CA1181522A - Assessment of life of duct - Google Patents
Assessment of life of ductInfo
- Publication number
- CA1181522A CA1181522A CA000408616A CA408616A CA1181522A CA 1181522 A CA1181522 A CA 1181522A CA 000408616 A CA000408616 A CA 000408616A CA 408616 A CA408616 A CA 408616A CA 1181522 A CA1181522 A CA 1181522A
- Authority
- CA
- Canada
- Prior art keywords
- value
- signal
- temperature
- duct
- pressure
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17D—PIPE-LINE SYSTEMS; PIPE-LINES
- F17D5/00—Protection or supervision of installations
-
- G—PHYSICS
- G07—CHECKING-DEVICES
- G07C—TIME OR ATTENDANCE REGISTERS; REGISTERING OR INDICATING THE WORKING OF MACHINES; GENERATING RANDOM NUMBERS; VOTING OR LOTTERY APPARATUS; ARRANGEMENTS, SYSTEMS OR APPARATUS FOR CHECKING NOT PROVIDED FOR ELSEWHERE
- G07C3/00—Registering or indicating the condition or the working of machines or other apparatus, other than vehicles
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measuring Fluid Pressure (AREA)
- Testing Or Calibration Of Command Recording Devices (AREA)
- Testing Of Devices, Machine Parts, Or Other Structures Thereof (AREA)
- Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method and apparatus are provided for assessing the overall lifetime tr until failure of a duct carrying fluid at elevated temperatures and pressures. Sensors located in the duct which can be a pipe sense the pressure of the fluid and its temperature at a multi-plicity of points along the pipe. The sensors emit voltage signals which are multiplexed and converted to pulses of a frequency proportional to voltage. The pulses are counted in a fixed time to determine a value for the pressure and temperature in a microcomputer. The microcomputer is programmed to calculate a value of hoop stress ? from the pressure values and the measured value of ? is used to determine the relevant value of the Larson Miller parameter P from values thereof stored as a calibration curve against ? in the microcomputer. The computer then calculates the value of tr from the equation:
tr=10(P/T-C)
A method and apparatus are provided for assessing the overall lifetime tr until failure of a duct carrying fluid at elevated temperatures and pressures. Sensors located in the duct which can be a pipe sense the pressure of the fluid and its temperature at a multi-plicity of points along the pipe. The sensors emit voltage signals which are multiplexed and converted to pulses of a frequency proportional to voltage. The pulses are counted in a fixed time to determine a value for the pressure and temperature in a microcomputer. The microcomputer is programmed to calculate a value of hoop stress ? from the pressure values and the measured value of ? is used to determine the relevant value of the Larson Miller parameter P from values thereof stored as a calibration curve against ? in the microcomputer. The computer then calculates the value of tr from the equation:
tr=10(P/T-C)
Description
z The pre~ent invention relates to the as~essment of the overall lifetime until failure of a duct9 particularly a pipe carrying a fluid such as a gas at elevated pressures and temperatures.
According to one aspect of the present inventlon there is provided a method for assessing the overall lifetime tr until failure of a duct ~arrying fluid at elevated temperatures and pressures, the method comprising sensing the pressure and temperature of the fluid at selected points in the duct, emitting ~ignals representative of the pressure and temperature, determining the values of pre~sure and temperature represented by the signals and deriving the value of tr from the values of pressure and temperature.
Preferably the temperature of the fluid is sensed at a plurality of points in the duct, a temperature signal is derived for each point and a value of tr is derived for each signal.
Suitably the signals are continuously sampled sequentially and the value of tr is derived Por each sampled pressure and temperature signal.
Conveniently each emitted signal is an analogue signal - '~
and is oonverted to a digital signal to enable the value of pressure or temperature to be determined.
Preferably the analogue signal is a voltage which is converted to pulses of a frequency analogous to the voltage.
Suitably the pulses are counted during the signal ~ampling period to provide values of the pre~sure or temperature.
In one embodiment of the invention, the fraction of life tF(m) of the duct used up between sequential signzl samples of the same temperature sensor is determined from the equation tF(m) = t1 - t2 tr1 Where t1 is the time of the last or current signal sample, t2 is the time of the last but one sample, tr1 is that value of tr derived from the current temperature signal and tF(m) is integrated to provide an overall fraction~t~(m) of the ll~e of the duct which has been used up in the period during which the duct h~s been carrying fluid.
I~ this case failure of the duct is indicated when tF(m) = 1.
In another embodiment of the invention the period L5~
during which the duct has been carrying fluid is subtracted from the currently derived value of tr ~o as to provide an indication of the period of the remai~ing use~ul life o~
the duct. In this case the highest currently derived value of tr is taken a3 the datum~ Preferably, however a factor i3 added to the value of pressure before tr i~ determined to provide a margin for safety in the currently derived value of tr. Suita~ly the factor ls 10S of the Yalue of pressure.
Conveniently the value of the stress to which the duct i~ being subjected is derived from th~ pressure value and is utilised together with the temperature value to derive the value of tr.
The du¢t may be a pipe and the derived stress i~ this case iQ desirably the hoop stress.
In a preferred embodimen* of the invention the relevant value of the Larson Miller parameter ~P) a~ herei~
defined is selected from a calibration curve of hoop ~tress against P for the pipe being tested and the value of tr i derlved ~rom the equation:-tr- IO(P/T~C) where C i a co~sta~t and T is the current temperature value in E.
4 ~ s~
Suitably the value of tr is printed out and~or displayed.
According to another aspect of the present invention there is provided apparatus for assessing the overall lifetime tr until failure of a duct carrying fluid at elevated pressures and temperatures, the apparatu3 comprising sensors for qensing the pressure and temperature of the fluid at selected points in the duct and for emitting signals representative of the pressure and temperature and means responsive to the signals for determining the values of the pressure and temperature from the signals and for deriving the value of tr from the values of the pre3sure and temperature.
