PT6 Fuel Nozzle Maintenance
By Peter Boissonneault
The Pratt & Whitney PT6 engine has proven itself to be one of most reliable and cost-efficient engines ever developed. Operators, and those of us in the repair and overhaul business, have become keenly aware that the ever-reliable PT6 is not immune to increasing maintenance costs. The challenge of maintaining high levels of quality while holding the line on costs is always present. Maintenance and repair organizations which are capable of meeting the ever-increasing quality demands of its customers, and at the same time controlling costs, will surely be in demand.
This article will address one aspect of hot section maintenance, those dealing with fuel nozzles and their proper servicing.
Design and installation
The fuel nozzle design and installation in the PT6 engine is simple and efficient. The installation consists of 14 adapter assemblies consisting of, primary, secondary, and inlet adapters of either simplex or duplex design. Unfortunately there exists some misunderstanding regarding the ongoing maintenance, recertification, and required overhaul of the fuel nozzles. We will make every effort to clear up some of the confusion on this topic and outline some of the choices available to you.
The Pratt & Whitney maintenance manual addresses recommended and approved fuel nozzle servicing procedures. The manual outlines recommended time intervals, inspection tasks, fits and clearances, cleaning procedures, leak testing, and flow checks.
Time intervals between fuel nozzle servicing should initially follow the Pratt & Whitney maintenance manual recommendations. Once an operator gains a history or trend on the condition of their fuel nozzles, adjustments such as time extensions or contractions might be warranted.
Dome thickness measurement of fuel nozzle sheath should not be less than .040 inches.
Inspection
The initial incoming inspection for damage to the tip, adapter, and sheath
should be completed first.
It's very important to carefully check each nozzle tip for the correct part number. Fuel nozzle sets with incorrect part numbers installed can not only be dangerous but cause extensive hot section damage. The installation of an incorrect tip on an adapter is possible since a variety of part numbers have the same physical and outward appearance. This problem is more prevalent with fleet operators using various engine models.
The ultrasonic cleaning procedures for fuel nozzles are outlined in the P&W maintenance manual. The use of a smaller, dedicated ultrasonic cleaner, works best. Cleaning one set of nozzles at a time avoids the possibility of co-mingling customer or engine sets. The proper cleaning of fuel nozzles takes time and is a procedure that should not be rushed. Improper cleaning procedures often result in unnecessary nozzle tip replacements.
Repeat cleaning
The P&W maintenance manual outlines that if nozzles fail to clean up, the procedure may be repeated. The repeat cleaning is an important consideration when it comes to holding the line on fuel nozzle expenditures. In fact, the second cleaning often results in the return of a normal flow pattern. Replacement nozzle tips are expensive and should only be required when multiple efforts to clean them fail.
The next step in the process is leak testing. The leak test reveals any problem between the nozzle tip, lock tab, and adapter assembly at the sealing areas. Also important to note at this time is external leakage at welds, and areas between primary and secondary ports.
Flow checking is next, making careful records of any irregularities in the spray pattern, and identifying its location. It's common to encounter two to four nozzles with streaking or flow problems at this time. The experience of the technician is key here. The technician must be able to recognize any irregularities in the flow pattern. Any streaking, dripping, or irregular flow are grounds for rejection of the tip. If there is any question about the flow pattern, it's best to margin on the safe side and reject the tip. On average, we encounter one or two nozzle tips that
ultimately fail (after the second attempt to clean them).
Sheaths
The last area to discuss about fuel nozzle servicing is sheaths. Sheaths
accompany almost all fuel nozzle sets we receive. There are three primary areas of concern with regard to sheaths: dome thickness, the area where the combustion liner comes in contact with the sheath, and the nozzle port diameter measurement.
Most sheaths require a minimum dome thickness of .040 to be considered serviceable. The measurement of the dome is taken with a calibrated micrometer. When the sheath comes in contact with the combustion liner the wear is limited to .010. Nozzle port diameter dimensions are taken with a dial caliper or a calibrated pin of correct size. This measurement is variable with each engine model. Sheath replacements take place on the average of one or two per servicing.
Damage from Fuel Nozzles
Poor fuel nozzle flows are a primary cause of reduced hot section life. Spitting or drooling nozzles can wreak havoc on the hot section components, especially the vane ring.
This photo of a CT vane ring shows the destruction that can be caused
by poor fuel nozzle spray patterns.
Smart maintenance
Poor fuel nozzle flows are one of the primary causes of reduced hot section life. That's why it is critical to provide the operator with a condition report. Using the report, the operator can identify and inspect their engine for damage.
