general purpose vs. special purpose steam turbine 비교 정리

 

steam turbine을 구분하는 여러 기준이 있는데 가장 일반적으로 사용하는 기준이 condensing type과 non-condensing (back pressure) type이 있다. 각각의 장단점을 이해하여 적절한 type을 선정하는 것이 중요하다.

 

 

condensing type은 turbine내에서 스팀 팽창의 효과를 최대한 높이기 위해 vacuum으로 운전을 하며 casing을 나오는 스팀은 후단 condenser에서 응축하여 이 응축수는 다시 hot well 펌프를 이용하여 boiler system으로 return시켜 cycle을 완성한다. steam의 inlet pressure 대비 매우 낮은 압력까지 떨어트려 운전을 하므로 non-condensing type 대비 효율이 높다. 반면 non-condensing type은 back pressure turbine이라고도 하며, turbine의 exhaust를 통해 let down되어 압력이 낮은 등급의 steam header에 연결이 된다. (steam balance상 매우 중요함.)

 

 

또 다른 분류기준이 service종류에 따라 general purpose와 special purpose가 있다. API code에 따르면 small size 혹은 power요구량이 크지 않고 non-critical service에는 general purpose steam turbine을 사용하며, big size이거나 power요구량이 크고 critical service인 경우에는 special purpose steam turbine을 사용한다.

API-611 Steam Turbines - Kessels

 

 

이에 따라 steam turbine의 기본설계, 재질, 윤활시스템, 제어, 보조설비, 악세서리 등의 요구사항이 다르다. general purpose는 API611에서 다루며 이는 주로 spare로 설치되는 수평형 혹은 수직형에 적용한다. 반면 special purpose는 API612에서 다루며 이는 spare가 없는 수평형에 적용한다.

 

 

아울러 steam압력이 48bar이하이고 400도이하에서는 API611을 적용하며 speed가 6000rpm이하에서 사용한다. 이보다 severe한 조건에서는 critical service로 간주하여 API612를 적용한다.

 

 

API611과 API612의 차이점은 아래와 같다.

 

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	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.
5	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.
6	Bearing	a)For smaller size turbine Anti-friction bearings (ball bearing) 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) (tilt-pad)
7	Shaft Grounding	Shaft grounding is not applicable	For condensing type steam turbine, brushes are kept on the shaft end for shaft grounding.
8	Overspeed trip protection	Mechanical pin type emergency	Electronic 2OO3
9	Rotor type	Built-up rotor	Solid rotor
10	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. Carbon ring seals are used only Up to the peripheral velocity of 50m/s.	Labyrinth seals are used for both interstage seal and shaft seal (Gland steam condenser required)
11	Trip and Governor valve	Trip and Governor valve in same body	separate mechanical latch type trip and throttle valve (T&T valve) with built-in low oil pressure trip cylinder and manual exerciser feature
12	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)
13	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)
14	Turning Gear	Turing gear are not required	Based on purchaser or manufacturer requirement, turning gears are available with the steam turbine
15	Governor	Oil relay governor of NEMA Class A is a standard governor. (Mechanical type) 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.
16	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
17	Curve and Report	Willans Line (Performance Curve)	a) Willans Line (Performance Curve) b) Rotor Response Analysis (Lateral) c) Campbell / Goodman Diagrams

 

 

API612에는 exhaust line에 full capacity safety valve를 설치하도록 요구하고 있지만 API611에는 이러한 requirement가 없다.

API612

 

API611에 relief valve가 언급되고 있지만 이는 oil system에 설치되는 relief valve이다.

 

 

API611을 적용하는 general purpose steam turbine의 경우, 위에 언급된 API612 requirement가 없어서 가끔 turbine exhaust line상 safety valve가 누락된 경우가 있으니 vendor 설계를 면밀히 검토해야 한다. (후단 밸브 blocked상태에서 casing 과압이 걸려 rupture가 발생한 사고가 있었음.)

 

API 611

 

 

Special Purpose Steam Turbine의 주요 vendor로는 아래와 같다.

• Siemens
• General Electric
• Shanghai Electric
• Dongfang Turbine
• Hangzhou Steam Turbine(HTC)
• MAN Energy Solutions
• Kawasaki Heavy Industries
• Mitsubishi Hitachi Power Systems
• Elliott
• Triveni Turbine
• Kessels
• KEPL
• CTMI
• Skinner Power Systems

#turbine#API611#API612#general_purpose#special_purpose#full_capacity#exhaust#back_pressure