Introduction to Flue Gas Piping (PDF)

Flue gases are a mixture of a variety of combustion products along with water vapor, carbon dioxide, particulates, heavy metals, and acidic gases generated from the direct/ indirect oxidation process. Various combustion sources like coal, fossil oil, wood, natural oil, etc. are producers of toxic gasses that may be hazardous to humans. However, flue gas is known to contain nitrogen oxide mostly, and the presence of carbon dioxide, sulfur dioxide, or carbon dioxide cannot be overruled.  Further, the use of flue gases is used for steam generation in industrial boilers and furnaces of power plants.

Requirement of Flue Gas Piping

If the enclosure of the boiler and furnace is not properly aerated, it may result in the accumulation of toxic gas like carbon monoxide. Further, the requirement of pipes or ducts becomes very essential. The pipes lead the toxic particles released into the open. Additionally, if not ventilated properly, it may lead to a hazard. Consequently, chimneys are installed to facilitate the harmful impacts.

Flue Gas Components

The main requirement for any kind of discharge to the atmosphere must be strictly converted hazardless before discharge. Safety comes first and every measure should be taken to maintain local regulatory laws. So, it would be per national or international pollution control laws. To adhere to the motive, both internal and external exhaust pipes, the exhaust pipe connecting the chimney, and internals of the boiler like expansion joints, silencers and dampers should be matched. However, improper combustion, noise, and increment of emission will result.

What are the Dampers in The Flue Gas System?

A damper is a piece of inline equipment installed in between the pipe running from the boiler to the chimney. Moreover, it facilitates storing of the heat energy even if the chimney is not working. Hence, a damper will possess a suitable open and close system.

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Strategy to Use Flue Gas Pipes

Heat up to 350℃ may be generated in boilers at times. Therefore, it would be highly recommended to use piping/duct materials that not only are resistant to flue gas but also heat. However, the design of the flue gas system may vary country-wise, for instance, the height of the chimney. But, the fundamentals are the same

  • Routing
  • Sizing

Explanation of Routing Fundamentals

The flue gas pipes are connected to the inlet of the chimney. Further, it would be ideal to route it straight traversing the smallest distance. Further, to avoid pressure and heat loss few elbows are needed to run the pipe. Moreover, it would not be suitable to narrow or widen the pipe while inserting a rather 30° isolation bend would be preferred. Furthermore, the flue gas pipe must be tapped at an angle of 30-45°to the chimney.

What Are Design Requirements at a Glance?

  • Criteria to maintain continuous combustion is to maintain design 0+/-1 MPA that is measured at the boiler end head.
  • To avoid heat and pressure loss, produce an aerodynamic design by inserting elbows.
  • To avoid heat loss, providing insulation will help.
  • To mitigate condensate issues steam traps can design along the pipe.
  •  Cleaning and inspection apertures should be there.
  • Not only corrosion but also temperature and condensate resilient piping commodities should be used.
  • EN 1856 should be the benchmark to ensure gas tightness.
Typical Flue Gas Network
Fig. 1: Typical Flue Gas Network

Normal Sizing Rule of Flue Gas Piping System

A chimney and boiler are interconnected employing pipe/duct in general. Further, all inline equipment like an expansion joint, silencer, fittings, etc is sized as per the nominal diameter of the pipe. Further, the boiler outlet temperature should not be exceeded while designing the system at 16.5 m/s. However, the speed mentioned is based on the operating flow rate.

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Why Stainless Steel Pipes Are Chosen For Flue Pipes?

The use of stainless steel pipes is being used these days. Further, because of the explicit properties of AISI 316L austenitic stainless steel. The main benefits are

  • Corrosive effects of acid condensation can be resisted
  • The flow of fumes is unrestricted
  • The phenomenon of condensation rises because the dew point is reached within a short time.

Why did Flue Gas Require to be Analyzed Frequently?

As such flue gas is toxic and can cause great harm any kind of leakage at flange, pipe, or boiler can cause safety concerns. Hence, power plants should handle flue gas with importance. Furthermore, what toxic materials need filtration. Not only small particles of N2 nitrogen but also carbon dioxide, water vapor, and oxygen, the exhaust pipe is run from the boiler end up to the chimney.

How to Minimize Content of CO

The most toxic component in flue gas is CO. However, under the strict laws of pollution control, the toxicity level in flue gas gets minimized. For this, the boilers are retrofitted with equipment to convert CO into a reusable form that can be used in different operations in the power plant. So, it can be concluded that power plants need to reduce the possibility of harm to the environment.

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What is The Percentage of Components in Flue Gas

It goes without any doubt how flue gas piping is important in a power plant. Furthermore, there may be questions for designing how much percentage of the components are expected in the flue gas. Please note

  • CO2: 8–10%
  • H2O: 18-20%
  • N2: 67-72%

Additionally CO contains one atom of oxygen and is flammable. Whereas, CO2 contains two atoms of oxygen and is non-flammable. Therefore, restriction of CO in flue gas is necessary. Again, CO2 is a greenhouse gas whose emission must be controlled. To ensure the safety of our environment, flue gas emissions to the atmosphere should be analyzed.

Bottom Line

Pollution control in recent times has introduced strict laws to add any more harm to the environment. The survey has been conducted to identify the main contributor. Further, among industrial entities, power plants were painted increasing carbon dioxide, a harmful component of flue gas exhaust to the atmosphere. Further, flue gases are by-products of boilers produced after the combustion of coal, natural gas, wood, etc. Further, stainless steel, thermal resistive, insulted, and non-corrosive pipes are used to mostly run up to the chimney. Moreover, to reduce the chances of pressure drop, a few elbows are introduced. The flue gas system is designed to withstand up to 350°.

Priyanko Mukherji

Education has no limitation, and I am Priyanko Mukherji a Mechanical Engineer( 2001 passed out of Shivaji University. Maharashtra) Have received a scope to share with you, whatever I learned throughout my career. I had an opportunity to work with some leading oil and gas, and petrochemical design companies mostly. Further, my field of work is piping layouts, proposals, material take off, 3d Modeling, and others.

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