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Fitness for Service of Cracked Valve on High Pressure Steam Line Header

Sofyan PURBA

Inspection Section – Technical Department PT Badak NGL Bontang; East Kalimantan, Indonesia

Phone : +62-548-55388; Fax : +62-548-552234; e-mail : sofyan@badaklng.co.id

Abstract Cracks were found on 24” valve at steam header line. This header connects high pressure steam header from Module I to Module II, and the valve was installed in 1989. The cracks occurred at valve body and bonnet with total of seven cracks were observed. Due to risk of valve failure; fitness for service needs to be conducted to ensure valve service ability. Alternating current potential drop technique was used for measuring crack depth. The crack dimensions and material properties are used as input for finite element analysis modelling. Fatigue growth analysis and fracture mechanic analysis were conducted also to obtain of valve properties. The assessment shows that the valve is still in safe region based on API 579 Level 3 assessment but need to monitor the valve service temperature. Keywords: valve, steam, crack, alternating current potential drop, finite element, fracture mechanic 1. Introduction Badak LNG operates eight LNG Process Trains, Train A-H, to produce LNG with maximum annual capacity of 22.5 MTPA. To support LNG production, a number of 11 water tube type boilers are operated with steam production of 295 ton/hr/boiler at 62 kg/cm2.g and 450 0C in Utilities-I, and 10 water tube type boilers with steam production of 379 ton/hr/boiler at 62 kg/cm2.g and 450 0C in Utilities-II. The steam from boiler is distributed thru the piping header system to process area as driver for compressor or pump, power generation and heating media. On January 27, 2011 several cracks were detected on one block valve of the high pressure steam header line, 31HS310-24”-JF2H, as showed in figure 1. The steam line is an interconnecting line between Utilities-I and Utilities-II. The crack found on the valve body had a length of 170 mm. Six (6) crack indications were also found on the valve bonnet. These had lengths ranging from 23 to 120 mm and were characterized as branched cracking. The location of cracked valve is showed on the Figure 2 below. Since the cracks exist, there is the risk that the valve may fail therefore detail assessment is required to ensure the cracked valve fit for the service. To replace the cracked valve is not easy task it is required well arrangement of Train E/F and Train C/D shutdown because the valve located at superheated steam interconnecting line of Utilities-I and Utilities-II. Besides that, the replacement valve is not available at that time so the replacement cannot be directly conducted. By doing the assessment, it is expected that there is comprehensive analysis of the valve condition and the remaining life prediction of the valve based on latest condition. For this assessment, Level 3 Assessment As per API 579 [1] was performed. Level 1 and 2 assessments because of complicated geometry and/or loading conditions, expectation of crack growth or has the potential to be active because of loading conditions, and high gradients in stress on valve material.

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic

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Figure 1. Crack observed on the valve

Figure 2. Location of cracked valve on Utilities area (Valve 6A) 2. Fitness for Service Approach The valve assessment was divided in to several stages as described below. Before discussing the assessment stage, the specification of the valve is described first. 2.1 Cracked Valve Specification The 24”cracked valve is gate type constructed using ANSI B16.34. This valve was installed in 1989 during construction of process train. Valve service is superheated steam with service

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pressure of 61- 62 kg/cm2 and service temperature of 450 oC (842 oF). To meet with high temperature service, the valve material is made from 1¼” Cr- ½” Mo material. Detail specification of the valve can be seen on table 1.

Table 1. Technical specification of cracked valve

Design code ANSI B 16.34 Size / ANSI Class 24” / 600 Valve type Gate flexible wedge type disc Year installed 1989 Material specification ASTM A217 WC6 Service High pressure superheated steam Design pressure 75 kg/cm2 (1066 Psig) Design temperature 450 o C (842 o F) Operating pressure 61- 62 kg/cm2 (882 Psig) Operating temperature 450 o C (842 o F) Insulation N/A For future operation the surface temperature on the West and East side is 109 o C and 77.9oC

2.2 Visual Inspection and Flaw Sizing Visual inspection of the cracked valve and obtain information about the crack appearance, location and orientation. To perform detailed flaw sizing using preferred method Alternating Current Potential Drop (ACPD) [2]. For the ACPD flaw sizing on the block valve we are therefore considering the thin “skin-effect”. For the flaw sizing the current is injected to the component using two current injectors and the surface potential over the crack and reference area is measured using a two point contacting probe. For flaw sizing accuracy determination it is important to use a reference block with cracks of accurately know depths of the same material as in the component where flaw sizing is to be performed. The basic principle for thin skin flaw sizing is illustrated in Figure 3 below.

