Business segment reporting (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Apr. 30, 2015 Segments	Apr. 30, 2014	Jan. 31, 2015
Segment reporting
Number of reportable segments	2us-gaap_NumberOfReportableSegments
Net sales	$ 37,674us-gaap_SalesRevenueNet	$ 59,524us-gaap_SalesRevenueNet
Gross profit	4,041us-gaap_GrossProfit	15,989us-gaap_GrossProfit
(Loss) income from operations	(4,800)us-gaap_OperatingIncomeLoss	5,709us-gaap_OperatingIncomeLoss
Piping Systems [Member]
Segment reporting
Segment Reporting Information, Description of Products and Services	Piping Systems, which engineers, designs, manufactures and sells specialty piping, leak detection and location systems
Net sales	20,277	42,354
Gross profit	2,355	13,458
(Loss) income from operations	(1,426)	8,017
Filtration Products [Member]
Segment reporting
Segment Reporting Information, Description of Products and Services	Filtration Products, which manufactures custom-designed industrial filtration products to remove particulates from air and other gas streams
Net sales	17,397	17,170
Gross profit	1,686	2,531
(Loss) income from operations	(1,230)	(544)
Corporate Segment [Member]
Segment reporting
(Loss) income from operations	$ (2,144)	$ (1,764)
Credit Concentration Risk [Member]
Segment reporting
Concentration Risk, Percentage	28.00%		31.00%
Sales Revenue, Net [Member]
Segment reporting
Concentration Risk, Percentage		21.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

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- Definition

Description of the types of products and services from which the reportable segment derives its revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 280

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8721-108599

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- Definition

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

+ References

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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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