Leak detection for refrigeration systems: a major challenge for the 21st century

1) Regulations

The regulation also sets down training requirements, prohibitions of use and bans on placing on the market depending on GWP (global warming potential) and by application, reductions in the quantities produced (quotas) and requirements: qualification, compulsory equipment and emission prevention with the obligation to carry out leak checks on installations containing fluorinated greenhouse gases.

Specific F-Gas section on leak checks:

Chapter II Article 4 of Regulation (EU) No. 517-2014 specifies the frequency of checks depending on the charge in tonnes of CO2 equivalent of the installations.

Charge of ≥ 5 t of CO2 equivalent: at least once every 12 months or only every 24 months if equipped with a fixed leak detector.

Charge of ≥ 50 t of CO2 equivalent: at least once every six months or only every 12 months if equipped with a fixed leak detector.

Charge of ≥ 500 t of CO2 equivalent: at least once every three months or only every six months if equipped with a fixed leak detector.

Operators are required to install a fixed leak detector that alerts the operator or maintenance company in the event of a leak for installations with a refrigerant charge equal to or more than 500 t of CO₂ equivalent (e.g. 128 kg of R-404A)

• • For stationary systems (refrigeration, air conditioning, heat pumps, fire suppression)

• • For Rankine cycle machines and electrical switchgear installed after 01/01/2017

For European member states, more stringent national regulations may be in place in addition to the F-Gas Regulation.

2) Leak detection according to standard EN 378 guidelines:

In addition to the environmental regulations, European standard EN 378 provides a supplement to ensure the safety of individuals.

It states that machine rooms must be equipped with detectors to ensure the safety of any users.

For explosive or hazardous gases, the detection threshold is set at less than 20% of the lower flammability limit and ammonia detectors must trigger ventilation, an alarm and disconnect the external power supply.

3) Types of detection: direct detection (portable detection; fixed leak detectors) and indirect detection (continuous monitoring through an expert system)

There are several types of detection:

• • Firstly, direct methods permit on-site leak checks and are used for periodic (portable detection) or continuous (fixed leak detectors) checks. Portable detectors can be used to locate the leak.

• • In addition to this method and to allow comprehensive monitoring of the installation’s operation, indirect methods have been developed. The use of sensors that take regular measurements together with algorithmic calculations allow this type of device to be placed inside the installation in order to identify and monitor its behaviour.

To comply with the F-Gas Regulation, the physical inspection of the installation must be carried out with a portable detector. As stipulated in European Regulation 517-2014, the frequency of checks may be reduced[1] if the installations are equipped with a fixed leak detector or a system based on indirect measurement methods.

Thus, in order to reduce the frequency of physical inspections, it is possible to use stationary detection systems, known as a “fixed leak detector” or “stationary detection system”, and indirect-measurement detection systems installed on the cooling unit that allow continuous monitoring of the installations and trigger an alert when a leak occurs.

Some countries, such as France and Belgium, have adopted the possibility of indirect leak checking with continuously monitored indirect detection systems (see section 5).

Thus, the three leak detection methods are complementary to each other: they guarantee compliance with the F-Gas 2 Regulation by contributing to the reduction of leaks and therefore to the preservation of the environment, while guaranteeing the safety of individuals.

4) Sensor technology

There are three forms of sensor technology: semiconductors (or heated diode sensors), electro-chemical sensors, and infra-red sensors. The choice of sensor depends on the gases or fluids to be detected.

a) Semiconductor sensors: for HFOs, HFCs, CFCs, HCFCs

This technology has the advantage of being the cheapest. However, it is sensitive to ambient temperature, humidity, solvents, cleaning agents, HCs (propane) and NOx (nitrogen oxides).

