Do a little research on Parker's website. They have a vast amount of information on how to pick the right seals, dimensions, etc.
http://ph.parker.com/us/en/seals-and-o-rings
You may work for a competitor, but I'll still help you. I work in oil and gas and do pretty much the same calculations on the same type of tools.
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You may need to refer to your analysis group if you have one and see if they have a standard they use. I am not aware of one for API. Check...