Preferably the sensors sense the temperature of the fluid at a plurality of points in the duct and emit a signal for each point, the signal responsive means being adaptad to derive a value of tr for each signal.
Suitably means are provided to continuously sample the signals sequentially and the signal responsive means is adapted to derive a value for tr for the sampled pressure and temperature signal.
Conveniently the sampling means is a multiplexerO
5 ~ 5~
Preferably each sensor is adapted to emit an analogue signal and means are provided to conver~ the analogue signal to a digital signal for processlng by the signal responsive means~
Suitably the analogue signal i8 a voltage and the converter means is adapted to convert the analogue signal to pulses of a frequency analogous to the voltage.
Conveniently the signal responsive means includes a counter for oounting the pulses during the signal sampling period to provide values of the pressure or temperature.
In one embodiment of the invention, the signal responsive means is adapted to determine the fraction of life tF(m) of the duct used up between sequential signal samples of the same temperature sensor using the equation tF~m) = t1 - t2 trl where t1 is the time of the last or current sample, t2 is the time of the last but one sample, tr1 is that value of tr derived from the current temperature ~ignal, the signal responsive means being adapted to integrate tF(m) to determine the overall fraction tF(m) of the life of the duct which has been used up in the period during which the duct has been carrying fluido In this case the signal responsive means ls adapted to indicate failure of the duct when tF(m) = 1.
6 ~ l5i~
Preferably the signal responsive mean~ i~ adapted to derive the value of the ~tres~ to which the duet i3 being sub~ected from the pressure value and to derive the Yalue for tr from the derived 3tress value and tbe kemperature value.
The duct itself may be a pipe and in this case the derived stresq is the hsop ~tre~s.
In a preferred embodiment of the invention means are provided to store a calibration of hoop stress aga~nst the Lar~on Miller parameter (P) a~ herein defined for the pipe being tcsted and the slgnal responsive means is adapted to select the relevant value of P corresponding to the derived hoop stress and to derive the value of tr from the equation:-~r ~o(P~-C) where C is a constant and T is the current temperature value in ~ .
In one embodiment of the invention means responsive to the signal responsive means are provided to print out the value o~ tr and in another embodiment mean~ may be provided to di~play the value o~ tr~
... .
7 ~ 5~
Pre~erably the signal re ponsive means and the store means comprise a computer.
Before the actual components of the apparatus are described in detail, it will it is believed bQ helpful to explain the steps required to calculate the time to failure tr of a pipe carrying fluid at elevated temperatures and pressure The time to failure tr of a material, usually by rupture, at a constant.stess is given by the equation:-~r- lO(p~-c) ., ............. ( 1 ) where tr is the time to fallure in hours measured from the instant when the material was first sub~ected to the stress, P and C are constants and T is the temperature in E at which the material is being held while being subjected to the stress.
P is the so called "Larson Miller parametern and ~rom equation (1):
P - T (logtr + C) ......... 0.. ~O.. (2) If for any given material, the stress required ~or rupture is plotted as a function of P, all the points are found to fall on a curve, the so called Larson Miller curve~
Thi~ relationship was first established by F.~. Larqon and James Miller who revealed their findings for a number of steel alloys in July 1952 in a paper published in the American Society o~ Mechanical Engineers. That paper contains a full explanation of the theories underlying the authors findings. The author3 also empirically discovered that constant C is equal to 20 for certain materials.
L
The applicants have discovered that the Larson-Miller 3 curve can be used to determins tr for a pipe carrying a 3 fluid at elevated pres~ures and temperatures. To do this it ~
is necessary first to establish a standard Larson-Miller 3 curve for the particular material from which the pipe is constructed by plotting the stress to rupture obtained experimentally against the corresponding parameter P. Once this curve has been established, the stress to which the pipe is being sub~ected in service can be determined and the parameter P corresponding to this stress can be read off from the curve and tr can be calculated from equation (1) above by inserting the relevant values of T and C. ,-For a pipe the relevant stre~s is the hoop stress ~, which can be determined from the equation:-= pD ................. ,....... (3) 2t where p i8 the pressure of the fluid carried by the pipe in psi, D is the external diameter of the pipe in inches and t 9 1 ~315Z~ ~
is the wall thickness of the pipe in inches.
Thus the time to rupture tr can be calculated by measuring the pres~ure and temperature of the fluid carried by the pipe in service, the time tr being the total time starting from the instant ~hen the pipe first began to carry fluid.
While the pressure at any point in a fluid ls constantt its temperature may vary so that in a length of pipe the temperature of the fluid may be very different at different points along the pipe length. Sirce the parameter P is directly dependent upon temperature, it will vary along the length of the pipe and thus the time tr will be different for different points in the pipe. It is therefore important for the temperature to be measured at as many points a3 possible along the pipe length to enable that point at which the rupture time is the shortest to be established.
...
Consequently the applicant measure temperature at a multiplicity of points along the pipe to provide an accurate overall indication of the rupture time tr.
~-Each calculated value of tr for each temperature r sensor point is then used to calculate the fraction of life tF(m) of the pipe used up between sequential signal samples of the same temperature sensor from the equation~
~.
. . .
1 ~ L5~
tF(m) - t1 - t2 ,............. (4) tr1 where t1 i5 the time of the last or current signal sample, t2 is the time of the last but one sample~ a~d tr1 i3 that value of tr derived from the current temperature signal.
tF(m) is integrated to provide an overall fraction ~tF(m3 of the life of the pipe which has been used up in the period during which the duct has been carrying fluid. In this case of course failure of the pipe is predicted ~hen ~tF(m) is equal to one.
