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FAILURE OF A VERTICAL AMMONIA TRANSFER PUMP SHAFT

Farabirazy Albiruni Technical Bureau of Inspection, PT. Pupuk Kalimantan Timur, Tbk.,

Jl. Ir. James Simanjuntak, Bontang 75313,Kalimantan Timur, Indonesia Email: frar@pupukkaltim.com

Abstract

A new upper shaft of a vertical ammonia transfer pump failed after 2000 hours of service. The new upper shaft material was AISI 410 martensitic stainless steel and replaced the original shaft made from AISI 4140 HT chromium-molybdenum steel while the lower shaft still in original condition. It was broken into four separated pieces with different fracture modes. The failed shaft was examined and tested by various methods. The result of the analysis indicated that the shaft failed by the improper mechanical properties specification of new shaft material. The hardness and yield strength of the new material were about 50% below the original material specification. In addition, these low hardness and yield strength were resulted from non heat treatment condition confirmed by metallography examination that revealed the anneal structures of shaft material. It is recommended that shaft material must be heat treated first to produce tempered martensite structure with hardness minimum 280 BHN. Key words: Shaft Failure, Martensitic Stainless Steel I. Introduction

Ammonia transfer pump is used to transfer the ammonia liquid from line pipe to the storage tank. This pump construction is vertical pump with two shafts, i.e. upper and lower shaft. After upper shaft replacement in January 2005, 2000 hours later there was flow losses indication in this pump during operation and found that the pump upper shaft fractured into 4 pieces. Table 1 below gives the technical data of this pump while the Figure 1 shows the pump construction schematically.

Table 1. Pump’s Technical Data

General Item : G 3204

Description : Urea Plant Feed Pump (NH3 Transfer Pump)

Manufacturer : Byron Jackson Holland Year of Manufacture : 1974

Hydraulic Data Max. Flow / Rated (l/s) : 18.5 / 20.3 Suction Head (m) : 40 Discharge Head (m) : 350 Differential Head (m) : 350

Performance Data Model : 300 VLT-18 stg/ C1-SB Speed (rpm) : 2950 Efficiency at Duty Point (%) : 75 Power at Shaft at Duty Point (kW)

: 63.5

Max. Power (kW) : 75 Max. Head (m) : 460 Max. Allowable Casing Press. (psi) (Suct./Disch.)

: 300 / 500

Min. Allowable Flow (m3/hr) : 20

Materials Casing, Barrel, Head : ASTM A 333 Gr.1 Bowls, Impeller : Cast Iron Shaft (Original/Modified) : AISI 4140 HT/AISI 410 Sleeve : 11-13 Cr

Motor Data

80 kW

3000 rpm Frame 500 V

50 Hz

3 Phase

Type V1 Ex (e)

G1 Fluid Data

Fluid Pumped : Anhydrous Ammonia Pumping Temp. (oC) : -33 Spec. Gravity at Pumping Temp.

: 0.682

Construction Data Casing Type : Vertical Can Casing Mounting : Vertical Impeller Type : Closed

Figure 1. Pump Construction

Upper Shaft

Lower Shaft

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II. Examinations and Results

Examinations to the failed shaft consist of: 1. Visual Examination 2. Positive Material Identification (PMI) 3. Hardness Testing 4. Tension Testing 5. Metallography Examination

II.1. Visual Examination Visual examination performed on failed shaft found that shaft broken into four pieces with different fracture modes. One piece of fractured shafts showed large plastic deformation and rubbing with ductile fracture, one had brittle fracture, while the others had reserved bending fatigue and rotational bending fatigue characteristic. Figure 2 to 5 show the four pieces of failed shaft.

Figure 2. Fractured Shaft No. 1 with Ductile Characteristic

Figure 3. Fractured Shaft No.2 with Brittle Characteristic

(Small Arrow: Fracture Origin)

Figure 4. Fractured Shaft No. 3 with Reserved

Bending Fatigue Characteristic (Small Arrows: Fracture Origins)

Figure 5. Fractured Shaft No.4 with Rotational Bending Fatigue Characteristic (Small Arrow: Fracture Origin)

II.2. Positive Material Identification (PMI) The shaft material conformed AISI 410 chemical composition requirement. Table 2 below gives the PMI result of shaft material using Alloy Analyzer Instrument.

Table 2. PMI Result of Shaft Material

Element Shaft Material AISI 410 Cr (% wt) 13.20 11.50 – 13.00 Ni (% wt) 0.05 0.00

II.3. Hardness Testing Hardness testing showed that the hardness of shaft was 150 BHN. The hardness specification of original shaft i.e. AISI 4140 HT was 280 BHN. This result was almost 50% below the required value. II.4. Tension Testing Table 3 below gives the tension test result. It is obvious from the table that the yield strength was almost 50% below the required value.

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Table 3. Tension Test Result

Shaft Material AISI 4140 HT Yield Strength (kgf/mm2)

34.021 73.815

Tensile Strength (kgf/mm2)

57.123 87.875

Elongation (%) 34 16

II.5. Metallography Examination Figure 6 below shows the microstructure of failed shaft. It had anneal structure consisted of ferrite (matrix) and chromium carbide dispersed in matrix. It had also indicated that no heat treatment applied to the failed shaft.

Figure 6. Microstructure of failed shaft consists of ferrite as a matrix (white area) and chromium carbide dispersed in matrix

(black dots). Mag: 200 x, Etch: Fry’s Reagent

III. Discussion From the examinations conducted, it was obvious that the mechanical properties, primarily hardness and yield strength of failed shaft were below the specification requirement. Mechanical properties related directly with the microstructure. Metallography examination shown the anneal structure consist of ferrite and chromium carbide. This microstructure has low fatigue strength. The best fatigue strength possessed by tempered martensite structure resulted from two sequences heat treatment quenching and tempering. IV. Conclusion The upper shaft of vertical ammonia transfer pump failed by the improper mechanical properties specification of new shaft material.

V. Recommendation Use the shaft material AISI 4140 or AISI 410 in heat treated condition with minimum hardness of 280 BHN. Make sure that shaft has tempered martensite microstructure. VI. Reference 1. R.A. Lula., Stainless Steel., American

Society for Metals., 1987. 2. Heinz P. Bloch, Fred K. Geitner.,

Machinery Failure Analysis and Troubleshooting 3

rd Ed.,Gulf Publishing

Company, 1997. Brief Biography Farabirazy Albiruni was born in Jakarta on November 8

th, 1978. Hold Bachelor of

Engineering from Department of Metallurgical Engineering, Faculty of Engineering, University Indonesia in 2002. After graduation, he works in PT. Pupuk Kalimantan Timur as Metallurgist and NDT Engineer.