CN102636075A - Selection method of pipe plugging for shell-sand-tube heat exchanger - Google Patents

Abstract

The invention relates to a selection method of pipe plugging for a shell-sand-tube heat exchanger and relates to the field of detection and evaluation of the shell-sand-tube heat exchangers in petrochemical industry. The method disclosed by the invention selects leakage heat exchange tubes based on a conventional hydrostatic test, and selects heat exchange tubes which are likely to leak within a next operational cycle by NDT (non destructive testing) and integrity assessment technologies. The NDT technology is used for finding out present defect sizes of the heat exchange tubes containing detects, the integrity assessment technology is used for obtaining a present maximum allowed defect size of the heat exchange tubes, and the maximum allowed defect size is a difference between a critical failure defect size and defect increment and NDT uncertainty in the next operational cycle. If the present defect size is smaller than the maximum allowed defect size, pipe plugging does not need to be implemented; otherwise, the pipe plugging needs to be implemented. The method disclosed by the invention can be implemented in the process of maintenance for selecting relatively dangerous heat exchange tubes and further plugging the tubes, thus effectively reducing temporary stop times of the heat exchanger in the operational cycle and then reducing enterprise accident potentials and improving enterprise economic benefits.

Description

A kind of shell-and-tube heat exchanger plugging system of selection

Technical field

The present invention relates to a kind of shell-and-tube heat exchanger when maintenance shell-and-tube heat exchanger plugging system of selection that is used for, belong to the petroleum chemical industry shell-and-tube heat exchanger and detect the evaluation field.

Background technology

Shell-and-tube heat exchanger is the maximum a kind of heat exchanger of use amount in the petroleum chemical enterprise, accounts for 25% of business equipment sum.Heat exchanger tube is the worst parts of shell-and-tube heat exchanger military service operating mode, often owing to (leaking in the heat exchanger) appears leaking in corrosion.Heat exchanger tube leaks will have a strong impact on product quality; Even generation serious accident; For example: on July 16th, 2011; 1,000 ten thousand tons of atmospheric and vacuum distillation unit heat exchangers of Dalian Petrochemical Industry Company of CNPC on-site take place to leak and cause big fire to cause heavy losses, therefore avoid heat exchanger tube to leak and the temporary parking that causes thus is the important topic that petroleum chemical enterprise faces.Because heat exchanger tube is in heat exchanger shell inside; When day-to-day operation and monitoring; Can only find the leakage rate of heat exchanger, overhaul can only be with carrying out after the heat exchanger temporary parking, but the stopping production that temporary parking causes loss is very big; So reducing the major measure of leaking in the heat exchanger is during the shut-down (breakdown) mainteance of heat exchanger plan, the method for restraining through plugging or integral replacing realizes.

Do not influencing under the heat exchange efficiency situation, plugging becomes main means.The most important content of plugging is how to select dangerous heat exchanger tube, how to confirm that promptly which heat exchanger tube need block.Existing plugging system of selection is to find the heat exchanger tube that leaks through the heat exchanger water pressure test, and which root leaks stifled which root.This method is simple, and detection speed is very fast, but the heat exchanger tube that has leaked can only find to overhaul the time, does not have the heat exchanger tube that leaks but have major defect during for those water pressure tests, then can't find out.And there is the heat exchanger tube of major defect in these, because factor such as corrosion causes the defective growth, before maintenance next time, leak probably, and this leakage is the interior temporary parking of the heat exchanger cycle of operation and the principal element of causing danger.Therefore; Leak economic loss and the potential safety hazard that causes for reducing heat exchanger tube, need badly on the basis of the existing water pressure test, find a more efficiently plugging system of selection; Make heat exchanger when this overhauls; Just can select leaked with the next cycle of operation in the heat exchanger tube that possibly leak, and with it stifled on, thereby significantly reduce the temporary parking number of times in the heat exchanger cycle of operation.

