Controlling Propane: Automation of ISC Valves
ISC valves are an integral part of safety when it comes to liquid propane tanks. These valves open and close the openings on the tanks to control liquid flow. They also provide automatic excess flow cutoff from the tank in the event of the piping or the valve itself shearing off. Smaller storage vessels generally have a manual handle and cable arrangement to allow attendants to hold the spring loaded valve open while the dispenser is active. These cables also have a fusible link installed which is designed to melt and close the valve in the event of fire.
This primitive arrangement was also used for large bulk transfer facilities in the past. In the event of human error, operators have been known to leave these manually held valves open all night. Piping fractures have happened at night due to extreme cold, causing tank or pipe supports to shift from frost action or other environmental causes. These breaks can allow a considerable volume of liquid propane to leak without actually exceeding the flow required to close the ISC. Fusible links have a minimal safety value. Having the ISC valves close after a fire, hot enough to melt the link, has already started is like closing the barn door after the horse has long gone.
Actuated ISC valves with pressurized nitrogen or air have been the norm on larger vessels for some time. This allows operators to dump the nitrogen and close all ISC valves in the event of an emergency. More recently, hazardous locations approved fail-safe electric actuators have become available for sites where nitrogen is inconvenient. However, many systems out there have been patched together without the proper integration and components a transfer facility requires leaving room for catastrophic incidents to occur. We have found large sites where emergency stop buttons do not close the ISC valves, pumps will run without the ISC valves open or ISC valves can be left open indefinitely without any warnings or indications to operators of this hazard.
Northridge Electric builds control systems which coordinate all bulk plant operations. Control stations automatically open ISC valves, start pumps, and monitor for pressure loss due to breakaways, fire and other causes. Emergency stop buttons cut power and close valves in a fail-safe manner. Gas detection systems and other means of shut down can also be integrated in these packages. These systems will automatically shut down your plant after pre-set periods of non-use as well. It is neither difficult nor expensive to do things right. These custom-built, pre-assembled packages are available for all applications, and can be installed by your local electricians who are capable to install basic systems but may not have the expertise to actually design a system of this magnitude.