Engineers who design safety systems are always mindful of a few basic truths: Anything can fail, so there should always be a backup. And sometimes, backups need backups. Looking at how this concept applies to pneumatically operated emergency shutdown (ESD) valves, and other similar on/off valves, is the main topic of my article in the November issue of Chemical Engineering. It’s important to pay attention to all the devices necessary to make these valves operate correctly, which includes following all the links in the compressed air supply chain.
While we pay a lot of justifiable attention to the valve actuator, it is also important to look at the relationship of the solenoid valve and airset regulator controlling the compressed air supply. The regulator serves as a filter and controls pressure, so the actuator gets the correct amount. In most ESD situations, the pneumatic actuator holds the valve closed by spring pressure and requires air pressure to open.
The valve should close for either of two reasons. First, because the solenoid valve has been triggered by the safety system to dump the air and close the valve. Second, in the event of a complete loss of air pressure, the ESD should close. Here’s where the backup to the backup comes in.
IEC 61511 requires the application of a three-way solenoid between the airset regulator and the actuator, but it is considered best practice to not rely solely on the solenoid to make sure the valve fails to the correct position. Instead, ESD applications require the end user to consider additional selection criteria when determining the correct airset regulator. Since airset regulators operate based on changes in downstream rather than upstream pressure, it can be problematic for SIS on-off applications if the airset is locked in position because there is no demand to the actuator and instrument supply pressure is lost. The regulator will not react because pressure downstream of the airset remains unchanged unless the solenoid removes the actuator pressure.
In other words, if the solenoid valve does not act correctly, the ESD valve will be held open due to compressed air trapped in the system. Can the airset regulator provide a mechanism to avoid this condition? The answer is yes, and there are two ways to do it. One is wasteful while the other is smart. The wasteful way is adding a small orifice on the outlet side of the regulator to bleed air out of the line constantly, releasing expensive compressed air, or natural gas for gas-operated valves. The smart way is adding an internal check valve to the airset regulator, which opens only when the supply air drops below the regulator’s setpoint.
Manufacturers have developed airset regulators with an integral check valve to address the drawbacks of the constant-bleed solution. This integral check valve is located inside the airset body and is closed during normal operation. In the event of a loss of supply pressure, or if supply pressure drops below the regulator setpoint by a preset amount, the check valve opens and allows the outlet pressure to quickly flow back through the regulator body to the upstream side. The orifice size of the check valve is larger than the constant-bleed orifice, which ensures fast valve reaction to losses of supply pressure. The check valve also provides a no-bleed solution, resulting in more efficient and safe operation with reduced emissions.
An example of the smart approach is Emerson’s Fisher 67C Series Instrument Supply Regulators. These are typically used to provide constantly controlled, reduced pressures to pneumatic and electro-pneumatic controllers and other instruments. They can be ordered with the Smart Bleed Internal Check Valve to avoid trapped air. Problem solved, and the backup has a backup.
What are you using at your facility? Are your actuator air supplies constantly bleeding away air or natural gas, along with money, all day long? Are your ESD valves protected against solenoid valve problems?
Visit the Precision Regulators & Valves pages at Emerson.com for more on technologies and solutions for fluid control and automation. You can also connect and interact with other engineers in the Oil & Gas and Chemical Groups at the Emerson Exchange 365 community.