Recommendations for the testing and certification of hydraulic proportional directional control valves

Status:

completed 12/2001

Aims:

The increasing use of hydraulic proportional directional control valves in safety-related hydraulic control systems made safety assessments of these valves an absolute priority. A number of valves produced by various manufacturers have already been subjected to theoretical assessment in recent years by the BG Institute for Occupational Safety (BIA). The results showed that proportional directional control valves may be suitable for the performance of safety-related functions. A comprehensive safety assessment was possible however only in a complete theoretical and experimental investigation. A test of this kind could not be performed, as the requisite test principles did not exist; neither ISO, EN or DIN standards, nor the body of regulations of the German institutions for statutory accident insurance and prevention (BGs) contain suitable test requirements. The objective was therefore to draw up, initiated by the expert commitee Chemistry appropriate recommendations for testing, into which the assessment criteria from the preliminary tests were also incorporated. Publication of the recommendations for testing in a suitable form is planned.

Activities/Methods:

The recommendations for testing were drawn up following literature surveys of requirements found in pertinent ISO, EN and DIN standards and the BG body of regulations of relevance to proportional directional control valves. The experience and assessment criteria gained during the preliminary tests were also taken into consideration. Requirements to be specified were to be confirmed where appropriate by experimental testing. The work was to be performed in close co-operation with the BG expert committee Chemistry and with the BIA certification body. The draft text of the recommendations for testing was also discussed with major valve manufacturers and interested BG expert committees, and presented to DIN working group UA 5. 3, responsible for hydraulic valves, and the BG working group responsible for hydraulics and pneumatics.

Results:

The test recommendation for electrohydraulic proportional directional control valves with switching off over contacts for the safety-related parts of control systems (PE-BIA-M01) formulates the safety requirements for the proportional directional control valves concerned. The requirements govern, in particular, the valve's hydraulic and electric/electronic part (e.g. design configuration, function, fault-mode behaviour), its ambient conditions (e.g. climatic conditions, mechanical stress, electromagnetic interference phenomena), and documentation of the valve. The test for the above valves was based upon the following major safety-related parameters/characteristics: the time to reset to the "safe" switching position following disconnection of the control voltage; remaining of the "safe" switching position with the control voltage switched off; and the leakage flow rate in the "safe" switching position. Assessment of the test results includes comparison with corresponding discrete-switching directional control valves (on-off switching valves). The test recommendations enable valve manufacturers and test institutions to determine, on a substantiated and transparent basis, whether a proportional directional control valve with switching off over contacts may be employed for the safety-related parts of control systems in the context of EN 954-1"Safety-related parts of control systems". These control systems can be realized more economically with proportional directional control valves capable of assuming safety functions than can control systems with additional on-off switching valves in overriding safety configurations.

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