Summary of the invention

The object of the invention is to provide a kind of heat exchanger plugging system of selection based on the water pressure test, Non-Destructive Testing and integrality assessment technique; Make shell-and-tube heat exchanger when this overhauls, leaked and overhauled the heat exchanger tube that fashion is not leaked but possibly leak in the next cycle of operation in the time of selecting maintenance.

The technical scheme that the present invention solves the problems of the technologies described above is following: a kind of shell-and-tube heat exchanger plugging system of selection may further comprise the steps: when heat exchanger overhauls; At first select the heat exchanger tube that has leaked through the heat exchanger hydraulic test method; Find out the heat exchanger tube that possibly leak in the next cycle of operation through Dynamic Non-Destruction Measurement and integrality assessment technique again, the heat exchanger tube that need block when said heat exchanger tube that has leaked and the heat exchanger tube sum that possibly leak in the said next cycle of operation are this maintenance.

On the basis of technique scheme, the present invention can also do following improvement.

Further; Said method of searching the heat exchanger tube that possibly leak in the next cycle of operation may further comprise the steps: when this maintenance of heat exchanger, at first find out the current flaw size that contains the defective heat exchanger tube through Dynamic Non-Destruction Measurement, obtain the current maximum allowable defect size of heat exchanger tube through the integrality assessment technique then; If said current flaw size is less than said current maximum allowable defect size; Then do not need plugging, otherwise, then need plugging.

Further, growth of defect amount in the current maximum allowable defect size of said heat exchanger tube=heat exchanger tube borderline failure flaw size-next cycle of operation-Non-Destructive Testing uncertainty.

Further, said heat exchanger hydraulic test method comprises that heat exchanger looses core, cleans, changes the pad and the step of pressure testing.

Further, said Dynamic Non-Destruction Measurement is an eddy detection technology.

Further, said method comprising the steps of:

Step 1: when this overhauls, select the heat exchanger tube that has leaked through the heat exchanger hydraulic test method;

Step 2: utilize Dynamic Non-Destruction Measurement, find out the heat exchanger tube that contains defective, and confirm the current flaw size S of each defective to the heat exchanger tube that does not leak, and the uncertainty S of definite Dynamic Non-Destruction Measurement _Unct

Step 3:, confirm that this overhauls the increased size Δ S of defective in the time difference between maintenance next time according to the technology of the residual life evaluation in the integrality assessment technique;

Step 4:,, calculate and under design conditions, to be unlikely to occur the borderline failure flaw size S that leaks or break in conjunction with specification, design conditions and the material property of heat exchanger tube according to the residual intensity assessment technique in the integrality assessment technique _Crit

Step 5: current maximum allowable defect size S when calculating this maintenance _Allw

Step 6: determine whether with containing on the defective heat exchange pipe plug, with current flaw size S of the heat exchanger tube of Non-Destructive Testing gained and current maximum allowable defect size S _AllwMake comparisons, if said current flaw size S is less than said current maximum allowable defect size S _Allw, then defective can be accepted, when this overhauls without plugging; Otherwise, need plugging.

Further, overhauled Non-Destructive Testing when record last time when existing, the computing formula of growth of defect size Δ S is in the said step 3:

In the formula, T _{This-last time}Be the time difference between this maintenance and maintenance last time; T _{The next time-this}Be the time difference between maintenance next time and this maintenance; Flaw size when S overhauls for this; S _{Last time}Flaw size when overhauling for last time.

Further, when not having Non-Destructive Testing last time data, the computing formula of growth of defect size Δ S is in the said step 3:

In the formula, T _{The next time-this}Be the time difference between maintenance next time and this maintenance; T is the heat exchanger tube active time; S is current flaw size.