Figure 3. Basic principles of thin skin flaw sizing using the ACPD technique

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2.2 Valve Material Testing and Characterization To obtain material tensile and fracture toughness at the operating temperature, it is not possible to do the test on the cracked valve. As the solution, a series of tests on similar grade material from a discarded valve originating from the same foundry as the assessed valve was conducted. The material testing was to include hot tensile, fracture toughness/fracture resistance and fatigue crack testing at the operating temperature of the cracked valve in view of the temperature gradients measured on the valve surface. The testing programme was to be designed to facilitate an accurate assessment of the remaining life of the valve based on applicable procedures input data requirements of the applied FFS standard ASME/API 579 [1]. 2.3 Finite Element Modelling The assessment included detailed finite element analysis (FEA) of the valve using the Abaqus software to develop a detailed finite element model of the block valve and the materials data established from the testing. The analysis was to be performed based on information about the operating pressure, temperature, surface temperature gradients, design stress/piping stress analysis and historic records showing fluctuations over time associated with operation of the steam system and the block valve, shut down history and changes to the operation of the valve and steam line in question due to commissioning of new trains. Further, for the remaining life prediction information about the future operation of the valve i.e. would it be kept open or closed and how would the pressure and temperature be expected to vary based on the operation of the steam turbines and the steam utility system is required. 2.4 Fitness for Service Assessment The fitness-for-service and remaining life assessment methodology applied for the analysis of the cracked block valve conforms to the API 579 procedures for crack assessment. As per API 579 Section 9.2.2.2 A Level 3 Assessment should be performed due to the following condition:

a. Advanced stress analysis techniques are required to define the state of stress at the location of the flaw because of complicated geometry and/or loading conditions.

b. The flaw is determined or expected to be in an active subcritical growth phase or has the potential to be active because of loading conditions (e.g. cyclic stresses) and/or environmental conditions, and a remaining life assessment or on-stream monitoring of the component is required.

c. High gradients in stress (either primary or secondary), material fracture toughness, or material yield and/or tensile strength exist in the component at the location of the flaw (e.g. mismatch between the weld and base metal).

For the block valve considered all above three conditions a), b) and c) apply therefore detailed finite element modelling of the valve to determine the operating stress in the valve bonnet and body at the crack locations, the flaws detected are expected to be in a sub-critical growth stage and on-line monitoring has been performed, and is required for extension of the remaining life. The block valve has been operating since 1989, and the maximum operating temperature has not exceeded the design temperature of 450 oC i.e. the super critical steam temperature. This implies that since the maximum valve body temperature is below ¼ of the steel’s melting point i.e. Tm = 1530 oC, the block valve body and bonnet should not have been subject to creep and creep fatigue. Further, the sub-critical crack growth to be considered for the

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remaining life assessment is crack growth associated with cyclic variations of the thermal and pressure stress i.e. fatigue crack growth. The fatigue crack growth parameters of the ASTM A217 WC6 grade cast steel were determined by testing at the anticipated future operating temperature of the valve, as described in above. 3. Test Result 3.1 Flaw Sizing of Crack in the Valve Body and Bonnet The results of 5 ACPD measurements performed at each location have been tabulated in Table 2 and Table 3 below. The measurement locations are seen marked using a blue paint marker on the crack body in Figure 4 and 5. The deepest flaw was found to have a height of 31.8 mm at location A4 of the body and 13.3 mm at location C6 of the bonnet. For assessing cracks in bonnet, the network is idealized as a single planar predominant flaw in accordance of API 579. Based maximum height of the branched network from the ACPD measurements, the idealized flaw height determined as 1.2 x maximum crack height from measurement, i.e.: 16 mm and 130 mm long. 3.2 Thickness Measurement Thickness measurements at the areas adjacent to the valve body crack and bonnet cracks were carried out at elevated temperature. The same reference block from the same manufacturer, same year of manufacture and which had been cast from the same material ASTM A217 grade WC6 as used for the crack height measurement trials was used as reference for the thickness measurements. The thickness of the reference block was measured at room temperature and also at elevated temperature 77.9°C (temperature measured at the time the thickness of the valve body was taken), and 63°C (temperature measured at the time the thickness of the bonnet) was taken. In Table 4 and 5 conservative estimates of the remaining ligament thickness are presented as the difference between the minimum wall thickness recorded for each location, and the greatest crack height recorded by ACPD. The numbering of the locations follows the same numbering convention as used earlier for the crack height measurements.

