Installation of Diesel Generator Exhaust Chimney's Flue Pipes

Large public buildings are equipped with emergency diesel generators to meet fire safety, emergency lighting, and production requirements, ensuring the protection of citizens' lives and property. One crucial component of the diesel generator's exhaust system is the high-temperature smoke flue. However, the installation process for this flue is often overlooked. Improper installation can lead to bracket deformation, reduced efficiency of the diesel generator set, and potentially hazardous safety incidents. Below is a brief guide on how to properly install the flue for diesel generator sets.

The Arrangement of the Smoke Exhaust System

According to the requirements of China's national standard for environmental protection, smoke flues should be directed to the roof for outdoor discharge at a high altitude, or treated for smoke and dust removal before discharge, to prevent environmental pollution.

1. The majority of diesel generator sets have a permissible back pressure of approximately 6.5 kPa. Therefore, the design of the flue, including its length and pipe diameter, must ensure that the smoke resistance does not exceed this permissible back pressure. Excessive smoke resistance can lead to decreased performance of the diesel generator sets, increased fuel consumption, higher flue gas temperatures, and prevent the unit from operating at its nominal rated power.

2. The muffler belongs to the diesel generator set accessories. Its resistance P1 can be found in the product samples; the general resistance value is between 1.1 and 1.4kPa.

3. The chimney outlet resistance is generally estimated by experience and the resistance value is about 0.5kPa.

4. To optimize the flue system, arrange the flue direction thoughtfully and aim to minimize its length, as this can help reduce resistance within the flue. Additionally, by creating a comprehensive pipeline diagram, you can avoid intersections between pipelines and decrease the number of elbows, which will lower the local resistance of the flue gas. This approach not only lessens the resistance in P4 but also saves on construction costs, a critical factor for projects based on lump-sum contracts.

5. The smoke flue over the wall should be added a protective sleeve, sticking out of the roof or side wall of the smoke flue outlet end, and should be installed with a rain cap.

6. Non-supercharged diesel engines and exhaust gas turbocharged diesel engines should be installed in the exhaust flue silencer. Two diesel engines should not share a silencer, which should be fixed separately.
   Eliminate Pipeline Expansion 1. The correct choice of metal expansion jointsIn diesel generator sets operating under rated conditions, the exhaust smoke temperature typically reaches around 550°C. In contrast, the room temperature in which the smoke flue is installed is about 30°C, resulting in a temperature rise of approximately 520°C. 

Based on relevant data, the coefficient of thermal expansion for metal flues is 1.13 mm/(m·100°C). This means that a 30-meter-long metal flue could expand by up to 176 mm during operation. Without the inclusion of expansion joints, this thermal expansion could potentially damage the equipment, support structures, and the building itself.

To mitigate these risks, it is essential to calculate the flue's axial expansion considering the flue's length, the expected temperature rise, and the thermal expansion coefficient. By selecting suitable metal expansion joints, we can effectively accommodate the temperature-induced expansion of the flue, ensuring the system operates normally and safely.

(1) Metal expansion joints

A metal expansion joint is commonly referred to as what is depicted in Picture 2. Various bracket arrangements for diesel generator set smoke flues typically include components such as metal bellows (expansion joints), mufflers, fixed brackets, guide brackets, sliding brackets, and other related parts, as shown in Picture 1.

(2) Fixed bracket

The primary function of it is to restrict the movement of the flue. It must support not only the weight of the flue but also withstand the axial stress caused by thermal expansion. Therefore, it is crucial to calculate the forces on the fixed brackets to ensure they are secure and reliable, thus preventing any safety incidents.

Expansion joint installation

(1) Expansion joints should be installed close to the fixed bracket. The opposite end of the first guiding bracket should be positioned at a distance of 4D from the expansion joint (where D is the diameter of the flue). The distance between the first and second guiding brackets should be 14D. Additionally, sliding brackets should be arranged in a manner that accommodates the expansion and contraction of the flue. The spacing for the sliding brackets must be determined in accordance with the relevant national standards, product specifications, and other specific conditions.
(2) Before installation, the expansion joint should be pre-stretched according to the calculated compensation amount to account for the flue expansion caused by temperature increases. To minimize tolerance issues with the expansion joint and ensure coaxiality of the flue, it is advisable to lay the flue first. Then, cut a section of the flue at the location where the expansion joint will be installed, using a wind welding technique. The length of this cut should equal the free length of the expansion joint plus the pre-stretching amount. Finally, install the expansion joint onto the flue and weld it securely in place. After installation, remove the transport tie rod, limit nuts, etc., to ensure that all the moving parts of the expansion joints are not jammed, so that the expansion joints can work normally.
(3) The waterproofing of the chimney outlet is one of the key points in the installation of the flue of diesel generator sets, and it is recommended to be constructed according to previous construction experience. Special reminder: due to the high temperature of the flue, the roof of the chimney outlet should be added around certain protective facilities, and conspicuously hanging the "high-temperature danger" words to prevent personal injury.

