Viktig information för transmittrar med option /A1 Gold-Plated Diaphragm Guldplätering kan aldrig helt stoppa genomträngningen av vätgas, men den får processen att gå långsammare. En tjock guldplätering med hög guldhalt får processen att gå långsammare. Yokogawa har gjort en förbättring av optionen /A1. Tidigare legerades guldet med Co och K för att öka hårdheten hos guldlegeringen (99,8% guld), men erfarenheten har visat att detta inte är nödvändigt. Yokogawa har därför sedan maj 2014 övergått till högrent (99,99%) guld för optionen /A1. Detta för att ytterligare förbättra transmitterns förmåga att stå emot vätgasgenomträngning. Det går inte på förhand att säga hur lång livslängd en transmitter i vätgasapplikationer har. Spridningen är stor, från 12 månader till över 10 år. Erfarenheten säger oss att det är lågtryckssidan som drabbas, vilket i sin tur ger en möjlighet att via HART titta på parametern P-LTD och därmed se hur långt processen har fortskridit. Parametern P-LTD visar hur mycket nollpunkten har förskjutits i förhållande till fabriksinställningen. På detta sätt kan man få en uppfattning om graden av vätgasgenomträngning. Nedan finns Yokogawas anvisningar kring hur parametern P-LTD i transmittern kan användas i diagnossyfte. Vi bifogar även Application Guide, Hydrogen Permeation (Doc.No: A90-TCS-179.) #1-1. P-LTD value in EJX och EJA Typ E (Hart) parameter P-LTD is a convenient and sole criteria for judgment of permeation or not. This value shows the shift of zero point with engineering unit compared with the default position at the production. So even if any technician has just repeatedly done zero trim without the record, he can confirm how much the shifted value is. The P-LTD value is not Hart parameter but EJX and EJA Typ E unique parameter. It is not available EJA- A series. #1-2. Details about P-LTD The P-LTD value soon after zero trim provides the user with a possibility to assume the drift value caused by H2 permeation. One caution point of P-LTD value is that the value is indicated with concrete engineering unit (example : kpa). So if same value of permeation might happen, the % value for the user s range will differ. It is depending on the user s range setting. OmniProcess AB
Example : In case P-LTD value is 2kPa, if the user s set range is 0-100kPa, it is 2% if the user s set range is 0 10kPa, it is 20%. So it is not possible to make judgement from the P-LTD value simply. The user is required to consider the ratio of P-LTD value for the set range. #1-3. Special note for judgement Even if there is 3% shift of zero point, sometimes the user can continue using the pressure cell without bad influence to the operation. On the other hand even with 2% shift, the user might have bad influence for the operation. So it is not realistic applying the same % shift value to all tag points. The shift % can be different tag point to tag point. Whether the user continues using the pressure cell or changes the pressure cell is their judgement based on their practice on that measuring point. Whether H2 permeation has happened or not is usually confirmed at the maintenance shop by vacuum static pressure test. (Both High and Low side of membrane will be pulled by vacuum pressure, then check the shift of output signal.) For user level, the user can consider that H2 permeation is happened when the shift % is more than double of the stipulated static shift % considering the accuracy and temperature, which is explained in General Specification sheet (GS). Example : Condition is at 13degC for ambient temperature / with EJX110A M capsule (Max Span) EJX110A performance is tuned up at 23degC in general for Yokogawa pressure transmitters. Now the ambient temperature is 13degC, so there is 10degC difference. Formula can be quoted from GS; Reference Accuracy [±0.04%] + Temperature effect [±(0.04% Span + 0.009% URL)] / 28degC 0.04 + (0.049)/ 28 X 10 = 0.0575 (%) When the shift is more than 0.115%, which is twice bigger than 0.0575, it can be said the hydrogen permeation has started with this pressure cell. #1-4. Usable or non-usable? Based on the points mentioned in #1-3, it can be said that H2 permeation has started. But as mentioned before, the tolerance limit of % value is different tag point by tag point. Still usable or need replacement has to be judged only from user s past practice at each tag point. OmniProcess AB