(98e) Material Selection for the Propane Systems, ITCS or CS?

One of the frequently asked questions during the material selection for the Propane system is whether to use Impact Test Carbon Steel (ITCS) or standard Carbon Steel (CS). In the absence of corrosion/erosion related factors, the material selection between ITCS and CS is dependent on fracture toughness at low temperatures. When selecting materials generally the minimum temperature limit of ITCS and CS is -46°C and -29°C respectively. Using ITCS will result in additional cost when compared with CS. Furthermore there are other issues to consider for the use of ITCS material such as: weld material and procedures, availability of material, maintaining a paper trail, etc.

Most LNG liquefaction and gas plants are equipped with an emergency shutdown (ESD) system and Emergency depressurization valve ( EDV) system as a means of preventing equipment brittle fracture during a fire. The material of construction for the equipment and piping must be selected to withstand the coldest temperature resulting from this depressurization, though not necessarily coincident with the system design pressure. Due to depressurization following an ESD, the equipment and the associated piping will experience a cool down due to the temperature reduction of the Propane fluids. The propane system can reach -45°C theoretically after opening the EDV to atmosphere pressure. The threat of brittle fracture is a concern when re-pressuring the equipment in cold conditions. It should also be noted that propane gas dispersion is detrimental and can initiate cascade explosions after ignition such as the Piper Alpha platform incident in which 167 people were killed.

This paper will cover the following:

? The selection of recommended material for propane systems including Propane Compressor suction (Low Low Pressure, Low Pressure, Medium Pressure and High Pressure), Propane compressor discharge, Propane de-superheater, Propane condenser and accumulator, the off site Propane storage and the De-Propanizer and reflux drum system

? Description of the operating scenarios (i.e start up, ESD or tripped, open EDV, etc) that could result in potential risk. ? The impact of the ?No Cold Repressuring? philosophy on operation of the plat. Some equipment and piping design could be based on a No Cold Repressuring philosophy. Under such a scenario, repressurisation of cold equipment is NOT ALLOWED until the equipment is warmed up to the minimum design temperature.

? A comparison of the code differences between the US standards/codes (ASME and ANSI), European codes and Australian standards/codes such as AS1210.

? An order of magnitude differential cost estimate between ITCS and CS propane systems for a 5.0 MTPA LNG plant

? Material selection for low ambient temperature locations such as North Sea, Russia and Norway.