When the steam system is operated after a shutdown it is important for the pressure to be gradually increased within the distribution network. This is to avoid a sudden thermal stress to the distribution piping which is cold. Thermal stresses will have a detrimental effect on the asset lifecycle including in extreme cases immediate failures like cracks in the pipes.
For most plants which have a process boiler generating steam at about 9barg care should be taken to start up the system and gradually raise the boiler pressure first to about 4 barg and then open the crown valve gradually (crown valve must not be opened fully). Steam should be allowed to fill the entire distribution system until a uniform temperature is attained in the distribution line. Steam trap bypasses should be kept open to evacuate the network till the pressure in the distribution lines reaches 4 barg. The bypass lines should then be closed (if the traps are sized for warm-up loads). The steam pressure in the boiler should then be raised gradually to 8 barg.
When condensate comes out of the traps on the process equipment, the crown valve can be opened completely.
This pattern of rise in pressure, or temperature in case of superheated steam, will remain the same. Only the time required for it will change due to the length and material of these pipelines.
So, if material of a pipe is alloy steel then its rate of heating will be slower and hold time will be more as compared to a carbon steel pipe.