Multi-measurement, deeper process insights

If you speak to the process operators in your plant they could probably tell you several ways in which they could run the process more efficiently with higher quality, greater yield, quicker batch runs, and less emissions had they only got a temperature profile instead of a single point measurement in their column, tower, reactor, furnace, kiln, or boiler etc. Multi-measurement transmitters make it easier than ever before for plants to deploy temperature profiling where it was not practical in the past. They are also ideal for leak detection in gas storage tanks such as in LNG processing plants, LNG floaters (FLNG), carriers, and terminals. So how can temperature profiles help and what should you deploy? Here are my personal thoughts:

The VDI/VDE process sensor 4.0 roadmap points out that an overarching development objective for process sensors is the determination of spatially distributed process information. From a single point measurement to one-, two- and even three-dimensional measurement and thus obtain a more detailed "insight" into processes.

Apart from process improvement and gas leak detection, temperature profiling can also be used to build a more accurate Digital Twin for equipment modeling for improved reliability and maintenance prediction, both on premise as well as through Industrial IoT (IIoT). This is possible thanks to digital networking, not practical with 4-20 mA signals.

In some applications, the temperature profile is captured using multipoint temperature probes with sensor arrays, while in other applications multiple single temperature sensors are used.

To condition the signals from all these sensors, you used to have to decide between either accuracy, using many single-point measurement transmitters, or low-cost, using control system temperature input cards or temperature multiplexers. Digital communication technology now enables multi-measurement temperature transmitters which provide both the precision of field mounted transmitters, and economy using digital networking technologies (fieldbus or wireless). Multi-measurement transmitters are an ideal solution and a logical transition to digital communications for a more efficient plant.

In older plans these temperature points were often not instrumented due to the high cost. However, increasing demands on process performance such as quality/yield, throughput, energy efficiency, and pollution abatement etc. now requires these points to be instrumented.

Another challenge in existing plants is that the old temperature multiplexers used in these applications are now failing, but due to obsolescence, spare parts are no longer available.

Multi-Point Temperature Applications

The right temperature is important for the operation of many processes. The wrong temperature will impact plant throughput, quality, and yield. Temperature is also important for maintenance, as high temperature is a leading indicator of problems in motors and machinery. If left unattended, improper temperatures can result in plant downtime and maintenance costs. Multi-measurement temperature transmitters are ideal for applications where there are many temperature measurements clustered together. Applications include:

• High resolution temperature profiles of storage tanks using multipoint temperature sensor arrays for computation of density to calculate volume and mass of the product.
• High resolution reactor temperature profiles captured using multipoint temperature sensor arrays to maintain a gradual and uniform temperature rise throughout the reactor without runaway hot-spots or channeling to prevent product or catalyst damage, and ensure reaction efficiency. The temperature profile is more accurate, enabling operators to better decide when the reaction process is complete, and stop before it runs too long, with a more repeatable result. That is, it is easier to operate the reactors.
• Column/tower temperature profile with sensors at each tray to optimize separation and product quality.
• Temperature profile of furnace or kiln to determine how efficiently the furnace uses energy to improve energy usage to reduce operating costs.
• Motor winding temperatures to ensure they are operating within specifications, thus extending service life and preventing unnecessary downtime. Particularly in pump-alleys where many motors are clustered together.
• Bearing temperature on compressors, pumps, fans, agitators, and conveyor belts etc. to alert when they exceed suggested operating temperatures to prevent potential damage, cascading into shutdowns of larger processing equipment.
• Heat exchanger efficiency by measuring inlet and outlet temperatures for steam and product to detect degradation due to fouling to determine if cleaning is needed. Particularly in heat exchangers with multiple bundles to determine which bundle is fouling.
• Analyzing boiler tube surface temperature to detect slagging or soot deposits hampering heat transfer and predicting fatigue to prevent boiler shutdowns due to tube ruptures, improving efficiency and plant availability.
• Monitor temperature profile alongside or within LNG tanks using multiple sensors to detect cold spots due to gas leak.

Old Solutions Fall Short

Several temperature profile solutions have evolved over the years. Older solutions include:

• Direct wired DCS temperature input card
• 4-20 mA transmitters
• Temperature multiplexer
• Remote-I/O temperature input card

On some infrequently used reactors such as for catalyst regeneration there may not even be any permanent monitoring solution, just empty thermowells due to the high cost of solutions available in the past. The operators had to hook up hundreds of temperature elements to portable chart recorders or data acquisition system each time the catalyst had to be regenerated - a time-consuming practice. Imagine how much time can be saved by personnel in your plant.

