis: state loads, diversity, redundancy, design life, ambient conditions, utilities, access and maintenance philosophy. Use limit-state or allowable-stress methods consistently.
- Material selection: match mechanical properties, temperature, pressure, corrosion resistance, fire performance, weight, cost and availability. Do not specify a generic family such as stainless steel without a grade.
- Marine atmosphere: 304 stainless steel can suffer pitting and crevice corrosion in chlorides; 316 or duplex grades are more resistant. Copper alloys, CuNi and titanium are common for seawater. Carbon steel needs coating and cathodic protection.
- Galvanic corrosion: separate dissimilar metals with insulating flanges, washers or sleeves; avoid a small anodic area coupled to a large cathodic area.
- Specification: use shall statements, measurable performance, referenced standards, submittal requirements, testing, spares and warranty. Avoid proprietary bias unless justified.
Worked example
A seawater cooling circuit is to serve a condenser. The options are carbon steel, 304 stainless steel, 90/10 CuNi and HDPE. Select and justify. Reasoning: carbon steel corrodes rapidly in seawater; 304 is prone to chloride pitting and stress-corrosion cracking; HDPE is limited by temperature and pressure and needs more support; 90/10 CuNi has good seawater resistance and is available in the required pipe sizes. Specify CuNi with a minimum and maximum velocity to avoid deposition and erosion, chlorination control, and insulating flanges at connections to carbon-steel equipment. Add a corrosion allowance only where it can be inspected.
Traps that cost marks
- Choosing on first cost alone and ignoring life-cycle maintenance.
- Creating a galvanic couple by using stainless fasteners on aluminium.
- Forgetting thermal expansion, support spacing and access for valve maintenance.
- Using chloride-bearing insulation on austenitic stainless steel.
- Writing a specification with no acceptance test or submittal requirement.