Fig. 1: post-weld heat treatment of butterfly valves in an electric furnace

Valve repair: welding, heat treatment and machining

Failure analysis shows that faulty valves are often the root cause of plant failures or even complete shutdowns. It is also true to say that the risk of valve failure increases with age. In this article, the author indicates how suspect valves can be identified and properly repaired, helping to maintain efficient plant operation.
 
^ Fig. 1: post-weld heat treatment of butterfly valves in an electric furnace

Article By Mr. N.S. Rao, EFCO
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Valves are the heartbeat of live piping systems and pipelines. Designed to work and perform 24x7, they must allow fluid flow inside the pipes without losing a microgram or millilitre to the environment. However, due to the chemistry and behaviour of flowing fluids, valve bodies, components, and other internal parts may undergo some or all of the following: (i) heavy corrosion (ii) erosion (iii) wear & tear (iv) expansion, contraction, pitting, chipping, and loss of mechanical properties. This can result in a weakening of the body structure and leakage to the atmosphere.

The good news is, that with a proper inspection and repair strategy, valves can be kept in good working condition. However, while doing preventive maintenance or valve annual maintenance, it is important to ensure that all service staff are properly qualified and API certified. They should perform the following inspection operations:

•  Thorough visual inspection
•  Magnetic particle (MT) inspection test on carbon steel valves and dye penetrant test or ultrasonic test (UT) on steel valves
•  Wall thickness survey on critical areas
•  If a discontinuity is observed on body shell areas/pressure carrying areas or seat areas then it must be attended / treated in most scientific way by adopting strict procedures
•  Cross-verify all the welding documents like WPS/PQR/WOPQ and relevant documents

Weld repairs

It is perfectly feasible to repair valve damage by welding, provided that the correct procedures are followed. Hence during the pre-welding phase, first remove the defect by grinding or gouging, possibly followed by a dye penetrant test. Ensure that defect-free mother material is visible. Engage a properly qualified and certified welder for the job and ensure that he adheres to the welding procedure specification (WPS) and the procedure qualification record (PQR). All welds should be made using calibrated welding equipment and with consumables as per the WPS/PQR.

During the actual welding, the techniques should be monitored and the pre-heat and inter-pass temperatures shall be recorded. Visual inspection and dye penetrant is required between two passes. Finally, do ensure a slow rate of cooling.

After welding, perform a weld inspection, dimensional check, and distortional check and also check for any defects.
Heat treatment

It is important that all weld repairs should undergo heat treatment. Do follow the recommended heating cycle. Note that full part heat treatment is important to ensure proper stress relief; localized stress relieving should be avoided. Also note that the heat affected zone (HAZ) is more susceptible to cracks than the weld zones.

If stress reliving is not properly performed, this can have a negative impact on functionality due to the formation of high/hot spots. These cannot be detected with dye penetrant tests and ultrasonic tests. High/hot spots may leak due to non-homogenised structures, they can impair any subsequent machining and cause damage to machining equipment.

Further, the mechanical properties of the weld material will defer from the material in the heat affected zone, leading to a weaker structure. 

Hydrogen induced cracking and sulphide stress cracking may also occur. It should be noted that finding the cause of a leak is an uphill task.

Qualified staff

As a final note, it is most important to deploy qualified servicemen in order to maintain the health of your valves. Selecting technicians on the basis of the lowest cost must be avoided. Instead, pay attention to the following issues when selecting the service vendor: (i) the available equipment (ii) the SOPs the technican(s) will follow (iii) the presence of international certificates. As a final piece of advice, do avoid simply checking for the presence of an ISO certificate which alone will not meet the product requirement. In all cases, give priority to API certified vendors.
 

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