Similarly, maintenance inspection of equipment like motors and machinery is done walking the plant with a temperature gun jotting down readings on a log sheet which is error prone, time consuming, and done much too infrequently to get an early warning of developing equipment problems. Just imagine the productivity improvement.

Direct wired DCS temperature input card

An old solution is to hardwire sensors point-to-point direct to the DCS I/O subsystem. This method has many drawbacks; requiring many expensive temperature input cards and large cabinet footprint. Connection requires either extension/compensating wire (for thermocouple) or multiple wires (3 or 4 for RTDs), conduit, and trays for long distances which is costly and labor intensive. In hazardous areas this method may require many barriers for intrinsic safety at additional costs. Moreover, the sensor’s low signal is prone to EMI/RFI interference noise pickup, resistance drift, and stray junctions (the thermocouple extension/compensating cable/leads are not identical to actual sensor wire so error due to intermediate junctions may be introduced) causing inaccurate readings.

Figure 1 Direct wired DCS temperature input card

4-20 mA transmitters

Another old solution is sensor head-mounted "puck" and "cookie" single-point temperature transmitters to condition the sensor signal to 4-20 mA in the field for better noise immunity, hardwired point-to-point to DCS AI cards. While more accurate than direct sensor wiring, this method also has drawbacks, especially when used for multiple temperature sensors. This requires many single-point transmitters plus analog input cards in the DCS and hardwiring with long cables, conduit, trays, and marshalling for each transmitter. The hardware, engineering, and installation is labor intensive and expensive. 4-20 mA requires careful matching of range in the system and every transmitter, plus five-point loop check at commissioning. The intermediate 4-20 mA signal conversions also impact accuracy.

Figure 2 4-20 mA transmitters

Remote-I/O temperature input card

A remote-I/O solution requires expensive modular hardware such as the temperature input cards, backplanes, remote I/O ‘head’ network cards, and fiber optic networking all of which should be in a redundant configuration. Additionally, the field cabinet is much larger and heavier than traditional junction boxes in order to house the remote-I/O subsystem, and the field cabinet responsibility is pushed to the systems group. Sensors are hardwired point-to-point to the field cabinet. Additionally, separate power supply is required, adding costs. Concerns with putting input cards outdoor include ambient temperature, humidity, airborne contaminants, vibration etc. affecting active electronic equipment, as well as hazardous area compliance, power distribution and grounding issues, open cabinet door maintenance procedures, system engineers have to work in the field instead of in the equipment room, requirement for shelter for maintenance to open field I/O cabinet while it is raining etc.

Figure 3 Remote-I/O temperature input card in the field

Temperature Multiplexer

Yet another old solution is temperature multiplexers (MUX). This method also has drawbacks; a MUX solution concentrates a large number of sensors into one piece of hardware. This creates unnecessary risk, as a single multiplexer failure would mean the loss of many measurement points. Sensors are hardwired point-to-point to the MUX with long cables, conduit, trays. Connection requires either extension wire (for thermocouple) or multiple wires (3 or 4 for RTDs), conduit, and trays for long distances which is costly and labor intensive. In hazardous areas this method may require many barriers for intrinsic safety at additional costs. Moreover, the sensor’s low signal is prone to noise pickup, resistance drift, and stray junctions causing inaccurate readings. Input channels are not individually isolated which leads to ground loop and surge challenges, because when there are many channels in one MUX, sensors tend to be wired from several places, having different ground potential, into the same MUX. Temperature multiplexers were common in the past. Many of these are coming to the end of their life. They typically get replaced by multi-measurement temperature transmitters.

Figure 4 Temperature Multiplexer

Digital Solution

Thanks to digital communication networks, multi-measurement temperature transmitters have emerged as a cost-effective solution. Each transmitter is installed close to the piece of equipment for which it shall capture the temperature profile, even in hazardous areas. Multiple equipment need not be wired to the same transmitter thus avoiding ground loop issues and enabling short sensor wiring. This reduces installation cost and improves the accuracy, as the cost and drift problems of long sensor wires are eliminated. A stable and accurate measurement translates into more accurate and higher integrity monitoring for improved plant performance.

Unlike 4-20 mA which is limited to one real-time variable per wire pair, digital networks communicate many process variables to the DCS. A single multi-measurement temperature transmitter accepts many sensor inputs. This reduces the cost per measurement point by drastically lowering the number of transmitters, system wiring, conduit trays, safety barriers, input cards, installation and commissioning time needed to complete a project.

Multi-measurement temperature transmitters share the same network together with other devices. A dedicated network is not required.