As an alternative to calculating tF(m), a direct indication of~the remaining useful life of the pipe can be obtained by subtracting the period during which the pipe has been carrying fluid from tr1. The disadvantage with this method is that the value of tr1 will only be reliable if the fluid has been at a relatively constant pressure and temperature during the period it has been carried by the pipe. However if this method is used the point at which the highest currentl~r derived value of tr has been calculated should be taken as the datum since this is the point at which the pipe will first rupture. To provide a margin for safety a factor is added to the value of pressure (or to the calculated stress)-before tr1 is determined and the applicants have found that a factor of 10~ of the measured pressure is an adequate margin.
15~;~
An embodiment o~ the present inventi on ~ill no~J be described with reference to the accompanying drawings in whioh:-Figure 1 iA a ~chematic block circuit dia~ram of the~pparatus, Figure 2 i~ a more detailed circuit diagram of a numbar of the components show~ in the dotted box in Figure 1, Figure 3a is a circuit diagram of the 2 to 4 line decoder shown schematically in Figure 2~
Figure 3b i9 a logic table for the decoder Figure 4 is a diagram of the signal selector shown in Figure 2 and, Figure 5 is a flow sheet of the sequential operations performed by a microcomputer in assessing tr until failure.
Referring to the drawings, block 1 include a multiplexer, amplifier and voltage to frequenay converter which supply in sequence a series of amplified frequency signals analogous to voltage signals supplied to the multiplexer by a number o~ sensors. The frequency signals are received by and processed in the central prooessing unit of a microcomputer 2 together ~ith data in the microoomputer store so as to provide inter alia information on the overall lifetime until failure tr of a duct carrying fluid at elevated temperatures and pressures. The data i~
supplied to the microcomputer 2 by way of a data teletype 3 and information is output to a printer 4~
Referring to Figure 2 the multiplexer comprises four analogue multiplexer chips 5 to 8 each of which is supplied with analogue voltage signals from eight sensors~ ¦
Chip 5 is connected to one pressure sensor P a~d to seven temperature sensors T1 to T7. Chips 5 to 8 however are each connected to eight temperature sensors T8 to T15, T16 to T23 and T24 to T31 respectively. Where the duct is a pipe or pipe work, the pressure sensor may be located at any convenient position in the pipe and the temperature sensors at conveniently spaced intervals along the pipe so as to record the temperature at a multîplicity of points along the pipe.
The multiplexer 1 also comprises a 2 to ll line decoder 9 and a signal selector 10 which together form parallel input-output ports to select one of the 32 signal inputs to the multiplexer chips 5 to 8. The 2 to 4 line decoder 9 receives two input control signals which are decoded to give four ~enable~ signals to sele-ct one of the four multiplexer chips 5 to 8. The signal selector 10 recei~es three input control signals which are supplied in parallel as three input select signals to the each of the four chips so as to select one of the eight sensor signals in that chip selected by the decoder 9.
Referring to Figure 3a, the decoder comprises two 13 ~ S~
pair~ of serially connected inverters 9 to 12 and four 'NOR' gates 13 tD 16 which each in turn produce a high output iD dependence on the level Or the two control input signals to the inverters as shown in the table in Flgure 3b. As sho~n in Figure 4 the signal selector comprise~
three parallel inverter~ 17 to 19 which respond to the eight combination~ of input signal~ to produce eight combinations of inverted output signals.
RePerring again to Figure 2, the decoder 9 enables each chip to be ~ampled or monitored for 4 ~econds and the signal selector samples or monitors each sensor ~or 0.5 second so that 32 multiplexed inputs of 0.5 second duration are monitored giving a cycle time of 16 seconds.
The output~ of the multlplexers 5 to 8 are wired ORed to the input of a low drift instrumentation ampli~ier 20.
The amplifier 20 is connected to the analogue input of a-voltage to frequency oonverter 21. The output o~ the converter 21 is connected directly to the counter tlmer circuit channel o~ the microcomputer 2. This circuit inter alia counts the number of frequency pulses over a ~ixed time period and hence enable the values of sensed pressure or temperature to be measured.
The microcomputer 2 is a Zilog Z8Q microcomputer which is programmed to calculate the time tr, the programme residing in 4K bytes of "read only" memory. The 14 ~ S~ ~
microcomputer 2 incorporate~ a teletype monitor 3 to provide teletype initialisation and a source code co~tained in ~everal modules as de3cribed below.
~al~ ~Q~lQ
Thi~ i.s the main control routine which calls the other routines. The data areas are initialised on re et (or power up) and the input parameters solicited from the teletype 2. The main programme loop is within this module;
supervising the analy~is Or the data, the ynchronisation of the oalculatlons with the input of new readings and the output of re~ults.
HQ~ule Pre~au~
From the counter time count~ the pres~ure of the fluid in the pipe is calculated and hence the pipe hoop strss3 O.
The parameter P is calculated ~rom pre-input values of P V9 log10 ~ by linear interpolation.
emPeratur,eO
-From the counter time count and the parameter P
calculated by the Pressure Module, tr is calculated for each o~ the 31 temperature qensors. The value of tr may be used either to calculate the fraction of life tF(m) which is integrated as previou3ly described to provide an integral ~tF(m) or the remaining useful life of the pipe may be directly calculated from tr. In this case tr may be ~ 5~
calculated with the value of P or ~ modified by the addition of a factor for safety.
~odul* Re~lt~
The results are converted to a standard code and output to the printer 3 shown in Figure 1.
Mo~u~ rr~Pt The central timer circuit is programmed to interrupt on two channels and both these interrupts are handled by routines in the main module.
(1) Timer In~errupt The timer is interrupted every 25 ms, the clock is incremented and the sen~or being monitored at the particular time is read and updated every 0.5 second.
According to one aspect of the present inventlon there is provided a method for assessing the overall lifetime tr until failure of a duct ~arrying fluid at elevated temperatures and pressures, the method comprising sensing the pressure and temperature of the fluid at selected points in the duct, emitting ~ignals representative of the pressure and temperature, determining the values of pre~sure and temperature represented by the signals and deriving the value of tr from the values of pressure and temperature.