Figure 4. Marking in the body

Figure 5. Marking in bonnet. Marking red location B, blue location C, green location D and yellow location E

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Table 2. Crack height and length measurement result on the body

Table 3. Crack height and length measurement result on the bonnet

Table 4. Thickness measurement result on the valve body

3.3 Material Characterization Result 3.3.1 Chemical Analysis Chemical analysis of the block valve material received for testing. The chemical analysis was performed using the Optical Emission Spectrography (OES). The detailed results of the analysis are tabulated in table 6. The cast valve body material is seen to meet the chemical requirements to ASTM Grade apart from silicon, which exceeds the maximum of 0.60% weight. 3.3.2 Tensile Testing Hot tensile testing of the block valve material was performed as per ASTM E21 – 2009 using a

Location Height Maximum (mm) A1 16.2 A2 23.6 A3 28.2 A4 31.8 A5 30.4 A6 22.2 A7 6.2

Crack length : 165 mm

Location Height

Maximum (mm)

Location Height

Maximum (mm)

Location Height

Maximum (mm)

Location Height

Maximum (mm)

B1 B2 B3 B4

2. 4 3.7 7.8 8.7

C1 C2 C3 C4 C5 C6 C7 C8

4.1 6.7 9.0 7.2

11.7 13.3 7.5 4.1

D1 D2 D3 D4 D5

6.2 11.7 6.9 5.2 5.3

E1 E2

7.3 10.5

Crack length : 33 mm Crack length : 120 mm Crack length : 70 mm Crack length : 23 mm

Location Average wall thickness (mm)

Max. Crack Height (mm)

Thickness of remaining ligament

A0 53 0 53 A1 55 16.2 38.8 A2 56 23.6 32.4 A3 59 28.2 30.8 A4 60 31.8 28.2 A5 63 30.4 32.6 A6 66 22.2 43.8 A7 69 6.2 62.8

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12.60 mm round bar test specimen with 50 mm gauge length. The test temperature was chosen as 130 oC, i.e.: approximately twice the temperature on the surface of the valve body and bonnet measured as 77 oC and 65oC, respectively according to the latest measurements. This test temperature conservatively, accounts for the temperature gradient in the valve material. For reference and for comparison with the ASTM A 217 WC6 standard requirements, testing was also performed at room temperature. It is seen from Table 7 that the cast valve meets the room temperature yield and tensile strength requirements to ASTM A217 grade WC6.

Table 5. Thickness measurement result on the valve bonnet

* No back wall echo

Table 6. Chemical composition analysis result on valve body

Element Element % weight

ASTM A 217 - WC6 % weight

Carbon, C 0.20 0.05 to 0.20 Silicon, Si 0.70 0.60 max Manganese, Mn 0.87 0.50 to 0.80 Phosphorous, P 0.033 0.04 max Sulphur, S 0.027 0.045 max Chromium, C 1.43 1.00 to 1.50 Molybdenum, Mo 0.94 0.45 to 0.65 Nickel, Ni 0.20 0.50 max* Iron, Fe balance Others 0.37 *Allowed as a residual element

Location Average wall thickness (mm)

Max. Crack Height (mm)

Thickness of remaining ligament

B1 53 2.4 50.6 B2 52.5 3.7 48.3 B3 52.6 7.8 44.7 B4 52 8.7 43.3 C1 35.75 4.1 31.6 C2 NBW* 6.7 - C3 56 9 47 C4 53 7.2 45.8 C5 52.55 11.7 40.8 C6 52 13.3 38.7 C7 51 7.5 43.5 C8 49.35 4.1 45.1 D1 49.7 6.2 43.2 D2 52 11.7 40.3 D3 53 6.9 45.1 D4 51.2 5.2 45.2 D5 52.25 5.3 46.7 E1 51.25 7.3 43.2 E2 50.6 10.5 39.9

