Preventing Steam/Condensate System Accidents
Department of Energy's Safety and Health Bulletin (Issue 95-1, DOE/EH 0437)
Summary: The following article is a part of National Board Classic Series, and it was published in the National Board. BULLETIN. This article appeared in the June 1995 Department of Energy's Safety and Health Bulletin. (Issue 95-1, DOE/EH 0437) (2 printed pages)
Water hammer, the unexpected release and associated shock wave of high-pressure steam/condensate, can cause death, severe injury, or extensive property damage. Unfortunately, not all employees who work near high-pressure steam/condensate systems realize the hazards associated with such systems.
Primary and contributing causes that may lead to these and related incidents have been compiled by the Department of Energy (DOE). However, one of the most important safety principles to remember is that steam and water cannot be safely mixed in a piping system (with the exception of specifically designed systems) without risking condensate-induced water hammer. Never mix steam with water, either by injecting water into a steam system or steam into a system that includes water (condensate).
There are two primary causes of steam/condensate incidents.
- Condensate systems must be sloped. Improper installation may cause pockets of water to accumulate.
- Inoperative or inadequate trapping in steam lines.
As shown in the sidebar at left, contributing causes are organized into three categories: design, maintenance, and management controls and procedures. Other factors may significantly compromise system integrity.
RECOMMENDATIONS FOR SAFE OPERATION
The above primary and contributing causes of water hammer and related incidents may be avoided through proper operation and maintenance procedures. All steam systems should be inspected to ensure proper distribution and sizing of cold traps. Steam traps should be located at all low points and inspected frequently for condensate accumulation. Valves that lack properly positioned steam traps should remain open at all times or be removed from the piping system (for sagging) and the piping insulation (for wear).
Before opening valves in steam lines, check for adequate placement and proper operation of steam traps. Fully open bleed valves, using reduced system pressure, to remove any remaining condensate. Activate cold steam piping slowly at reduced pressure and with trap bleed valves continuously open. To prevent possible condensate accumulation, place blowdown valves before and after a vertical rise. In addition, all isolation valves must have bypass systems, though such systems will not prevent water hammer if condensate is present.
Finally, do not "crack open" valves to avoid condensation-induced water hammer; the formation of a condensation-induced water slug can occur at very low condensate flow conditions.
Improperly designed steam/water systems must have problems corrected. Once properly designed systems and operations are in place, maintenance should be the highest priority.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.