Fuel Nozzle Inspection

July 1, 2000

Fuel Nozzle Inspection

By Greg Napert

July 2000

As aircraft gas turbine engines become more sophisticated both in power-thrust capacity and mechanical design, it is increasingly important to review the overhaul and repair process as utilized for critical engine components, such as the fuel injector. Large thrust engines combine compact and complex external dressings for nacelle installations. As a result, many critical components such as the fuel nozzles become inaccessible until a major inspection is mandated. The expense of an engine major inspection requires that inspection, analysis, and assessment of repair/discard decisions of critical components such as the fuel nozzle must be regarded as highly sensitive procedures to be followed by overhaul and repair and OEM suppliers.

Basic principles

There are many types of fuel nozzles on the market, but the primary objective of the fuel nozzle is to atomize and vaporize the fuel for efficient combustion in the engine. This task becomes paramount to achieve efficient, consistent combustion performance and to attain required long-term service life. Dirty or damaged fuel nozzles can result in inefficient engine operation, damaged engine components, and premature engine removal.

JT9D-7 Duplex nozzle with water injection option

Although at present there are many variations of fuel nozzle designs, a start to understanding the basics of fuel nozzle maintenance and overhaul would be to look at a very popular design used on engines in the past, referred to as a dual-orifice nozzle. This pressure-atomizing nozzle assembly is, in effect, a nozzle within a nozzle. Fuel is supplied to the primary and secondary orifices through a manifold system. The primary nozzle provides adequate spray for ignition and idle while the secondary manifold supplies fuel only when engine fuel flow demand is high. When both orifices are delivering fuel, their output is blended into a single-pressure atomized spray.

These principles of staged fuel supplies apply to 50 percent of the nozzles that are in turbine engines today. Through the years, design modifications have improved the ability to atomize the fuel, while also bringing about cleaner and more durable nozzles.

PW2000 air blast atomization nozzle

A new fuel atomization technology incorporates "airblast" nozzles. New designs feature large fuel passageways and swirlers in which high-velocity compressor air is introduced and fuel is incorporated into the swirling columns of air. The result is a more efficient air-fuel mixture for combustion. Airblast technology offers improved atomization and a more fuel-efficient burn. It lowers emissions as well as noise, making it easier to meet Stage 3 noise requirements.

Even with these new designs, there are persistent design-oriented problems associated with normal wear and tear in the field that need to be addressed at overhaul.

Woodward FST, a fuel nozzle manufacturer and repair facility located in Zeeland, MI, is an example of a facility that offers a multitude of repairs and services related to fuel nozzles. Most of the nozzles that FST manufactures and overhauls are related to Pratt & Whitney engines, primarily the JT8 and JT9 as well as PW2000 and PW4000 Series engines. However, it also manufactures and repairs nozzles for General Electric and IAE (International Aero Engines, a consortium between Pratt, MTU, and Rolls Royce).

Woodward FST's Technical Publications Coordinator Ken Houtman, says, "With nozzle overhaul and maintenance, you can't emphasize enough the physical handling aspects of overhaul. You need to put protective closures on the nozzles as soon as you get them, maintain them, and keep them in separate trays to minimize handling damage.

"We've seen cases where some of our customers who test nozzles just toss the ones that don't pass inspection into a pile. Then, after a period of time, they just pack them up in a box and send them to us. What they don't realize is that by being careless with the nozzles and not packing them properly for shipping, they are unknowingly making what may be salvageable nozzles into scrap."

Handling damage

Phil Rogers, director of Product Support for Delavan Gas Turbine Products Division of BFGoodrich, a facility located in West Des Moines, IA which manufactures and overhauls nozzles, says, "Whether mechanics are performing some basic cleaning procedure themselves or preparing the hardware for return to the manufacturer for service, extreme care should be taken when handling the part. The fuel nozzle tip and any sealing surfaces or threads are prone to damage with rough handling. Packaging is very important. Proper care and handling can contain repair costs and possibly be the difference between a salvageable part and one that becomes scrap."

Salvaging damaged nozzles

Fuel nozzles with streaky sprays that cannot be repaired by simple cleaning may need a new nozzle body or may require refinishing of the nozzle orifice, depending on if there is any internal or external damage as well as how deep the damage is. It may be possible to restore the nozzle finish within tolerance. FST says it can usually refinish any surface with scratches below five-thousandths of an inch. If the damage is in the orifice angle area, however, you may not be able to save it. By the time you remove scratches, etc., the angle may be locally altered and the final orifice diameter is opened up beyond blueprint specification limits.

A new life for turbine fuel nozzles

Manufacturers do not typically spring to mind when speaking of competitive aftermarket repair and overhaul services, but that has changed significantly in this industry over the last couple of decades.

The sharp downturn in aviation during the early 1980s forced many aircraft manufacturers to look seriously at providing competitive aftermarket services.