Preferably the temperature of the fluid is sensed at a plurality of points in the duct, a temperature signal is derived for each point and a value of tr is derived for each signal.
Suitably the signals are continuously sampled sequentially and the value of tr is derived Por each sampled pressure and temperature signal.
Conveniently each emitted signal is an analogue signal - '~
and is oonverted to a digital signal to enable the value of pressure or temperature to be determined.
Preferably the analogue signal is a voltage which is converted to pulses of a frequency analogous to the voltage.
Suitably the pulses are counted during the signal ~ampling period to provide values of the pre~sure or temperature.
In one embodiment of the invention, the fraction of life tF(m) of the duct used up between sequential signzl samples of the same temperature sensor is determined from the equation tF(m) = t1 - t2 tr1 Where t1 is the time of the last or current signal sample, t2 is the time of the last but one sample, tr1 is that value of tr derived from the current temperature signal and tF(m) is integrated to provide an overall fraction~t~(m) of the ll~e of the duct which has been used up in the period during which the duct h~s been carrying fluid.
I~ this case failure of the duct is indicated when tF(m) = 1.
In another embodiment of the invention the period L5~
during which the duct has been carrying fluid is subtracted from the currently derived value of tr ~o as to provide an indication of the period of the remai~ing use~ul life o~
the duct. In this case the highest currently derived value of tr is taken a3 the datum~ Preferably, however a factor i3 added to the value of pressure before tr i~ determined to provide a margin for safety in the currently derived value of tr. Suita~ly the factor ls 10S of the Yalue of pressure.
Conveniently the value of the stress to which the duct i~ being subjected is derived from th~ pressure value and is utilised together with the temperature value to derive the value of tr.
The du¢t may be a pipe and the derived stress i~ this case iQ desirably the hoop stress.
In a preferred embodimen* of the invention the relevant value of the Larson Miller parameter ~P) a~ herei~
defined is selected from a calibration curve of hoop ~tress against P for the pipe being tested and the value of tr i derlved ~rom the equation:-tr- IO(P/T~C) where C i a co~sta~t and T is the current temperature value in E.
4 ~ s~
Suitably the value of tr is printed out and~or displayed.
According to another aspect of the present invention there is provided apparatus for assessing the overall lifetime tr until failure of a duct carrying fluid at elevated pressures and temperatures, the apparatu3 comprising sensors for qensing the pressure and temperature of the fluid at selected points in the duct and for emitting signals representative of the pressure and temperature and means responsive to the signals for determining the values of the pressure and temperature from the signals and for deriving the value of tr from the values of the pre3sure and temperature.
Preferably the sensors sense the temperature of the fluid at a plurality of points in the duct and emit a signal for each point, the signal responsive means being adaptad to derive a value of tr for each signal.
Suitably means are provided to continuously sample the signals sequentially and the signal responsive means is adapted to derive a value for tr for the sampled pressure and temperature signal.
Conveniently the sampling means is a multiplexerO
5 ~ 5~
Preferably each sensor is adapted to emit an analogue signal and means are provided to conver~ the analogue signal to a digital signal for processlng by the signal responsive means~
Suitably the analogue signal i8 a voltage and the converter means is adapted to convert the analogue signal to pulses of a frequency analogous to the voltage.
Conveniently the signal responsive means includes a counter for oounting the pulses during the signal sampling period to provide values of the pressure or temperature.
In one embodiment of the invention, the signal responsive means is adapted to determine the fraction of life tF(m) of the duct used up between sequential signal samples of the same temperature sensor using the equation tF~m) = t1 - t2 trl where t1 is the time of the last or current sample, t2 is the time of the last but one sample, tr1 is that value of tr derived from the current temperature ~ignal, the signal responsive means being adapted to integrate tF(m) to determine the overall fraction tF(m) of the life of the duct which has been used up in the period during which the duct has been carrying fluido In this case the signal responsive means ls adapted to indicate failure of the duct when tF(m) = 1.
6 ~ l5i~
Preferably the signal responsive mean~ i~ adapted to derive the value of the ~tres~ to which the duet i3 being sub~ected from the pressure value and to derive the Yalue for tr from the derived 3tress value and tbe kemperature value.
The duct itself may be a pipe and in this case the derived stresq is the hsop ~tre~s.
In a preferred embodiment of the invention means are provided to store a calibration of hoop stress aga~nst the Lar~on Miller parameter (P) a~ herein defined for the pipe being tcsted and the slgnal responsive means is adapted to select the relevant value of P corresponding to the derived hoop stress and to derive the value of tr from the equation:-~r ~o(P~-C) where C is a constant and T is the current temperature value in ~ .
In one embodiment of the invention means responsive to the signal responsive means are provided to print out the value o~ tr and in another embodiment mean~ may be provided to di~play the value o~ tr~
... .
7 ~ 5~
Pre~erably the signal re ponsive means and the store means comprise a computer.
Before the actual components of the apparatus are described in detail, it will it is believed bQ helpful to explain the steps required to calculate the time to failure tr of a pipe carrying fluid at elevated temperatures and pressure The time to failure tr of a material, usually by rupture, at a constant.stess is given by the equation:-~r- lO(p~-c) ., ............. ( 1 ) where tr is the time to fallure in hours measured from the instant when the material was first sub~ected to the stress, P and C are constants and T is the temperature in E at which the material is being held while being subjected to the stress.
P is the so called "Larson Miller parametern and ~rom equation (1):
P - T (logtr + C) ......... 0.. ~O.. (2) If for any given material, the stress required ~or rupture is plotted as a function of P, all the points are found to fall on a curve, the so called Larson Miller curve~
Thi~ relationship was first established by F.~. Larqon and James Miller who revealed their findings for a number of steel alloys in July 1952 in a paper published in the American Society o~ Mechanical Engineers. That paper contains a full explanation of the theories underlying the authors findings. The author3 also empirically discovered that constant C is equal to 20 for certain materials.
