Proper functioning of instruments and control systems is pivotal to ensuring safety, environmental compliance and productivity. Mishaps caused by malfunctioning instrument systems could tarnish a company’s public image, credibility and even survival.
Today, an engineer typically can choose from a vast array of instruments for a given application. For instance, hundreds of different techniques can accurately measure flows, levels, temperatures and pressures. In addition, smart instruments now available offer self-diagnostics that streamline troubleshooting efforts. Unfortunately, though, problems with instrument selection and installation still persist.
For example, consider this small sampling of selection/installation mishaps:
• An orifice-plate/differential-pressure (dP) cell in steam flow service failed to function due to poor installation. The inability to automatically control steam flow forced operators to operate in manual mode, leading to sub-optimal performance.
• A thermocouple didn’t respond to temperature variations because of improper location.
• A misapplied radar level transmitter plagued operators with false level alarms.
A best-in-class plant may have specific procedures for instrument application. However, these usually focus on the selection and installation of a particular type of device and lack the flexibility to handle new or complex instrumentation. So, here, we’ll look at a generalized approach based on a systems methodology that, in conjunction with specific procedures, should help minimize instrument selection and installation mishaps. Although this article looks specifically at sensors and transmitters, the approach applies to other equipment as well.
The Systems View
Figure 1 shows an example of systems to consider in the selection and installation of an instrument. You can assign as many systems as you deem necessary for a specific problem. Let’s look at the systems depicted in Figure 1.
Figure 1. Considering the overall environment in which the sensor and transmitter function is important.
Objectives and environment. These are the top-level considerations. The objectives, for example, may be monitoring, operational control, process safety or environmental compliance; the stated aims collectively will influence cost and justification. Objectives also should address lifecycle, which includes long-term issues such as maintenance and calibration; special training or tools, if any, required; compatibility with the existing distributed control system (DCS) and future upgrades; and minimizing the cost and environmental impact of the system’s eventual retirement.
Environment aspects include, for example, area classification and weather extremes. You should consider top-level issues such as:
• purpose of the instrument system;
• input/output (I/O) and power needs, and other utility requirements;
• interface with your DCS;
• physical space demands;
• necessary maintenance and upkeep, including the need for calibration and testing tools — safety-critical sensors require regular testing and accurate record-keeping;
• service and support from the manufacturer or vendor;
• training, if any, for operators and maintenance personnel, including any special calibration or regulatory requirements;
• electrical area classification;
• necessary special protection, e.g., shelters or housing, from weather extremes such as snow, heavy rains, hurricanes, and excessive heat or dust;
• the maturity of the instrument technology — nascent technology could present a relatively high risk of failure while long-established technology could soon become obsolete; and
• lifecycle cost and economic justification.
Process. This system considers the impact of the process fluid, operating conditions and the immediate environment on the integrity of the process signal reaching the instrument. Experience shows that most sensor and transmitter problems stem from inadequate attention to process systems.
Simply put, the main function of the sensor and transmitter is to provide an accurate and precise (repeatable) signal as soon as practicable. This demands an instrument system compatible with the needs of the process fluid and ambient environment.
The near-infinite number of combinations of process conditions and instruments leads to correspondingly vast opportunities for improper installation. Table 1 presents a typical list of process variables that could impact the function of a sensor and transmitter.
As stated before, inadequacies related to the process system account for a large proportion of problems with instruments. The key to avoiding mishaps is to ensure instrument (sensor) compatibility with the entire range of process conditions. Seemingly minor omissions often have resulted in major consequences, such as:
• An orifice-plate/dP cell meter for measuring flow through the convection section of a furnace began to alarm frequently on low flow. The liquid contained a moderate amount of particulates, which prompted the need for frequent meter maintenance. The operators believed the high frequency of the alarms meant they were just nuisance alarms (this was prior to the advent of the alarm management systems) and, hence, reflexively “silenced” these alarms. Low flow eventually resulted in a tube leak that caused significant damage to the furnace.