And this is particularly true for many of the manufacturers of turbine fuel nozzles.

One such manufacturer, Delavan Gas Turbine Products Division of BFGoodrich, a manufacturer of many fuel nozzles for Pratt & Whitney Canada products such as the JT15D (used on Cessna Citation) and PW100 Series engines (used on many regional and commuter aircraft), dedicates a significant portion of their facility to overhaul and repair services.

According to Phil Rogers, director of product support, Delavan has a number of programs that it offers to its customers to include overhaul, cleaning, repair, and exchange pool services.

Rogers says that there can be significant advantages to having your nozzles overhauled at the manufacturer. For one, the nozzles are usually "zero-timed" meaning that the nozzles return to service under new limits with no recorded time on them. Second, there are a substantial number of repair schemes that the manufacturer can use to salvage nozzles that would otherwise be scrapped.

For instance, Delavan uses an orifice repair scheme for JT15D nozzles that do not flow properly. The orifice is actually sealed inside the nozzle base and can only be replaced by machining an access hole at the base of the nozzle.

Since this is part of the manufacturing process to begin with, Delavan simply applies the manufacturing techniques necessary to drill out and re-insert the orifice in the base of the nozzle.

Rogers says a recent re-structuring of their division has resulted in being able to turnaround PW100 nozzles in around five days, and all other nozzles in around 20 days. He emphasizes that they do keep an exchange supply of many nozzle models for AOG purposes.

Removing the guesswork

When it comes to accurately measuring the spray pattern of fuel from a nozzle, most involved would agree it's as much of a black art as a science.

Many factors inhibit the process of determining how much fuel is sprayed and the pattern of that spray - the two factors needed to know that a newly machined nozzle or a retrofitted one is acceptable. Proper lighting is critical in order to see the spray cone, but even properly lighted, a spray image is often obscured by droplets, fuel mist, and other reflections. For this reason, the traditional "visual" methods of inspection and acceptance of fuel nozzle spray have relied heavily on operator training and interpretation, and they also have their limits. As the volume of output of fuel nozzles increases, and with the tightening of tolerances required on fuel nozzle performance, OEMs of nozzles are looking for improved methods for capture.
Enter vision technology, or software that functions as an electronic "camera" to capture an image graphically.

"When we first looked at this problem, it appeared a natural fit for a vision system approach," said Dave Plis, technical sales manager at Habco Inc., the designer and manufacturer of the "AngleMaster®"nozzle spray system.

The system uses a small camera in conjunction with an image capture board to take measurements digitally and import the image into Lab Windows software. The spray cone is illuminated from within using flexible fiber optic light tubes. Video imaging, while effective, still does not relieve the problem of clarifying an actual spray cone since the visual "noise" of droplets, mist, and reflection still remain. Normal edge detection techniques yielded poor results, especially under varied lighting conditions due to this visual "noise." A better method was needed to filter the data and find the theoretical line which represents the exact edge of the spray.

The AngleMaster® software accomplishes this by dividing up the spray angle field into a two-dimensional plane of coordinates that represent the intersection of the cross sections with the actual cone edge. The software then performs multiple linear fits, picking points and then discarding points that do not appear to be on the line. By finding all the "real" points that determine a line, on both sides of the spray cone, the software then calculates the equation for determining true angle spray. The system has been tested in production use, and accuracy and repeatability of the spray angle measured have been within 1 degree, well within the tolerance limit.

The AngleMaster® concept is a good example of new technology being applied to help an old process problem, making fuel nozzle acceptance testing more accurate and less hazardous for all.

General Nozzle Inspection

By Eric Blickley Woodward FST [email protected]

Cracking in the web area can be repaired by welding an insert and remachining.

For general nozzle repairs, the directions set forth in the Component Maintenance Manual (CMM) should always be followed.

Preliminary inspection generally consists of the following steps:

• Remove all protective enclosures

• Clean

• Visually inspect fuel injector assemblies prior to testing. A complete internal and external cleaning of the fuel injector in an assembled condition is available in the CLEANING portion of the CMM.

• Visually inspect the fuel injector assembly for obvious external defects such as cracks, dents, or gouges. Examine inlet connector mounting nut threads for damage.

• Pressure test. Proper pressure testing of the fuel injector will determine if any leakage is occurring using specified limits.

• Flow Test. Proper flow testing of the fuel injector will determine if the fuel injector is performing within specified limits.

Use caution when removing the O-ring on the bottom groove of the nozzle. It has a 32 microfinish which must not be damaged. If you need to pry, use a soft toothpick or plastic pick.

If all testing and visual inspection is satisfactory, the fuel injector can be returned to service. If the inspections or tests demonstrate any defects, or operating parameters outside of the specified limits, further overhaul and/or repairs will have to be performed to bring the fuel injector back into service.

The CMM or Service Bulletins will specify any repairs or modifications that may be performed on the fuel injector.