Economic efficiency of airborne thermal infrared imaging

Underground district heating networks are a characteristic element of urban infrastructure. In most cities, their total length amounts to hundreds of linear kilometres. In Moscow it exceeds 5 500 km (on average, more than 4.2 linear km of twin-pipe heating mains per 1 km² of urban area). Heating-pipe diameters range from 50 mm to 1400 mm; installation depth is usually 1.5–2 m, rarely 3–5 m or less than 1.0 m. The total flow rate of circulating network water is tens of thousands of m3/h. In Moscow the flow rate exceeds 250 thousand m³/h, while total network-water leakage during the heating season is 5000–6000 m³/h.

The high flow rate of district heating water, high temperatures (70°–150°C) and pressure (up to 1.5 · 106 Pa) make leakage one of the most intense sources of environmental impact. This impact takes several forms:

• Thermal: changes to the thermal regime of groundwater, soils and the upper part of the ground;
• Mechanical: erosion of ground in contact with heating pipes, forming collapse sinkholes (anthropogenic suffosion);
• Chemical: dissolution of karst-forming rocks and changes in groundwater chemistry;
• Hydrodynamic: changes in the groundwater level.

The extensive length of the networks and the specific features of their functioning and operation make high-productivity condition-monitoring methods important. The main objective is not limited to detecting failed sections. When a heating main ruptures, such locations are usually detected quite rapidly by parameter-monitoring systems, promptly localised, and appropriate measures taken. Far more important is predicting the development of emergencies to prevent severe and sometimes tragic consequences. During severe frost in the winter heating season, network temperature and pressure are at their highest, so the probability of rupture in severely corroded pipe sections is very high. As a result, a building, an entire block or even a district (if a trunk main ruptures) loses its heating. Long-lasting continuous leaks are particularly dangerous: they erode the ground beneath asphalt and cause sudden collapse sinkholes, into which equipment often falls and in which people may even die.

One remote sensing method successfully used to diagnose the condition of heating networks since 1992 is airborne thermal infrared imaging (TIKAS). The survey methodology was developed at GNPP Aerogeofizika during 1982–1991. Its main advantages are rapid operation and high productivity, allowing extensive areas to be examined in a relatively short time.

Surveying cities with areas of up to 100 km2 requires 5–6 flight hours. Preliminary materials are delivered to the client on the day after the survey.

Infrared imaging means recording electromagnetic radiation from the ground surface or various objects in the infrared (IR) spectral region and converting it into a visible image. Of particular interest is imaging in the middle (3–5 μm) and far (8–14 μm) ranges of the IR spectrum. In these ranges, the intensity of IR radiation is largely governed by the thermal state of objects at the ground surface. The survey is therefore called thermal infrared imaging, reflecting the essence of the method. Thermal infrared imaging for monitoring heating-pipe condition can be performed from the ground or from an aircraft.

The thermal field is extremely dynamic. Its formation is influenced by numerous anthropogenic and natural factors: weather conditions (air temperature, cloud cover, wind and precipitation), the position of the sun above the horizon, and the thermophysical properties and condition (primarily moisture) of the underlying surface and near-surface soil layer.

A presentation at a similar seminar in 2000 set out the basic methodological principles of TIKAS and techniques for interpreting the resulting materials, demonstrating the suitability of this method for remote monitoring of underground heating-network condition. Feedback indicates that the work attracted interest among specialists directly responsible for network operation and repair.
The suitability of TIKAS for heating-network condition monitoring stems from the high reliability of the data and their dependable use in engineering and management decisions, which ultimately determines the high economic efficiency of the method.

The information value of remote observations of heating-network condition increases substantially when surveys are conducted in a monitoring programme (twice a year, at the beginning and end of the heating season). An end-of-season survey provides indispensable information for planning repairs, enabling the most rational use of limited funds and shortening repair schedules. A survey at the beginning of the heating season provides an objective assessment of the condition of the networks with which operating services enter winter.

Since 1992, the State Unitary Enterprise Mosteploenergo has fully implemented comprehensive monitoring of heating-network condition, using TIKAS data to plan repairs and preventive work. Both Mosteploenergo data and Aerogeofizika materials provide grounds for confidently stating that a stable trend towards overall improvement of the entire heating infrastructure has developed in the monitored areas. The most representative data on the economic efficiency of TIKAS have come precisely from the close, long-standing collaboration between GNPP Aerogeofizika and the State Unitary Enterprise Mosteploenergo.

