In this post, we will see the difference between API 611 and API 612.
While selecting a steam turbine as a driver for compressors or pumps for refinery/petrochemical industries. The first question faced by the design engineer is which standard steam turbine as to be selected either API 611 or API 612. The basic difference is highlighted below.
API 611 Vs API 612
| S.no | Description | API 611 | API 612 |
| 1 |
Application |
General Purpose Steam Turbine | Special Purpose Steam Turbine |
| 2 |
Design Life |
Turbine Design life is 20 years and uninterrupted period of operation will be 3 years | Turbine design life is not defined in the standard. Also, the uninterrupted period of operation will be as per purchaser requirement. |
| 3 |
Operating condition Variation & Steam Purity |
Condition of Operating steam is as per NEMA-SM 23 | Condition of Operating steam is as per NEMA-SM 24 |
| 4 |
External Force and Moment |
As per NEMA-SM 23 | As per NEMA-SM 24 |
| 5 |
Safe operation Limit of the rotor (momentary) without damaging |
The rotors are designed for the safe operating limit of 110% of the trip speed | Steam Turbine used for process Application -127% of rated speed. In the case of generator Application – 121% of synchronous speed. |
| 6 |
Integral Flinger |
Integral flingers are not applicable | The rotor shaft will be designed with Integral flinger between the bearing housing and shaft seal. This to avoid the moisture enter into the bearing housing. |
| 7 |
Bearing |
a)For smaller size turbine Anti-friction bearings are used take for both Radial and Thrust load b)For a medium-size turbine, the hydrodynamic bearing is used for Radial load and Anti-friction bearing is used for Thrust load c)Large size turbine hydrodynamic bearings are used for both radial and thrust load. (6.10) |
Hydrodynamic bearings are used for both radial and thrust load (10.1) |
| 8 |
Shaft Grounding |
Shaft grounding is not applicable | For condensing type steam turbine, brushes are kept on the shaft end for shaft grounding. |
| 9 |
Shaft Seals |
a) Labyrinth type seals are used as an interstage seal of steam turbine b) Carbon ring seals are used as Shaft end seals. Caron ring seals are used only Up to the peripheral velocity of 50m/s. |
Labyrinth seals are used for both interstage seal and shaft seal |
| 10 |
Material of Construction |
Pressure containing parts shall be Cast iron and steel (6. 12.1.3) | Pressure containing parts shall be of steel. (11.1.10) |
| 11 |
Coupling Selection |
Coupling shall be rated to Rated power of turbine with service factor as per AGMA 922. For these types of steam turbine coupling shall be selected as per API 671 based on purchaser requirement. |
Coupling shall be as per API 671 (15.1.2) |
| 12 |
Turning Gear |
Turing gear are not required | Based on purchaser or manufacturer requirement, turning gears are available with the steam turbine |
| 13 |
Governor |
Oil relay governor of NEMA Class A is a standard governor. If the purchaser needs an Electronic governor then NEMA Class D will be installed on the turbine. It is an optional requirement.) |
Governor shall be of electronic type. It will meet the requirement as per NEMA Class D. |
| 14 |
Inspection and testing |
a) 1-hour uninterrupted mechanical test (8.3.3.2 e) b) Measure Oil Temperature (IN) c) Measure Oil Pressure (IN) d) Measure Steam Pressure e) Inlet and Exhaust f) Measure Steam Temperature g) Inlet and Exhaust h) Standard Rotor Balance |
a) 4-hour uninterrupted mechanical test (16.3.3.2 -f) b) Rotor Dynamics Testing c) Amplitude vs. Frequency d) Amplitude vs. Speed e) Phase Angle vs. Speed f) Prove Lateral Critical (N/A for stiff shaft rotors) g) Measure Oil Temperature (IN / OUT) h) Measure Oil Pressure (IN / OUT) i) Measure Oil Flow j) Measure Steam Pressure k) Inlet and Exhaust l) Measure Steam Temperature m) Inlet and Exhaust n) Check rotor unbalance response (if the coupling is furnished by manufacturer) o) Incremental Rotor Balance |
| 15 |
Curve and Report |
Willans Line (Performance Curve) |
a) Willans Line (Performance Curve) b) Rotor Response Analysis (Lateral) c) Campbell / Goodman Diagrams |