Smart Plant Modernization of Core Process Control - Part 2

Reducing the workload: Maintenance

There are thousands of instruments in a plant, so it is critical they are reliable, requiring minimal attention and that any necessary maintenance is made easy. This is over and above what digital transformation can do for equipment reliability.

The mechanical servo and float-and-tape tank gauges, displacer interface level, turbine and PD flowmeters, turbine flowmeters, and mechanical controllers have moving parts and therefore are susceptible to wear and failure. When they do fail, the maintenance team must replace the failed device, resulting in increased workload and maintenance cost as well as causing production downtime. The recommendation is to upgrade these positions to radar level gauges; GWR level and interface level transmitters; magnetic, vortex, or Coriolis flowmeters; and electronic controllers, respectively.

Float-level switches used to detect high or low level also have mechanical moving parts. The recommendation is to upgrade these positions to vibrating-fork-level switches without moving parts.

Turbine flowmeters may also be used for custody transfer. Also, for these positions, the recommendation is to upgrade to custody-transfercertified ultrasonic or Coriolis flowmeters without moving parts.

Displacer transmitters may also be used for density measurement. For these positions, the recommendation is also to upgrade, in this case to vibrating fork density transmitters.

Lastly, old-style control valve positioners and position transmitters have mechanical position feedback linkages, which are susceptible to wear and failure. The recommendation is to upgrade these valves to positioners and position transmitters with non-contact position sensing with no moving parts. As a result of more reliable instrumentation, plants enjoy reduced maintenance cost and reduced process downtime

Replacements like temperature sensors in thermowells or pressure transmitters with manifolds cause little downtime. However, replacing an inline flowmeter, bottom-mounted level transmitter, an instrument on a pressurized tank, or a valve positioner causes significant downtime. For these positions, it is even more important to have a reliable instrument with no moving mechanical parts.

Flowmeter calibration is labor-intensive because the flow sensor is installed inline and must be pulled. Often, the flowmeter must be shipped to an external lab, possibly overseas. This incurs high maintenance cost and production downtime during the swap. The recommendation is to upgrade to flowmeters with smart meter verification (SMV) to predict calibration drift to assess the need for flowmeter calibration or whether calibration can be scheduled later, all without interrupting the process. By optimizing the calibration schedule, plants reduce maintenance cost and production downtime.

As explained previously, with analog control valve positioners or I/P converters, there is no telling if control loop setpoint deviation or hunting is due to the control valve. There is a risk of unnecessarily overhauling the valve, adding to the workload and maintenance cost as well as causing production downtime. The recommendation is to upgrade all control valves with smart valve positioners with valve performance analytics. Valve issues can be verified on the positioner display or over Bluetooth from a mobile device. By verifying valve issues, plants can avoid unnecessary overhaul or optimize a necessary overhaul. Plants reduce maintenance cost and production downtime.

Devices connected to the DCS through hardwiring provide very limited information, for example, in the case of drives and motor starters. Maintenance technicians must go to the marshalling cabinet or motor control center (MCC) to check from the local display. Similarly, interaction becomes very slow when RS-485 networking is used in cases such as instrumentation through HART multiplexers. This adds to the workload.

Another challenge is that as servers go obsolete or fail, new servers do not support RS-485/RS-232. Many new replacement devices no longer support RS-485. The recommendation is to upgrade devices and DCS to use Ethernet networking with the appropriate protocol like PROFINET for motor controls and HART-IP for instrumentation and infrastructure. As a result of the complete set of information from the device, plants reduce maintenance cost and production downtime.

Improving plant sustainability

Automation has a key role to play in the sustainability of industrial plants. Solutions for several challenges such as loops in manual and comfort zones are already mentioned. But even more can be achieved by modernizing automation. And this is over and above what digital transformation can do for sustainability.

Direct spring pressure release valves (PRVs) start lift and reseat well below set pressure. This, in turn, causes emissions, flaring, and product loss. The recommendation is to upgrade to pilot operated PRVs (Figure 5), which have more precise pressure handling thus reducing release and flaring. As a result, plants minimize product loss and enhance sustainability. They are also less sensitive to back pressure.

Manual gas leak inspection with portable testers for greenhouse gas (GHG) emissions like methane is time consuming. This in turn leads to delayed detection and response and therefore excessive emissions. It is also labor intensive, which leads to field operator stress. The recommendation is to deploy ultrasonic, infrared (IR), or non-dispersive infrared (NDIR) sensors to detect gas leaks. As a result, plants improve safety and sustainability.

Simple proportional-integral-derivative (PID) control does not handle multiple interacting loops, multiple constraints, and difficult process dynamics such as long deadtime, long time constants, and inverse response. Production operators are forced to make frequent manual adjustments, which leads to operator stress and mistakes. This in turn causes off-spec product, high energy consumption, and low throughput. Combustion control is one such example. The recommendation is to upgrade to a DCS with advanced process control (APC) software for model predictive control (MPC) based on dynamic matrix control (DMC) supporting multiple inputs, outputs, and constraints. For selected loops, this will reduce process variability and keep the process within constraints. As a result, plants reduce operator stress, reduce off-spec product, and improve sustainability and throughput.

PRV in applications with back pressure such as from flare headers often use internal bellows. When the bellows eventually fail, the PRV will vent process gas straight to the atmosphere contributing to emissions and potentially causing fires and explosions. The recommendation is to upgrade to PRV with backup piston. The backup piston can reduce emissions by more than 90 percent in the event of a bellows rupture, and it ensures balanced operation. As a result of the better back pressure handling, capacity, and reliability, plants reduce emissions and product losses, while improving safety and sustainability. Bellows leak detection can also be added.

Improving plant safety

Both functional safety as well as occupational safety and health will benefit from automation modernization. Many modern instruments recommended in this article like the tank gauging system, radar level, GWR interface level, level switch, ERS, temperature and position sensors, as well as Coriolis, magnetic, and vortex flowmeters are available as safety integrity level (SIL)-rated. This means they have a low failure rate and high diagnostic coverage, making them ideal for safety instrumented functions (SIFs). As a result, plants can improve safety.

High aerodynamic noise is created by turbulence in gas, steam, or vapor flow in high pressure-drop valves. Traditional valve trims and insulation do not help sufficiently. Noise is an occupational health issue, so requirements have become more stringent. The recommendation is to upgrade to control valves with valve trims and cages designed and built using additive manufacturing for low noise with minimal pressure drop. Additionally, a modal attenuator (Figure 6) can be inserted to act as a silencer with no pressure drop. As a result, plants improve occupational safety.

A smart plant modernization workshop

Your plant may have some but not all these problems. And it may have other problems. Conduct a smart plant automation modernization workshop to uncover challenges around manual loop intervention, instrument maintenance, sustainability, and safety. Based on the workshop findings, replace old automation components across the plant before they fail. But remember: Each modernization campaign and application should be validated. Do not replace just because there is something new.