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Failed Steam Trap Damage

CONDENSATION INDUCED WATER HAMMER STRIKES AGAIN

IDENTIFIER: 1995-RL-WHC-0048        Date: November 7, 1995

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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