The typical time frame required to complete a fuel nozzle service should be around four shop hours.
When is it wise to overhaul fuel nozzles?
Fuel nozzle overhaul is expensive and sometimes confused with recertification. The overhaul of fuel nozzles requires each tip to be overhauled or replaced with an overhauled tip. The overhaul process for the nozzle tip includes checking the flow pattern. Also required during the overhaul is the measuring of the flow rate and cone angle. The P&W overhaul manual is used for a guideline during this procedure. The cost to overhaul a set of fuel nozzles (14) can reach well over $1,000. When is it wise to overhaul fuel nozzle sets? Some reasons for suggesting overhaul include: fuel nozzles with a questionable history, i.e. purchase of a serviceable or core set of nozzles, fuel that turned out to be from a questionable source, a hot section with a problem history, MORE program requirements, and engine overhaul.
Operators that are interested in obtaining optimum performance for their
engines should seek a fuel nozzle vendor that specializes in that service. The training, equipment, and work environment are all-important aspects of fuel nozzle servicing. The fuel nozzle condition report goes a long way to stopping minor hot section problems before they become too serious. Be sure your fuel nozzle vendor supplies you with a written report on how your nozzles performed.
Adjustments to fuel
nozzle service times can be effectively dealt with after reviewing condition reports. If your condition reports indicate a higher than normal rejection rate for nozzles perhaps it would be wise to reduce the time interval for servicing.
Each fuel nozzle recertification should be accompanied by the proper documentation. The documentation should include either a serviceable tag or a FAA form 8130-3 for return to service.
The development of a comprehensive fuel nozzle maintenance program can produce noticeable savings in hot section costs. Those operators with on-condition hot section maintenance programs will see a longer life. Operators using a hard time hot section life will notice the savings when the bill arrives.
When selecting a fuel nozzle vendor be sure to inquire about turn times, availability of exchange kits, and what type of a condition report you will be receiving outlining the condition of your fuel nozzles. This is also a good time to order the fuel nozzle gasket kit that will be required for the re-installation of the fuel nozzles.
The shipping of fuel nozzle sets should be planned carefully as the delicate nature of the nozzle tips warrants careful protection. Each fuel nozzle should be packaged individually and their position as installed on the
engine noted.
Inspection measuring sheath for port diameter with calibrated pin.Inspection to check sheath for combustion liner contact wear of greater than 0.010 inch depth.
Fuel Nozzle Specifics
Definitions
Streaking - is defined as a variation in spray quantity between different parts of the spray cone at the reference flow points, and is specified as a percentage variation from nominal. For convenience, the spray cone may be divided into 10 equal wedge-shaped areas for comparison.
Spitting - is a condition which exists when large drops of unatomized fluid occur intermittently and is usually dispersed outside of the spray cone.
Drooling - is defined as large drops of unatomized fluid on the nozzle face.
Typical Test Procedure
Opening pattern should be set to fuel flow as specified in the maintenance manual. An open spray free from spitting and drooling must be observed. Note that an onion shape may be evident between the nozzle tip and the open spray, however, this is not a requirement.
Adjust the pressure regulating valve to 125 psig. Flow must be within specified manual limits.
Spray Cone Angle Test
1. Set pressure to 125 psig.
2. If measuring device has blades, set them parallel to spray boundary while maintaining a gap of approximately 1/16 inch between edge of blade and boundary of spray.
3. If measuring device has pointers, set tips of pointers so that they just meet spray boundary.
4. Measure half angle of spray cone angle. Maximum and minimum spray cone half angle must be within specified limits. Half angle is the angle subtended from vertical normal to the nozzle spray cone outer surface, at a distance of 1/2 inch from the nozzle orifice along the spray cone. The cone angle is applicable to all planes cut vertically through the spray cone.
5. The angle can be best measured by rotating the nozzle around the vertical axis and observing the position of the maximum and minimum angle.
6. Observe spray cone and ensure streakiness is within specified limits.
7. Spitting and drooling should also not be present in the spray pattern.
Leakage Test
1. Install each fuel manifold adapter assembly in pressure test fixture. For inlet adapters, a blanking plate is required.
2. Supply calibrating fluid at 500 psig at room temperature. Leakage is not permitted.
3. Remove fuel manifold adapter assembly from test fixture.
Peter Boissonneault is the shop manager for Hyannis, MA based Heritage Turbines, Inc., Their toll free help line is (888) 621-778
