Kiln Connection: Ensuring Burner Accuracy
My clients have heard me say that “bad data is worse than no data at all.” Upon reviewing kiln data, I often find errors that are either designed into a system or operator induced. Many of these issues can prevent burner setup accuracy. Small details are often the cause of big problems, and an accurate setup of the combustion system relies on good procedures, in addition to proper design.
Correct Burner Setup
Burner setup is a key part of proper kiln management and affects both efficiency and accuracy. While flow errors occur due to many different problems, the result is usually increased fuel consumption and lower product yield. Problems can exist in the following areas:
Air and fuel pressures are normally measured with a digital manometer, with readings typically in inches of water column (WC), which is standard in the U.S. On the other hand, metric units (mm WC, millibars of WC or kilopascals) are common in other advanced countries.
Inaccurate measuring techniques can result in errors exceeding 10%. Always calibrate your manometer, or at least compare it to a known pressure to be sure you are starting out on the right foot. An inexpensive U-tube water manometer is always a good source for accurate readings. In addition, make certain that the tubing has no leaks.
Know What You Are Measuring
When measuring the air and gas flows with in-line orifice plates, remember that the flow is proportional to the area of the orifice plate and the square root of the change in pressure. Figure 1 shows the pressure within the gas or air piping; a slight rise in line pressure occurs before the orifice, and then a decline in pressure just after it. The positioning of the pressure taps (up and downstream) are important, but commercial orifice plates take care of that part of the measurement.
It is also important that the orifice is located in a straight run of piping (ideally with 10 pipe diameters upstream and 5 diameters downstream) so that the flow is laminar and pressure readings are not subject to turbulence. If the upstream run has a valve or elbow a short distance up or downstream from the orifice plate, the accuracy will be seriously affected. It is tough for kiln designers to incorporate these long runs in their piping, but accuracy will be compromised if uninterrupted up and downstream runs are short.
Be certain to correct the flow curve for temperature, specific gravity and pressure. All commercial orifices have specifications for these factors. Make sure to correct for field conditions; an orifice plate designed for 0 psig will read 15% low if the line pressure is 5 psig.
Burner Pressure Readings
If line orifice plates are not available, you may have to rely on burner pressure or change in pressure readings. In general, these readings are never as accurate as separate metering orifices that have been properly installed.
One burner manufacturer has changed their flow curves for a standard high-velocity burner several times, and it is hard to know whether the catalog values are accurate (most times they are not). When in doubt, purchase an air and gas metering orifice and use it to calibrate one burner to determine how accurate the built-in metering is.
Burner Fuel Regulation
Most combustion systems employ ratio regulators to proportion the fuel to the air flow. As the air valve opens, the pressure in the air line increases along with the air flow to the burner. This pressure increase is directed to the ratio regulator by an impulse line, and the ratio regulator output increases in direct proportion to the impulse line pressure.
This is a simple, accurate system, but it often becomes inaccurate when installed in the field. The impulse line location and method of installation is crucial, since this value regulates the gas flow. Therefore, it should be a very accurate signal. The impulse line should be in a straight run of piping, with at least 5 (and preferably 10) diameters of upstream piping without any valves or fittings, and 5 diameters downstream. The connection to the piping must be made via a coupling welded on the pipe diameter (not threaded into the pipe), and the hole should be clean and de-burred.
If the connection protrudes into the pipe, the impulse pressure will not maintain exact proportionality with flow because of turbulence, and the entire ratio system will be inaccurate. I have worked with systems that have inaccuracies of 15% due to this error.
Accurate burner setup is a cornerstone of good kiln operations and management. If you are buying a kiln, be certain to specify how flow equipment should be installed. If you are designing equipment, keep the standard guidelines in mind to ensure a successful and accurate installation.
Any views or opinions expressed in this column are those of the author and do not represent those of Ceramic Industry, its staff, Editorial Advisory Board or BNP Media.