L
The applicants have discovered that the Larson-Miller 3 curve can be used to determins tr for a pipe carrying a 3 fluid at elevated pres~ures and temperatures. To do this it ~
is necessary first to establish a standard Larson-Miller 3 curve for the particular material from which the pipe is constructed by plotting the stress to rupture obtained experimentally against the corresponding parameter P. Once this curve has been established, the stress to which the pipe is being sub~ected in service can be determined and the parameter P corresponding to this stress can be read off from the curve and tr can be calculated from equation (1) above by inserting the relevant values of T and C. ,-For a pipe the relevant stre~s is the hoop stress ~, which can be determined from the equation:-= pD ................. ,....... (3) 2t where p i8 the pressure of the fluid carried by the pipe in psi, D is the external diameter of the pipe in inches and t 9 1 ~315Z~ ~
is the wall thickness of the pipe in inches.
Thus the time to rupture tr can be calculated by measuring the pres~ure and temperature of the fluid carried by the pipe in service, the time tr being the total time starting from the instant ~hen the pipe first began to carry fluid.
While the pressure at any point in a fluid ls constantt its temperature may vary so that in a length of pipe the temperature of the fluid may be very different at different points along the pipe length. Sirce the parameter P is directly dependent upon temperature, it will vary along the length of the pipe and thus the time tr will be different for different points in the pipe. It is therefore important for the temperature to be measured at as many points a3 possible along the pipe length to enable that point at which the rupture time is the shortest to be established.
...
Consequently the applicant measure temperature at a multiplicity of points along the pipe to provide an accurate overall indication of the rupture time tr.
~-Each calculated value of tr for each temperature r sensor point is then used to calculate the fraction of life tF(m) of the pipe used up between sequential signal samples of the same temperature sensor from the equation~
~.
. . .
1 ~ L5~
tF(m) - t1 - t2 ,............. (4) tr1 where t1 i5 the time of the last or current signal sample, t2 is the time of the last but one sample~ a~d tr1 i3 that value of tr derived from the current temperature signal.
tF(m) is integrated to provide an overall fraction ~tF(m3 of the life of the pipe which has been used up in the period during which the duct has been carrying fluid. In this case of course failure of the pipe is predicted ~hen ~tF(m) is equal to one.
As an alternative to calculating tF(m), a direct indication of~the remaining useful life of the pipe can be obtained by subtracting the period during which the pipe has been carrying fluid from tr1. The disadvantage with this method is that the value of tr1 will only be reliable if the fluid has been at a relatively constant pressure and temperature during the period it has been carried by the pipe. However if this method is used the point at which the highest currentl~r derived value of tr has been calculated should be taken as the datum since this is the point at which the pipe will first rupture. To provide a margin for safety a factor is added to the value of pressure (or to the calculated stress)-before tr1 is determined and the applicants have found that a factor of 10~ of the measured pressure is an adequate margin.
15~;~
An embodiment o~ the present inventi on ~ill no~J be described with reference to the accompanying drawings in whioh:-Figure 1 iA a ~chematic block circuit dia~ram of the~pparatus, Figure 2 i~ a more detailed circuit diagram of a numbar of the components show~ in the dotted box in Figure 1, Figure 3a is a circuit diagram of the 2 to 4 line decoder shown schematically in Figure 2~
Figure 3b i9 a logic table for the decoder Figure 4 is a diagram of the signal selector shown in Figure 2 and, Figure 5 is a flow sheet of the sequential operations performed by a microcomputer in assessing tr until failure.
Referring to the drawings, block 1 include a multiplexer, amplifier and voltage to frequenay converter which supply in sequence a series of amplified frequency signals analogous to voltage signals supplied to the multiplexer by a number o~ sensors. The frequency signals are received by and processed in the central prooessing unit of a microcomputer 2 together ~ith data in the microoomputer store so as to provide inter alia information on the overall lifetime until failure tr of a duct carrying fluid at elevated temperatures and pressures. The data i~
supplied to the microcomputer 2 by way of a data teletype 3 and information is output to a printer 4~
Referring to Figure 2 the multiplexer comprises four analogue multiplexer chips 5 to 8 each of which is supplied with analogue voltage signals from eight sensors~ ¦
Chip 5 is connected to one pressure sensor P a~d to seven temperature sensors T1 to T7. Chips 5 to 8 however are each connected to eight temperature sensors T8 to T15, T16 to T23 and T24 to T31 respectively. Where the duct is a pipe or pipe work, the pressure sensor may be located at any convenient position in the pipe and the temperature sensors at conveniently spaced intervals along the pipe so as to record the temperature at a multîplicity of points along the pipe.
The multiplexer 1 also comprises a 2 to ll line decoder 9 and a signal selector 10 which together form parallel input-output ports to select one of the 32 signal inputs to the multiplexer chips 5 to 8. The 2 to 4 line decoder 9 receives two input control signals which are decoded to give four ~enable~ signals to sele-ct one of the four multiplexer chips 5 to 8. The signal selector 10 recei~es three input control signals which are supplied in parallel as three input select signals to the each of the four chips so as to select one of the eight sensor signals in that chip selected by the decoder 9.
Referring to Figure 3a, the decoder comprises two 13 ~ S~
pair~ of serially connected inverters 9 to 12 and four 'NOR' gates 13 tD 16 which each in turn produce a high output iD dependence on the level Or the two control input signals to the inverters as shown in the table in Flgure 3b. As sho~n in Figure 4 the signal selector comprise~
three parallel inverter~ 17 to 19 which respond to the eight combination~ of input signal~ to produce eight combinations of inverted output signals.
RePerring again to Figure 2, the decoder 9 enables each chip to be ~ampled or monitored for 4 ~econds and the signal selector samples or monitors each sensor ~or 0.5 second so that 32 multiplexed inputs of 0.5 second duration are monitored giving a cycle time of 16 seconds.
