SLAC National Accelerator Laboratory, USA

SLAC National Accelerator Laboratory, USA
Improving the Reliability of Accelerator Magnets Learning from our failures Presented at the Accelerator Reliability Workshop, Cape Town, 14th April 2011 Cherrill M. Spencer SLAC National Accelerator Laboratory, USA

Improving Magnet Reliability-Spencer 2
Overview of this talk Decision to hold a magnet reliability survey Description of on-line survey Example questions Who took the survey Kinds of failures magnets suffer from- ranked Correlate materials & practices with failures Advice based on survey responses Continue as open discussion Raffle for bottle of SA wine- for survey takers Improving Magnet Reliability-Spencer

Decision to hold an on-line magnet reliability survey-to date a 12 year story
Anticipating designing and building a 20 Km long e+e- linear collider (ILC) with over 13,000 magnets of about 100 different styles: Estimated the MTBF & MTTR of typical SLAC magnets from historical records Calculated that to reach desired availability could not continue to make magnets in same way Did a Failure Modes & Effects Analysis (FMEA) of a typical water cooled electromagnet and saw there were MANY ways such magnets could fail, at all life stages Decided to consult with & learn from other magnet engineers at other particle accelerators

Decision to hold an on-line magnet reliability survey- could learn from each others’ problems Through interacting with magnet engineers from other labs at PAC and Magnet Technology conferences realized would be useful to all of us to Collect detailed technical information about electromagnets in use at accelerators all over the world Find out what materials and fabrication techniques are being used How these magnets are operated and the ways they fail Do this through a web-based survey, March 2011 Send out an to over 150 accelerator engineers & physicists inviting them, or their magnet colleagues, to fill in the survey for their institution’s magnets Improving Magnet Reliability-Spencer

Goals of the web-based survey- a work in progress
Use a web-based survey program available to all countries (chose “Survey Monkey”) Analyze the survey data to look for correlations amongst failure modes & rates, fabrication materials & techniques, operating conditions and maintenance practices Goals : to identify the more reliable materials (e.g. insulating tapes, epoxies, LCW hoses) , more reliable fabrication techniques (e.g. coil winding methods) more reliable operating conditions (e.g. properties of the LCW) more reliable maintenance practices seek other experts’ opinions and advice on these topics Publish the analysis results for all to learn from Improving Magnet Reliability-Spencer

Summary description of the survey
Total of 64 questions, of which 9 had to be answered 6 identifying questions, e.g. institution name, accelerator/beam line name, age of magnets 12 questions about magnet design standards in effect when these magnets were designed 15 questions about materials used & associated problems, e.g. epoxy resin & fillers; cracks in potted coils 4 questions about types & frequencies of failures 21 questions about their LCW system supplying LCW 6 questions about other failure types & their advice Improving Magnet Reliability-Spencer

Example of material used question
Many questions had extra windows for free-form answers that didn’t fit with the given list of answers Improving Magnet Reliability-Spencer

Example of material question plus problem and their solution info
Improving Magnet Reliability-Spencer

List of countries represented in the 28 survey takers. Some institutions answered for multiple machines Countries where accelerators situated No. of Accelerators No. of Institutions Australia Canada China France Germany Japan Russia South Africa Sweden Switzerland United Kingdom USA TOTALS countries accelerators institutions Improving Magnet Reliability-Spencer

Main uses of these 28 machines
Main use of the particle beams Number High Energy Particle Physics Research: 10 Synchrotron Radiation Light Source: Nuclear Physics Research Proton Therapy & Radioisotope Prod Educational Improving Magnet Reliability-Spencer

Surveys completed for these age ranges of sets of magnets (28 surveys)
No. 1 8 13 6 Improving Magnet Reliability-Spencer

Surveys completed for magnets in 3 different running modes
Number 19 8 1 Improving Magnet Reliability-Spencer

Nine common magnet failure types: rank how frequently your magnets suffer each type Based on my SLAC experiences I categorized nine types of magnet failures. Survey taker had to find out how often their magnets suffered such failures and then RANK them for their frequency. Chose 1st for the failure type that happened the most over past 4 years, 2nd for the second-most etc. Chose “9th” for all remaining types they’ve had at least once in past 4 years. Improving Magnet Reliability-Spencer

