MECHANICAL REPAIR PROCESSES

This Section provides guidance on the applications of a range of commonly-used mechanical repair processes. For many techniques, achieving repairs of an integrity appropriate to the high-risk application of explosion-protection demands additional care beyond that encountered in basic workshop practice.

Any reclamation should be carried out by competent personnel, skilled in the processes to be employed and using good engineering practices. Operators of reclamation techniques (e.g. welding, metal spraying) must show their level of competence by undertaking a practical skills test in the technique before being permitted to utilize the technique for the first time, then every three years thereafter. If the operator has not used the technique in the previous six months they should undertake a re-test.

If any proprietary process is used, the instructions of the originator of the process should be followed.

All reclamation should be properly documented and records retained. Such records include:

(a) Identification of the component part.

(b) Method of reclamation.

(c) Details on any dimensions that differ from those in relevant certification/approval documents or the original dimensions of the component part.

(d) Drawings showing reclamation details including material removed and replaced.

(e) Date.

(f) Name of the organization carrying out the reclamation.

If the reclamation is carried out other than by the owner/operator or service facility, the owner/operator and service facility should be provided with a copy of the record.

A reclamation procedure that would result in dimensions or other integral aspects affecting explosion-protection integrity being different from those given in relevant certification/approval documents is considered inappropriate without additional certification.

In the event of any uncertainty regarding the permissibility (from an explosion-protection safety point of view) of an intended reclamation procedure, the advice of the manufacturer or certifying authority should be sought. It will be necessary to carry out tests afterwards to verify that the reclamation procedure is acceptable.

3.13.2 Welding 3.13.2.1 General

Workshops are reminded that Ex equipment enclosures may be subjected to extreme service conditions. Weld repairs must ensure that the mechanical properties of the repaired item are not significantly altered by the methods of repair. Weld repair is a common technique used for both rectifying defects and imperfections and for deposition of additional metal prior to machining.

However, welding techniques can have a significant impact on the properties of the repaired item. Every effort must be made to ensure controlled welding conditions. In particular, the heat employed in welding can cause internal stresses that cause distortion in adjacent structures. Welding on mounting structures as well as on enclosures can cause distortion that impacts on the integrity and functionality of enclosures, and close tolerance dimensions should be rechecked after the completion of welding.

3.13.2.2 Materials of construction

Materials of construction will determine the approach to repair of explosion-protected enclosures. Some materials, such as cast iron, SG iron and phosphor-bronze have limited weld-ability. Before attempting any weld repair the suitability of the materials of construction must be established from relevant certification/approval documents.

3.13.2.3 Quality welding principles

High-integrity welded construction is qualified through a two-step process. The procedure (combining configuration, consumables and welding parameters) is qualified through the creation of a test piece that is destructively tested. Other welding personnel may then be qualified (by using the same qualified procedure) to create a test piece that is examined non-destructively, through macroscopic examination or through destructive testing. This is detailed in AS/NZS 3992:1998.

Some configurations have been used so extensively that further weld qualification is not required, just welder qualification—refer to Table 2.1 in AS/NZS 3992. Because of the unique nature of many weld repair configurations, the following recommendations set out a means of qualifying welding in a manner equivalent to that used in the fabrication Standards

The threaded hole repair configuration recommended by AS/NZS 3800 requires some form of qualification.

The use of weld metal as an overlay deposit prior to machining should be addressed in a similar manner. (See AS/NZS 3992, Section 8.)

3.13.2.4 Welding procedure qualification

For most weld repairs it will be necessary to create a job-specific repair procedure that includes procedures to qualify and control welding. Workshops should qualify the proposed procedure by creating a defect equivalent to that being repaired, in similar material, perform the proposed weld repair and test to establish that suitable properties have been achieved.

The tests applied should be agreed on by the parties concerned. Examples of suitable tests may include destructive tensile, bend and impact tests, macroscopic examination of the weld deposit, with hardness traverse, or stud pull-out tests, depending upon the nature of the repair. All welders doing the job must repeat this procedure using either the agreed test or a cut specimen demonstrating full fusion to qualify as a welder for the process.

