Steam System Pressurization Rate

[DMeier5950]

Good evening,

I am writing a procedure for operation of a steam system at a combined utility plant. I am working on a cold iron start-up procedure, whereas they will pressurize the steam header through an 18 inch carbon steel pipe through approximately 2 miles of steam tunnel at 175 psig and will use that pressurize a 24 inch by 20 foot header, which then branches off into multiple 6 - 8 inch runs to different reducing stations.

I am trying to figure out the proper pressurization/heat up rate limit for bringing steam into the tunnel and then into the header. The tunnel I think is more critical due to the amount of piping. Unfortunately none of the available drawings provides me any more information on the piping system aside from the pipe O.D., so I was attempting to use the most limiting stress factors I could find for the system and apply them throughout.

The individual headers are equipped with 1 inch warm up lines, so I assume that over-stressing the header isn't plausible if they simply use the warm-up line. However I cannot verify that the tunnel valves have warmup valves installed, so I want to provide a pressurization rate.

When I was an operator, we had pressurization rates for our steam header (1.45 hrs from ambient to 0 psi, 1 hr from 0 - 100 psi, etc.,). Our system was much higher pressure and more complex, but I imagine it is still an important factor. Am I overthinking this?

Is there a way to use the Tensile and Yield stresses from ASME B.31.1 to calculate the maximum allowable thermal stress due to rate of change? Or at least in the ballpark? Or is it not as important as I think it is?

Thanks!

 

[Ehsan.m]

Steam System Pressurization Guidelines
When performing a cold start-up, it is crucial to control the rate at which the steam system is
pressurized and heated to prevent thermal stresses that can cause damage. The following steps
should be considered:
1. Initial Heating (Ambient to 0 psig):
- Slowly introduce steam into the system through the warm-up lines if available, allowing the
temperature to rise gradually. This minimizes thermal shock and ensures even heating.
- Typical rates for a controlled heat-up could be in the range of 1-1.5 hours from ambient to 0 psig.
2. Pressurization from 0 to 100 psig:
- Increase pressure at a controlled rate to avoid rapid thermal expansion. A guideline rate is to
take approximately 1 hour to reach 100 psig. Monitor temperature gradients across the piping to
ensure even heating.
- Utilize the warm-up lines for gradual pressure increase if installed on the tunnel valves. If not,
manual control of the main valves will be necessary.
3. Further Pressurization to 175 psig:
- Continue to increase the pressure gradually, allowing at least 30 minutes to an hour depending
on the system's response and temperature monitoring.
Thermal Stress Considerations
Thermal stress is calculated using the relationship:
Stress = E · Alpha · Delta T
Where:
- Stress is the thermal stress,
- E is the modulus of elasticity of the pipe material (temperature-dependent),
- Alpha is the coefficient of thermal expansion,
- Delta T is the temperature change.
Compare the calculated stress to the allowable stress in the ASME B31.1 code to ensure it remains
within safe limits. The material properties from ASME B31.1 will help guide the maximum allowable
stress for your specific system configuration.
Practical Application
Given the lack of detailed information on the piping, applying the most conservative estimates for
the entire system is prudent. The absence of warm-up valves in the tunnel suggests that a very
conservative pressurization rate should be used.
References
1. Pipeng Toolbox, "ASME B31.1 Test Pressure Calculators".
2. Arveng Engineering, "Stress, Flexibility & Supports of a Steam Piping System | ASME B31.1".
3. Eng-Tips Forums, "Steam System Pressurization Rate - ASME (mechanical) Code Issues".