Further, borderline failure flaw size S in the said step 4 _CritComputing formula do

$S_{\mathrm{crit}}=\left\{\begin{array}{cc}\frac{69t}{2/3[({\mathrm{\σ}}_s+69)-\frac{{\mathrm{\σ}}_s}{M}]}& \mathrm{if}\frac{L^2}{\mathrm{Dt}}\≤20\\ \frac{69t}{S_{\mathrm{flow}}}& \mathrm{if}\frac{L^2}{\mathrm{Dt}}>20\end{array}\right.$

In the formula, t is the original wall thickness of heat exchanger tube, mm; L is the axial length of corrosion default, mm; D is the heat exchanger tube external diameter, mm; σ _sBe the yield strength of heat exchanger tube material, MPa; S _FlowBe flow stress, MPa is taken as S _Flow=σ _s+ 69; M is the bulging coefficient, and the computing formula of said bulging coefficient M does

$M={(1+0.8\frac{L^2}{\mathrm{Dt}})}^{0.5}$

If calculate S _CritSurpass 80% of original wall thickness, then get S _CritBe 0.8t.

Further, current maximum allowable defect size S in the said step 5 _AllwComputing formula be S _Allw=S _Crit-Δ S-S _Unct

The invention has the beneficial effects as follows: the present invention has been leaked on the basis of heat exchanger tube when existing dependence hydraulic test method is selected maintenance; The heat exchanger tube that does not leak when using Non-Destructive Testing and integrality assessment technique to find out maintenance but possibly leak in the next cycle of operation, the heat exchanger tube that need block up when heat exchanger tube that has leaked and the heat exchanger tube that possibly leak are confirmed as this maintenance.Compare existing plugging system of selection; Enforcement of the present invention can be selected more dangerous heat exchanger tubes when maintenance; After these heat exchanger tubes are blocked, can effectively reduce the temporary parking number of times in the heat exchanger cycle of operation, thereby reduce enterprise accident hidden danger and improve the business economic benefit.

Description of drawings

Fig. 1 concerns sketch map for heat exchanger tube tube wall defect size in the shell-and-tube heat exchanger of the present invention.

The specific embodiment

Below in conjunction with accompanying drawing principle of the present invention and characteristic are described, institute gives an actual example and only is used to explain the present invention, is not to be used to limit scope of the present invention.

Heat exchanger tube wall thickness t of the present invention, borderline failure flaw size S _Crit, current maximum allowable defect size S _Allw, growth of defect size Δ S and detect uncertainty S _UnctBetween relativeness as shown in Figure 1.

For realizing above-mentioned purpose; The technical scheme that the present invention adopts is: when this maintenance of heat exchanger; At first select the heat exchanger tube that has leaked according to existing heat exchanger hydraulic test method; Select the heat exchanger tube that possibly leak in the next cycle of operation through Non-Destructive Testing and integrality assessment technique afterwards, the heat exchanger tube that need block when two parts sum is overhauled for this.The lookup method that possibly leak heat exchanger tube in the next cycle of operation is: when this maintenance of heat exchanger; At first find out the current flaw size that contains the defective heat exchanger tube through Dynamic Non-Destruction Measurement; Obtain the current maximum allowable defect size of heat exchanger tube through the integrality assessment technique then, if the former less than the latter, does not then need plugging; Otherwise, the then stifled pipe that needs.The current maximum allowable defect of heat exchanger tube is of a size of the poor of heat exchanger tube borderline failure flaw size and interior growth of defect amount of the next cycle of operation and Non-Destructive Testing uncertainty.Heat exchanger plugging system of selection basic step of the present invention is summarized as follows:

Step 1: when this overhauls, loose core, clean, change steps such as pad, pressure testing through heat exchanger, select the heat exchanger tube that has leaked.

Step 2: utilize Dynamic Non-Destruction Measurement,, find out the heat exchanger tube that contains defective, and confirm the current flaw size S of each defective to the heat exchanger tube that does not leak, and the uncertainty S of definite Dynamic Non-Destruction Measurement (being generally EDDY CURRENT) _UnctBecause heat exchanger tube quantity is more in the heat exchanger; If it is tighter that maintenance duration requires; Can be rule of thumb the heat exchanger tube of One's name is legion be carried out region ordering, preferentially detect the heat exchanger tube in seriously corroded zone, thereby detect defective as much as possible by the seriously corroded degree.