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Table 7. Tensile testing result

YS (MPa) TS (MPa) El (%) RA (%) Room temp 23 oC 287 587 14 13 Elevated temp 130 oC 225 487 16 23 Room Temp Requirements to ASTM A 217 Grade WC6

Minimum 275 485–655

3.3.3 Fracture Toughness (CTOD) Testing Three crack tip opening displacement (CTOD) test specimens were extracted from the discarded valve body of cast steel ASTM A 217 WC6. The specimens were machined to square section B x B = 40 mm x 40 mm specimens as per BS 7448 Part 1 [4], and notched by mechanical notching and pre-cracked by fatiguing in three point bending. The resulting total crack height to specimen width was a/W = 0.33. Testing was performed at the test temperature of 130oC i.e. the same test temperature as for the hot tensile test. The test results obtained have been summarized in Table 8. It is seen that the three CTOD fracture toughness values are fairly similar, with a minimum value of the three tests of 0.38 mm. Since two of the specimens i.e. Nos PS24#1 and PS24#3 exhibited instability close to the maximum load plateau, the minimum critical value of 0.38 mm was applied for the fracture assessment.

Table 8. CTOD test results obtained by testing

Test specimen

No.

Notch & pre- crack height,

Ao (mm)

Plastic component of clip gauge opening,

Vp (mm)

Load P (kN)

CTOD (mm)

δ Comment

15600 PS 24 #1 14.88 0.82 75.72 0.38 Critical event, δc

15600 PS 24 #2 15.02 0.96 72.11 0.43

Max. load plateau, δm

15600 PS 24 #3 14.02 0.84 77.42 0.40 Critical event, δ

3.3.3 Fatigue Growth Testing Fatigue crack growth testing was performed on a single edge notched tension specimen (SENT) with B = 9.6 mm and W = 29.6 mm. The initial notch and fatigue pre-crack height was a =5.4 mm. The testing was performed at the same temperature as the CTOD and hot tensile testing i.e. at 130 oC. The fatigue crack growth testing was performed at 5 Hz varying the stress range between 20 MPa, 40 MPa, 50 MPa, 75 MPa, 100 MPa and 150 MPa until the crack exhibited stable propagation. The crack extension was monitored using ACPD and visual measurement of the crack height on the side of the fatigue crack growth specimen. The results of the fatigue crack growth testing of the block valve material in terms of crack height versus number of stress cycles, are showed in Figure 6. Also showed in this figure are the crack growth parameters A = 3.1E-17 and m = 4.4 at 130 oC obtained from the curve fitting. The fatigue threshold was found from the tests to be approximately ∆Ko = 184 Nmm-3/2.

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Figure 6. Results of fatigue crack growth testing of the 24” block valve material 3.4 Finite Element Modelling Result The objective of the analysis was to study the stress distribution on the 24” block valve in the closed condition, at the two crack locations (i.e. West Side valve body and bonnet crack) and to provide the stress values as input data to the fitness-for-service and remaining life estimation. The 3D finite element model of the block valve was developed based on the 2D engineering design drawing for the valve and manufacturer specification brochure was also reviewed for information purpose as well as thickness measurement. In the current model, the dimensions of the bonnet “cap” have been assumed since the curvature is anticipated to affect the stress distribution at the bonnet “cap”. The model is showed on the figure 7. The material properties input for the finite element analysis was taken from the tensile test results obtained from testing performed at high temperature (i.e. 130˚C). In order to input the elastic-plastic material properties to Abaqus, it is important to generate best fit smooth Ramberg-Osgood stress strain curve since stress strain data obtained from the testing will normally have a number of irregularities which may not be acceptable for the FEA. The engineering stress-strain curve generated does not give a true indication of the deformation characteristics of a metal because it is based entirely on the original dimensions of the specimen, and these dimensions change continuously during the test. Therefore the true stress-strain curve should be adopted in the finite element analysis. Fixed boundary condition (BC) is assigned to West Side of the block valve model while simply supported boundary condition is assigned to the East Side. In simply supported BC, the model is free to move axially. The BCs considered in the current analysis were based on the piping isometric drawing as showed in Figure 8 and it is anticipated that the 24” block valve is not rigidly fixed in all directions at both end of the pipe. To simulate the operating condition of the 24” block valve in closed condition, internal pressure is applied to the West Side (62.25 bar = 6.225 MPa), bonnet (61 bar = 6.1 MPa) and East Side (61 bar = 6.1 MPa) of the 24” block valve model.