¹ See table on page 1

The semiconductor sensor, also known as a metal oxide sensor (MOS), detects toxic products, fuels and refrigerants. It is sensitive to changes in gas concentration (decrease in oxygen concentration), humidity, and temperature changes. Its sensitivity is low and it has to be calibrated. It is an economical technology with a long lifetime (around five years).

copyright bacharach

 Detection principle: a thin metal oxide film is deposited on a silicone surface. The oxidising catalytic reaction in contact with the target gas and the heated metal oxide surface changes the electrical resistance and alters conductivity. This change in resistance is correlated with the measured gas concentration.

b) Electrochemical sensors: NH₃

Cette technologie de capteurs est utilisée pour la détection du NH₃. C’ est une technologie ultra-précise, qui assure la sécurité des personnes grâce à une détection à faible concentration de ce produit toxique.

This sensor technology is used for NH₃ detection. It is a highly accurate technology which ensures the safety of people thanks to the detection of low concentrations of this toxic product.

This technology is suitable for a wide variety of toxic gases, and is very accurate at very low ppm concentrations. It responds quickly to a leak and it is a selective technology that only searches for the target gas. As a result, there is no risk of cross-sensitivity. This technology has to be calibrated to the target gas and the sensors have a lifetime of three to five years.

The sensor consists of a working electrode, a counter electrode, a reference electrode and an electrolyte. Electrochemical sensors work like batteries. When the target gas is present, a chemical reaction on the working electrode generates a small electrical charge between two electrodes that is proportional to the gas concentration.

c) Infra-red sensors: detection of HFOs, HFCs, CFCs, HCFCs, NH₃, Propane, CO₂.

Infra-red sensor technology is highly sensitive. It can detect a wide range of refrigerants, while being completely insensitive to other products, and has a long lifetime. It can detect HFOs, HFCs, HCFCs, CFCs, NH₃, propane (R-290) and CO₂.

The infra-red sensor is highly sensitive with very low cross-sensitivity with other gases. This technology is not sensitive to pollutants (silicones, lead, etc.) and can calibrate itself automatically. Response times are short, with detection thresholds possible at very low concentrations (sensitivity of 1 ppm). It is a highly accurate, if somewhat slightly more expensive, detection technology; the average lifetime of the sensor is five to seven years.

The sensor consists of a light source, an anti-interference filter, a detector and a chamber into which the target gas is diffused after being drawn in. Only the green colour of the light beam is filtered and analysed by measuring its intensity. If a gas is present, the green colour of the luminous radiation is reduced by a measurable amount.

5) Detection devices and descriptions

Portable detectors:

Leaks are indicated by an audible and visual alarm and a display screen. This type of detector allows the precise location of the leakage area to be found.

Before each use, a check using a product such as the mini-check, which simulates a calibrated leak of R-134a of 5 g/year, enables the calibration of the device to be verified.

Bubble detection :

After searching for a leak with a portable detector, an aerosol such as Prestobul can be used on pipes, where the suspected leak is located, to pinpoint the source through the presence of bubbles.

Fixed leak detectors:

stationary leak detector, also known as a fixed leak detector, is a refrigerant leak detector. Depending on the model, it is an independent sensor-transmitter with alarms, which can be used either as a stand-alone detector or connected to a control system (e.g. BMS) via a Modbus link. They are generally equipped with one or more relays to activate external safety equipment such as valves, fans, general alarms, etc. They require the installation of sensors, also called “probes”, in accordance with the specific characteristics of the installations and the fluids to be detected. They must be selected and adapted to the fluids that are to be detected.

Positioning of the sensors:

Protected from splashes of water and potential hazards, away from draughty or ventilated areas (high traffic area with strong air flow, extraction area (fan)), and away from radio frequency identification systems or transmitters for these systems.

In the interest of human safety, position the sensors in the living area (oxygenation).

Integrated predictive – DNI: Belgian authorization + energy efficiency

With average leakage rates of around 20% to 25% per year, condensing units installed in food retail outlets or the food industry require permanent leak detectors for the continuity of processes, to improve performance and comply with environmental regulations.