The output~ of the multlplexers 5 to 8 are wired ORed to the input of a low drift instrumentation ampli~ier 20.
The amplifier 20 is connected to the analogue input of a-voltage to frequency oonverter 21. The output o~ the converter 21 is connected directly to the counter tlmer circuit channel o~ the microcomputer 2. This circuit inter alia counts the number of frequency pulses over a ~ixed time period and hence enable the values of sensed pressure or temperature to be measured.
The microcomputer 2 is a Zilog Z8Q microcomputer which is programmed to calculate the time tr, the programme residing in 4K bytes of "read only" memory. The 14 ~ S~ ~
microcomputer 2 incorporate~ a teletype monitor 3 to provide teletype initialisation and a source code co~tained in ~everal modules as de3cribed below.
~al~ ~Q~lQ
Thi~ i.s the main control routine which calls the other routines. The data areas are initialised on re et (or power up) and the input parameters solicited from the teletype 2. The main programme loop is within this module;
supervising the analy~is Or the data, the ynchronisation of the oalculatlons with the input of new readings and the output of re~ults.
HQ~ule Pre~au~
From the counter time count~ the pres~ure of the fluid in the pipe is calculated and hence the pipe hoop strss3 O.
The parameter P is calculated ~rom pre-input values of P V9 log10 ~ by linear interpolation.
emPeratur,eO
-From the counter time count and the parameter P
calculated by the Pressure Module, tr is calculated for each o~ the 31 temperature qensors. The value of tr may be used either to calculate the fraction of life tF(m) which is integrated as previou3ly described to provide an integral ~tF(m) or the remaining useful life of the pipe may be directly calculated from tr. In this case tr may be ~ 5~
calculated with the value of P or ~ modified by the addition of a factor for safety.
~odul* Re~lt~
The results are converted to a standard code and output to the printer 3 shown in Figure 1.
Mo~u~ rr~Pt The central timer circuit is programmed to interrupt on two channels and both these interrupts are handled by routines in the main module.
(1) Timer In~errupt The timer is interrupted every 25 ms, the clock is incremented and the sen~or being monitored at the particular time is read and updated every 0.5 second.
(2) Counte~ I~te~rupt The counter is interrupted every 256 pulse counts a~d the total is incremented and the counter restarted.
~Q~lQ ~RO~ StQ~
This module contains the permanent data which accurately de~ines the system and includes the Data Module.
The Data Module contains values of log10 ~ and the corresponding value of P which are input before the fluid is introduced into the pipe.
Referring to Figure 5, the computer oparations numbered are as follows:-30) Start - swit¢h on power 31) Input M = 1 where M is the temperature sensor to be sampled (normally a thermocouple)~
32) Read L and N where L - number o~ thermcouples being sensed and N = number of data points to plot or describe the Larson-Miller curve.
33) Input tF (m) m_1, L = O where tF(m) is the fraction of life used up between signal ~amples from the same temperature sensor.
34) DIMENSION ~(N); P(N) 35) (i) Input ~D (I) data where ~D is the stress data obtained experimentally to constru¢t the Larson-Miller curve anc. I = 1,2,3,....N.
(ii) Input PD (I~ data where PD is the Larson-Miller parameter correspo.nding to ~D(I) and I - 1,2,3 .... N~
Statement 34) sets aside a storage area to contain the ~D~I~ and PD(I) data.
The ~D(I) and PD(I) values may be stored permanently if the system is to be dedicated to a particular component material, 36) PD = N
37) Read ~ and T (input from multiplexer) for M
38) I = 2 39) J = I
40) Is stress o~< ~D(I) 41) Does I = PD
42) I ~
4 ) p p ~ ~ _ g(J 1) x PD(J) - PD(J-13 ~ J) ~ ~(J-1) This equation enables the rele~ant value of P to be determined by linear interpolat~on.
44) tr = 10(P - 20) 45) Print/diqplay rupture time tr 6) tF(m) = t1 - t2 tr where t1 is time of the present temperature measurement and t2 is the time of the last but one temperature measurement.
47) Does ~tF(m) = 1 48) Does M = L
49) M - M+1 50) M = 1 51) Sound Alarm 52) -Stop.
While not shown, tr in 44) can be calculated from a P
value derived from the pressure or qtress value to which 10% of that value has been added to pro~ide a margin for safety~ The remaining use~ul life of the pipe can then be calculated by subtracting the period during which the pipe 1d ~ L5~2 ha~ been carrying the fluid ~r~m tr.
It will be appreciated that while not de~cribed there are other method~, apart frvlD the Lar30n-Miller relationship, for derivirg the value of tr.
~Q~lQ ~RO~ StQ~
This module contains the permanent data which accurately de~ines the system and includes the Data Module.
The Data Module contains values of log10 ~ and the corresponding value of P which are input before the fluid is introduced into the pipe.
Referring to Figure 5, the computer oparations numbered are as follows:-30) Start - swit¢h on power 31) Input M = 1 where M is the temperature sensor to be sampled (normally a thermocouple)~
32) Read L and N where L - number o~ thermcouples being sensed and N = number of data points to plot or describe the Larson-Miller curve.
33) Input tF (m) m_1, L = O where tF(m) is the fraction of life used up between signal ~amples from the same temperature sensor.
34) DIMENSION ~(N); P(N) 35) (i) Input ~D (I) data where ~D is the stress data obtained experimentally to constru¢t the Larson-Miller curve anc. I = 1,2,3,....N.
(ii) Input PD (I~ data where PD is the Larson-Miller parameter correspo.nding to ~D(I) and I - 1,2,3 .... N~
Statement 34) sets aside a storage area to contain the ~D~I~ and PD(I) data.