The ability to add new innovative devices is one of the many reasons plants are built with digital networks in the first place. Lastly, the temperature readings are transmitted as real engineering unit values avoiding range mismatch between 4-20 mA transmitter and DCS, thus eliminating five-point loop tests, and speeding up commissioning. Diagnostics for individual sensors and real-time PV validity indication for operator confidence, as well as measurement over the full sensor limits (not restricted to 4-20 mA range) etc. are other reasons plants go digital.

Fieldbus Multi-Measurement Temperature Transmitters

Multi-measurement temperature transmitters based on FOUNDATION fieldbus support eight isolated and individually configured temperature sensors each. When a very high number of points are required, several transmitters are mounted inside the same field enclosure. Fieldbus transmitter use only a single pair of wires to the junction box for all 8 measurements and are two-wire loop powered making wiring easy. Since fieldbus transmitters are bus-powered, separate power supply cabling is not required. Plants that already have FOUNDATION fieldbus networks can easily add digital devices with no additional system upgrades. Depending on the hazardous area protection scheme preferred in the plant, use either intrinsically safe or non-incendive transmitters, or mount inside a flame/explosion-proof junction box.

FOUNDATION fieldbus devices have computational ability to calculate the average of multiple sensors, select the highest or the lowest temperature, or choose the valid value from redundant sensors etc. FOUNDATION fieldbus supports peer to peer communication directly from one device to another. That is, temperature readings can be combined with readings from other transmitters to compute density, corrected level, or volume etc.

Figure 5 Fieldbus Multi-measurement temperature transmitters

Readings from transmitters in inaccessible points such as on columns and towers, or tall furnaces and boilers etc. can be communicated to a local indicator at eyelevel. One local field indicator can receive up to eight variables from any transmitter or any other device on the network.

Wireless Multi-Measurement Temperature Transmitters

Wireless multi-measurement temperature transmitters based on WirelessHART is another digital solution which accepts four temperature sensors in the same device. Plants that already have wireless sensor network infrastructure can easily add digital devices. A wireless transmitter connects quickly and easily since no wires have to be run back to the system, thus shortening project time.

This enables temperature transmitters to be installed on points previously not monitored by the DCS, to enhance plant performance and reliability. A wireless multi-measurement temperature transmitter shares the same WirelessHART network infrastructure as other wireless sensors. Make sure to use transmitters which are intrinsically safe, suitable for use in hazardous areas. Since they are wireless, no safety barriers are required and entity parameter engineering is not required.

Figure 6 Wireless multi-measurement temperature transmitters

Digital Transformation of Existing Systems

Modern DCSs have native support for wireless and fieldbus. However, any old DCS can also make use of multi-measurement temperature transmitters or any other WirelessHART and FOUNDATION fieldbus device using a wireless gateway or fieldbus linking device available from many suppliers, which convert to Modbus/RTU, Modbus/TCP, or OPC. Fieldbus or wireless support in the DCS engineering console is not required as the configuration is done through a web server embedded in the gateway or through Intelligent Device Management (IDM) software. Once the network is in place, more devices can be added at will for all kinds of measurements. One wireless gateway supports a wireless network with up to 100 transmitters (400 sensors). A fieldbus linking device supports 4 fieldbus networks with 16 devices each (512 sensors).

Digital Transformation of Plant Operation

Temperature sensors on seldom used reactors and other processing equipment may initially not have been connected to the DCS. Similarly, temperatures of motor windings and bearings on various assets have not been monitored continuously as many plants were designed with the minimum amount of instrumentation, due to the high cost of hardwiring. However, increased focus on reliability, energy efficiency, shorter shutdown periods and safety now requires additional measurements to enable digital transformation of how the plant is run and maintained. Fieldbus and wireless are ideal solutions. Digital multi-measurement temperature transmitters have made the monitoring of large concentrations of temperature points simple and cost effective. The combination of transmitters with multiple inputs and digital networks enable any plant to improve plant performance, increase productivity and safety, and reduce downtime.

The future is digital

In addition to multi-measurement transmitters the VDI/VDE roadmap describes composite measuring systems with sensors talking among themselves, valve positioner talking to sensors, 3D profiles for more measurements, mutual plausibility checks, sensor firmware update, new business models, and more. Such enchanted devices are possible thanks to digital networking, not practical with 4-20 mA signals. Well, that’s my personal opinion. If you are interested in how the digital ecosystem is transforming process automation click “Follow” by my photo to not miss future updates. Click “Like” if you found this useful to you and share it with others if you think it would be useful to them.