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Two (2) different operating temperature distributions were analysed, such as: 1. Low temperature case: measured at the block valve surface.

a. West Side: 114.4˚C b. Bonnet: 73˚C c. East Side: 70.6˚C

It is noted that there is no information available regarding the temperature distribution inside the block valve. Hence, in the current analysis assumption has been made to increase the temperature inside the block valve by 100˚C.

2. High temperature case: temperature distributions applied in at the outer surface of the

model were based on updated information / surface temperature measurement i.e. 415˚C for the West side, 412˚C for the East side, 402˚C for the bonnet cap and 375˚C for the valve body). For this case, the operating temperature inside the block valve is set to 440˚C due to the flow of superheated steam inside the block valve.

Figure 7. Isometric view (left) and front view (right) of finite element model

Figure 8. Piping isometric drawing

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In Abaqus, to perform a thermal analysis an initial temperature condition needs to be defined. In the current analysis, ambient temperature of 25˚C was assumed as the initial condition at the outer and inner surface of the 24” block valve. Based on the summary of 3D finite element results listed in Table 8, it can be seen that the increase in temperature between the low and high temperature will further increase the stress istributions at the valve body and the bonnet (i.e. ~72 MPa stress increase at valve body and ~74 MPa stress increase at the bonnet). For the low temperature case at the valve body crack location it is seen that the stress does not decrease significantly if the temperature is reduced by 10˚C (i.e. reduction of stress of 1 MPa). If the temperature is increased by 20˚C, then the stress increases by 4.5 MPa. Hence, the stress range between low temperature -10˚C to low temperature + 20˚C for the valve body is 4.5 MPa. Similarly we find that for the valve bonnet the stress range between Low temperatures -10˚C to Low temperature + 20˚C is 9 MPa. The cyclic stress and number of cycles assuming temperature fluctuation once a day are showed in Table 10.

Table 9. Summary of FE results

Case Location Von-Mises stress (MPa)

Average (MPa)

Low temperature Valve body 78 to 129 103.5 Bonnet 57 to 122 89.5

High temperature Valve body 157 to 194 175.5 Bonnet 132 to 195 163.5

Low temperature minus 10˚C

Valve body 77 to 128 102.5 Bonnet 54 to 116 85

Low temperature plus 20˚C

Valve body 84 to 130 107 Bonnet 61 to 127 94

Table 10. Cyclic stress and number of cycles assumed per year

Case Location Stress range, ∆σ(MPa) Cycles per year Low temperature – 10oC + 20oC

Valve body 4.5 365 Bonnet 9 365

Low temperature - High temperature

Valve body 72 365 Bonnet 74 365

3.5 Fitness for Service Assessment Result The failure criteria at Level 3B is determined as per API 579 [1], from the failure assessment diagram (FAD). For the block valve assessment the Level 3B i.e. the material specific FAD is established from the engineering stress-strain curve obtained from the hot tensile test. The material fracture toughness is established by fracture mechanics testing of similar ASTM A 217 WC6 grade material from the discarded valve of same rating, same manufacturer and year of manufacture, and which has been operating in the same service in the steam system. By implementing procedure in API 579 [1], the failure assessment diagram (FAD) for 24” valve is showed on the Figure 9.

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Figure 9. Material specific Failure Assessment Diagram for valve body/bonnet at 130 oC

For the valve body, the assessment was conducted using API 579 Section 9 [1] procedures at Level 3B. It is seen from Figure 10 shows that the flaw is safe based on the maximum von-Mises stress for the low temperature case +20 oC, derived from the finite element analysis. Critical flaw sizes (heights versus lengths) were determined for cracks in the valve body subjected to tensile stress due the temperature gradient in the valve assuming different wall thicknesses reported in the vicinity of the valve body crack. The results presented in Figure 11 show that the 31.8 mm height and 170 mm long flaw is safe contained within the boundaries of the FAD.

