Inspection of Crack and FFS Assessment of Isomeric RPV
Yung-How Wu1, Chu-Chung Hsiao2, Chih-How Chen3
1, 2Industrial Technology Research Institute
3Energy Research Center
ABSTRACT
Weld cracks were found on the inner surface of an isomerization reactor in a scheduled outage. According to regular maintenance operation, surface cracks were ground or weld repaired once found on refinery RPV. Since it is full of pearl powder catalyst inside, vessel open and repair process may affect the plant's operations and cost a lot. PAUT was then applied to acquire carefully the size and relative position of the crack and the local vessel structure. FFS of the reactor was assessed in terms of FEM model based on PAUT results, fracture mechanics and damage mechanism analysis. Results confirmed the crack in safe margin with safety factor SF as a benchmark and the isomerization plant was then returned to service. Compared to former maintenance, great maintenance and downtime cost were saved in this case referring to API 579 standard.
Keywords: RPV, Weld, PAUT, Fracture Mechanics, FFS, Isomerization
1. 異構化反應器破裂檢查及適用性評估
吳永豪1、蕭祝螽2、陳志豪3
1工業技術研究院資深員
2工業技術研究院工程師
3能源研究中心研究員
摘要
某石化廠的異構反應器於定期檢修時在銲道內壁處發現疑似裂縫。以往的檢修 常規要求反應器一旦出現裂縫就會磨除或銲補。該槽體內部充滿珍珠粉觸媒,是否開 槽檢修將影響工廠營運至鉅。是故,本研究先以相控陣超音波法精確量出裂縫及局部 結構尺寸及相對位置,再結合檢測果建構的 FEM模型、破裂力學及破損機制來評 估該反應器的適用性。綜合分析係以安全係數SF為基準,推斷該反應器的內壁裂縫 尚處於安全許可範圍並判定異構化廠可重啟運轉。相較於以往的在役檢修作業,本案 參酌API 579標準所採取的決策為廠方節省了巨額的開槽檢修及停機損失。
關鍵字:反應器、銲道、相控陣超音波、破裂力學、適用性、異構化
Inspection of Crack and FFS Assessment of Isomeric RPV
Yung-How Wu1, Chu-Chung Hsiao2, Chih-How Chen3
1, 2Industrial Technology Research Institute
3Energy Research Center
ABSTRACT
Weld cracks were found on the inner surface of an isomerization reactor in a scheduled outage. According to regular maintenance operation, surface cracks were ground or weld repaired once found on refinery RPV. Since it is full of pearl powder catalyst inside, vessel open and repair process may affect the plant's operations and cost a lot. PAUT was then applied to acquire carefully the size and relative position of crack the local vessel structure. FFS of the reactor was assessed in terms of FEM model based on PAUT results, fracture mechanics and damage mechanism analysis. Results confirmed the crack in safe margin with safety factor SF as a benchmark and the isomerization plant was then returned to service. Compared to former maintenance, great maintenance and downtime cost were saved in this case referring to API 579 standard.
Keywords: RPV, Weld, PAUT, Fracture Mechanics, FFS, Isomerization
5. (a) (b)
圖六:(a)周向銲道裂縫模型深度a、銲冠高度d 及(b)C1 銲道裂縫與周邊結構組合。
(2) 考慮破裂力學理論之應力分析
以破裂力學處理裂縫尖端附近的應力在數學上是定義尖端附近具一奇異應力場,並
