Valve World Americas | April 2017 • www.valve-world-americas.net 28 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 Repairs In 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