CASE STUDY #1 Zinc Recovery Plant No.2 SHAL Tank Agitator Blade Failure Report & Recommendations

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1 CASE STUDY #1 Zinc Recovery Plant No.2 SHAL Tank Agitator Blade Failure Report & Recommendations By: Justin Taylor Photo 1 Report Summary The purpose of this document was to put forward an initial recommendation to the client for the successful repair and remediation of the failed agitator in the Plant Sulphuric Acid Mixing Tanks and provide further information on how to retrofit the existing impellers in order to avert further failures. Prior to the recommendations, the reasons and the cause of the failure are discussed. It was known that the environment that the agitator operated was very aggressive in terms of, density of the media to be pumped, chemical nature of the media and the abrasiveness of the media, these all having adverse effects on the impellers. Scope of the Investigation The scope of this review was to assess and highlight the mechanisms that were responsible for the degradation of the impellers. We then evaluated the alternatives available that would inhibit the degradation of other impellers, and finally we discussed our recommendations. Upon the inspection of the failed impeller it was seen that chemical and mechanical degradation originated from various locations on the blade surface. This illustrated that the point of initiation had a random component to it which acted as the starting point of the crack. More specifically, of the three blades inspected, each blade had started to fail in the region where the blade connected onto the central connecting hub (between the bolts). But within this area, each blade showed signs of crack initiation at different locations. This was also evident when the blade trailing edges were inspected for erosion, this also appeared to occur at random areas. For these reasons, the attention of our report focussed on the area where the blade bolted onto the hub. Following the successful modification and commissioning of the failed impeller under review, these improvements could be replicated on existing impellers currently in operation at the plant.

2 Assumptions It was assumed that the blade material was a 904L Stainless Steel because no material documentation was available and this is a commonly used material in this environment. In the absence of empirical data on the loading of the blade and not having the original design of the agitator, we assumed that the failure was a High Cycle Fatigue failure combined with the material properties of 904L Stainless Steel. These aspects were used in our theoretical analysis in order to determine what the loading on the blade was. We further assumed that original design had a safety factor of 2. Findings Upon our initial inspection of the impeller blade we witnessed that the point of fixture, namely the bolting up area had failed catastrophically. The blade parent material had been torn away from its point of fixture through the bolt holes. There was also widespread evidence of severe pitting on the blade surface, some pits were up to 1.5mm deep and 1 mm wide. The trailing edge of the blade also showed signs of chemical attack and erosion. Of the four bolts that secured the blade to the hub connection, one had failed but the remainder were intact. Processes of Degradation As stated previously, two methods of degradation have been identified, these are individually discussed below: Chemical Attack The material used for the impeller blade was a Stainless Steel the grade of which could not be confirmed but it was assumed that a grade 904L Austenitic stainless steel was used. This stainless steel has extreme corrosion resistance in this application, superior to that of 316. Even though the material has inherent corrosion resistant properties, it did not mean that no corrosion would occur, it merely occurred at a lower rate. The rate of corrosion in stainless steel is highly dependant on the protective oxide layer that covers the material and prevents corrosion. In an environment where abrasion occurs, this oxide layer is subsequently removed and the unprotected material is then exposed to the chemical agents that attack the surface. For this reason it is imperative to have a material in this environment that has extreme inherent corrosion resistance. Abrasive Attack The media in which the agitator operated was extremely abrasive with hard particle solids that were kept in suspension by the mechanical action of the agitator. These hard particles mechanically eroded the material surface and literally tore away small parts of the blade in a continuous process. This could be clearly seen on the blade surface as it had a rough shot-blasted-like appearance. The rate of erosion is dependant on two properties of the material, its hardness and its toughness. The harder and tougher the material is, the lower the rate of erosion. Unfortunately these two mechanical properties are seldom found together in a single material. For example, Bennox (Abrasion Resistant) plate and rubber are both used as wear linings, the Bennox plate is extremely hard but has low toughness (brittle) whereas the rubber is very tough but is soft with low strength. For this reason the blade material should be very hard in order to exhibit good abrasive qualities. A rubber blade alternately would not have sufficient strength as a blade material.

3 Failure Mechanism The processes outlined above are only responsible for minor exterior damage to the impeller blade. This damage although minor in comparison the remainder of the material left on the blade is significant in the sense that it was responsible for the initiation of another failure mechanism called fatigue. Simply put, abrasion and corrosion were responsible for the creation of small pit holes on the surface of the blade. These pit holes (however small) caused stress concentrations that when the blade was loaded, caused the nominal stress that were seen by the parent blade material to be multiplied to almost 300% at the bottom and sharpest point of the pit holes. This meant that even though the blade was lightly loaded, there were extreme loads occuring at the pit holes that were in excess if the ultimate tensile strength of the material. This was where a crack initiated and continued to grow from. Evidence of this was clearly seen from the failed blades viewed on site. When the blade failure face was closely inspected, three classic periods of failure could be seen. Firstly there was the point of initiation, the pit; secondly there were the so called Beach Marks that are a sequential set of arcs emanating outwardly from the pit; lastly there was the catastrophic failure region, this was where the material had been torn away in one action. It was further explained that during the crack propagation period (formation of Beach Marks) the blade had not yet failed and it was still sustaining the load satisfactorily. Only when the crack had become so deep that the blade could no longer sustain the load did the blade fail and the material was torn apart in one action. This was the catastrophic failure area. Failure Zone Photo 2 Pitt Hole Beach Marks Catastrophic Failure Zone Photo 3

