LESSON LEARNED STATEMENT:
Pressurized dead legs without functioning traps or periodic manual
blowdowns must be assumed to have condensate present. The affected
section of piping MUST be isolated, depressurized and drained before
restoring steam to the system.
Always check the operation of main line steam traps before initiating
steam to a branch header. When a trap fails, condensate can build up in
a system in a very short time.
A condensation induced water slug can form at very low condensate flow
conditions. "Cracking Open" valves in lines with condensate in them is
NOT safe and can increase the severity of water hammer under some
conditions.
Winterization activities may result in abnormal system lineups that can
lead to condensate in dead legs. Extra caution should be used when
restoring systems to service to insure that no condensate is present in
any portion of the piping.
DISCUSSION OF ACTIVITIES:
Summary:
On Thursday, October 26, condensate induced water hammer occurred in a
main steam line in the Plutonium Finishing Plant (PFP) while attempting
to blowdown the end of a 350 kPa (50 psig) DN50 (2") steam line in which
a trap had failed. No injuries or system damage occurred.
Details:
A Power Operator and a Cognizant Engineer were preparing to perform a
leak test on a D-7 branch line steam trap. Before initiating steam to
the branch, they checked the main header trap located in the east end of
the facility. The main header trap was not functioning and was cold.
Forty eight hours earlier, the same main header trap had been checked
and was found to be operating.
The Power Operator felt branch isolation valves located just off the
main line with his hand and noted they were slightly warm to touch.
They decided to drain the liquid in the main line through a mudleg which
bypassed the failed trap. When the Power Operator cracked open the
drain valve, V-102, condensate induced water hammer was heard in the
main header. Valve V-102 was immediately closed. The water hammer
continued at irregular intervals but seemed to be gaining in intensity
so the operator proceeded to close the main isolation steam valve to
Building 241-Z. The water hammer stopped as soon as that valve was
closed.
ANALYSIS:
Condensate had accumulated in the long slightly sloping main header
while the main header steam trap was not working. When the trap bypass
valve was cracked open, steam flow across the surface of the condensate
was initiated, causing condensation induced water hammer.
RECOMMENDED ACTIONS:
RECOMMENDED SAFE OPERATIONAL AND DESIGN PROCEDURES TO PRECLUDE WATER HAMMER
SAFETY PRINCIPLE:
Do not mix steam with water, either by injecting water into a steam
system or steam into a system containing water. Steam and water cannot
be mixed safely in a piping system without risking the occurrence of
condensation induced water hammer. Condensate should be assumed to be
in all low points and dead legs until proven otherwise.
Following are 13-recommendations for Safe Operational and Design
procedures for Steam Systems that should be observed:
1. Review and inspect all steam systems to insure proper distribution
and sizing of steam traps for startup, and operation; also that all
low points have steam traps and blowdown valves. Give maintenance
the highest priority.
2. Frequently inspect all steam traps to insure that they operate
properly and that no condensate accumulates. Immediately repair or
replace erratic steam traps. Use thermocouples where feasible to
locate condensate accumulation.
3. Do not use the method of "cracking open" valves with or without
bypass systems to avoid condensation induced water hammer. This
will not guarantee safe operation. The formation of a Condensation
induced water slug can occur at very low condensate flow
conditions.
4. Valves in pipe lines which lack properly positioned steam traps or
drain valves should remain open at all times or preferably should
be removed from the piping system.
5. Before opening valves in steam lines, certify that the steam traps
operate properly. Fully open the blowdown valves to remove any
condensate, leave open until condensate is not noticeable or for a
minimum of three minutes.
6. Where feasible, operate the valves remotely using mechanical
extension linkage, reach rods or adequately controllable power
operated valves.
7. Inspect the piping system for sagging, where necessary install
steam traps or repair the sagging.
8. Check and repair the piping insulation, it will save energy and
reduce condensate accumulation in the piping system.
9. Warm up of cold steam piping should be performed slowly with trap
blowdown valves continuously open. Do not introduce steam into a
piping system without verifying water is not present. If a
condensate removal system does not exist, do not introduce steam
until the system is corrected.
10. The above list of recommendations should be followed irrespective
of piping size. Do not exclude small pipe sizes without an
appropriate analysis.
11. All isolation valves are to have bypass systems, however, bypass
operation will not prevent water hammer if condensate is present.
12. Placement of blowdown valves before and after a vertical rise (such
as over-the-road) is required to prevent possible condensate
accumulation.
13. Improperly designed steam/water systems should not have the
incorrect features overcome by operational methods. The systems
must have the improper design corrected.
ORIGINATOR: Westinghouse Hanford Co.
CONTACT: John Bickford; (509)373-7664 [Telephone], (509)373-6120 [FAX], John_C_Bickford@rl.gov [e-mail]
AUTHORIZED DERIVATIVE CLASSIFIER: Terry Vail, (509)373-2092
REVIEWING OFFICIAL: J. C. Bickford, (509)373-7664
PRIORITY DESCRIPTOR: Yellow/Caution
FUNCTIONAL CATEGORY: 430 Life Cycle Facility Operations
REFERENCES: Occurrence report number: RL--WHC-PFP-1995-0059
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