Valve leakage


 Valves are installed at intervals along pipelines and can be closed to stop the flow of gas for maintenance, ruptures or leaks.

Common causes of valve leakage

Valves can leak for a variety of reasons, including:
                      i.        The valve is not fully closed (e.g., due to dirt, debris, or some other obstruction).
                     ii.        The valve is damaged. Damage to either the seat or the seal can cause leakage.
                    iii.        The valve is not designed to close 100%. Valves that are designed for precise control during throttling may not have excellent on/off capabilities.
                    iv.        The valve is the wrong size for the project.

Solution: Leaking valve

1.    Use of line blinds (Valves can be left open or leak through in the closed position). Blinds provide positive isolation and eliminate the possibility of damaging devices.
2.     Applying a sealant so that gas will not leak past the plug even when the valve is in closed position (refer to OSHA 1910.147 for lockout/tagout of hazardous energy sources). 
3. Replacement with a new valve during shutdown maintenance
4.    Refer to MSS Standard SP-61, API Standard 598 , ANSI/FCI Standard 70-2 

Sampling Devices

Do not use a combustion flue gas analyzer; use a good quality, recently calibrated LEL (lower explosive limit) meter during natural gas introduction and removal. A four gas meter can be used to monitor oxygen levels during Nitrogen post repair purging. Make sure that two LEL meters are available



Protection against damage caused by valve leakage


1.    Regular valve testing and inspection

2.    Selection of appropriate valve size
3.    Insist on the amount of leakage tolerance for the pipeline project. Zero leakage is not always anticipated


Valve leakage test


The maximum allowable leakage (MAL) for a specified test depends on the valve size.The following test could be conducted every twelve months

a.    Hydrostatic test
b.    Pneumatic test

Also the following test may be performed.
a.    Valve seat leakage tests may be performed for pressure relief valves;
b.    Closure tests to check the closure mechanism of valves such as globe, gate, plug, check, and ball valves,
c.    Backseat tests for valves with backseat element such as globe valve, and gate valve;
d.    Shell leakage tests may be performed for valves in “full open” and “full closed” service eg. Check valve, stop valve, isolation valve.


Reference


[1]   Allied Valve INC. 2015. https://alliedvalveinc.com/the-valve-expert/valve-leakage-tests-explained/ (accessed November 2, 2017).
 

Comments

  1. sneha6 July 2019 at 08:27

    Thnq for sharing Good Info Stack Gas Analyzer
    based on telecom laser technology for measuring right at the heart of industrial processes.

    ReplyDelete

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