I haven't taken valuim in more then 6 months,if i take 20-30 mgs for severe back pain and have 10 days till a urine drug test, is it likely i'll pass?

Yes, probably: Half-life of valium in your urine is up to 60 hours, with metabolites up to 100 hours. If your urine drug test is 10 days after your valium use, that is 240 hours. Likely you will pass, but depends on how sensitive lab test is. Consider other treatment for your back spasms, like ice packs, or heat (later), massage. Using less (or no) valium will improve your odds of passing drug screening test

Odds in your favor : Valium is a long acting benzodiazepine with active metabolites. It gets metabolized by the liver. It's clearance from the body depends on the condition of your liver and the presence of competing substances. Also, detection of valium in the urine is also based on the sensitivity and specificity of the test. Usually clears in 4 to 7 days. Best wishes!