Another view of failures ranked by frequency data
Most labs have <5 magnet failures a year. Improving Magnet Reliability-Spencer

Analyze survey data associated with LCW hose & fittings failures. Part 1 MOST POPULAR FAILURE TYPE: 20 machines ranked failures of LCW hoses or fittings as 1st, 2nd or 3rd in frequency. But 7 machines ranked them as 9th = hardly any hose or fittings failures compared to eight other types. What about their hose make? Filter data to look at just these 7 machines and their inner hose material & fitting types- NOT conclusive, except none used nylon inner hose. Improving Magnet Reliability-Spencer

Analyze survey data associated with LCW hose & fittings failures. Part 2 Compared nylon and EPDM inner hose tubes for hose failures and blocked passages: Nylon: 5 machines ranked 1st and 2nd for hose leaks EPDM: 2 machines ranked 1st and 2nd for hose leaks Nylon: a 2nd, 3rd & 4th rank for blocked passages EPDM: zero ranks for blocked passages My conclusion: LCW hoses with EPDM innermost tube are better than nylon for avoiding hose failures & blocked passages Improving Magnet Reliability-Spencer

Analyze survey data associated with LCW hose & fittings failures. Part 3 Compared permanently crimped hose couplings with “other methods” with respect to hose & fittings leaks- No conclusion as to which is best; they had equal numbers of 1st & 2nd ranked failures Improving Magnet Reliability-Spencer

Analyze survey data associated with LCW hose & fittings failures. Part 4 Q56: Do you only replace hoses after they or their fitting has leaked? Yes or No. Filter data on Q56 to answer :Does replacing hoses before they fail help reduce the frequency of leaking-hose failures? 15 machines replace hoses before they leak, 6 of them ranked hose leaks as 9th (recall only 7 machines with 9th) 9 machines only replace hoses after they have leaked, just 1 of them ranked hose leaks as 9th My conclusion: One should replace LCW hoses on a schedule even if they have not yet leaked. Improving Magnet Reliability-Spencer

Do newer magnets have longer MTBFs?
Water cooled magnet MTBFs vary widely, 13 surveys reported from 220,000 to 12,600,000 hours. Chart shows ages of magnets . Conclude: cannot say newer magnets (<30 yrs) will fail less often than old magnets (>30 yrs) [in last 4 years] Improving Magnet Reliability-Spencer

Why have magnet design standards?
Q6: Did your institution have a written set of magnet design standards/rules when these magnets were designed [e.g. limiting velocity of LCW flowing in coils; not allowing internal braze joints in coils] ? Second-most popular failure type: water leaks at braze joints (14 machines ranked 1st , 2nd or 3rd) A leaking internal braze joint has more severe consequences than an external one: turn to turn short Can one see a difference in the frequency of braze leaks correlated with an institution having a design standard forbidding internal brazes or not? Improving Magnet Reliability-Spencer

Do magnets at machines with a rule forbidding internal brazes have less braze leaks? Filter on Q17- either have a rule or don’t, and look at the frequency rankings of water leaks at braze joints: Rule? 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Tot # No rule Yes My conclusion: having rule helps reduce frequency of braze leaks [look at 1st & 2nd ranks numbers] Improving Magnet Reliability-Spencer

Third most popular failure type: those caused by human error
Q60: Root cause of many magnet failures is human error. What are you lab’s strategies for minimizing human errors; describe strategies used at design , fabrication and operation stages? This was one of the 9 questions you had to answer in order to complete the survey (and win a prize). Some people wrote very brief answer for only one stage, others wrote long paragraphs and even added “after maintenance period” strategies. On next few slides: summary of strategies Improving Magnet Reliability-Spencer

Strategies at DESIGN stage for minimizing human errors
Have written magnet design standards Specify standard set of materials: Cu, insulation tape, epoxy components. Insist vendors use them. Refer to past experience. Hold design reviews with “external” reviewers Do computer modeling Design in safety factors for field strength, delta T Build a prototype and measure it Improving Magnet Reliability-Spencer