以一破裂力學參數:應力強度因子(stress intensity factor, SIF)來代表該奇異點應力場的大
小,而具裂縫材料之可容許應力強度因子(critical SIF)訂為材料參數:破裂韌度(fracture
toughness)。當材料受力之裂縫尖端應力強度因子值超過破裂韌度時,裂縫會失穩而成
長。對於韌性材料,若其應變能釋放率(strain energy release rate)小於該材料之臨界值時,
裂縫即停止進展而不致於整體破壞(global failure or fracture)。
假設裂縫座標系統如圖七所示,在一般荷重下,以極座標表示之裂縫尖端附近之應
力場可寫為:
0 ,
2 2
I I II II
ij ij ij ij
K K
r f f O r
r r
, (1)
其中, I K 、II K 分別為I、II型應力強度因子,i, j x, y, 0
ij 為非奇異應力項,O r
代表高階項, I
ij f 、 II
ij f 為角函數。考量裂縫以張裂模式(mode I)為主控,經座標
轉換後可將周向應力寫為:
,
2
K
r O r
r
, (2)
其中,K 稱為等效應力強度因子(effective SIF):
3 3
cos sin cos
2 2 2 I II K K K
. (3)
6. x
y
r
θ
crack
圖七:裂縫尖端座標系統
假定裂縫會沿最大周向應力垂直方向擴展,則從式(3)之極大值,可得裂縫擴展方向
為:
2 2
1 8
2 tan
4
I I II
c
II
K K K
K
. (4)
且當裂縫開始擴展時,等效應力強度因子即為破裂韌度( Ic K ):
max Ic K K . (5)
將式(4)代入式(3)可得最大等效應力強度因子為 :
3 2 2
3
2 2 2 2 2
4 2 3 8
12 8
II I I II
eff
I II I I II
K K K K
K
K K K K K
, (6)
其中, 0 II K 。因此,在一般荷重狀況下,若式(6)計算值大於或等於材料之破裂
韌度值時,裂縫不穩定,裂縫會擴展。本研究檢視在不同銲冠高度及不同裂縫深度下之
最大等效應力強度因子,而安全係數則可利用下式定義:
SF Ic
eff
K
K
. (7)
本案假設材料為線彈性,應力分析採用的各項參數如表一。
表一:應力分析採用參數
參數 數值
楊氏係數* (E) 200 GPa
泊松比 ( ) 0.3
操作壓力 (P0) 10 MPa
銲冠高度 (d) 0.10 mm ~ 10 mm
裂縫深度 (a) 0.10 mm ~ 15 mm
破裂韌度* (KIc) 50 MPa/m0.5
*http://en.wikipedia.org/
7. (3) A107 反應器計算結果與分析
圖八為Case A 在銲冠高度4 mm 時,裂縫深度分別為4 mm 及10 mm 下容器整體之
von Mises 應力分佈。可以發現,在裂縫較淺的案例(4 mm)中,尖端附近的應力(尖端位
置除外)並非最大,甚至較部分銲道邊緣小,顯示淺裂縫對整體壓力結構受力影響不大,
反之,在裂縫較深的案例(10 mm)中,尖端附近的應力為最大,會影響應力分佈狀態,
顯示該裂縫已不可忽視。
σmax
σmax
(a) (b)
圖八:Case A 之von Mises 應力分佈(σmax 指最大位置、單位: Pa),裂縫尺寸分別為(a) a=4
mm、d=4 mm,(b) a=10 mm、d=4 mm。
周向裂縫Case A 之等效應力強度因子分佈圖如圖九所示,其中安全係數(SF)定義如
式(6)(不考慮材料塑性變形)。結果顯示應力強度因子與銲冠高度呈反向關係,與裂縫深
度則呈正向關係。依據定義,當SF < 1 時,裂縫不穩定,會開始延伸擴展,結果呈現現
有裂縫狀況仍處穩定範圍,在不改變操作壓力下,此裂縫是安全的,尚不致延伸破壞。
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014
a (m)
0
0.002
0.004
0.006
0.008
0.01
d (m)
2<SF<5
SF>5
圖九:Case A 在不同銲冠高度d 及裂縫深度a 下之最大等效應力強度因子分佈(單位:
MPa)。
8. 4、結論
(1) 本案採用相控陣超音波法精確量出異構化反應器局部結構及銲道裂縫的尺寸與相對 位置,並結合破裂力學及參考API 579規範來確認該反應器的適用性。藉此提供業 主採取重啟運轉的決策,節省了巨額的開槽檢修及停機損失。
(2) 台灣勞檢單位及石化廠至今仍沿用多年的檢修常規,一旦發現結構出現裂縫就採取 磨除或銲補作業。國外對於反應器等構件的在役檢測已普遍採用FFS規範來判定適 用性,對於類似異構化反應器的案例而言開槽檢修與否將影響工廠營運至鉅。在 當今能源產業的安全與營運效益必須兼顧環境下,本案例值得國內管制及技術相 關單位做參考。
5、參考文獻
(1) DAVID S. J. & PETER R. PUJAD´O, Handbook of Petroleum Processing, Springer, 2006, pp. 400-416.
(2) API 579 Recommended Practice for Fitness For Service, API, 2006.
(3) API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, API, 2006.
(4) 吳永豪、蕭祝螽,「中油吸附槽銲道相控陣超音波檢測」,工業技術研究院報告,中 油公司,2012年 4月。
(5) Hanshell RD, Shaw KG. Crack tip finite elements are unnecessary. International Journal for Numerical Methods in Engineering, Vol. 9, pp. 495~507, 1975.
(6) Boresi, A.P., Chong K.Elasticity in engineering mechanics, John Wiley & Sons, New York, 2000.