Question: “I have a charge of about 30,000lbs of ammonia and I just added 2,000lbs to my charge. It’s been 2 years since we’ve added any ammonia and our High Pressure receiver was getting low. Do I have to account for those 2,000lbs I seem to have lost over the past 2 years? “
Answer: Yes. See, that was easy! The tough question is “How do I account for those 2,000lbs?”
It’s difficult to answer this question without specifics but let’s discuss a typical system along these lines to show you how it’s done. In the following example, we’ll show a typical justification for a 2,000lb loss over 2 years:
Take a look at your incidents: Do you have any with releases? Let’s say that in this case we have two incidents over the past two years and between the two of them they released 150lbs. That means we still have to justify about 1,850lbs of loss – or 925lbs a year. Where could this other ammonia have gone? According to the IIAR, it’s not uncommon to expect a loss of 5-10% of the whole system charge. Here’s a quote from the February, 2012 Condenser (Page 40) where the Technical Director; Eric Smith of IIAR says:
“…while it is likely an effort in futility to try to calculate exact losses, it is a good idea to make some reasonable estimate of refrigerant that was lost throughout the year and include that loss in annual reports. Typical sources of minor refrigerant loss are: purgers, compressor shaft seals, valve stem packing, oil draining operations, and minor repairs. An often-cited rule of thumb is 5%-10% loss annually of the total system charge, depending upon the age, size and condition of a system.”
Garden City Ammonia Program has always said that even a well maintained ammonia system could lose up to 10% of their charge annually without having any minor or major leaks.
Here is what the math looks like in our case: The Loss Amount per Year is 925lbs and the Total System Charge is 30,000lbs. The Loss rate = Total System Charge / Loss Amount per Year or 0.0308 which we convert into a percent by shifting the decimal point two parts to the right. That means our loss here is about 3.08% which is well under what the IIAR says we should expect as typical. We know we’ve done things over the past two years which have involved very minor releases to the atmosphere such as oil draining and pumping down valve stations for repair. Furthermore, we have an auto-purger which is always releasing very small amounts as it operates. We also know that we have shaft-seals on our compressors and valve-stem packings which are also not 100% vapor-tight. Is it reasonable that all the above items would add up to a 3.08%?
It would not be difficult to write up a “Letter to File” explaining the above and place it in your Process Safety Information file with the copy of the delivery slip from the ammonia supplier.
While in the “freon world”, the EPA’s GreenChill has published that the synthesized refrigerants can lose up to 25% of their annual charge each year.
Note to think on:
If one lost 99.9lbs of NH3 per day for 365 days straight how much ammonia would be lost?
