One of the great things about conferences such as last week’s Emerson Exchange Americas conference is reconnecting face-to-face with people where some time has passed. I had the chance to connect with Emerson’s Riyaz Ali, whose expertise I’ve highlighted in several digital valve controller and safety-related posts.
He shared a story where he was recently asked about having local valve diagnostics access in the production process. These diagnostics come from digital valve controllers and displayed through local control panels and HART-based handheld communicators or computer-based asset management software or host control systems.
The valve diagnostics required included dynamic error band, drive signal, output signal, step response, and signature curve. It would allow configuration, testing and troubleshooting from the automation system level or asset management software level in the maintenance station or locally by the handheld device, which has better visibility than having screens located on the field devices since these devices would have more limited viewing areas.
Some diagnostics are not accessible at the device. Riyaz explained that local key push button have only limited parameters to configure as compared with a handheld device or computer-based asset management software. These options have access to complete device description (DD) parameters. Also, local key push buttons are locked after the commissioning and installation of control valves. Riyaz has not seen anyone using local device push buttons for emergency shutdown, isolation or blow down valves. This is because On-Off valves or shutdown valves are generally in one static position.
For local display on a device, the utility of the information is limited specifically for SIS shutdown valves, which are in dormant states until a safety demand, unlike basic process control system (BPCS) valves that continuously move in response to process demands. Generally, large shutdown valves use rotary actuators, with NAMUR shaft and positioners that are shaft mounted. This makes visibility of such information, if not impossible, at least cumbersome to see from top, and may require access via a ladder or platform.
Riyaz noted that for safety applications, a safety instrumented system (SIS) field device, the display of parameters as a requirement is not driven by IEC 61508 and IEC 61511 global safety standards, SIS requirements, or any regulatory requirements of which he is aware. He noted that display variables at the field device level are optional but not a mandatory requirement and may possibly by available but its use in SIS applications is limited.