SQL Server 2012 :: Convert Parts Of String To Int

I have log files that I am loading into a table with duration data in the format "xx hrs xx min xx sec". Only the parts that are required will be there so if duration is only 2 seconds , the column will show "2 sec".

I am trying to get the duration into in to do some analysis on it and I have come up with this query so far which returns the correct data but i am wondering if there is a way to do what I am trying in a more readable format.

CREATE TABLE #tmp(duration VARCHAR(20))

INSERT INTO #tmp
VALUES ('1 hrs 20 min 12 sec'), --4812 sec
('48 sec'), --48 sec
('39 min 1 sec'), --2341 sec
('11 hrs 1 min 1 sec'), --39661 sec
('59 min 0 sec'), --3540 sec
('2 min 50 sec') --170 sec

And this is what I have so far

SELECT CASE
WHEN CHARINDEX('hrs', duration, 1) <> 0 THEN CAST(SUBSTRING(duration, 1, CHARINDEX(' hrs', duration, 1) - 1)AS int) * 3600 + CAST(SUBSTRING(duration, CHARINDEX(' hrs', duration, 1) + 5, CHARINDEX(' min', duration, 1) - 7)AS int) * 60 + CAST(SUBSTRING(duration, CHARINDEX(' min', duration, 1) + 5, CHARINDEX(' sec', duration, 1) - 14)AS int)
WHEN CHARINDEX('hrs', duration, 1) = 0

[Code] ...

I ultimately plan on converting this to a SSIS expression so that is why I am looking to simplify it.