John Rezabek recently discussed how to make more effective use of the infrastructure we likely already have installed, and in particular the investment in positioners (“Can things of yore deliver gold?”). Here are some suggestions on how easy it is to capture the benefits John alluded to in July.
Let me start with an example. While working in the oil sands, our control system analog cards didn't support digital (HART) communications. However, one of our team members identified that, rather than run another set of wire pairs out to our HART positioners, we could simply read that signal in the interface room with one of several devices that convert the digital message to an analog input (AI) signal. This AI signal then provided positive feedback on actual versus output valve position by using the analog output (AO) signal cable at the cost of a few feet of cable and an AI termination. Much less expensive than running new cable to the positioner. If we wanted, we could also set an alarm on this feedback message to serve as a limit switch. Or, depending on the type of HART monitoring device purchased, the device itself can perform this task, and change the state of a dedicated contact that could be used for an interlock or similar purpose.
Other examples of ways we could use the secondary, tertiary and quaternary values for sampling devices include but aren't limited to:
- Ambient transmitter temperature—body temperature to confirm that heat tracing is working or not too hot;
- Differential pressure transmitters—bulk line pressure (upstream pressure sensor), thus alleviating the need for a second pressure transmitter;
- Bidirectional flowmeters—(ultrasonic, magnetic), positive and negative flow;
- pH and conductivity meters—electrode performance/damage;
- Coriolis flowmeters—depending on whether mass flow, volumetric flow or density is the AI signal, the other two are prime candidates, as are process temperature, Brix/Baume or total flow;
- Radar level transmitters—can also provide interface volume/mass, percent range or calculated product volume/mass; and
- Valve positioners and damper operators—In addition to stem position, air supply pressure and actuator pressure (the original example).
Because each device has its own unique set of parameters, there will be more options available, but this isn't a bad collection of “free” information that's likely already sitting in any device installed this century. If you want to investigate taking advantage of these capabilities, below are a few items to consider.
Because a HART loop can have only one primary master (normally the control system’s analog card) and one secondary master, reserved for use by the handheld communicator, monitoring devices should leave these modes for their intended use even if the host system doesn't support HART. The same priority holds for fieldbus systems: primary is the host and secondary is for maintenance. Signal monitoring devices should be configured in guest/visitor/listen mode, so they connect passively, and can continuously sample data from a smart instrument without affecting normal loop operation.
When WirelessHART was introduced, one of the use cases commonly found in whitepapers was to install a WirelessHART adapter to your device (and, of course, the unmentioned associated network infrastructure) to bring all the intelligence from your devices to your control or asset management system. This is still a valid option, but you could also replace your existing analog card. Or, if you can’t isolate a full card at once, you could perform a hot cutover of the single loop to install the loop monitoring device.
As John implied in May, things of yore can deliver gold. All it takes is a little effort and ingenuity to find the nuggets amidst all the dross—and maybe a good team to find the place to look for those nuggets in the first place.
