When evaluating a centrifugal compressor for a potential bid, there are several factors to consider. It is important to check any deviations from specified standards to ensure they are acceptable for the application. Additionally, performance curves must include details such as capacity at surge limit, capacity at the choke point, and power versus capacity pressure versus curves. The model offered must be a developed model that has been tested and in operation for two years or more, and the compressor manufacturer must have guaranteed pressure at the battery limit specified in the specifications.
Furthermore, the surge limit should typically be below 75% of the specified rated capacity, and the choke point must not be near the operating point. Tip speed and Mach number for all stages should also be checked, as well as critical speeds and bearings. Couplings and gears should be suitable for higher speed, and the oil system should conform to API 614 requirements. In addition, the cooler should be a removable bundle, and heat loads and surface area available for heat exchangers should be obtained.
All major rotating parts should also be dynamically balanced, and guaranteed vibrations should be within limits as per API standards. Finally, positive sealing should be provided by the vendor, and controls and protection must be as per requirements specified in datasheets. When deciding on motor rating, the highest supply frequency to the electric motor should be taken into account, as well as power consumption at the lowest inlet temperature and worst gas compression.
When assessing a potential compressor, it is important to check any deviations from specified standards. These should be evaluated to ensure that they are acceptable for the application.
Technical Bid Evaluation for Centrifugal Compressor
Performance Curve Requirements
The performance curve should include details such as capacity at surge limit, capacity at the choke point, and power versus capacity pressure versus curves.
Model Development and Testing
It is essential to confirm that the model offered is a developed model, and that it has been tested and in operation for two years or more.
Pressure Guarantee
Ensure that the compressor manufacturer has guaranteed pressure at the battery limit specified in the specifications.
Surge and Choke Limits
The surge limit should typically be below 75% of the specified rated capacity, and the choke point must not be near the operating point. Additionally, check the surge limit at the minimum supply frequency of the electric motor.
Tip Speed and Mach Number
Check the tip speed and Mach number for all stages. Lower Mach numbers provide superior efficiency.
Critical Speeds
Critical speeds should be a minimum of 30% away from the operating speed. In the case of turbine drive, any critical speed should not be close to the speed trip value of the turbine.
Bearings
Bearings should be tilting pads, and hydrodynamic thrust bearings should be suitable for thrust in both directions.
Gear & Couplings
Gears and couplings should be spacer type, and should be suitable for higher speed (due to an increase in frequency in case of electrical motor drive and up to trip speed for turbine drive). The service factor should be at least 1.4 for motors drive and 1.6 for turbine drives.
Oil Requirements
The oil system should conform to API 614 requirements, if not specified. It is generally preferred to have the same capacity as Main oil/Aux.oil/Emergency oil pumps. Oil piping downstream of filters should be SS. Oil pumps should preferably be positive displacement screw pumps.
Cooler Requirements
The cooler should be a removable bundle, water through tube design, conforming to ASME Sec VIII, TEMA ‘C’, and with auto drain traps (for air coolers). Coolers tubes should be a minimum of 19″ OD, 18BWG; waterside fouling factor should be as per design basis with water velocity which should be 1.5-1.8 m/s.
Heat Exchangers
Heat loads and surface area available for Heat Exchangers should be obtained from vendors. The coolers should be provided with 10% excess area. The quantity of cooling water required should be checked from heat loads given by vendors.
Dynamic Balancing
All major rotating parts should be dynamically balanced. Balancing speed and procedure should be obtained from vendors for records and review.
Vibration Guarantees
Guaranteed vibrations should be within limits as per API standards.
Inlet Guide Vanes
The entire single-stage compressor should be preferably provided with inlet guide vanes. IGV is effective only at part load operations. The effect of IGV on multistage compressors is very small, hence, to compare power consumption at part loads, capacity vs. power curves should be obtained for compressors with and without guide vanes. The selection of guide vanes for multistage compressors should be based on techno-commercial considerations.
Positive Sealing
For all process compressors where leakage of a medium cannot be allowed to lead into the atmosphere, positive sealing should be provided by the vendor.
Controls and Protection
Controls and protection provided by the vendors must be as per requirements specified in datasheets.
Motor Rating Considerations
When deciding on motor rating, the highest supply frequency to the electric motor should be taken into account. Additionally, power consumption at the lowest inlet temperature and worst gas compression (in case of varying composition of gas constituents) should be assessed.