Fire Tube Boiler Tube

The fire tube boiler is a simple and economical boiler that has been used since the 18th century. It relies on burning fuel in the furnace to heat water in the fire tube to generate steam. The fire tube boiler consists of a furnace body, tubes, flue and control system, among which the boiler tube is an important part of the fire tube boiler.

Fire tube boiler tube structure

The fire tube boiler tube structure is relatively simple, usually composed of straight tubes, elbows and expansion joints.

1. Straight tube

The straight tube is the most basic tube form in the fire tube boiler. The straight tube is usually made of medium-thick or thin-walled seamless steel pipe or mild steel tubes. The outer layer of the straight tube is usually covered with an insulation layer to reduce heat loss and reduce the surface temperature of the boiler and improve safety.

Material selection: 20# carbon steel/304 stainless steel

Wall thickness standard: 3mm/5mm/8mm

Insulation layer: aluminum silicate fiber + galvanized iron sheet (anti-scalding design)

2. Bend tube

Bend tubes are mainly used to form a water tube membrane shell structure. An important function of pipe bending is to make water flow smoother instead of being hindered by the inside of the pipe. Pipe bending usually adopts cold drawing manufacturing process to ensure the finish and dimensional accuracy of the inner wall of the pipe.

Bending radius specification: ≥3 times the pipe diameter (ASME standard)

Welding requirements: Argon arc welding full welding process (key points for leak prevention)

3. Expansion joint

The expansion joint is composed of two pipe flanges and a hose in the middle, which is mainly used to compensate for the linear expansion of the pipeline during temperature changes. Expansion joints are usually made of metal materials and can withstand high temperatures and high pressures.

Compensation calculation: 5mm expansion space needs to be reserved for every 100℃ increase in temperature

Replacement cycle: 3 years/5000 working hours

Fire tube boiler tube features

1. Small size and light weight

The pipe structure of the fire tube boiler is simple and relatively small. The density is relatively small under the same working pressure, so it is light in weight, which has the same advantages as the lightweight design of some hydraulic tubes.

2. Fast heat transfer speed

By injecting high-temperature gas directly into the water tube, the fire tube boiler tube can quickly heat the water to the evaporation temperature, thereby increasing the heat transfer speed.

3. Compact structure

The tube layout in the fire tube boiler is very compact, with almost no spare space, so it can efficiently complete heat exchange in a small space.

4. High fire risk

Since the tubes in the fire tube boiler are in direct contact with the fuel, the fire risk is relatively high. In addition, due to the complex internal structure of the tube, if it is not regularly inspected and maintained, the risk of tube bursting will increase.

Common failures of fire tube boiler tubes

1. How to deal with the problem of water leakage in fire tube boiler tubes in an emergency?

Emergency symptoms: water seepage in the tube wall > 200ml/min, steam pressure drops by 15%

Emergency plan:

Turn off the combustion system and reduce the pressure to 0.2MPa.

Wrap 3 layers of high-temperature resistant epoxy tape (3M 2216 recommended).

External reinforcement stainless steel clamp (spacing ≤150mm).

2. How to check the decrease in heat transfer efficiency of fire tube boilers?

Warning indicators: Fuel consumption increased by 20% | Steam production decreased by 30%

Emergency plan:

Internal scale: Use endoscope measurement method to detect, thickness ≥ 2mm needs pickling (hydrochloric acid concentration 10%).

External ash accumulation: Use U-type differential pressure gauge to detect, and blow when the flue gas pressure difference is > 150Pa.

Insulation failure: Scan with infrared thermal imager, surface temperature rise > 40℃ needs to be replaced.

3. What are the signs before the boiler tube bursts?

Abnormal sound characteristics:

Intermittent metal knocking sound (> 85 decibels)

Accompanied by 2-5Hz low-frequency vibration (detectable by mobile phone APP)

Abnormal temperature:

Sudden increase in temperature difference between adjacent pipe sections > 30℃

Local overheating and reddening (> 450℃)

Summary

The fire tube boiler tube is one of the most important components of the fire tube boiler, which directly affects the performance and service life of the boiler. Through a deep understanding of the structure, material and characteristics of fire tube boiler tubes, we can better understand the operating principle of this traditional boiler, and can also maintain and service it more accurately to extend its service life.

Read more: How To Weld Boiler Tubes? or Material of Boiler tube