Figure 10. Location of failure assessment point for flaw in valve body

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Figure 11. Location of failure assessment point for flaw in valve body compared to the calculated

critical flaw heights versus flaw lengths

Figure 12. Valve body flaw profile and wall thickness profile established from similar valve cross section and minimum wall thickness derived from UT and measurement

In Figure 12 the valve body flaw has been plotted against the minimum wall thickness measured in the area around the flaw. The minimum wall thickness is estimated by UT thickness measurements in taken in vicinity of the valve body crack, where back wall echo could be detected. It is apparent that the thickness values derived by the UT measurements are incorrect due to the problems of obtaining the back wall echo at the exact crack location. Is believed that the minimum thickness measurement is taken at the location of the valve where there is a recess for the valve seals. The ‘t2’ values showed in Figure 10.4 are based on the updated information, which represents the wall thickness estimated by sectioning a similar (same manufacturer and grade) discarded valve, at the same position where the valve body crack was located and also by correlating this with the thickness derived from the manufacturer’s drawing. The measurement have some inherent uncertainty, but are considered far more reliable than the thicknesses derived from UT, which were associated

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with loss of back wall echo in the vicinity of the crack due to the thickness transition and measurement at locations that are associated with thinner wall, presumably due to the presence of the recess for the valve seals. The valve is intended to operate the valve in the closed condition, under the defined low temperature case, with minimal temperature excursions i.e. Low temperature -10oC and Low temperature + 20oC. The remaining life of the valve can be estimated assuming the valve remains in the closed position, and the operation of the steam system causes maximum temperature swings between 10 oC below and 20 oC above the low temperature case, with a frequency of once a day. Under these circumstances the remaining life can be estimated by calculating the time for the flaw to propagate to become critical or grow through the wall thickness, eventually causing a leak before break (rupture) at location A4. Calculations were performed using the stress range 4.5 MPa, one cycle per day and using the crack growth parameters showed in Table 9. The results of this analysis showed that under these conditions no crack extension would be expected, as showed in Figure 13.

Figure 13. No crack growth is estimated for the valve body crack for thermal stress cycles

Similar analysis was run for illustration, assuming the temperature swings between the low and high temperature case once a day. The analysis showed that if the valve experiences temperature fluctuations between the low temperature case and the high temperature case (inside 440 oC superheated steam) the body crack would grow as showed in Figure 14, and the flaw would eventually become unsafe. The expected failure mode is leak before rupture.

For the crack in the bonnet, the same procedure is applied. It is seen from Figure 15 that the 16 mm height and 130 mm equivalent flaw is safe for the low temperature case i.e.: for a membrane stress of 127 MPa as per the finite element analysis. The critical flaw size was estimated for the low temperature case as showed in Figure 16. Fatigue crack growth analysis was performed to check if the flaw in the bonnet would grow to become critical. Based on the results of the finite element analysis it was found that the primary membrane stress range associated with fluctuating temperature in the bonnet 10 oC

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below to 20 oC above the low temperature case was 9 MPa. It is seen from Figure 17 that the flaw would not be expected to grow under these conditions, if the frequency of the cyclic stress change associated with the temperature fluctuation is once a day.

Figure 14. Crack growth estimated for the valve body crack for thermal stress cycles associated with temperature fluctuations between the low and high temperature once a day

Figure 15. Location of failure assessment point for flaw in valve bonnet

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Figure 16. Critical flaw sizes estimated for the valve bonnet for the low temperature operation case

Figure 17. Results of fatigue crack growth calculations for the re-categorized flaw for the low temperature case

The fatigue calculation was also performed for illustration assuming a stress range of 74 MPa associated with a swing between the low and high temperature case, once a day, as showed in Figure 18. The analysis shows that if the valve experiences temperature fluctuations between the low temperature case and the high temperature case (inside 440 oC superheated steam) the branched bonnet cracks would grow and eventually become unsafe. The failure mode is uncertain due to the nature of the branched cracking. Potentially this could cause fracture before leak by interaction of the branched cracking.

