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Turbomachine design and arrangement is often complex and optimization must be made between the mechanical requirements, electrical specification, auxiliary equipment design, operation, reliability, commercial and maintenance.
2 OUTDOOR OR INDOOR
An important decision for every turbomachine could be outdoor or indoor installation. At first glance, indoor installation seems safer, more reliable and better, but the best recommendation as a first option is to consider OUTDOOR installation. It makes operation and maintenance much easier. For some projects enclosure or station design could be very expensive and complex.
Sometimes conflicting requirements for enclosure design, noise control, ventilation fan, HVAC system, civil design, foundation design, piping arrangement, support design, various maintenance access and different crane access could result in an extremely costly system and enclosure.
In many projects, turbomachine is ordered based on an indoor installation concept assuming an enclosure for noise control and protection (with overhead crane, ventilation/HVAC, proper area classification, etc) will be designed in future. Enclosure volume should be minimized to keep cost down, however it compromises access and other requirements. Heat generation calculation and proper ventilation are usually complex issues. Most importantly access requirements for installation and major overhaul could be problematic. Various new HSE and risk issues are introduced by an indoor concept.
From the author’s point of view, OUTDOOR installation would be the best solution. A package should be designed for OUTDOOR, without shelter. Noise protection based on initial noise study (with proper margin) should be included in vendor scope (with machine localized noise protection). All maintenance (routine maintenance, overhaul, etc) should be done by proper mobile cranes. In future, if a temporary shelter will be installed, it will improve package reliability. Of course there are always some exceptions to the above rule, but it is recommended to consider a less problematic OUTDOOR solution as a first option.
Overhead crane (or mobile routine maintenance crane) should be sized with respect to train component maximum weights for routine maintenance (such as rotor, gear unit, upper casing, etc). Crane service factors 1.2-1.4 are commonly used. Three crane capacities should be identified: 1- Overhead crane (or mobile crane) for routine maintenance. 2- Maintenance mobile crane. 3- Maximum skid installation weight (for installation).
Lifting lugs should be indicated for skids / equipment. Lifting lug details and lifting study are critical part of machinery design review.
Sufficient space should be allowed for withdrawal of components, cooler bundle removal and lay-down.
3 TURBOMACHINERY PIPING AND AUXILIARY EQUIPMENT
The thermal piping design often requires that flexibility be added to the system which is counter to the requirement for more support and increase stiffness to meet vibration design. These analyses should be conducted by the same party to optimization design iteration and result in an overall optimized system. Piping configuration and spacing should ensure that expanding lines do not touch adjacent lines or other components.
During the early stage of piping layout design, following criteria are recommended:
1-Minimizing point of shaking forces generation in equipment and piping such as elbow, tee, size variation, etc.
2-Extra supports are needed near concentrated masses.
All auxiliaries (vessel, separator, etc) are in the vicinity of the turbomachine, which are subjected to vibrations and shaking forces. For auxiliary fixed equipment, proper support (such as skirt support) is strongly preferred. Leg type supports or other relatively flexible supports should be avoided. Engineering practices for vessels: 1- For all connection WN (Welding Neck) and especially for small connection integrally forged LWN (Long Welding Neck) are recommended. 2- All pressure and integral parts should be full penetration welded. 3- Nozzle connections with counter (self reinforced with lip type nozzle) are recommended for critical applications to make 100% Radiography (or X-ray test) possible and reduce stress concentration.
If necessary, low cycle fatigue analysis should be carried out following the procedure described by ASME VIII Div. 2 Appendix 5. Stress concentration factors and proper safety factors can decrease cyclic stress limit to around 20 Mpa peak to peak (pp). Special care should be taken for equipment with thickness more than 50 mm. In many cases auxiliary equipment and vessels (drums, separators, filters, heat exchangers, etc) have caused delay in package delivery. Following items should be reviewed carefully: 1- Material selection. 2- Attachments and supports. 3- Nozzles and nozzle orientation. 4- Internal piping and connections.
4 ELECTRICAL PULSATION ISSUE
Today large numbers of turbomachines are electric motor driven. On the other hand, many gas turbines, steam turbines or turbo-expanders are used to generate electric power. Pulsation on electric machine load (current pulsation) is an important issue especially for plant that has a relatively weak electric system. Irregularity degree in the train should be limited to below acceptable levels based on mechanical and electrical requirements. Proper inertial capacity should be assured for the train.
Amin Almasi is senior rotating equipment consultant in Brisbane, Australia.
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