Business segment reporting (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jul. 31, 2013	Jul. 31, 2012	Jul. 31, 2013 Segments	Jul. 31, 2012
Segment reporting
Number of reportable segments			2
Net sales	$ 61,665	$ 42,882	$ 117,249	$ 86,504
Gross profit	13,693	7,583	26,660	14,329
Income (loss) from operations	4,770	(603)	7,660	(2,465)
Piping Systems [Member]
Segment reporting
Net sales	43,478	22,905	79,536	42,649
Gross profit	10,590	4,869	21,034	8,267
Income (loss) from operations	6,493	1,443	11,873	1,598
Filtration Products [Member]
Segment reporting
Net sales	17,324	19,276	35,957	42,252
Gross profit	2,842	2,552	5,117	5,664
Income (loss) from operations	501	(292)	18	(96)
Corporate and Other [Member]
Segment reporting
Net sales	863	701	1,756	1,603
Gross profit	261	162	509	398
Income (loss) from operations	$ (2,224)	$ (1,754)	$ (4,231)	$ (3,967)