Step 3:, confirm growth of defect size Δ S in the next cycle of operation according to the technology of the residual life evaluation in the integrality assessment technique.The method of confirming is divided into two kinds:

1. when having the Non-Destructive Testing record of last time maintenance, use this defects detection result with last time the defects detection result poor, growth of defect size Δ S when adopting the linear method prediction to overhaul next time.Predictor formula does

In the formula, T _{This-last time}Be the last time cycle of operation, i.e. the time difference between the maintenance of this maintenance and last time; T _{The next time-this}Be the next time cycle of operation, i.e. the time difference between maintenance next time and this maintenance; S is meant current flaw size, i.e. flaw size during this maintenance; S was meant the flaw size when overhauled last time last time.

2. when not having the Non-Destructive Testing data of maintenance last time; When finding certain defective as using for the first time this method or the first time; Can confirm corrosion rate according to heat exchanger tube active time T and current flaw size S earlier, confirm Δ S according to corrosion rate and next periodic duty time then.Predictor formula does

Step 4:,, be unlikely to occur the borderline failure flaw size S that leaks or break under the calculation Design operating mode in conjunction with the data such as specification, design conditions and material property of heat exchanger tube according to the residual intensity assessment technique in the integrality assessment technique _CritService Environment by heat exchanger tube can know that its major defect is the corrosion thinning defective, for corrosion thinning defective, S _CritBe meant the maximum corrosion depth of borderline failure.With reference to the 1st grade of judgement of ASMEB31G-2009 " Manual for Determining the Rema ining Strength of Corroded Pipelines ", S _CritThe calculating principle be the flow stress that the circumference stress that heat exchanger tube bore that contains defective is no more than heat exchanger tube, the computing formula of derivation gained does

$S_{\mathrm{crit}}=\left\{\begin{array}{cc}\frac{69t}{2/3[({\mathrm{\σ}}_s+69)-\frac{{\mathrm{\σ}}_s}{M}]}& \mathrm{if}\frac{L^2}{\mathrm{Dt}}\≤20\\ \frac{69t}{S_{\mathrm{flow}}}& \mathrm{if}\frac{L^2}{\mathrm{Dt}}>20\end{array}\right.---\left(3\right)$

In the formula, t is the original wall thickness of heat exchanger tube, mm; L is the axial length of corrosion default, mm; D is the heat exchanger tube external diameter, mm; σ _sBe the yield strength of heat exchanger tube material, MPa; S _FlowBe flow stress, MPa is taken as S _Flow=σ _s+ 69; M is the bulging coefficient, and its computing formula does

$M={(1+0.8\frac{L^2}{\mathrm{Dt}})}^{0.5}---\left(4\right)$

If according to formula (3) calculate S _CritSurpass 80% of original wall thickness, then get S _CritBe 0.8t.

Compare the 1st grade of judgement of ASME B31G-2009, the special character of formula (3) is following 3 points: (1) formula is a limit flaw size criterion, is not the limit stress criterion; (2) according to the designing requirement of GB151-1999 " shell-and-tube heat exchanger ", the heat exchanger tube allowable stress has 1.6 times safe clearance at least than yield stress; And when detecting, the present invention considered the detection uncertainty; So when formula (3) was derived, safety coefficient was no longer considered, is taken as 1.0; (3) because mostly the heat exchanger tube material is No. 10 steel, No. 20 steel and some steel alloys, the difference of its yield strength and tensile strength is far above 0.1 σ _s, and the temperature that heat exchanger tube is on active service is about below 300 ℃, and the creep effect that temperature causes can be ignored, so avoid flow stress S _FlowGet undue conservative data 1.1 σ that ASME B31G-2009 recommends _s, and data S between getting wherein _Flow=σ _s+ 69.