Procedures should be documented in a manner equivalent to AS/NZS 3992, Appendix B.

3.13.2.5 Typical considerations in welding activities 3.13.2.5.1 General

There are many considerations in establishing a suitable weld procedure and service facilities are advised to consult with persons having appropriate expertise prior to undertaking and welding an item of explosion protected.

3.13.2.5.2 Welding joint preparation

Items for repair must be gouged to remove all traces of cracking. This should be checked using a suitable non-destructive testing method, such as magnetic particle testing in accordance with AS 1171, Non-destructive testing—Magnetic particle testing of ferromagnetic products, components and structures, or dye-penetrant testing in accordance with AS 2062, Non-destructive testing—Penetrant testing of products and components for non-magnetic materials.

Particularly when using weld metal deposition to fill holes or gouges, care must be taken to ensure that a suitable bevel is applied to promote fusion of weld metal to parent metal.

While commonly used as a means of restoring damaged threaded holes, fill-and-tap techniques present particular problems regarding side-wall fusion and slag inclusions. Slag-less techniques (such as GMAW) are recommended.

Surfaces must be cleaned immediately prior to commencement of welding.

3.13.2.5.3 Welding position

In large enclosures it may be necessary to weld in a position other than the preferred flat orientation. This should be considered when qualifying welding procedures.

3.13.2.5.4 Welding energy input

In some instances, thin-walled enclosures may distort under high heat input. Heat input can also change the properties of adjacent parent metal. This is particularly relevant in metal deposition, such as gouge-and-hole filling activities, where heat is more concentrated than normally occurs in fabrication welding.

3.13.2.6 Post-weld testing

Each item repaired should be subjected to relevant post-repair testing. Depending on the nature of the repair, this could include a non-destructive test (NDT), possibly radiographic examination in accordance with AS 2177, Non-destructive testing—Radiography of welded butt joints in metal or ultrasonic testing in accordance with AS 2207, Non-destructive testing—Ultrasonic testing of fusion welded joints in carbon and low alloy steel.

Acceptance criteria should be as per AS 1554 Part 1 grade SP. Weld-repaired enclosures should be subjected to a suitable and relevant mechanical test, such as a hydrostatic pressure test, or possibly a pull-test on a threaded stud.

3.13.3 Metal spraying

Thermal spray techniques are increasingly used to build up worn or eroded surfaces prior to re-machining to the desired surface finish. Thermal spraying melts a filler or feeder wire to droplets that are deposited onto the target surface. These droplets key onto the surface, with additional bonding provided through diffusion and other chemical/physical processes.

The bonding of the sprayed droplets to the target surface, and to previously deposited material, is influenced by—

(a) target cleanliness;

(b) surface finish/profile;

(c) temperature of the deposited spray and cooling rates;

(d) deposition velocity; and

(e) physical and chemical properties and reactions between the target surface, the molten droplet and the entrainment media (typically air or inert gas).

Cleaning and grit blasting are important for substrate preparation. A rough and clean target area provides the surface needed for good bonding. Both chemical and mechanical bonding is relevant.

Sprayed material cools rapidly, however the applied temperature remains important in promotion of diffusion bonding. In some cases, bonding can be improved through application of a pre-heat to the target surface, although the formation of oxidation products may impair the bonding.

An increase in thermal and kinetic energy increases chances of metallurgical bonding.

Poor deposition can result in a number of failure modes including—

(i) porosity;

(ii) stress;

(iii) thermal shock; and (iv) cracking.

3.13.4 Sleeving

Should the fitting of a sleeve be considered a form of reclamation, serious consideration must be given to the process and materials to be used because incorrect procedures can lead to development of a second flamepath or distortion of casing through effects of heat from brazing.

Once the sleeve has been securely fitted, the surface is to be machined.

AS/NZS 60079.1 states that the surfaces of joints should be such that their average roughness Ra does not exceed 6.3 μm.