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Figure 18. Results of fatigue crack growth calculations for the re-categorized flaw on the bonnet

for temperature fluctuations between the low and high temperature cases

4. Discussion

ACPD crack height measurements were performed of the 24” block valve. The results showed that the 170 mm flaw located at the valve body on the West Side had a semi-elliptical shape with a maximum height of 31.8 mm. ACPD measurements were also performed on the valve bonnet and the greatest flaw height for the network of branched cracks was determined to 13.3 mm. Applying the API579 [1] procedure for re-categorisation of branched cracks the characteristic idealized flaw or equivalent flaw size was determined to be 16 mm in height and 130 mm long. Based on hot tensile testing, crack tip opening displacement (CTOD) testing of a discarded valve of the same material ASTM A217 grade WC6, and finite element modelling of the valve in the closed position, the subsequent fracture mechanics analysis, showed that both the body crack and the equivalent bonnet flaw was safe for the low temperature case considered. Fatigue crack growth analysis showed that the flaw would not grow under the current low temperature operating conditions, assuming temperature swings between 10 oC below the low temperature and 20 oC above the low temperature case for the closed condition. Crack growth analysis showed that sub-critical crack growth could take place and the flaws could become unsafe depending on the operation of the steam system. If the valve experiences significant temperature changes (between the low and high temperature case), once a day, a leak (no rupture) could develop. The valve is intended in the closed condition and under the defined low temperature case, with minimal temperature excursions i.e. Low temperature -10 oC and Low temperature + 20 oC. Under such strictly controlled conditions, the finite element analysis showed that the valve body would experience a stress range of 4.5 MPa at the crack location. Fatigue crack growth analysis was performed imposing a stress range of 4.5 MPa once a day. The results of the analysis showed that under these conditions no crack extension would be expected.

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If the valve is continued operated under controlled conditions it is recommended that crack monitoring of the body and bonnet flaws is performed to confirm that no crack extension is taking place as the current fatigue crack growth analysis indicates. Since the analysis indicates that the main growth occurs in the thickness direction hence, there may be difficult to detect any crack growth on the valve surface. Therefore, the temperature monitoring shall be conducted to avoid any high swing beyond lower temperature case. 5. Conclusion Based on the above the results of the ACPD flaw height measurements, materials, CTOD, fatigue crack growth and finite element modelling, Follows are the conclusion: a. The results showed that the 170 mm long flaw located at the valve body on the West Side

had a semi-elliptical shape with a maximum height of 31.8 mm. b. Failure assessment based on the minimum thickness and the stress derived from the finite

element analysis showed that the flaw is in the safe region of the Level 3B failure assessment diagram.

c. Crack growth analysis showed that the crack in the valve body is not likely to grow if the

valve experiences limited temperature cycling (between 10 oC below to 20 oC above the low temperature case) but sub-critical growth of the body crack will occur if the valve experiences temperature cycling between the low and the high temperature case (superheated steam at 440 oC inside), and the valve is likely to develop a leak before break at the valve body crack location.

d. The largest flaw height associated with the network of branched cracks was 13.3 mm and idealized or equivalent flaw size was determined to be 16 mm in height and 130 mm long.

e. The fracture mechanics analysis performed based on the API 579 Level 3B procedures

showed that for the current low temperature case, the crack is in the safe area of the failure assessment diagram.

f. Fatigue crack growth analysis performed based on the stress range derived from the finite

element analysis assuming temperature fluctuation from 10 oC below to 20 oC above the low temperature, showed that the equivalent crack in the bonnet would not grow. But, if the temperature swings between the low temperature and the high temperature case (440 oC superheated steam inside) the analysis showed that the equivalent bonnet flaw would experience sub-critical crack extension.

g. Under the current low temperature case the valve can be operated safely, it is

recommended to perform crack monitoring to confirm that there is no crack extension using ACPD. Close monitoring of temperature shall be conducted to prevent any temperature swing beyond lower temperature case.

Acknowledgements We express our gratitude to DNV Singapore for assistance in conducting this assessment.

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References 1. American Petroleum Institute, ‘Fitness for service’, API 579-1/ASME FFS-1, JUNE 5,

2007. 2. Martin C Lugg, ‘An Introduction to ACPD’, TSC Technical Bulletin TSC/MCL/1146

Rev. 18, February 20, 2002. 3. British Standards Institution, ‘Guide to methods for assessing the acceptability of flaws in

metallic structures’, BS7910: 2005. 4. British Standards Institution, ‘Fracture mechanic toughness test. Method for

determination of KIc, critical COD and critical J values of metallic material’, BS7448 Part 1: 1991.

5. DNV, ‘Fitness for service and remaining life assessment of block valve in high pressure steam system PT Badak NGL Bontang ’, Technical Report no CTC_R_2011029, October 2011.