Step 5: current maximum allowable defect size S when calculating this maintenance _AllwS _AllwFor considering to detect uncertainty S _UnctAfterwards, equal borderline failure flaw size S _CritPoor with Δ S.S _Allw, S _Unct, S _Crit, the relativeness between Δ S and the wall thickness t is shown in Figure 1, figure intermediate recess place representes defective, solid line is represented current flaw size, the predicted boundary of defective when dotted line representes that detect next time.Can set up the current maximum allowable defect size S of heat exchanger tube according to following formula _Allw

S _allw＝S _crit-ΔS-S _unct (5)

Step 6: determine whether with containing on the defective heat exchange pipe plug.With current flaw size S of the heat exchanger tube of Non-Destructive Testing gained and current maximum allowable defect size S _AllwMake comparisons, if the former is smaller or equal to the latter, then defective can be accepted, when this overhauls without plugging; Otherwise this heat exchanger tube can leak in the next cycle of operation, need plugging when this overhauls.Promptly determine whether plugging according to following formula

Below in conjunction with specific embodiment the present invention is further specified.

600,000 tons of/year coking gasoline and diesel hydro-refining units of certain petro-chemical corporation U type heat exchange of heat pipe, come into operation in January, 2000, and the last replacing tube bank is in June, 2006.Medium is the reaction distillate in the pipe, and out temperature is 55 ℃ and 45 ℃, and design pressure is 6.87MPa.The heat exchanger tube material is No. 10 steel, and bore is Φ 19 * 2mm.Managing outer medium is recirculated water, and out temperature is 28 ℃ and 40 ℃, and design pressure is 0.38MPa.In June, 2011, this heat exchanger overhauls with its corresponding device.To this heat exchanger, plugging system of selection practical implementation step of the present invention is following:

Step 1: the completion heat exchanger carries out pressure testing to tube bank after loosing core, clean, change the pad operation, and finding out the heat exchanger tube that has leaked has 5.

Step 2: heat exchanging organ pipe Shu Jinhang EDDY CURRENT, because heat exchanger tube quantity is more, rule of thumb, the two circle corrosion of heat-exchanging tube bundle outermost are more, therefore the heat exchanger tube to outermost two circles carries out Eddy Current Nondestructive Testing.Find that respectively there is following defective in 3 heat exchanger tubes: No. 1 there is axially long L in heat exchanger tube ₁Be 34mm, horizontal wide C ₁Be 5mm, the darkest S ₁Corrosion thinning defective for 0.32mm; No. 2 there is axially long L in heat exchanger tube ₂Be 16mm, horizontal wide C ₂Be 3mm, the darkest S ₂Corrosion thinning defective for 0.51mm; No. 3 there is axially long L in heat exchanger tube ₃Be 6.4mm, horizontal wide C ₃Be 10.2mm, the darkest S ₃Corrosion thinning defective for 1.01mm.The uncertainty of EDDY CURRENT is 10%.

Step 3: confirm the increased size Δ S of defective in the next cycle ₁, Δ S ₂, Δ S ₃Owing to there was not data accumulation when overhauling last time, can calculate the corrosion depth increment according to formula (2).Known heat exchanger tube active time T is 5 years, and next cycle of operation plan is 3 years, so can get

$\left\{\begin{array}{c}\mathrm{\&Delta;}{\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="120409160650.png"\; state="\{\{state\}\}">S</figure-callout>}}_1=\frac{3}{5}{\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="120409160650.png"\; state="\{\{state\}\}">S</figure-callout>}}_1=0.19\\ \mathrm{\&Delta;}{S}_2=\frac{3}{5}{S}_2=0.31\\ \mathrm{\&Delta;}{S}_3=\frac{3}{5}{S}_3=0.61\end{array}\right.---\left(7\right)$