This may be readily checked through the use of a surface roughness comparator.

3.13.5 Thread repair 3.13.5.1 General

AS/NZS 60079.1 requires that the quality of threads be of medium or fine tolerance according to ISO 965-1 and ISO 965-3

In addition threaded fasteners are required to be in accordance with AS/NZS 60079.0:2008, Clause 9.2.

These ISO Standards for thread form are reproduced in AS 1721.

Thread form measurement is a technically involved activity demanding a deep understanding of the thread phenomena. AS 1721 recommends thread forms be verified by gauging in accordance with AS 1014. However, it is impractical for workshops to hold gauges for all thread forms likely to be encountered.

As an alternative verification measure, the workshop should follow these steps:

(a) Establish that the threaded fasteners provided with the enclosure are all similar and show appropriate markings.

(b) Conduct a visual examination for damage to the thread form.

(c) Confirm the depth of penetration and numbers of threads engaged.

(d) Perform a physical test to establish that the threads engage without seizing.

(e) Establish that the engaged fastener does not allow axial movement.

If any doubt exists to the suitability of threaded fasteners arising from these checks, the final arbiter should be gauging. Where uncertainty over the suitability of a threaded fastener exists, it is recommended that the fastener be replaced.

3.13.5.2 Reclamation of threaded holes

Particularly when using weld metal deposition to fill holes or gouges, care must be taken to ensure that the hole is suitably prepared. This should involve the following Steps:

(a) Removal of metal in excess of original hole depth.

(b) Ensuring a suitable bevel is applied to promote fusion of weld metal to parent metal.

(c) Thorough cleaning of the hole immediately prior to welding.

Although commonly used as a means of restoring damaged threaded holes, fill-and-tap techniques present particular problems regarding side-wall fusion and slag inclusions.

Instead, GMAW techniques are recommended.

Tapped holes must comply with the depth requirements of AS 60079.1 in order to accommodate threaded fasteners.

After tapping, threaded holes should have their quality of reclamation verified through non-destructive testing for weld faults (typically inclusions, cracking and lack of side-wall fusion) and undergo a physical test involving the insertion of a stud and application of a tensile pull-out load. The applied load when testing a reclaimed threaded hole should reflect the load that would be placed on that hole during the conduct of the relevant hydrostatic test for the enclosure.

3.13.6 Bolt fit—Fixing bolts, studs and nuts

Broken or missing fasteners should be replaced with the fasteners described in the certification/approval documents, or equivalent new fasteners where fastener information is not detailed in the certification/approval documents.

Stud or bolt holes that could pass into the flameproof enclosure should always be blind holes, with a thickness of metal at the bottom of the hole of not less than 3 mm or one-third of the hole diameter, whichever is the greater. If, for reasons of construction, holes have to penetrate such enclosure walls, they should be plugged for not less than 6 mm or the diameter of the hole, whichever is the greater, by a screwed plug complying with thread pitch requirements, which is permanently fixed in place.

Permanently attached studs should be screwed in place and securely fixed. No washers (plain or lock) should be placed under bolt heads, screw heads or nuts unless they form part of the original approval.

For coal mines, bolt heads, nuts and the like used on flameproof equipment and enclosures should be suitably shrouded or designed (e.g. with a button or pyramid head so that they can only be loosened and removed with the aid of a special tool). A pyramid or button-headed bolt should only be used if the surface around the hole has been spot machined to ensure that the axis of the bolt is normal to the surface. Where replacement bolts are used, they should be of the same type, diameter, pitch and length, and at least the same tensile strength.

Any broken or damaged attachment, which could affect the flameproof properties of an enclosure, should be replaced with a part that does not void the current certification.

For Group II equipment, shrouding is no longer a requirement, however for Group 1 enclosures, shrouding is still a requirement.

3.13.7 Distortion repair

Bent and distorted enclosures may be restored to their original shape. However, cold forming can change the mechanical properties of the parent metal, particularly in cast structures. Enclosures subjected to cold forming repairs should be subjected to a hydrostatic pressure test after repair.

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