If you want to be an API-510 inspector working the petrochemical inspection circuit, it seems to be one of those industries that you have to know someone to get into. You need to work in the industry for a certain amount of time in order to take the test to get certified. You usually need to be certified in order to get hired on to work. Its a chicken and egg situation. If you know an inspector, you can probably get them to get you hired on as a helper.
Your other option is to break in from the NDE side. You could get an NDE certificate from a tech college (2-3 semesters?) and sign on with an inspection company, then 1-5 years later take your API 510 certification exam.
The upside is reasonably competent API 510 inspectors can work as many months of the year as they choose to accept jobs for, work 12 hour shift with overtime 7 days a week through most of the year, and make $30 to $35 an hour + 1.5X overtime.
The downside is living on the road out of hotels and long hours.