The Matelex Intelligent Level Detector (“DNI” –  Intelligent Level Detector) is a patented fixed leak detector based on the indirect measurement method. This method involves the continuous analysis of the operating parameters of the installation. The DNI, by means of a specific algorithm and associated metrology, learns the so-called “normal” operation of an installation thanks to the permanent analysis of its operation. It then identifies deviations from this operation and is thus able to detect the presence of a leak.

The DNI, compatible with 120 fluids and different types of HP tanks (vertical, horizontal, inclined), uses the principle of communicating vessels to measure the level of fluid in the tank without the need for any major modifications to the installation. A 1″ 5/8 copper column, adapted to the geometry of the tank, is weighed with a strain gauge. Protecting this column will allow this type of method to be installed outdoors.

Real-time analysis of the weight/pressure/temperature metrological data collected and consideration of the thermodynamic data of the fluid used will indicate the presence of leaks locally (dry contact on BMS) or remotely (Sentinelle online monitoring, email alerts). The aim of continuous monitoring is therefore to detect leaks early on, allowing operators to react more quickly.

To ensure the accuracy of the measurements, an annual check is performed to replace the weight/pressure measurement chain and calibrate the temperature probes so that the impedance of the cables and the drift of the PT100 probes can be taken into account.

There are two types of alarm, allowing continuous monitoring and feedback which is essential for the operation of a refrigeration process.

• • Low level alarm: similar to the stationary “Kubler” levels found on a large number of tanks, the DNI enables continuous and configurable monitoring of the low level in the tank. Its configuration allows the trigger threshold to be adjusted and the operator to be alerted quickly in the event of a significant and rapid drop in fluid level.

• • “Statistical” alarm: algorithmic analysis requiring a learning period to work. The learning period – seven days during which the measurements are specifically analysed – allows the reference levels of the installation to be defined. Used by the real-time algorithm, the reference levels are compared with the average data to make the “statistical” alarms reliable while adapting the real-time analysis to each condensing unit.

The expert monitoring system, Sentinelle, displays the values recorded by the DNI in the form of detailed curves, allowing a visual and documented analysis of the installation’s operation. Many other assessments are available with the aim of improving the performance of the installations and reducing their environmental impact by reducing direct and indirect greenhouse gas emissions.

As well as addressing the direct environmental impact of a refrigerant leak, Matelex is also working on reducing the indirect impact of energy consumption. The analysis carried out by the DNI and Sentinelle enables action to be taken against excess energy consumption due to a low level of refrigerant in the cylinder to be addressed.

Finally, in addition to leak detection, the energy module, added to the DNI, allows the enthalpic cycle of the installation to be found in real time, thanks to additional measurements. Compressor operating cycles are, for example, studied to warn of risks of premature wear leading to a risk of compressor failure, but also to improve the compressors’ management (reduction of short cycles, monitoring of start-up frequencies, etc.).

Access to detailed and historical analysis is intended to help installers improve the settings of their installations with the aim of optimising the operation of the installations.

6) How to ensure your detection program is successful

• • Correct installation of the equipment by trained and qualified installers.

• •  Servicing of the equipment and annual maintenance.

• • Test the proper functioning of the devices and set the alarm thresholds appropriately (calibration kit, smartphone application) to avoid significant refrigerant losses.

• • Test and calibrate with the desired gas

• • Check the sensors and alarms are working.

• • Calibration maintains the accuracy of the sensor. It is compulsory every year in some European countries.

7) Benefits of leak detection:

– Reduction in the amount of fluid used = financial savings

– Lower greenhouse gas emissions = environmental benefit

– Reduced energy consumption = financial savings and environmental benefits

– Improved performance of the installations due to good containment and adequate refrigerant charge

– Compliance with standards and regulations (EN 378, F-GAS II, ASHRAE, etc.) = safety

– Leak prevention = Protection of people and improved comfort = safety & health

– Preservation of goods and equipment = Savings and safety

This article was produced jointly by Climalife and Matelex, two key players in energy performance.