Step 4: calculate borderline failure corrosion depth S according to formula (3), formula (4) _Crit1, S _Crit2, S _Crit3, the SMYS of known No. 10 steel is 205MPa, can calculate

$\left\{\begin{array}{c}{\mathrm{<figure-callout\; id="1"\; label="S\; crit"\; filenames="120409160650.png"\; state="\{\{state\}\}">S</figure-callout>}}_{\mathrm{crit}}=0.50\\ S_{\mathrm{crit}2}=1.07\\ S_{\mathrm{crit}3}=16.7\end{array}\right.---\left(8\right)$

Step 5: the current maximum corrosion depth S that allows that confirms each defective _Allw1, S _Allw2, S _Allw3According to formula (5), can calculate

$\left\{\begin{array}{c}{\mathrm{<figure-callout\; id="1"\; label="S\; allw"\; filenames="120409160650.png"\; state="\{\{state\}\}">S</figure-callout>}}_{\mathrm{allw}}=0.50-0.19-10\%\&times;0.32=0.28\\ S_{\mathrm{allw}2}=1.07-0.31-10\%\&times;0.51=0.71\\ S_{\mathrm{allw}3}=1.67-0.61-10\%\&times;1.01=0.96\end{array}\right.---\left(9\right)$

Whether step 6: confirming to contain the defective heat exchanger tube needs plugging.Can get according to formula (6)

$\left\{\begin{array}{c}{\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="120409160650.png"\; state="\{\{state\}\}">S</figure-callout>}}_1=0.32>{\mathrm{<figure-callout\; id="1"\; label="S\; allw"\; filenames="120409160650.png"\; state="\{\{state\}\}">S</figure-callout>}}_{\mathrm{allw}}=0.28\\ S_2=0.51<S_{\mathrm{allw}2}=0.71\\ S_3=1.01>S_{\mathrm{allw}3}=0.96\end{array}\right.---\left(10\right)$

So No. 1, No. 3 heat exchanger tubes need block up, No. 2 heat exchanger tubes are without plugging.

Through the plugging scheme that the present invention proposes, except 5 pipes that leak, No. 1, No. 3 heat exchanger tubes that contain defective also need block up, so this maintenance should be blocked up 7 heat exchanger tubes altogether.

The above is merely preferred embodiment of the present invention, and is in order to restriction the present invention, not all within spirit of the present invention and principle, any modification of being done, is equal to replacement, improvement etc., all should be included within protection scope of the present invention.

Claims (10)

1. shell-and-tube heat exchanger plugging system of selection; It is characterized in that: said method comprising the steps of: when heat exchanger overhauls; At first select the heat exchanger tube that has leaked through the heat exchanger hydraulic test method; Find out the heat exchanger tube that possibly leak in the next cycle of operation through Dynamic Non-Destruction Measurement and integrality assessment technique again, the heat exchanger tube that need block when said heat exchanger tube that has leaked and the heat exchanger tube sum that possibly leak in the said next cycle of operation are this maintenance.

2. shell-and-tube heat exchanger plugging system of selection according to claim 1; It is characterized in that: said method of searching the heat exchanger tube that possibly leak in the next cycle of operation may further comprise the steps: when this maintenance of heat exchanger; At first find out the current flaw size that contains the defective heat exchanger tube through Dynamic Non-Destruction Measurement; Obtain the current maximum allowable defect size of heat exchanger tube through the integrality assessment technique then, if said current flaw size less than said current maximum allowable defect size, does not then need plugging; Otherwise, then need plugging.

3. shell-and-tube heat exchanger plugging system of selection according to claim 2 is characterized in that: growth of defect amount in the current maximum allowable defect size of said heat exchanger tube=heat exchanger tube borderline failure flaw size-next cycle of operation-Non-Destructive Testing uncertainty.

