Thermal Shock and How to Avoid It

Thermal Shock and How to Avoid It

If you have ever jumped into a cold pool on a warm summer day, you probably have an adept understanding of thermal shock.

Thermal shock, as it relates to steam boilers, happens as a result of the resistance of the boiler structure to movement caused by thermal expansions and contractions within the boiler. These stresses occur every firing cycle in varying degrees.

Thermal shock can lead to boiler failures such as leaks at the tube-to-tubesheet joints, cracked tubesheet ligaments, or broken stays. Failures of this type are costly in downtime and repairs.

What Causes Thermal Shock?

1. Short cycling – this most commonly happens when your boiler is oversized for the application

2. Poorly tuned burner controls

3. Low temperature return water to the boiler – if the temperature of the return water is low, it creates a cyclic cooling and reheating of the shell and tube surfaces. In ideal conditions your boiler will maintain a constant temperature and steam output It is required for the boiler manufacturers to set guidelines for the system-and-controls designers to use, in order to minimize these effects. •

These guidelines include:

• A minimum water return temperature to the boiler

• A minimum water flow rate through the boiler

• Recommendations as to how to set burner controls to maximize the boiler shell temperature for a given operating pressure, and minimize the number of operating cycles and the burner firing rate, for a given load condition

How Do You Reduce Thermal Shock?

When concerning thermal shock, you should address two major criteria - reducing the magnitude of the stresses induced (reducing furnace-metal temperature to shell-metal temperature) and reducing the number of stress cycles (burner firing cycles).

This can be accomplished through:

Burner Tuning - Correctly setting the burner combustion and maintaing optimal feed water temperature.

Boiler Sizing - It is critical to have the appropriate boiler size for your application in conjunction with a proper turndown rate. You should also pay particular attention to seasonal changes as it relates to the sizing of your steam system. Too large a system and your boiler will be subjected to unnecessary short cycling, inversely, if your boiler is too small you will not have adequate steam for your given application.

Failures caused by thermal shock are fatigue failures caused by thermal stress cycling. In almost every case there is not an indication of boiler design or manufacturing deficiencies, but more commonly issues arise due to the manner in which the boiler has been sized, controlled or operated.

WARE has a team of experienced technicians who have sized and tuned boilers for a wide variety of applications. Our decades of field experience give us the confidence to quickly troubleshoot your unique steam application. For assistance in boiler sizing or burner controls tuning – call us today!

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