Plant managers always are looking for ways to improve operational performance or reduce costs. This effort involves many elements; much hinges on the condition and performance of process equipment. Getting a handle on relevant metrics requires effective instrumentation and data analytics — which often can pose challenges due to a lack of measurement points and the complexity of data analysis software.
Most plants consist of a collection of process units, many custom designed for the particular service, with supporting subsystems, often purchased as modules or skids. Here, let’s focus on such subsystems. In many instances, they are left out of an evaluation entirely or considered secondarily. However, these subsystems frequently provide significant opportunities.
Types Of Subsystems
We can divide subsystems into two categories: active and passive.
Active subsystems have their own automation capabilities, even if they are on a small scale. For example, consider a chemical mixing and injection skid to draw product from two chemical tanks, mix them in a specific proportion and pump the mix into the larger process unit at a desired rate. It usually would have its own instrumentation (level for the tanks, flow meters, pressure transmitters, etc.) plus a programmable logic controller (PLC) to read the instruments and regulate the functions. The distributed control system (DCS) running the larger unit tells the PLC what it should be injecting into the process. The PLC handles the internal functions of the skid.
This relieves some of the loading from the DCS because it doesn’t have to manage all the smaller functions of the skid. On the other hand, quite possibly neither the DCS nor the plant’s maintenance department get any substantial amount of diagnostic data from the skid. Presumably the skid, at a minimum, does send an alarm to the control room if it can’t do its job because a pump has failed or one of the chemicals has become depleted. That may be the extent of its condition monitoring and reporting capabilities, though. Some skid builders can build in more-sophisticated monitoring capable of sending more-granular information, such as the level of the chemical containers, condition of the pumps and so forth. However, this adds expense and usually won’t be done unless the customer specifically requests such capability.
In contrast, passive subsystems have no internal automation or control capabilities. A heat exchanger controlled entirely by the DCS is a good example. The subsystem itself is essentially mechanical.
In both situations, getting the kind of detail necessary to make useful evaluations will require adding instrumentation if more data are needed, and then analyzing the data from existing and new instruments to create actionable information.
What Data Are Available?
Returning to our chemical injection skid, let’s say the control room wants to know how much of each chemical remains in each tank. Several considerations play into this:
• What kind of level measuring device did the builder put on the skid? Is it simply a switch that sends a low-level alarm or does it monitor the whole scale from full to empty?
• Does the PLC running the skid look at the full scale or does it only care about crossing a critical line?
•Is the PLC set up to report the level measurements to the control room or does it lack that functionality?
So, you must ask if the variable of interest is being measured and if accessing the data is possible. The answer could be no —on one or both counts.
In such a case, you many need a path to talk to the field device or additional instrumentation. If the skid has a full-scale level device, such as a differential pressure (DP) or a guided-wave radar (GWR) instrument, it may be possible to add a wireless converter. If the transmitter uses 4–20-mA plus HART, you can install a wireless adapter to send the relevant process data to the plant’s WirelessHART network. This need not interfere with the connection to the PLC, so the level transmitter can continue to do its job in terms of process control.
If the level devices on the skid aren’t suitable, you may need to add an entirely new instrument; a good choice usually is a DP or GWR device with native WirelessHART (Figure 1). Other technologies are also adaptable using 4–20-mA-to-WirelessHART adapters. If you have a network in place but it doesn’t have spare capacity, it’s fairly simple and inexpensive to expand it; this almost always offers a cost-effective option.
Considering skids and modules more broadly, dozens of different performance and condition-monitoring applications may be pertinent, depending upon the nature and complexity of the subsystem. If the PLC or other controller can’t readily deliver information as to what’s happening, you will have to reach in from the outside —WirelessHART can help accomplish this task. Hopefully, you only may need to install wireless adapters because all points of measurement already are in place. If adding new points of measurement is necessary, this will be costlier and may require downtime for process penetrations. However, you at least will find a growing roster of native wireless instruments from a variety of vendors.