• A turbine flow meter with a totalizer was the culprit behind difficulties in consistently making in-specification batches. It seldom gave proper quantities of liquid for a batch. Investigation into the meter installation revealed that, between batches, the meter was exposed to vapor that caused erroneous readings of flow and its totalization. A simple piping change to re-orient the meter (so it always remained submerged in liquid) solved the problem.
Sensor and transmitter. Briefly put, the key objective of the sensor, which is the component in contact with process conditions, is to send a representative signal to the transmitter as quickly as possible. The transmitter then normalizes this signal to 4–20-mA, a digital value or another output. Today, a smart (digital) transmitter can provide, in addition to the current value of the process variable, a wealth of data such as device status, diagnostics and transmitter-specific information. Moreover, it requires fewer calibrations than an analog counterpart and facilitates remote calibration and troubleshooting. Figure 2 shows a schematic of a smart transmitter.
Figure 2. Such a device can provide a wealth of diagnostic and other information besides the current measurement.
Some aspects to consider include:
• accuracy specification, generally stated as % span or % value of the process variable — typically, accuracy at the ends of the span or range is lower than at the middle;
• electrical area classification (API-500 or zones) for the transmitter;
• the proper application as well as limitations of the device, as detailed by its manufacturer or vendor — including all relevant information in your specification will help minimize application mishaps;
• calibration and testing needs — a device involved in custody transfer requires periodic calibration, one that’s part of a safety instrumented system (SIS) must undergo specified proof testing, and even smart transmitters need occasional calibration;
• suitable location and orientation to ease calibration or proof-testing;
• grounding — necessary for some instruments such as magnetic flow meters, it can mitigate noisy or erratic signals from other devices; and
• the manufacturer’s specifications on voltage requirements (typically, 24-V DC) and allowable variations in voltage at the transmitter.
Signal and power systems. A number of issues can afflict these systems, including:
• electromagnetic interference (EMI). Such interference, generated, for example, by AC/DC motors, power cables, welding and lightning, can affect signals sent by the transmitter. To combat EMI, use twisted and shielded wiring. In addition, keep signal wiring some distance from power cables to minimize interaction of the magnetic fields generated by power current and those by signal wires;
• protection of signal and power cables from vibration and weather extremes, including snow, rain, excessive heat and dust; and
• voltage variation. Manufacturers provide detailed specifications on voltage requirements (typically, 24-V DC) and allowable variations in voltage at the transmitter.
DCS and cloud system. Signals from the transmitter go to the DCS, which then responds to such inputs with outputs to control valves, solenoids, etc. With today’s modular systems, I/O or power availability for a control loop generally aren’t issues. Nonetheless, the DCS architecture should consider redundancy of I/O and power supplies as well as backup power. Diagnostic utilities available with most I/O systems ease configuration and troubleshooting.
The DCS now usually is part of an organization’s connected network. It communicates not just to the plant but also to corporate (e.g., enterprise resource planning), cloud storage and internet systems. This mandates close coordination between the operating technology and information technology groups.
This connectivity has raised the DCS’ vulnerability to cyberattacks. With an existing DCS system, addition of a sensor, transmitter or loop generally doesn’t require a comprehensive security review because the security system already is in place. For more-extensive efforts, follow some cybersecurity best practices such as:
• provide a greater level of security for any device that’s part of an SIS;
• always include DCS system vendors in cybersecurity discussions; and
• establish access control (both electronic and physical) and training along with patch management and defense-in-depth.
Do Instrumentation Right
A systems approach forces us to methodically consider all potential issues that could impair the functionality of process instruments. Using this approach along with instrument-specific procedures for selection and installation should help minimize application problems.
GC SHAH, PE, CFSE, CSP, CFPS, is a Houston-based consultant specializing in process safety, including hazard analysis and fire protection services. Email him at [email protected].