(2a) Mechanical Challenges in Very Large Compressors and Mechanical Drive Steam Turbines

Ethylene Crackers have always chased increasing economies of Scale.  Larger trains provide better returns on investment as increasing compressor frame sizes generally provide significant increases in capacity without the same increase in cost.

 This leads to challenges as manufacturers try to increase the size of the machines in Propylene and Charge Gas Service.

 The lecture will outline some challenges manufacturers have in building very large machines.

 Pressure Ratings on large machines:  The ultimate pressure rating of any compressor tends to decrease as you increase the diameter of the casing.   Many compressors designed for extremely large volumes were originally designed to compress air or for Catofin service which required pressures of less than 50-100 psig.  As the need for extremely large in LNG and Ethylene Service has occurred all suppliers have increased the ratings on their largest frames however in many cases experience is limited and weights are increasing.

 Weight:  Very large compressors are now well over 300,000 lbs.  This limits the ability of some manufacturers to lift fully assembled machines.  Even moving the top and bottom half can challenge some crane capacities.  In many cases these machines must be split just to move them off the test stand.  

 General Manufacturing:  These machines can challenge the size of the largest casing machine tools and at-speed balance facilities.  

 Plant Capacity:  Generally there are only a few machine tools capable of doing these large machines.  If 4 compressors and 2 turbines all need to go through the same machining centers then schedule can easily be impacted by minor upsets and the ability to recover schedule is very limited.

 Testing:  Test stands must often be expanded and enlarged to test these units.  Industry standards for testing require straight runs of pipe which are multiples of the inlet diameter.  As inlet sizes increase the machines must be mounted higher and higher to meet testing standard.  Steam capacity can be a limiting factor even to provide a part load string test.

 Shipping:  These machines have exceeded the ability to ship assembled over the road.  Most must be disassembled to go by road and even then some must go by rail or barge. 

 None of these challenges is insurmountable.  Experience for these large machines does exist in the large power Generation market.  In many cases large Steam Turbine or Gas Turbine facilities at sister locations can be used to manufacture large components or balance rotors.   However large frame experience at the required pressures, assembly, testing and ability to ship can tend to limit traditional manufacturing locations for Compressors and Mechanical Drive Turbines.