Five days before Christmas this year, a new wave of process plants will begin to fall under emissions limits set out in European law.
The Medium Combustion Plant Directive controls sulphur dioxide, nitrogen oxide and dust pollution from the burning of fuels in plants with a rated thermal input between 1 and 50 megawatts. It is thought to cover more than 30,000 sites across the UK.
New plants of this size will have to meet stated emissions limits from 20 December 2018, with staggered deadlines for existing sites to comply. Larger and smaller facilities are already covered by the Industrial Emissions Directive and the Ecodesign Directive respectively.
Marcus Pattison, the organiser of air quality and emissions event AQE 2018 which takes place this November in Telford, warns: “It is essential for manufacturers and process operators to keep abreast of the latest regulatory developments, in addition to the standards, methods and technologies that relate to the control and monitoring of emissions.”
Andrew Dixon, from Gasmet Technologies UK, says permits issued in accordance with the Medium Combustion Plant Directive require certification of emissions monitoring equipment.
“It is vitally important to take future requirements into account when choosing appropriately certified technology,” he adds. “Where possible, the monitoring requirements should be assimilated into the plant design at an early stage.”
Steve Billingham, chief executive at Oxfordshire-based Duvas Technologies, points out that as well as the pollutants covered by the directive, other gases including benzene, ammonia and formaldehyde could be released by process plants.
“The presence of such gases may mean that the process is inefficient, or that equipment is not operating effectively,” he says.
It is vitally important to take future requirements into account when choosing appropriately certified technology
Andrew Dixon, Gasmet Technologies UK
As well as a valuable indicator of problems within the plant, an escape of gas can be extremely dangerous in its own right.
“The aforementioned gases are highly toxic to humans and the environment, if released in high quantities,” says Billingham. “Benzene, for example, is a well-known carcinogen. As gas travels with wind direction, these impacts can affect areas much wider than just the point of pollution.”
This dispersal of emissions is one of the key challenges when it comes to detecting a leak. “Most gases move rapidly away from the point of pollution. As such, if static monitoring equipment is used, detection can only be made if the pollutant travels near to that point.”
Even if modelling software is put in place, this is limited by the accuracy of the algorithm. As such, mobile emission-detection methods are evolving.
“Technology is seeking to become ever more mobile; allowing real-time readings at exact locations,” says Billingham. “This increases accuracy of data and the understanding of the true situation significantly.
“The Duvas DV3000, for example, delivers low-level, real-time, mobile, multi-species readings, thanks to the use of ultraviolet spectroscopy.”
Edward Naranjo, director of fire and gas systems at Emerson Automation Solutions, points out the immediate dangers of certain unwanted emissions.
“Large unwanted escapes of combustible gases lead to fire and explosions,” he warns starkly. “Even at low gas concentrations, combustible airborne contaminants may cause acute and chronic effects on plant personnel, and if gas is dispersed beyond the fence line, affect residents of the surrounding community.
“Toxic gas releases may result in injury to people and animals or loss of life. Gas escapes can also result in degradation of the environment, as pollutants are introduced into the soil and waterways often far from the point of release.”
Naranjo explains that in the complex make-up of modern process plants, there can be a multitude of potential leak points.
“On loss of containment, lighter-than-air gases can quickly travel to the atmosphere,” he says. “Without a structure to stop their travel, gases disperse and don’t reach detectable or alarm level concentrations. By contrast, heavier-than-air gases may pool in low lying areas like sumps and stratify. Depending on detector placement, a device may not detect the gas.
“There is also consideration for pressurised gas release. Gas may jet out at considerable distance from its source.”
Technology is improving on two major fronts to increase detection efficiency, he says. “First, manufacturers are developing area monitors like open path gas and ultrasonic gas leak detectors. Because these devices provide large areas of coverage, they stand better odds at detecting a gas release regardless of wind velocity and orientation.”
These systems can also be more economical in some spaces if fewer devices are required per target area.
“Second, computer-aided design tools, field mapping software, and fluid mechanics modelling programs are enabling end users to identify the optimum number and placement of detectors to achieve a detection coverage commensurate with the level of risk.”
Technology increases accuracy of data and the understanding of the true situation significantly
Steve Billingham, chief executive, Oxfordshire-based Duvas Technologies
Such instruments rely on process safety methods such as those published by the National Fire Protection Association and the Society of Fire Protection Engineers, he adds.
Looking ahead, Naranjo believes process manufacturers will come to put their faith in gas imagers as the emerging technology improves.
Infrared gas cloud imaging allows a scene, either illuminated by infrared laser radiation or infrared radiation from natural sources, to be captured by a specialised camera that allows rapid identification of leaks.
“Fixed gas imaging systems offer the potential for characterising material releases in ways few other detection technologies can,” says Naranjo.
“They can identify the source of the leak, measure gas concentration across a gas cloud and monitor the evolution of the gas as it disperses in air. In addition, these systems can resolve multiple-leak scenarios.”
Fixed gas imagers can also reduce monitoring costs, he says. “Emissions monitoring often depends on routine inspections with specialised equipment. Some of the surveillance is done from an aircraft, a vehicle or on foot. It is labour intensive. Installing fixed gas monitors to detect emissions reduces the need for frequent plant inspection.”
Process safety or environmental monitoring instrumentation is no substitute for inherently safe plant design and good maintenance practices
Edward Naranjo, director of fire and gas systems, Emerson
Current obstacles to be overcome include the limitations of resolution, according to Naranjo. “Regrettably, devices with large areas of coverage generally detect only large leaks – for instance those with a mass flow rate in excess of 1kg per second over five minutes,” he says. “They lack the resolution to detect small quiescent releases that are common in many process plants.
“By contrast, many handheld or transportable systems, while suited to detect small releases, lack the coverage to monitor large sections of a process area. “Further, because they are not designed for continuous operation, they cannot be used to provide early warning when a release has occurred.”
He says these difficulties illustrate just how tricky it is to confidently detect all emissions from process plants – and calls for dedicated work on the subject.
“Fugitive, low-pressure releases are perhaps the most difficult to detect as gas is quickly diluted in open modules. Programmes that seek to improve detection accuracy while reducing the rate of false positives contribute toward reducing plant emissions.”
However, as well as driving improvements in cutting edge leak detection technology, process manufacturers should remember the importance of doing the basics.
“Process safety or environmental monitoring instrumentation is no substitute for inherently safe plant design and good maintenance practices,” says Naranjo.
“Equipment sized appropriately and maintained is less liable to leak. Establish and follow maintenance programs for field instruments and monitor the integrity of pipework and vessels. Because of their high failure rates, stakeholders should pay particular attention to the maintenance of control valves and pumps.
“Last, one should install smart field devices where practical. Smart field devices offer significant advantages for tracking and maintaining plant assets and are equipped with diagnostics that help improve device availability.”