3　Design Condition and Design Criterion

3.1　Design Condition

3.1.1　The power piping shall be designed based on the pressures，temperatures and characteristics of the medium carried inside，in combination with the ambient conditions and loadings.

3.1.2　The design pressure of piping components shall not be less than the maximum sustained operating pressure（MSOP）.

3.1.3　The design pressure of piping components in special applications shall comply with the following requirements:

1　Piping components used for conveying gasified fluid with low gasification temperature:the design pressure shall not be less than the maximum possible pressure at the maximum ambient temperature causes by gasification when the valve is closed or the fluid does not flow.

2　Piping components at the outlet of centrifugal pump:for the constant-speed pump，the design pressure of piping components shall not be less than the sum of the pressure at the highest point of rated work characteristic curve and the pressure at the pump suction；for the varialbe-speed pump，the design pressure of piping components shall not be less than the sum of the pressure at the highest point of rated speed characteristic curve and the pressure at the pump suction.

3　If no safety valve is installed downstream of pressure relief device，and the fluid may be shut off or blocked，the design pressure of piping components shall not be less than the possible maximum pressure of the fluid before it enters the pressure relief device.

4　For piping provided with safety valves，the design pressure of piping components shall not be less than the lowest setting pressure of any safety valve.

3.1.4　The design pressure of commonly used piping shall comply with the following requirements:

1　For units with supercritical and below parameters，the design pressure of main steam piping shall be taken as the rated working pressure at the superheater outlet under BMCR.

2　For ultra-supercritical parameter units，the design pressure of main steam piping shall be the larger of the following two values:

1）105% of the design pressure at the main steam valve inlet of turbine；

2）the design pressure at the main steam valve inlet of turbine plus the pressure drop of main steam piping.

3　The design pressure of reheat steam piping shall be 1.15 times the high pressure casing exhaust pressure in heat balance under the valve wide open（VWO）condition of turbine.

4　The design pressure of extraction piping of turbine shall comply with the following requirements:

1）The design pressure of non-adjustable extraction piping shall be 1.1 times the extraction pressure under the VWO condition of turbine，and shall not be less than 0.1 MPa.

2）The design pressure of adjustable extraction piping shall be taken as its maximum working pressure.

3）The design pressure of back pressure turbine exhaust piping shall be taken as its maximum working pressure，but shall not be less than 0.1 MPa.

5　The design pressure of steam/water piping connected with the startup separator of once-through boiler shall be taken as the possible maximum working pressure under all operating conditions of the separator.

6　The design pressure of high-pressure feedwater piping shall comply with the following requirements:

1）For the outlet piping section of constant-speed feedwater pump from booster pump to main feedwater pump or from main feedwater pump to boiler economizer inlet，the design pressure shall be the pressure corresponding to the highest point on the booster pump or the main feedwater pump characteristic curve plus the pressure at the inlet side.

2）For the outlet piping section of variable-speed feedwater pump from the feedwater pump outlet to the first shut-off valve，the design pressure shall be equal to the pressure corresponding to the highest point on the characteristic curve under the rated pump speed plus the pressure at the inlet side；for the section from the first pump outlet shut-off valve to the boiler economizer inlet，the design pressure shall be 1.1 times the pressure to be boosted by the pump under the rated speed and design flow plus the pressure at the inlet side.

3）The design pressure of high-pressure feedwater piping shall be determined giving consideration to the influence of inlet water temperature on pressure.

7　The design pressure of low-pressure feedwater piping shall comply with the following requirements:

1）For the constant pressure deaeration system，the design pressure shall be the rated pressure of deaerator plus the static pressure（water column）corresponding to the highest water level of deaerator.

2）For the sliding pressure deaeration system，the design pressure shall be 1.1 times the deaerator heating extraction pressure under the VWO condition of turbine plus the static pressure（water column）corresponding to the highest water level of the deaerator.

8　The design pressure of condensate piping shall comply with the following requirements:

1）For the piping at the inlet side of condensate pump，the design pressure shall be taken as the static pressure（water column）between the pump suction center line and the plane at turbine exhaust hood port（at which point the pressure inside the condenser is to be considered as atmospheric pressure），and shall not be less than 0.35 MPa.

2）For the piping at the outlet side of condensate pump，the design pressure shall be equal to the pressure to be boosted by the pump plus the pressure at the water inlet side when the pump outlet valve is closed；the pressure at water side shall be taken as the static differential pressure（water column）between the highest water level in condenser hotwell and the pump suction center line.

9　The design pressure of heater drain piping shall be 1.1 times the extraction pressure under the VWO condition of turbine，and shall not be less than 0.1 MPa.If the pressure increases as a result of the drain static pressure in the piping is above 3% of the steam extraction pressure，the static pressure shall also be taken into account.

10　The design pressure of boiler blowdown piping shall comply with the following requirements:

1）Piping located upstream of the boiler blowdown valve:（a）for periodic blowdown piping，the design pressure shall not be less than the sum of the lowest setting pressure of any safety valve on the boiler drum and the static differential pressure（water column）between the highest water level of boiler drum and the lowest point in the piping；（b）for continuous blowdown piping，the design pressure shall not be less than the lowest setting pressure of any safety valve on the boiler drum.