4 Bolt Failure From the inspection of the failure and if photo 1 is studied it could be seen that only 1 bolt failed of the 4 that secure the blade to the central hub. This bolt was also situated on the blade tip side of the flange on the opposite side of the crack from the other bolts. The bolt when examined showed signs of cracks on the threads above the failure. It could be deduced from the fact that only this bolt failed and by inspecting its location that the overload that caused the failure was a result of this bolt having to sustain the majority of the loading on its own after the formation of the crack had isolated the other bolts. The three remaining bolts showed no signs of fatigue deterioration. In the case where the blade material was able to carry the entire load effectively, this load would be spread over all the bolts and localised loading of one area would be significantly reduced. Thus the beefing up of the blade plate as illustrated below would spread the load to all the bolts so that one bolt was not overloaded and the rest saw almost no load. Theoretical Analysis Material Stainless Steel grade Ultimate Tensile Stress, S ut Yield Stress, S y : 904L : 490 MPa : 220 MPa K - Factors The maximum allowable stress in the material is reduced by the factor K. In order to establish what the value of K is, the following components of K are applied: K a surface factor = 0.6 (very rough surface) K b size factor = x d where d = (hb) 0.5 K c reliability factor = (99 % reliability) = (12 x 245) 0.5 = mm = x = 0.82 K d temperature factor = 1 (negligible as temp is below 450 C) K e stress concentration factor = 1/k f where k f = 1 + q (k t -1) K f other effects = 0.9 (assumed) K = K a x K b x K c x K d x K e x K f = 0.6 x 0.82 x x 1 x x 0.9 = 0.18 Endurance limit under this loading condition: = (3 1) = 2.2 = 1/ 2.2 = 0.454

5 S e = S y x K = 220 x 0.18 = 40 MPa This is the maximum stress allowed to be reached in the parent material in order prevent the critically loaded areas on the blade from exceeding the blade material s yield strength. Calculation of bending moment on blade: M y I The face of the failure was measured at 245 mm long by 12 mm wide 3 b h I e 8m 3 Finding the bending moment on the blade we use the material yield stress divided by a safety factor of 2: σ = S y / 2 = 220 / 2 = 110 MPa I M y 110e e Nm From our previous analysis we know that the maximum allowable stress that the blade can sustain is only 40 MPa, we recalculate the required thickness h of the blade. M y y = h/2 I h e6 19.9mm b h I 12 3 It was seen from the above calculation that the required thickness of the blade must be at least equal to 20 mm in order to sustain the loads under these working conditions. Repair Process It was our recommendation that the remaining blades be removed from the agitator shaft hub and that they be repaired. The repair process would entail the welding up of all the surface defects on the blade like pit holes and erosion pockets. The welding would then be ground flush with the remaining surface of the blade (to be done in accordance with an approved procedure). In addition, we recommended that in order to increase the blade thickness up to the minimum required 20 mm that a compensation plate of equal thickness be fixed to the impeller blade. This can be seen on the accompanying illustration. The exact dimensions of this plate are not covered here but are part of a separate procedure. The final thickness would then be 24 mm overall.

6 In order to fit this compensation plate flush, it would be need to be shaped to the same contour of the existing blade and the keel plate would have to be removed. This keel plate would then be fixed to the compensation plate. The connection between the blade and the compensation plate would be made via a bolting method. An additional bolt would also be required at the opposite side of the keel. An additional advantage of this compensation plate arrangement was that this plate would act as a sacrificial wear plate and could be replaced when it had been worn down, this was a more cost effective option than replacing the entire blade when corroded or cracked. The compensation plate would also act as a wear plate as it was the contact surface the pumped the material inside the tank. Shaft Connection Plate Impeller Blade New Compensation Plate Keel Figure 1

7 Conclusion The findings of this report indicated that the original assumptions regarding the sizing of the impeller blade thickness were in all likelihood incorrect. This was because of the severe operating conditions both chemically and abrasively that caused the stress concentrations. These stress concentrations severely reduced the allowable yield stress eg. the yield stress of the material is 220 MPa but under these conditions it is reduced to 40 MPa, a reduction of 5.5 times. The addition of a compensation plate would serve to reduce the stresses in the blade to the required level and it would protect the blade from abrasion degradation. The compensation plate was much smaller than the blade which would also act as a sacrificial component that was more cost effective to replace than the blade itself. Recommendations Whilst our calculations attempted to simulate the actual conditions that the agitator was subjected to, we further recommended: 1. That the blades were removed and repaired in terms of surface irregularities and cracks; 2. A compensation plate arrangement was adopted and installed, so that the new overall thickness was double that of the existing i.e. 24 mm with the plate. 3. This modification could be carried out on other agitator blade arrangements that experienced similar problems. 4. Following the modification to the blade, the dynamic conditions of the loaded system should be studied to identify possible resonance problems, the level of loading in terms of system torque on the shaft and motor current during startup and running should form part of this study. 5. The optimum level of agitator blades should be confirmed that will deliver the best efficiency. 6. A scheduled maintenance programme be followed to inspect the condition of these blades that may identify when repairs were required and when the compensation plates needed replacement before irreparable damage was suffered by the blade and it had to be replaced.