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
(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 and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.