Machinery safety control systems and fluid power - Part 5
This article is the fifth of a six part series providing an overview of requirements, principles, applications and technology for pneumatic and hydraulic safety systems of machinery. In the last issue we overviewed the application of part 1502 (validation) of the new AS4024.1 Safety of machinery. Our fifth topic deals with the practical application of appropriate control methods of the safety related parts of control system which have been identified in the hazard identification / risk assessment.
Prior to commencement of design of a safety related control system the hazard identification / risk assessment would have highlighted the safety related parts of control system that need to be addressed within the design criteria. The application reviewed will be a brick stacker and palletizer. As described in part 1 of this series the risk matrix is the essential tool for the designer. The assessment will in most circumstances only identify the level of risk and therefore determine a risk category, the designers responsibility is to apply correct product for the application including identification of known hazards such as load holding. For example the application of block & bleed system would not be appropriate for runaway loads or horizontal load holding application such as a conveyor in which case a Block & Block valve system would be more appropriate in light load circumstances. Vertical load holding applications should incorporate the existing technology of pilot operated check valves, counter balance valves etc and not monitored spool valves due to spool leakage and potential failures between the valve and cylinder.
SEVERITY OF INJURY ( S )
S1 slight (normally reversible) injury
S2 serious (normally irreversibly) including death
FREQUENCY OF EXPOSURE ( F )
F1 Seldom to quite often and / or the exposure time is short
F2 Frequent to continuous and / or the exposure time is long
POSSIBILITY OF AVOIDING THE HAZARD ( P )
P1 Possible under specific circumstances
P2 Scarcely possible
BRIEF DEFINITION OF RISK CATEGORIES
RISK CATEGORY B & 1 Do not require the application of monitored fluid power valving.
RISK CATEGORY 2 A single fault in the control system may lead to the loss of the safety function but the fault must be detected. A single monitored valve fulfill these requirements
RISK CATEGORY 3 No single fault may lead to the loss of the safety function but multiple faults may lead to the loss of the safety function. Dual series ported monitored valves fulfill these requirements
RISK CATEGORY 4 No single or multiple failures may lead to the loss of the safety function. Dual series ported monitored valve fulfill these requirements.
Risk categories 3 & 4 do not discriminate and require dual monitored valves.
Hydraulic & Pneumatic Circuits
Brick Stacker & Palletizer Hazard identification/Risk Assessment
Tyne Stacker: S2 F2 P2 Category 4 Monitored Dual Block & Bleed Hydraulic
Tyne Clamps: S2 F2 P2 Category 4 Maintain System Pressure on Clamps (Over Centre Clamp)
Conveyor: S2 F2 P1 Category 3 Monitored Dual Block & Block Hydraulic
Pusher: S2 F1 P2 Category 3 Monitored Dual Cartridge Valves
Pallet Rotate: S2 F1 P1 Category 2 Monitored Single Block & Bleed Hydraulic
Plastic Wrapper: S2 F1 P1 Category 2 Monitored Single Block & Bleed Pneumatic
Strapper: S2 F2 P2 Category 4 Monitored Dual Block & Bleed Pneumatic
Originally written by Murray Hodges, director of Fluidsentry Pty Ltd. Murray is a member of the Sf041 Technical Committee for AS4024.1 Safety of Machinery and is the fluid power design representative for the Safety and Environmental Risk Consultants of Australia.