Neil Arlidge wrote: > Bruce Napier wrote: >> On 16 Jan 2009, at 10:19, Adrian Stott wrote: >> >>> Why would a more complex (i.e. adjustable) one be needed? >> >> >> If there is no barrier between the air and the water, the air will >> all tiptoe gradually away as it dissolves in the water. Hence there >> is usually some sort of diaphragm or bladder to stop that happening. >> >> No matter what the arrangement, the air pressure is bound to reduce >> over time from loss one way or another. Thus most accumulators have a >> tyre valve in the top so that you can pump it up. >> >> Because the optimum pressure is either half the pump cut out pressure >> or a little less than the cut in pressure (depending who you believe, >> and often comes to much the same thing), and because pumps vary in >> these pressures, you need to be able to adjust the accumulator >> pressure. >> >> The Cleghorn Wareing ones come pressured to 5 bar, so that you can >> bleed off as much as you want when installing. If you don't realise >> this, you are wasting your time installing it, as the pump will cut >> out long before water starts entering the flask. >> >> As a final ironic note, having started this thread: after installing >> the new pump, and having had to wait to get the new accumulator, when >> I came to install the latter, I discovered that the old one was now >> performing perfectly. Applying the NB=DF rule, I have therefore >> retained the new accumulator as a spare. > > RONALERT...RONALERT! > > ISTR that mine was initially over pressurised with nitrogen and you > let gas out until it was at the required pressure. > Is nitrogen less likely to escape through a rubber diaphram than air?
Unlikely, since air is 78% nitrogen anyway and is a smaller molecule. It will probably reduce "aging" - there's all sorts of things in rubber, and with the gas at a high pressure, it's likely that the oxygen will very slowly react with some of the components of the rubber - probably making it less flexible and more likely to crack with age. -- Ron Jones Process Safety & Development Specialist Don't repeat history, unreported chemical lab/plant near misses at http://www.crhf.org.uk Only two things are certain: The universe and human stupidity; and I'm not certain about the universe. ~ Albert Einstein