4. shell-and-tube heat exchanger plugging system of selection according to claim 1 is characterized in that: said heat exchanger hydraulic test method comprises that heat exchanger looses core, cleans, changes the pad and the step of pressure testing.

5. shell-and-tube heat exchanger plugging system of selection according to claim 1 is characterized in that: said Dynamic Non-Destruction Measurement is an eddy detection technology.

6. shell-and-tube heat exchanger plugging system of selection according to claim 1 is characterized in that: said method comprising the steps of:

Step 1: when this overhauls, select the heat exchanger tube that has leaked through the heat exchanger hydraulic test method;

Step 2: utilize Dynamic Non-Destruction Measurement, find out the heat exchanger tube that contains defective, and confirm the current flaw size S of each defective to the heat exchanger tube that does not leak, and the uncertainty S of definite Dynamic Non-Destruction Measurement _Unct

Step 3:, confirm that this overhauls the increased size Δ S of defective in the time difference between maintenance next time according to the technology of the residual life evaluation in the integrality assessment technique;

Step 4:,, calculate and under design conditions, to be unlikely to occur the borderline failure flaw size S that leaks or break in conjunction with specification, design conditions and the material property of heat exchanger tube according to the residual intensity assessment technique in the integrality assessment technique _Crit

Step 5: current maximum allowable defect size S when calculating this maintenance _Allw

Step 6: determine whether with containing on the defective heat exchange pipe plug, with current flaw size S of the heat exchanger tube of Non-Destructive Testing gained and current maximum allowable defect size S _AllwMake comparisons, if said current flaw size S is less than said current maximum allowable defect size S _Allw, then defective can be accepted, when this overhauls without plugging; Otherwise, need plugging.

7. shell-and-tube heat exchanger plugging system of selection according to claim 6 is characterized in that: overhauled Non-Destructive Testing when record last time when existing, the computing formula of growth of defect size Δ S is in the said step 3:

In the formula, T _{This-last time}Be the time difference between this maintenance and maintenance last time; T _{The next time-this}Be the time difference between maintenance next time and this maintenance; Flaw size when S overhauls for this; S _{Last time}Flaw size when overhauling for last time.

8. shell-and-tube heat exchanger plugging system of selection according to claim 6 is characterized in that: when not having Non-Destructive Testing last time data, the computing formula of growth of defect size Δ S is in the said step 3:

In the formula, T _{The next time-this}Be the time difference between maintenance next time and this maintenance; T is the heat exchanger tube active time; S is current flaw size.

9. shell-and-tube heat exchanger plugging system of selection according to claim 6 is characterized in that: borderline failure flaw size S in the said step 4 _CritComputing formula do

$S_{\mathrm{crit}}=\left\{\begin{array}{cc}\frac{69t}{2/3[({\mathrm{\&sigma;}}_s+69)-\frac{{\mathrm{\&sigma;}}_s}{M}]}& \mathrm{if}\frac{L^2}{\mathrm{Dt}}\&le;20\\ \frac{69t}{S_{\mathrm{flow}}}& \mathrm{if}\frac{L^2}{\mathrm{Dt}}>20\end{array}\right.$

In the formula, t is the original wall thickness of heat exchanger tube, mm; L is the axial length of corrosion default, mm; D is the heat exchanger tube external diameter, mm; σ _sBe the yield strength of heat exchanger tube material, MPa; S _FlowBe flow stress, MPa is taken as S _Flow=σ _s+ 69; M is the bulging coefficient, and the computing formula of said bulging coefficient M does

$M={(1+0.8\frac{L^2}{\mathrm{Dt}})}^{0.5}$

If calculate S _CritSurpass 80% of original wall thickness, then get S _CritBe 0.8t.

10. shell-and-tube heat exchanger plugging system of selection according to claim 6 is characterized in that: current maximum allowable defect size S in the said step 5 _AllwComputing formula be S _Allw=S _Crit-Δ S-S _Unct

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