First, let’s DEFINE what we mean when we use the term “remotely operated shutoff valve.” I have come to love the UK’s HSE definition, which is:
A valve DESIGNED, INSTALLED, and MAINTAINED for the primary purpose of achieving rapid isolation of plant items containing hazardous materials in the event of a failure of the primary containment system (including, but not limited to, leaks from pipework, flanges, and pump seals). Closure of the valve can be initiated from a point remote from the valve itself. The valve should be capable of closing and maintaining tight shutoff under foreseeable conditions following such a failure (which may include fire).
Manual valves should never be used when the employee affecting the isolation would be in danger. This is a significant consideration in deciding when to use Remotely Operated Shutoff Valves (ROSOVs). Manual valve isolation may be acceptable when rapid isolation is NOT required to prevent a major accident. However, manual valves are often fitted mainly for maintenance work and are unlikely to be the safest or most effective option for emergency isolation.
ROSOVs can be manually activated through push buttons located a distance from the valve. Automatic Shut-Off Valves (ASOVs) activated by a detection system (e.g., a toxic or flammable gas detector) can provide a more immediate response.
One advantage of manual activation is that an intelligent assessment of the most appropriate measure for dealing with a release can be made. Claims are sometimes made that manual activation is necessary to avoid spurious trips associated with automatic systems; however, the root cause is often a poorly designed system rather than any inherent weakness in an automated response.
Manual activation must be justifiable, and the location of push buttons must NOT ENDANGER the employee. They should be accessible and in a safe and suitable place in relation to the hazardous event that may occur. There should generally be AT LEAST TWO (2) ALTERNATE ACTIVATION POINTS, which should be READILY IDENTIFIABLE both
- in the plant (e.g., labeling) and
- in all relevant operating procedures
Advantages of ASOVs include more rapid isolation and a reduction in the frequency of some modes of human error.
For example, facilities for manual activation on emergency escape routes should be provided as a backup to automatic activation and can result in more rapid response in some circumstances.
A remotely operated valve can be operated by various methods, such as
- pneumatic,
- hydraulic or
- electrical energy sources
ROSOVs should continue to be capable of performing their function in the EVENT OF FAILURE OF THE PRIMARY POWER SUPPLY (e.g., mechanical springs or pressurized fluid reservoirs). It may be possible to convert existing manual isolation valves to remote operation by incorporating an actuator and a suitable control system. The suitability of such a conversion is beyond the scope of this guidance, and advice should be sought from the valve manufacturer and actuator.
The correct sizing of the actuator is crucial to meeting the safety requirements specification of the ROSOV. Undersizing may result in the valve not operating on demand, while oversizing may damage the valve or actuator assembly. The design must understand the safety requirements and be based on the complete system characteristics. This would include taking into account:
- the static/dynamic forces of the assembly;
- the effect of the process application on these forces;
- the frequency of exercising the system;
- the minimum/maximum range of gas/hydraulic pressures used for actuation; and
- the fact that actuators are manufactured in discrete sizes
Excess flow valves
An excess flow valve is designed to remain open in normal operation, but if the flow through the valve exceeds a preset maximum, the valve closes. These valves allow flow in either direction but normally only trigger for excess flow in the specified flow direction. The valve setting MUST exceed the maximum flow rate foreseeable in normal operation. Depending on the particular design of valve a setting significantly higher (perhaps as much as 50%) may be required to avoid ‘chatter’ and damage to the valve.
A catastrophic failure downstream of the valve will result in increased flow and a pressure drop across the valve, causing it to close where the downstream failure is more limited (e.g., a hole or a crack, or there is the crushing of the pipework, then the restricted flow may not be sufficient to cause the valve to shut, and the release will continue. Other factors are relevant when considering the use of an excess flow valve including foreign matter that can lodge in the valve and prevent it from closing.
Advantages claimed for excess flow valves include the relative simplicity of a mechanical system and their automatic action – eliminating some potential human errors. In the context of this guidance, excess flow valves would not normally be considered equivalent to ROSOVs for emergency isolation. Many of the same issues surrounding retrofitting apply, but in some cases, lower costs overall may mean that an excess flow valve is reasonably practicable to retrofit when a ROSOV is not.