#1 Report bugs and suggestions for improvement » Test périodique avancé -Periodic Test with Repair » 2016-08-20 09:59:54

rgeng
Replies: 1

Hi,
I'm trying to build a fault tree with components (repairable) subjected to periodic test .

I've discovered a strange comportament. (Bug?)   I enclose .opsa file  and some screenshoot.

Link to the files:  https://www.dropbox.com/sh/wq5s4rw6cdm9 … UNUda?dl=0

(I've checked also with XTFTA manual and it seems to me that input data for the model are correct)

Suppose to have:
-A component subjected to periodic test every 48h, with MTTR=3 h --> mu=0,33333 , lambda dangerous detected between two test equal to 2.5 e-5
-Suppose the simplest model --> final event, or port, component     (See files at link above).

Strange things discovered (I've chosed XFTA calculation):

- if mission time > 48 h (time between two tests), the calculation seems correct BUT NOT in the cases of mission time 480, 960, ecc. (Seem correct instead in case of 48, 96, ecc.)

-in particular, see enclosed files at the link above, with mission time 479..and perhaps 481.., the probability seems correct. BUT instead with mission time 480 the probability is...1 (???) - The same with 960, etc.

-in case of mission time 8760 the red diagram is quite strange (not regular, some peaks higher/lower than others).

Bye   Roberto

#2 Re: Questions about Arbre-Analyste » PFD avg question » 2016-06-22 10:24:42

Thank You, Emmanuel.

I agree, of course: infact that wasn't a program's bug, but a my mistake.

So I've learned to be care before to input a value, in particualr if 'optional'. Infact, if I've understood well, the optional time is a 'feature' (not a bug!) of the program  that permits to input a fixed value of probability, based on a selected time t, and so the probability is fixed to P(time) at that time also in case of proof test.  In that case the probability is not a PFD avg (PFDavg is infact meaningless with this option) but an imposed probability at that time, and so that value is imposed at every time step.
If this consideration could be useful for someone else...

Anyway, Arbre Analyste  is a excellent software !

Regards      Roberto

#3 Re: Questions about Arbre-Analyste » PFD avg question » 2016-06-20 13:48:57

PROBLEM SOLVED!

I've found my error !

Infact, in the E001  I've previously put  the last value in the cell  "Time" (optional)  equal to 1.

Instead, if I omitt that value (blank), the results are correct.

#4 Re: Questions about Arbre-Analyste » PFD avg question » 2016-06-20 13:30:01

Hello Emmanuel,
it's strange, maybe I'm doing something wrong, infact Your results seem to be correct.
I've tryed also to change language, from italian, to english and then to french, but my results are the same.

Since I do not know how to attach the files to this message, i've created a link on dropbox
https://www.dropbox.com/sh/x2c4nll6aaf7 … EeuDa?dl=0

What am I doing wrong?

Thank You in advance for Your help        Roberto

#5 Questions about Arbre-Analyste » PFD avg question » 2016-06-19 10:55:32

rgeng
Replies: 5

A question concerning PFD avg.

Suppose to make a simple calculation, starting from a base event with the law "Test périodique simple" (Same considerations with other laws based on exponential probability), a gate OR, with this only base event. So you have E001 ad P001.
Suppose to have a mission time = 1 and time test =1 (proof test). Suppose lambda=0.12
The program gives the following results, with XFTA calculation:

Date   Pr/Pfd      Pfdavg
0       0.11308    0.11308
0.01   0.11308    0.11308
.....
1       0.11308    0.11308

These values are strange for the following reasons:

1)  According PFD definition (Qs(T) - ref p.7.2 XFTA manual) it is strange that Pr/Pfd = 0,11308.  Then it is also strange that pfdavg has the same value for the different times (spet times), for the reason  2) below.

2) Second, but most important, PFD avg is the 'average'. So, we could have roughly half of these values, if we appoximate the probability function as linear/ triangular .
  But, more exactly, If we have exponential law P(t) = 1 - e^(-labda*t) , the indefinite integral is: t + 1/lambda*e^(-labda*t) + c.  So, integrating P(t) from 0 to t , where t= proof test time, we have:   PFD average at time t is :   t + 1/lambda*e^(-labda*t) - 1/labda .

So, if we have for example lambda = 0,12,  proof test time = 1,
we should have  PFD avg=0,05767   
(for step 0,01 --> 0,00059976, for step 0,02 --> 0,001199  and so on..)

We could verifiy these values with a simple spreadsheet.

Or, alternatively,  using LOGAN program  (demo version full functioning up to 30 events..)  at link http://loganfta.com/download/download.html   (for example assuming Probability Type Unrevealed , lambda=0.12 (called in this case as 'frequency') and prrof test time =1)


Regards

#6 Re: Report bugs and suggestions for improvement » SIL in the report » 2016-05-31 12:14:46

Hi Emmanuel, thank You.
Infact in the documentation of XFTA - see page number 27 - the values of PFD are correct, according IEC 61508 (for example from 0.01 to 0.1 for SIL 1, from 0.001 to 0.01 to SIL 2 and so on).

Instead, in the Report generated by the program , it appears:

SIL

La tabella riporta l'andamento SIL nel tempo.
SIL    limite inferiore    limite superiore    sojourn-time    rapporto
1    0.001                         0.01               181.743    0.020747
2    0.0001                0.001              16.7648            0.00191379
3    1e-005                0.0001                     0    0
4    0                         1e-005             0

To Note:  the values of the legend in the table of the report -see table above - are ten times greather than PFD tha we could have
I.e in the table above, SIL 1  from 0.001  to 0.01 - Instead, the values of the limits of PFD for SIL 1  should be from 0.01 to 0.1, according to the standards.

Is it just a problem of the values of the legend in the table?

Regards

#7 Report bugs and suggestions for improvement » SIL in the report » 2016-05-30 18:03:59

rgeng
Replies: 4

Good evening,
first of all, Thank You for Your great software.
I've downloaded it this afternoon and now I'm trying to understand the use of the program with the standard example.

Generating the report with that example, I've notice also few rows concerning SIL in the report.
Concerning SIL, I saw  SIL 1  from 0.001 to 0.01,  SIL 2 from 0.0001 to 0.001, etc.
But, if we consider PFD, (probability of Failure on demand - ref.IEC 61508), these values should be 10 times greater: so, SIL 1 from 0.01 to 0.1 (RRF=risk reduction factor = 1/PFD --> from 10 to 100), SIL 2 from  0.001 to 0.01 (RRF from 100 to 1000) and so on.
Or didn't i understand well, and perhaps is there another meaning for the SIL for the quantities in the report? I searched something concernig Sil in the help, but i've found no information concerning that part of report.

Anyway, many thanks for Your fantastic software.

Roberto (from Italy)