The ~D(I) and PD(I) values may be stored permanently if the system is to be dedicated to a particular component material, 36) PD = N
37) Read ~ and T (input from multiplexer) for M
38) I = 2 39) J = I
40) Is stress o~< ~D(I) 41) Does I = PD
42) I ~
4 ) p p ~ ~ _ g(J 1) x PD(J) - PD(J-13 ~ J) ~ ~(J-1) This equation enables the rele~ant value of P to be determined by linear interpolat~on.
44) tr = 10(P - 20) 45) Print/diqplay rupture time tr 6) tF(m) = t1 - t2 tr where t1 is time of the present temperature measurement and t2 is the time of the last but one temperature measurement.
47) Does ~tF(m) = 1 48) Does M = L
49) M - M+1 50) M = 1 51) Sound Alarm 52) -Stop.
While not shown, tr in 44) can be calculated from a P
value derived from the pressure or qtress value to which 10% of that value has been added to pro~ide a margin for safety~ The remaining use~ul life of the pipe can then be calculated by subtracting the period during which the pipe 1d ~ L5~2 ha~ been carrying the fluid ~r~m tr.
It will be appreciated that while not de~cribed there are other method~, apart frvlD the Lar30n-Miller relationship, for derivirg the value of tr.
Claims (31)
1. A method for assessing the overall lifetime tr until failure of a duct carrying fluid at elevated pressures and temperatures, the method comprising sensing the pressure and temperature of the fluid at selected points in the duct, emitting signals representative of the pressure and temperature, determining the values of pressure and temperature represented by the signals and deriving the value of tr from the values of pressure and temperature.
2. A method as claimed in Claim 1 in which the temperature of the fluid is sensed at a plurality of points in the ducts a temperature signal is derived for each point and a value of tr is derived for each signal.
3. A method as claimed in Claim 1 in which the signals are continuously sampled sequentially and the value of tr is derived for each sampled pressure and temperature signal.
4. A method as claimed in Claim 1 in which each emitted signal is an analogue signal and is converted to a digital signal to enable the value of pressure or temperature to be determined.
5. A method as claimed in Claim 4 in which the analogue signal is a voltage which is converted to pulses of a frequency analogous to the voltage.
6. A method as claimed in Claim 5 in which the pulses are counted during the signal sampling period to provide values of the pressure or temperature.
7. A method as claimed in Claim l in which the fraction of life tF(m) of the duct used up between sequential signal samples of the same temperature sensor is determined from the equation Where t1 is the time of the last or current signal sample, t2 is the time of the last but one sample, tr1 is that value of tr derived from the current temperature signal and tF(m) is integrated to provide an overall fraction .SIGMA.tF(m) of the life of the duct which has been used up in the period during which the duct has been carrying fluid.
8. A method as claimed in Claim 7 in which failure of the duct is indicated when .SIGMA.tF(m) = 1.
9. A method as claimed in Claim 1 in which the period during which the duct has been carrying fluid is subtracted from the currently derived value of tr so as to provide an indication of the period of the remaining useful life of the duct.
10. A method as claimed in Claim 9 in which the highest currently derived value of tr is taken as the datum.
11. A method as claimed in Claim 10 in which a factor is added to the value of pressure before tr is determined to provide a margin for safety in the currently derived value of tr.
12. A method as claimed in Claim 11 in which the factor is 10% of the value of pressure.
13. A method as claimed in Claim l in which the value of the stress to which the duct is being subjected is derived from the pressure value and is utilised together with the temperature value to derive the value of tr.
14. A method as claimed in Claim 13 in which the duct is a pipe and the derived stress is the hoop stress.
15. A method as claimed in Claim 14 in which the relevant value of the Larson Miller parameter P as herein defined is selected from a calibration curve of hoop stress against the LMP for the pipe being tested and the value of tr is derived from the equation:-tr=10(P/T-C) Where C is a constant and T is the current temperature value in 0K.
16. A method as claimed in Claim 1, 7 or 15 in which the value of tr is printed out and/or displayed.
17. Apparatus for assessing the overall lifetime tr until failure of a duct carrying fluid at elevated pressures and temperatures, the apparatus comprising sensors for sensing the pressure and temperature of the fluid at selected points in the duct and for emitting signals representative of the pressure and temperature and means responsive to the signals for determining the values of the pressure and temperature from the signals and for deriving the value of tr from the values of the pressure and temperature.
18. Apparatus as claimed in Claim 17 in which the sensors sense the temperature of the fluid at a plurality of points in the duct and emit a signal for each point, the signal responsive means being adapted to derive a value of tr for each signal.
19. Apparatus as claimed in Claim 17 in which means are to provided to continuously sample the signals sequentially and the signal responsive means is adapted to derive a value for tr for the sampled pressure and temperature signal.
20. Apparatus as claimed in Claim 19 in which the sampling means is a multiplexer.
21. Apparatus as claimed in Claim 17 in which each sensor is adapted to emit an analogue signal and means are provided to convert the analogue signal to a digital signal for processing by the signal responsive means.
22. Apparatus as claimed in Claim 21 in which the analogue signal is a voltage and the converter means is adapted to convert the analogue voltage to pulses of a frequency analogous to the voltage.
23. Apparatus as claimed in Claim 22 in which the signal responsive means includes a counter for counting the pulses during the signal sampling period to provide values of the pressure or temperature.
24. Apparatus as claimed in Claim 17 in which the signal responsive means is adapted to determine the fraction of life tF(m) of the duct used up between sequential signal samples of the same temperature sensor using the equation:- where t1 is the time of the last or current sample, t2 is the time of the last but one sample, tr1 is that value of tr derived from the current temperature signal, the signal responsive means being adapted to integrate tF(m) to determine the overall fraction .SIGMA.tF(m) of the life of the duct which has been used up in the period during which the duct has been carrying fluid.
25. Apparatus as claimed in Claim 24 in which the signal responsive means is adapted to indicate failure of the duct when .SIGMA.tF(m) = 1.
