April 10, 2017

Chinese Power Plant Demonstrates Criticality of Effective Level Measurement in High Pressure/High Temperature

by Robin Hudson, Rosemount Level Product Manager, Emerson Automation Solutions

An interesting case history from a 600 kW coal fired power plant in China has implications for any safety-critical environment. This application involved drain pot alarms on super critical steam lines.

As you know, any liquid entering a steam turbine will cause damage. This is costly and even more important, can be dangerous to personnel due to the extreme temperatures and pressures involved in the steam turbine. Generally, steam pots (also called condensate traps or steam traps) are used on steam lines to drain off condensate before it can enter the turbine. These drain pots use level sensors to detect high levels of condensate buildup and then open valves to discharge it.

The problem that arises, however, is that the extreme temperatures and pressures in the steam lines can cause many level detection devices to fail. The plant in China had been using float switches but their performance was found to be unstable over 2900psi pressure and 932˚F. Components had to be replaced frequently. In addition, the devices had no self-diagnostics or status outputs so the operators couldn’t detect when the floats might fail or if they were working properly. This high and unpredictable failure rate increased maintenance which added costs and upped the chances of a significant accident.

To resolve the issue, the plant replaced all of their float switches on the supercritical steam lines with Mobrey Hydratect 2462 Water and Steam Detection Systems. Unlike most detection systems, the patented-design Mobrey Hydratect 2462 functions reliably in steam/water detection environments up to 4350psi pressure and 1,040˚F – easily accommodating the requirements of the Chinese plant. In addition, the electronic controller gives a visual indication and relay output to indicate steam, water or a fault condition. Extensive self-monitoring within the system assures that any component failure results in a fail-safe condition.

To sum up the benefits of using this type of detection system, the Mobrey Hydratect 2462 lets you –

  • Reliably detect water or steam in lines, columns, and condensate pots
  • Eliminate the need for routine testing through superior reliability
  • Prevent turbine water damage with Turbine Water Induction Protection (TWIP)
  • Saves time with self-checks that make routine testing unnecessary
  • Configurable steam/water thresholds adjustable according to water quality

The combination can significantly reduce maintenance costs, and even more important, greatly improve protection of people and property.

Do you deal with level measurement in a safety critical environment? What type of system do you use?

April 5, 2017

Application Snapshot: Optimizing Leak Detection

Leak detection at large chemical sites can be both difficult and expensive. At one Belgian plant, operators were making twice-daily inspection rounds using portable leak detectors — but new and stricter government regulations required continuous monitoring instead. So, plant management decided to implement an online leak-detection system.

In 2013, after research on suppliers and technology, the plant installed a system that uses hydrocarbon fast-fuel sensors and sensor cables, and wireless discrete transmitters. An IEC-standard WirelessHART field network communicates leakage data from the remote transmitters to the plant control system.

The leak detection project saved around 60% compared to a conventional wired approach. Moreover, the system took only one day to commission. Besides ensuring compliance with regulations, the installation has enabled the plant to expand the operators’ view and has enhanced its existing leak-detection practices, helping to improve site safety.

Challenges for Plant Operators

The primary drivers for implementing real-time leak detection were the flammable nature of the product itself and the desire to reduce compliance issues, increase employee safety and provide constant monitoring of the plant’s condition. Specific challenges included:

  • Difficulty in detecting/monitoring hydrocarbon leaks around tanks, valves and pipe flanges
  • Absence of installed instruments on tanks to detect abnormal changes in level due to leaks
  • Fluctuation of tank level with changes in process demand
  • Lack of ready-to-use options to detect leaks at valves and flanges

The plant’s management wanted to avoid the issues posed by traditional wired online monitoring (e.g., requirements for extensive cabling), together with the associated input/output, signal converters and installation costs. So, an evaluation team began a search to select the right technology. It reviewed different solutions and then tested a number of potentially viable available options:

  • Gas detection using a catalytic probe
  • Optical monitoring system (spectral analysis)
  • Analyzer for lower explosion limit (LEL) detection
  • WirelessHART hydrocarbon leak detection
  • WirelessHART fast fuel sensors for leak detection

Figure 1

Leak Detection Photo 1

The team found most promising an approach that integrated sensors that detect liquid hydrocarbons with WirelessHART communications technology. It put that option through further laboratory trials with simulated leaks. Successful test results prompted a decision to implement that approach at the plant.

The leak detection system uses eleven wireless discrete transmitters in combination with fast fuel sensors and sensor cables. When one of the sensors detects xylene or benzene, the associated transmitter wirelessly transmits an alarm signal to a smart wireless gateway for relay to the control room where operators monitor and record the status for accurate incident reporting and time stamping in the host control system. The discrete input/output devices are part of IEC-standard WirelessHART field networks (See Fig. 1).

Figure 2

Leak Detection Photo 2

The sensor cables are attached to the concrete bases surrounding tanks to detect leaks from the tank wall. They also are laid underneath connecting piping and draped around the tank flanges and manholes as well as valves. The fast fuel sensors detect hydrocarbons in sump wells and pump skids. Leaking xylene or benzene upon touching the sensor cable or sensor completes a circuit. This sends a discrete input to the input/output device that transmits it wirelessly to a gateway that relays data to the control room (See Fig. 2).

Key Performance Improvements

The plant team found that wireless technology provides a reliable, affordable way to gather information from hydrocarbon sensors that continuously monitor storage tanks, valves and pipelines throughout the plant, and to deliver that information to the control system to notify operators of any leaks. This automated monitoring system has enabled the site to meet the latest government regulations for storage and handling of flammable liquids.

Installation by the plant’s maintenance team was quick and easy. Smart wireless technology eliminated the need for new instrumentation cabling and associated trenches and ducting, saving an estimated 50% of the cost and 90% of the time required to put in a conventional wired system.

Combining wireless technology with hydrocarbon sensors and sensing cable has provided effective leak detection for the large site. It illustrates how a plant can take advantage of wireless technology to gain business benefits — including enhanced safety and regulatory compliance — in applications beyond the process itself.

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