2）Piping located downstream of the boiler blowdown valve:（a）if the installation of valves or blind flanges results in the pressure increase in the piping medium，the design pressure shall be determined according to the principle for determining the design pressure of piping located upstream of the blowdown valve；（b）if no valve or blind flange is installed and，therefore，the medium pressure will not increase，the design pressure of periodic and continuous blowdown piping shall be determined according to Table 3.1.4.

11　The design pressure of feedwater recirculation piping shall comply with the following requirements:

1）If the unit system is used，the design pressure for the last shut-off valve before the deaerator and the piping located upstream of it shall be taken as the design pressure of corresponding high pressure feedwater piping；the design pressure of piping located downstream of the last shut-off valve shall be taken as:（a）the rated pressure of deaerator for constant pressure deaeration system；and（b）1.1 times the heating extraction pressure of deaerator under VOW condition for sliding pressure deaeration system.

2）If the header system is used，the design pressure for the throttling orifice plate and the piping located upstream of it shall be taken as the design pressure of corresponding high pressure feedwater piping；the design pressure of the piping located downstream of the throttling orifice plate shall be taken as the rated pressure of deaerator in case that no valve is installed or it is impossible to shut off the valve on medium outlet.

12　The design pressure of exhaust piping located downstream of the safety valve shall be determined based on the hydraulic calculation results.

3.1.5　The design temperature of piping components shall not be less than the maximum working temperature of the medium in the piping under continuous operation condition.

3.1.6　The design temperature of piping components in special applications shall comply with the following requirements:

1　For piping connected with the heat exchange equipment such as boiler and heaters，the design temperature shall be determined by considering any possible temperature deviation of the heat exchange equipment.

2　For piping lined with non-metallic materials，the design temperature of the lining materials shall be taken as the maximum working temperature of the fluid，and that of the outer metal layer can be determined through heat transfer calculation or test.

3.1.7　The design temperature of commonly used piping and piping components shall comply with the following requirements:

1　The design temperature of main steam piping shall be the rated working temperature of steam at superheater outlet plus the allowable temperature deviation under normal operation condition，which may be taken as 5℃if not specified by the boiler manufacturer.

2　The design temperature of reheat steam piping shall comply with the following requirements:

1）The design temperature of the hot reheat steam piping shall be the rated working temperature of steam at boiler reheater outlet plus the allowable temperature deviation under normal operation condition，which may be taken as 5℃if not specified by the boiler manufacturer.

2）The design temperature of the cold reheat steam piping shall be taken as the temperature derived from isentropic analysis by using the steam exhaust parameters of high pressure casing under the VWO condition of turbine and the design pressure of piping.

3　The design temperature of turbine extraction piping shall comply with the following requirements:

1）The design temperature of non-adjustable extraction piping shall be taken as the temperature derived from isentropic analysis by using the steam extraction parameters under the VWO condition of turbine and the design pressure of piping.

2）The design temperature of adjustable extraction piping shall be taken as the maximum working temperature of extraction steam.

3）The design temperature of back pressure turbine exhaust piping shall be taken as the maximum working temperature of exhaust steam.

4　The design temperature of steam piping located downstream of the desuperheater shall be taken as the maximum working temperature of steam at desuperheater outlet.

5　The design temperature of steam/water piping connected with startup separator of once-through boiler shall be taken as the maximum working temperature of piping medium under all possible operating conditions of separator.

6　The design temperature of high pressure feedwater piping shall be taken as the maximum working temperature of high pressure feedwater leaving the high pressure heater.

7　The design temperature of low pressure feedwater piping shall comply with the following requirements:

1）For a constant pressure deaeration system，the design temperature shall be taken as the saturation temperature corresponding to the rated pressure of deaerator.

2）For a sliding pressure deaeration system，the design temperature shall be taken as the saturation temperature corresponding to 1.1 times the extraction pressure of deaerator under the VWO condition of turbine.

8　The design temperature of condensate piping shall be taken as the maximum working temperature of condensate leaving the low pressure heater.

9　The design temperature of heater drain piping shall be taken as the saturation temperature corresponding to the design pressure of extraction piping of the heater.

10　The design temperature of boiler blowdown piping shall comply with the following requirements:

1）The design pressure of piping located upstream of the boiler periodic or continuous blowdown valve shall be taken as the saturation temperature corresponding to the lowest setting pressure of any safety valve on the boiler drum.

2）Piping located downstream of the boiler blowdown valve:（a）if the installation of valves or blind flanges results in pressure increase in the piping medium，the design temperature shall be determined according to the principle for determining the design temperature of piping located upstream of the blowdown valve；（b）if no valve or blind flange is used and，therefore，the medium pressure will not increase，the design temperature of periodic or continuous blowdown piping shall be determined according to Table 3.1.7.

11　The design temperature of feedwater recirculation piping shall comply with the following requirements:

1）For a constant pressure deaeration system，the design temperature shall be taken as the saturation temperature corresponding to the rated pressure of deaerator.

2）For a sliding pressure deaeration system，the design temperature shall be taken as the saturation temperature corresponding to 1.1 times the extraction pressure of deaerator under the maximum calculated output condition of the turbine.

12　The design temperature of exhaust piping located downstream of the safety valve shall be determined based on the corresponding data in the hydraulic calculation of the exhaust piping.