Monitored valves - how safe are they?

During recent years there have been many developments within the safety industry with regard to hydraulics and in particular pneumatic valves. These systems have predominately been applied to press applications and primary fluid power machine supply.

General industry has been on a trend to treat fluid power directional control valve monitoring whether it be hydraulic or pneumatic with the same regard as to a three-phase electrical contactor by monitoring an auxiliary contact of the device and incorporating this contact within the feedback control loop of a certified safety relay.

What has been overlooked in these applications is that a three-phase contactor applied in a safety application is generally of a positive guided device where three separate contacts along with an auxiliary contact are being switched. Should one contact stick or become welded, the other contacts will break.

There is also additional redundancy within this form of monitoring by the added security of either a thermal overload or motor circuit breaker, whereas should a contactor fail, the contactor may permit one phase to remain connected, in which case the motor protection would provide the additional redundancy therefore shutting down the control system with a feedback monitoring contact to indicate a system failure.

Unfortunately both hydraulic and pneumatic valves do not have multiple circuits for external monitoring devices and certainly do not have any form of positive guidance for protection and are of a single spool or poppet configuration. They are similar to a gate or guard circuit requiring full monitoring as there is no further redundancy to protect the system from failure.

There have been three primary forms of valve monitoring techniques used; the first being an internally statically monitored type where two valves within the one housing cross monitor each other for faults. These valves have an extremely high level of integrity and are usually supplied with a declaration of conformity to the relevant safety standards, thus ensuring the failure modes of the valve are able to be detected without the loss of the safety function. As these valves are not monitored electrically, they sometimes have an optional pressure switch but have no safety certification and are of single-circuit configuration.

Another form of monitoring, more common within hydraulic valves, is the use of inductive proximity switches placed within the pressurised areas of the valve. Even though these monitoring devices are pressure-rated at 7000 psi, the basis for a category 3 or category 4 application as per AS4024 is to be able to detect any single component failure that can lead to the loss of the safety function.

This component failure must also include the monitoring device itself and with their own internal circuitry we would need to assess each component within the monitoring device. These products have only one channel of feedback and are unable to be monitored directly into a dual-channel, certified, safety relay normally utilised in category 3 or 4 applications.

The third is a positive-driven plunger-switch, monitoring a poppet or spool. There are already several versions on the market - some with accreditation and some without. The placement and mounting of the monitoring switch is critical, especially where normally-closed circuits are used as there have been several incidents where a switch has been mounted externally of the valve and has come adrift of the valve body. In this situation, the primary valve switches. Whether it be in a monitored feedback loop or to a dedicated certified safety relay, this fault cannot be detected where the wiring is in series between two valves.

ISO-monitored valves are now entering the market. Where extreme care should be taken on the plumbing of these valves as the sub-base for these devices are available in either single ported, block form or a bolt-together manifold where, if utilised, would bypass the series redundant function of the primary valve.

A number of valve manufacturers claim to have approved or certified valves and have been unable to produce any supporting documentation. These valves also happen to be the type failing in industry. One particular valve claiming CE certification was found to be CE certified for EMC (electro magnetic conformity) with no relevant safety standards applied.

Valves are failing time and again, and in each application the end-user has been under the belief that they were using an approved product which would have meant that the valve meets the criteria of standards such as AS4024.

Approved valves sometimes fail but it can be assured that they will fail to safety and not to danger.