In 2003 we received a question from a reader that drew a lot of attention. He wrote:
We would like to avoid the cost of separate valves by using existing modulating valves for safety shutdown. Is this an acceptable practice under any circumstances? If so, where and how can it be done?
Our experts stepped up and offered their expertise. Some said "just don't do it. There is not an acceptable practice under any circumstances." Others said that it all depended on the danger.
"As long as single-component failure doesn't jeopardize the safety of the personnel, it is allowable to use the control valve as a safety shutdown system," said Imtiyaz Mohammed Arab from Bechtel National, Waste Treatment Plant.
Fred Porth from Metso Automation siad that S84 sort of says no. "ISA-S84.01 section 18.104.22.168 specifically prohibits the use of a control valve from the basic process control system (BPCS) as the only final element for SIL 3," Porth wrote.
Also, Dennis Hablewitz from Eurotherm/Barber-Colman said that you had the option to pay now or pay later. Hablewitz listed several reasons for using separate valves for safety shut down.
To read Hablewitz's list and the entire responses our experts offered, read the article "Can We Use Control Valves for Safety Shutdown?"