Unfortunately, large-scale, integrated use of TIKAS data is significantly hindered by the departmental fragmentation of municipal bodies managing urban heat supply. In Moscow, for example, heating networks are administered by the State Unitary Enterprise Mosteploenergo, the Heating Networks branch of JSC MOSENERGO, the State Unitary Enterprises Mosgorteplo and Teploremontnaladka, and dozens of industrial enterprises that own their own heating infrastructure. The non-optimal configuration of survey areas consequently reduces the economic efficiency of the survey and interpretation process itself.

As stated in our previous presentation, we assess heating-network condition using four grades. This is, of course, a qualitative assessment, since remote measurement of absolute thermodynamic temperature is impossible in principle.

We use the following heating-network condition grades:

• standard heat losses (pipeline in normal condition, intact and dry thermal insulation);
• increased heat leakage (wet or damaged insulation, increased intensity of corrosion processes);
• high heat leakage (small pinhole leaks, damaged and wet insulation);
• emergency condition (rupture of the heat pipeline with discharge of the heat-transfer fluid).

Experience shows that these grades provide a sufficiently comprehensive description of network condition. Intermediate or new grades can be used if necessary.
The effectiveness of the survey and interpretation process is clearly demonstrated by summary Table 1:

Table 1.
Results of checking airborne thermal IR survey materials for 1995–2001
(according to the State Unitary Enterprise Mosteploenergo)

Analysis of these data shows that, on average, each km² of monitored area contains 2–3 network sections with high heat losses. Approximately 800 such sections are identified across the entire area in each survey cycle on average. The exception was the surveys in 2000*, when operational restrictions limited coverage to only part of the area (17%).

Ground verification by enterprises of the State Unitary Enterprise Mosteploenergo indicates that the causal relationship between heating-pipe condition and its expression in the thermal field is confirmed in the overwhelming majority of cases: 65–96% (79% on average). High heat losses are often caused by ducts flooded with groundwater or water from adjacent utilities (15 to 34%). Such sections undergo accelerated corrosion. Between 12 and 20% of anomalies correspond to shallow heating-main installations.

Naturally, the greatest interest concerns sections where pinholes develop: corrosion has reduced pipe-wall thickness to a critically low value and produced through-wall defects 1–2 mm in diameter. Heat-transfer fluid leakage here is small and therefore not registered by parameter-monitoring systems. However, because the leaks are continuous, their total volume is comparable to the average groundwater runoff. According to V. M. Lipovskikh and N. A. Voronkova (JSC MOSENERGO), the average continuous leakage across the city is 7.2 m³ per hour per 1 km², and near utilities it exceeds the specific groundwater runoff by a factor of 5. Thus, make-up water demand is largely determined by persistent small leaks. Particularly dangerous is the high probability of rupture at thinned-wall sections as loads increase during the winter heating season.

Pinholes account for 14 to 27% of all heat-loss sections. Early detection and elimination can prevent major failures during the heating season. Timely repair of defects identified by airborne thermal imaging reduces make-up water demand by an average of 10% of the standard value, equivalent to 60 m³/h for enterprises of the State Unitary Enterprise Mosteploenergo. Heat losses are reduced by an average of 5% of the standard values.

The cost of a single airborne thermal IR survey is 4–5 thousand roubles per square kilometre of urban area. For remote cities, the cost increases substantially because of transport expenses (helicopter ferry flights and personnel transport), but the benefits of TIKAS in all cases exceed the associated costs many times over.

Economic calculations by the State Unitary Enterprise Mosteploenergo show that using airborne thermal imaging data together with the ground-based acoustic method saves 20–30 million roubles per survey cycle over an area of 350 km².

Based on survey data for Ryazan (1997), promptly completed repairs saved approximately 2 million roubles against survey costs of 400 thousand roubles (in redenominated prices). It should also be borne in mind that this amount excludes the costs that would inevitably have been incurred in dealing with the potential accidents that were prevented.

It is important to note that, owing to incomplete and fragmented technical documentation, notorious secrecy requirements and the considerable complexity of repair work, operating organisations in many Russian cities often lack reliable data on the horizontal position of heating networks. Thermal survey data can correct and refine this information and can be used to create new heating-network layout plans, an exceptionally urgent task.

Ultimately, the economic efficiency of TIKAS for remote diagnosis of heating networks depends on clients’ ability to assimilate the information obtained and translate it into engineering and management decisions. Our work in various Russian regions (Arkhangelsk, Ryazan, Tula, Kaluga, Lipetsk, Stary Oskol, Gubkin, Tyumen and most towns in the Moscow Region) indicates considerable inertia in the thinking of specialists and managers. Overcoming it is the key to substantially improving the efficiency of capital investment and a guarantee of safe and trouble-free