Strategies at FABRICATION stage for minimizing human errors
Monitor commercial vendor’s operations closely Hipot (“megger”) coils to steel core Do hydrostatic pressure tests of coils Measure LCW flow at operating pressure Design & use a traveler: step-by-step instructions for every fabrication task with space for pertinent test results to be written in and signatures of tech. Train technicians to braze hollow-core conductor into the terminal block; wind coils Do full magnetic measurement of every magnet Improving Magnet Reliability-Spencer

Strategies at OPERATION stage for minimizing human errors
Put thermal switches on every LCW return cond. Double check power connections before power up Check magnetic polarity with gauss meter at few A Software limits on current allowed Flow switch alarms to indicate low LCW flow [keep checking switches are functioning] Train technicians in installation tasks Use male & female power connections Use bayonet power connections [completely avoiding lugs needing tightening] Improving Magnet Reliability-Spencer

Strategies after a maintenance period for minimizing human errors
Go through a check list and inspect mechanical items like “LCW turned back on” Use micro-switches on all water valves, read out by control system Run magnets at intermediate power, turn off, enter tunnel with infra-red thermal camera to look for hot spots on power terminals Train technicians, give them time to do the job right Improving Magnet Reliability-Spencer

Conclusions for talk for Accelerator Reliability Workshop
No institution has failure-free magnets Very old magnets can continue to function The actual numbers of magnet failures that bring down the beam are quite small, but each one can be costly The most common failures are associated with the LCW cooling the magnets; The survey will be re-opened for more labs to complete it and further analysis of the survey data will be made, to be presented at MT 22 [I hope] Improving Magnet Reliability-Spencer

Open discussion of magnet reliability
The characteristics of the Low Conductivity Water (LCW) flowing through our magnets have impact on the operation of the magnet and it appears from my survey that a large fraction of labs do not appreciate this. I had 13 questions about LCW properties and the components of an LCW system and this section generated the most “don’t knows” Improving Magnet Reliability-Spencer

Summary of survey responses to some LCW system questions page 1

Studied the LCW answers of the 2 (a) surveys and can see they have too much dissolved oxygen in their LCW Improving Magnet Reliability-Spencer

Useful Articles on Effects of LCW in Copper & Optimum Parameters
“Corrosion of Copper by De-Ionized Cooling Water” by H.Scholer and H. Euteneur, Institute fur Kernphysik, Mainz FRG. EPAC 1988 paper “Accelerator Magnet Plugging by Metal Oxides: A theoretical investigation, remediation and preliminary results” . By W.W.Rust, Jefferson Labs, USA. Published in PAC2003 “Low Conductivity Water Systems for Accelerators” by R. Dortwegt, APS, Argonne National Lab, USA Published in PAC 2003 Improving Magnet Reliability-Spencer

AVAILABILITY: DEFINITIONS for 1 & N devices
Availability: Average ratio of the time that the system or component is usable to the total amount of time that is needed. MTBF (Mean Time Between Failure): MTBF is a basic measure of reliability for repairable items. It can be described as the number of hours that pass before a component, assembly, or system fails. MTTR (Mean Time To Repair): MTTR is the average time required to perform corrective repair on the removable items in a product or system. Maximum value for A = 1 (or 100%) Overall availability of system of 3 dis-similar devices = A1 x A2 x A3 Overall A of N identical devices= A1 xA1 x A1 ….x AN = AN Availability of N magnets = (Availability of one magnet) N Improving Magnet Reliability-Spencer

MANUAL ON HOW TO DO A FMEA ON AN ELECTROMAGNET:
Ensuring/improving magnet availability: Various things must be done. What, depends on stage of a project you are in. If you have existing magnets- different approach Reliability engineering must be started as early as the conceptual design stage of a project and involve all engineering disciplines and accelerator physicists. If large numbers of new magnets are needed- multiple commercial vendors to provide, so Magnet design standards will be imposed to ensure reliable magnets by staying within well-recognized limits on magnet parameters to ensure reliable magnets by using high quality materials Common review, QC and maintenance processes will be followed to ensure reliability But in order to have a significant impact on the MTBF and MTTR values of your magnets I recommend you carry out a detailed Failure Modes and Effects Analysis (FMEA) to learn how to revise your future magnet designs, fabrication techniques and operating conditions to make more reliable magnets and reduce their repair time. MANUAL ON HOW TO DO A FMEA ON AN ELECTROMAGNET: Improving Magnet Reliability-Spencer

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