26. Apparatus as claimed in Claim 17 in which the signal responsive means is adapted to derive the value of the stress to which the duct is being subjected from the pressure value and to derive the value for tr from the derived stress value and the temperature valve.
27. Apparatus as claimed in Claim 26 in which the duct is a pipe and the derived stress is the hoop stress.
28. Apparatus as claimed in Claim 27 in which means are provided to store a calibration of hoop stress against the Larson Miller parameter (P) as herein defined for the pipe being tested and the signal responsive means is adapted to select the relevant value of P corresponding to the derived hoop stress and to derive the value of tr from the equation:-tr=10(P/T-C) where C is a constant and where T is the current temperature value in 0K.
29. Apparatus as claimed in Claim 17 in which means responsive to the signal responsive means are provided to print out the value of tr.
30. Apparatus as claimed in Claim 17 in which means responsive to the signal responsive means are provided to display the value of tr.
31. Apparatus as claimed in any of claims 28 to 30 in which the signal responsive means and the store means comprise a computer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8123818 | 1981-08-04 | ||
| GB08123818A GB2103801B (en) | 1981-08-04 | 1981-08-04 | Assessing lifetime of duct by measuring fluid pressure and temperature within the duct |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1181522A true CA1181522A (en) | 1985-01-22 |
Family
ID=10523681
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000408616A Expired CA1181522A (en) | 1981-08-04 | 1982-08-03 | Assessment of life of duct |
Country Status (7)
| Country | Link |
|---|---|
| EP (1) | EP0072105B1 (en) |
| JP (1) | JPS5838834A (en) |
| CA (1) | CA1181522A (en) |
| DE (1) | DE3276015D1 (en) |
| DK (1) | DK339182A (en) |
| GB (1) | GB2103801B (en) |
| ZA (1) | ZA825050B (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3314181A1 (en) * | 1983-04-19 | 1984-10-25 | Kraftwerk Union AG, 4330 Mülheim | METHOD FOR MONITORING THE FATIGUE OF COMPONENTS, e.g. IN NUCLEAR POWER PLANTS |
| US4525763A (en) * | 1983-11-30 | 1985-06-25 | General Electric Company | Apparatus and method to protect motors and to protect motor life |
| US4733974A (en) * | 1986-07-29 | 1988-03-29 | Qualitrol Corporation | Transformer life consumption indicator |
| DE3824003A1 (en) * | 1988-07-15 | 1989-02-23 | Nsq Hauk Ges Fuer Qualitaetssi | METHOD FOR TESTING PIPELINE SYSTEMS |
| FR2680416B1 (en) * | 1991-08-12 | 1995-06-30 | Sames Sa | FLUIDIZED POWDER FLOW MEASURING METHOD AND FLOW MEASURING DEVICE USING SUCH A METHOD. |
| DE19703709C1 (en) * | 1997-01-23 | 1998-08-27 | Mannesmann Ag | Method for detecting the actual state of a container, in particular composite compressed gas containers for vehicles |
| AU752024B2 (en) * | 2000-04-14 | 2002-09-05 | Kabushiki Kaisha Toshiba | Method and equipment for assessing the life of members put under high in-service temperature environment for long period |
| US6556924B1 (en) * | 2000-07-27 | 2003-04-29 | Hydroscope Canada Inc. | Maintenance optimization system for water pipelines |
| DE102009032859A1 (en) | 2009-07-13 | 2011-01-20 | Robert Bosch Gmbh | Device for determining wear variable or remaining service life of hydro pneumatic pressure accumulator, has computing unit determining wear variable or remaining service life of accumulator under application of wear variable function |
| WO2017194369A1 (en) | 2016-05-09 | 2017-11-16 | Haldor Topsøe A/S | Monitoring of heated tubes |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5434501B2 (en) * | 1974-02-15 | 1979-10-27 | ||
| US4298955A (en) * | 1976-04-01 | 1981-11-03 | The Insurance Technical Bureau | Method of and apparatus for the detection and analysis of hazards |
| DE2658818A1 (en) * | 1976-12-24 | 1978-07-06 | Bea Baugesellschaft Fuer Elekt | Diagnostic computer for drive units - has A-D converter feeding data to store and operating panel pre-programming all modes of operation |
| US4184205A (en) * | 1977-11-25 | 1980-01-15 | Ird Mechanalysis, Inc. | Data acquisition system |
| JPS5616372A (en) * | 1979-07-20 | 1981-02-17 | Nippon Telegr & Teleph Corp <Ntt> | Receiver of variable scanning facsimile signal |
| JPS5764141A (en) * | 1980-10-07 | 1982-04-19 | Hitachi Ltd | Method and device for foreseening life of apparatus consisting of metallic structure |
-
1981
- 1981-08-04 GB GB08123818A patent/GB2103801B/en not_active Expired
-
1982
- 1982-07-12 DE DE8282303636T patent/DE3276015D1/en not_active Expired
- 1982-07-12 EP EP82303636A patent/EP0072105B1/en not_active Expired
- 1982-07-15 ZA ZA825050A patent/ZA825050B/en unknown
- 1982-07-29 DK DK339182A patent/DK339182A/en not_active Application Discontinuation
- 1982-08-03 CA CA000408616A patent/CA1181522A/en not_active Expired
- 1982-08-04 JP JP57136208A patent/JPS5838834A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| DK339182A (en) | 1983-02-05 |
| GB2103801A (en) | 1983-02-23 |
| GB2103801B (en) | 1985-05-22 |
| EP0072105A2 (en) | 1983-02-16 |
| ZA825050B (en) | 1983-04-27 |
| EP0072105B1 (en) | 1987-04-08 |
| JPS5838834A (en) | 1983-03-07 |
| DE3276015D1 (en) | 1987-05-14 |
| EP0072105A3 (en) | 1984-07-18 |
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