It’s not unusual to see combustion installations malfunctioning due to incorrectly sized pipes.

A common solution for this is an extensive pipe reorganization, which leads to significant refurbishment costs and production downtime.

In this article we’d like to talk to you about the most important requirements when seeking to ensure an appropriate pipe network in your combustion systems.

Article content:
Factors to review
Branch pipe design

Factors to review


We advise you to pay attention to the following in your combustion pipework:

Premix pilots and pilot nozzles: Those connected to air/gas mixers with excessively short elbows or nipples are often very unstable or require out-of-ratio air/fuel adjustments to avoid shutdown.

Mix pressures: These should be kept lower than desired to avoid flame instability. This can cause overly weak flames that can go out, often causing unwanted shutdowns due to flame failure.

Incorrectly installed impulse lines: Can give the wrong pressure signals, causing failures in the air/fuel ratio control system, thus not following the same air/fuel ratio throughout the operating range; this can lead to burner instability, incorrect furnace atmosphere and poor temperature uniformity, to mention just a few issues.

Gauges and pressure measurement devices: Placed in the wrong locations can give false readings with considerable margins of error, causing confusion to furnace operators attempting to set combustion settings or air/fuel ratios.

Branch pipe design


The following sections of straight pipe – without turbulence caused by direction changes such as reducing elbows or accessories–  should be considered when installing a pipe for a combustion system (D refers to the pipe diameter; 5D in a 1” pipe would be 5"):

1.- To ensure complete mixing and flame stability: between the mixer and the premix burner nozzle.

2.- To avoid a false signal to a ratio regulator or controller: between the impulse line tap point and the upstream control valve and the downstream accessory or valve. And consider the same center line as the axis of the butterfly valve; also, it is recommended that you never remove the intake at the bottom, in order to prevent any dirt entering the impulse line and plugging it up.

3.- To achieve the same flow and pressure distribution in all downstream branches: between the control valve and the first branch of the downstream pipeline.

4.- To achieve the same flow and pressure distribution in all descending branches: between the supply elbow or T and the first descending branch or downpipe.

5.- To avoid false readings due to flow turbulence: between the pressure gauge or pressure switch tap, before or after the control valve.

6.- Correct and error-free measurement requires non-turbulent, laminar flow: between the measurement orifice and the upstream or downstream valve or accessory. Accurate and repeatable measurement requires a smooth flow.

7.- To avoid an error in the flow and pressure of the oil in the burner atomizer, due to the effect of oil falling inside the tube due to gravity, intermittently forming bubbles in the flow: between the Ratiotrol™ (air/oil ratio regulator) and the burner’s central line.

8.- See the table for some rules of thumb, covering different fluid velocities in the pipe.

At Nutec Bickley we are specialists in industrial furnaces and their diverse components. Contact our expert advisers so they can let you know about all the solutions that we have for you.

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