The Flue Insulation

1. Choosing materials of insulation

When diesel generators operate at full load, the flue gas temperature can reach around 550°C. Many domestic projects often use glass wool flue shells for exhaust flue insulation. However, for exhaust flues that undergo water spray treatment, the lack of adequate cooling measures makes glass wool unsuitable. At high temperatures, it can quickly become brittle and disintegrate into powder, significantly compromising its insulation effectiveness.
In contrast, both rock wool and hydrated calcium silicate can withstand high-temperature environments up to 650°C under normal conditions. Among these, hydrated calcium silicate offers the lowest heat transfer coefficient, resulting in optimal thermal insulation performance, while also being more cost-effective. Therefore, for the design of diesel generator set flues, it is recommended to use hydrated calcium silicate flue shells as the insulation material. 
The selection of flue insulation thickness is provided in the table below.

2. Insulation Construction Guidelines

According to China's national standards, any surface of a device that is under 2 meters and may be touched by people must be insulated if it exceeds 70 °C. After insulation, the surface temperature of the device should not exceed 40 °C. 
When installing the flue, it should be as straight as possible, with minimal bends and no downward angles. The installation of the smoke flue should avoid curves, U-shaped turns, and downward bends to ensure the most direct path for smoke evacuation. Additionally, the design should consider minimizing contact with areas where people may be present.

(1) Exhaust flue external surface temperature ≤ 450 ℃ when the insulation layer uses a layer of rock wool felt, exhaust flue external surface temperature ≥ 500 ℃, the insulation layer using 2 layers, that is, contact with the wall of a layer of aluminosilicate fibre felt, outsourcing a layer of rock wool felt, aluminosilicate fibre felt thickness for the numerator in the table, the thickness of the rock wool felt for the denominator in the table.
(2) Physical properties of rock wool: density 80~100kg/m³, thermal conductivity into = 0.03~0.0407W/(mxK). Physical properties of aluminosilicate fibre: density 150~250kg/m3, thermal conductivity into = 0.14~0.174W/(mxK).
(3) In the process of insulation installation, in the smoke flue axis upward every interval of about 2m should be set 5 ~ 10mm expansion of the reserved seam, and filled with asbestos rope, for the elimination of axial expansion, to avoid the flue warming and expansion damage to the insulation layer. Tie a layer of fine wire mesh on the outer wall of the flue, so that the hydrated calcium silicate insulation between the semicircular shell forms a gap of 3-5mm, filled with asbestos rope, used to eliminate radial expansion, to avoid the flue temperature expansion damage to the insulation layer.

Other Considerations in Construction

1. To ensure proper drainage and prevent rainwater from accidentally entering the diesel generator set, the horizontal exhaust flue should maintain a slope of 0.5% towards the riser. Additionally, at the bottom of the riser, install DN20 all-metal valves to facilitate water drainage and inspection after rainstorms.

2. Diesel generator sets should be installed separately rather than trying to save costs by having two or more sets share a common smoke exhaust system. Even if the generator sets are of the same model, each one must have its own separate smoke exhaust system.
3. The chimney outlet of the roof should not be directed towards other buildings or walkways. To minimize pollution emissions, the color of the smoke must be lightened before it is discharged. This can be achieved through the use of a smoke purification system or water spraying. Additionally, the setup must be approved by the relevant state departments.
4. Each diesel engine exhaust flue should be led out of the outdoors separately, it is suitable for overhead laying, and can also be laid in the trench. The exhaust flue and muffler should be set up separately to provide support; they should not be directly connected to the diesel engine exhaust flue or fixed to any other parts of the diesel engine. A flexible connection should be used between the exhaust flue and the main flue. The brackets on the flue must allow for expansion and contraction, or a roller-type bracket can be used. Additionally, a short flexible flue or expansion bellows should be installed between the two fixed racks of the long flue, allowing the entire system to function as a cohesive unit.
5. The length of the exhaust flue and the matching requirements with the flue diameter should be determined according to the data provided by the manufacturer. When the smoke exhaust flue needs to pass through the wall, it should be equipped with a protective sleeve. When the flue is laid vertically along the wall, the outlet end of the flue should be equipped with a rain cap or cut into a bevelled angle of 320~450. The wall thickness of all smoke pipes should be not less than 3mm.
6. The direction of the exhaust flue should be able to fire prevention, out of the outdoor part of the appropriate slope of 0.3% ~ 0.5%. Slope to the outdoor, easy to smoke condensate and condensate discharged outdoors. When the horizontal flue is long, install a drain valve at the low point.
7. The smoke flue in the machine room using overhead laying, the indoor part should be set up to protect the thermal insulation layer and 2 metres from the ground below the part the thickness of the insulation layer should not be less than 60 mm; when the flue overhead laying below the fuel or trench laying needs to be through the fuel, should also be considered when the security measures.
8. When the flue is long, the natural compensation section should be used, and if there is no condition, the compensator should be installed.
9. The smoke exhaust flue should have minimal bends. The degree of each bend should be greater than 900, and there should generally be no more than three bends. Exceeding these guidelines can cause the diesel engine's smoke exhaust to be obstructed, negatively impacting the power output of the diesel engine set.

Picture 1 Installation of diesel generator exhaust flue

Picture 2 Location of diesel generator bellows

Table 1 Flue pipe insulation thickness (mm) selection