IIAR

Recent political figures have resurrected interest in the space program and the conversation invariably highlights all of the great technology spinoffs of NASA, from memory foam, freeze-dried foods and HACCP to Tang (maybe – not so much!). In addition to ammonia heat pipe cooling in the space shuttle, there is another link to our industry, Weibull analysis. Bear with me here. In the pioneering days of the Mercury and Apollo programs, every mission pushed the technical envelope beyond the current knowledge base. There was precious little time for extensive R & D or multiple prototypes. This is where the math of Prof. Waloddi Weibull (1887-1979) comes into play. Using his techniques, a failure rate of just a few samples could be reliably predicted and applied to a large number of similar components. The aerospace industry of the early 1960’s were the early adopters of this technology. Pratt & Whitney Motors also used Prof. Weibull’s methods.

In 1972, Dr. Weibull (on the left) received an ASME Medal and his photo was captured with Dr. Richard Folsom (in the middle), along with former Astronaut Neil Armstrong, the first man to step on the moon (on the right). Segue to the present day…

5 year replacement of relief valves is the norm. Does it have to be? Bulletin 110 (June 2007 revision) states ”…2) An alternate to the prescriptive replacement interval, ie., five years, can be developed based on documented in service relief valve life for a specific applications using industry accepted good practices of relief valve evaluation…”. To that end, the IIAR Research Committee has developed a testing standard for safety relief valves. The Ammonia Refrigeration Foundation then sponsored a follow-on research project to prove the viability of this test procedure. Testing was successfully conducted by the Industrial Refrigeration Consortium and the standard was deemed workable and accepted by IIAR. Now that the industry has a standardized procedure to test safety valves, how do we use the data? Thanks to the ARF and Dr. Fred Elder, Dr. Weibull’s methods live on.

The most recently completed research project of the ARF is the SRVCalc–Statistical Pressure Relief Replacement Interval Calculator. This PC based analysis software will be available free to IIAR members. It will allow you to take the data from a batch, (7 minimum) of similar, tested relief valves and predict the useful lifespan.

Mark your calendars – The IIAR Research Forum held Tuesday, March 20, 2012 at 1:30 PM in the Convention Center (Frontier Airlines Center), Room 202D will feature the Author of SRVCalc, Dr. Fred Elder.

Here is a “peek behind the curtain.” The software is intuitive and easy to use, once you have tested a batch of relief valves with the procedure noted above. The required input values are:

Valve Details; Scheduled Replacement interval, NB#, Manufacturer, Model number, Manufactured date, installed date and valve set point.

Installation location details; Facility, Equipment name and P & ID Tag number.

Removal from Service; Date Removed, Bench test #, Pop Pressure, Condition notes and Removal reason.

Once a group of valves have been entered into the SRVCalc, they can be analyzed with a click of a mouse. The output of the Weibull probability plot will then display the predicted life span of that group of valves.

It is this plot that will then provide the plant with suitable documentation to alter the five year replacement interval.

We hope to see you there. Remember more data – better results!