Valve World Americas | April 2017 • www.valve-world-americas.net28

CASE STUDY

By Barry Messer & Gobind Khiani – Fluor Canada

During the fabrication of valve bod-ies, a valve manufacturer drilled mis-aligned bolt holes into the body of the flange. These bolt holes were used to connect the valve body to the flange. We became aware of these repairs af-ter the assembly of the actuators and the valves were already complete and almost ready to ship. This was a time sensitive issue because the valves were required at site as soon as possible.

Introduction to bolt misalignment: So what is misalignment of bolts, we con-sider various aspects of the coordina-tion hole tolerance stacking problem. They arise when trying to mate two parts such as stringers to skins, skins to skins and body to adaptor, or pan-els to frames etc., by placing temporary fasteners through matching hole pairs or hole triplets. Usually there will be K such hole pairs or triplets, where K>2 and K may be as large as 60 or higher.

When such fasteners are temporary, their function is to act as clamping de-vices that hold the parts in position while intermediate holes are match-drilled and riveted. Since such riveting fixes the po-

Casting Weld RepairsIn this article we will discuss misalignment of bolt holes into the body of the valve flange casting (body to adaptor). How manufacturers review code interpretation on repair of these.

sition of the parts relative to each other, these coordination holes also serve, the much more important role of defining the geometry or final position of the two parts relative to each other. The tempo-rary fasteners are removed after riveting the intermediate holes. The coordination holes are then cleaned out, i.e., match-drilled with larger, full-sized holes and bolted/riveted.

Defining the relative geometry of parts by coordination holes differs from pre-vious practices of using massive tools for joining parts. The tools carry the geometry information which is trans-ferred to the parts by holding them in place in the tools while match drilling and bolting them. This process results in variation in the geometry as defined on the toll and in variation in fixturing the parts on the tool. The variation in the geometry as defined on the tool acts more like a bias, fixed effect, or mean shift which does not lend itself very well to statistical variation cancel-lation. Such biases would repeat them-selves time and again, unless there is some slow drift in the tool geometry,

which represents another systematic variation component.

Although the coordination of the whole scenario was the original motivation for looking at this problem, the analysis methods proposed here have a wider scope. Namely, they are relevant when-ever several particular features point to locations on another part. In the above motivating example these feature point locations would be the hold centers. Typically they are subject to variations which lead to mismatches when more than one pair need to be aligned. As-sessing the maximal size of this mis-match (over all feature point location pairs) from a statistical and worst case

perspective is our main goal (figure 1).

Current Practices:In order to correct the placement of the bolt holes, a weld repair is required, all the bolt holes need to be filled in and then the material re-drilled.

1. The filling is namely stated by manu-facturers as weld build up not a weld repair since only a handful of bolt

Figure 1

holes require repair. As a result, they were conducted on each of the valve bodies differently. A typical sketch showing bolt holes with welded ma-terial and then re-drilled (figure 2).

2. During this process if the method found is inefficient manufacturers occasionally remove a complete cir-cumference of the affected area of the valve body, replace with welded material and then re-drill the holes as required (figure 3).

Code Interpretation:The code requirements as per API6D and ASTM A350, PWHT (post weld heat treatment) are required on weld re-pairs. However manufacturers may fol-low ASME Section IX approved welding procedure and perform the weld repair/machining error (and not a material defect, which shall not be done as per API6D). Therefore, they are in compli-ance with the requirements.

Though there is a subtle difference be-tween weld repair and weld build up, the API6D has been treating this as one

Figure 2

Figure 3

CryogenicService Valves

Tested in our Non Destructive Test Laboratory a perBritish Standard BS 6364

by immersion in liquid Nitrogen at a temperature of -196 and Helium is introduced to guarantee no leakage.

Authorized Distributor

www.walworth.comE-mail: sales@twcousa.com

Phone: (281) 566-120013641 Dublin Court, Stafford, Texas 77477

www.valve-world-americas.net • April 2017 | Valve World Americas 29

CASE STUDY

and the same. The manufacturer occa-sionally submit welding procedures, PQRs (procedure qualification records), ultrasonic procedures, valve drawings, MTRs (material test report) and non-conformance reports for the weld re-pairs (if asked by user).

According to ASME if one has to repair a casting then in layman terms one exca-vates the cavity, cleans it and fills it with layers of weld deposits, and depending upon the degree of repair, the weld has

to be heat treated after the repair (PWHT).

According to API6D weld build up does not require a PWHT (post weld heat treatment) as it is not treated as weld repair but manufacturers commonly refer this to as weld build up. For ex-ample a dimensional error due to cast-ing shrinkage or improper machin-ing specifically around the flanges is termed as weld build up and this can be achieved by minimum preheat to 100 degrees Fahrenheit and multi pass

welds. Once again, the PWHT is not re-quired per API6D which according to us conflicts with ASME VIII Div 1 repair re-quirements, which calls for PWHT over 1.5 inches. This is based on whole set of rules and as per ASME experience.

To note B31.3-2014 the updated code has changed the PWHT requirements by waiving this provided the casting surface has been preheated to 100 de-

grees Fahrenheit on multi-pass welds.

Conclusion:

One should keep in mind the severe sour service where PWHT (post weld heat treatment) is a must. Hence, if a user does not specify the PWHT re-quirements, the valve castings are be-ing repaired as standard manufactur-ing practice without PWHT. Assuming this is done mostly with 100 degrees Fahrenheit post welds, same should be specified in engineering notes.

AboUT ThE AUThorS

Barry Messer is Technical Director and Senior Fellow with Fluor Corp. and also manages the Fluor Canada Ltd Metallurgy and Welding Engineering Group in Calgary, Alberta. He is a Director with the Canadian Welding Bureau. Barry has over 35 years experience in metallurgy, welding, materials selection and NDE development. He is regularly involved in the analysis and mitigation of fabrication and in-service failures

for the chemical, petroleum, power, pipeline and mining industries. Bar-ry is an active member of NACE and ASME.

Gobind Khiani is a professional engineer with more than 23 years of experience in the petroleum indus-try. Gobind has a Bachelor’s from the University of Pune, India and a Master’s from the University of Calgary in Materials and Pipeline Engineering. He has served as Lead Engineer, in Engineering and Project Management roles, for operating companies and Engineering, Procurement, and Construction

(EPC) companies. During this time Gobind has 75+ publications and pre-sentations in his field for magazines journals and conferences. Over the past 4 years, Gobind has been a Global Valve Subject Matter Expert in the Piping Department at Fluor.