The compressor bleed control system on the PW500 series engines provides surge-free operation throughout the entire engine operating range. This is accomplished by controlling the high pressure airflow through the compressor. The key components involved are the bleed off valve (BOV), EEC or BOV controller, torque motor, TØ sensor and the N2 speed sensor.
The TØ sensor, N2 speed sensor and the Linear Variable Differential Transformer (LVDT, part of the BOV) are three inputs that the EEC or BOV controller use to regulate the position of the BOV. As the RPM of the N2 increases, the EEC or BOV controller signals the torque motor to begin closing the BOV. The air from the P3 to the torque motor is modified by the motor and then sent to the BOV, causing the BOV to close.
The LVDT will send a signal to the EEC or BOV Controller with the position of the BOV. The BOV is spring loaded open with P2.8 air from the engine side of the BOV trying to open the valve at all power settings. If the higher pressure P3 air isn’t modified the BOV would always be closed so, as mentioned before, the P3 air is modified to allow the P2.8 air to overcome the modified P3 air from the torque motor and allow the BOV to open when N2 RPM is reduced. The torque motor uses an Orifice Fitting to help in modifying the P3 air to “Fine Tune” the performance of the Bleed Control System for each engine. There are 31 classes of Orifice Fittings ranging from 0.019” to 0.101” in diameter. The Orifice class is determined at Test Cell but if the torque motor is replaced or if troubleshooting of the Bleed Control System is required, the Orifice Reclassification test will be required to determine the new orifice class.
This test is critical because the difference in pressure of the modified P3 air and P2.8 air acting on the BOV is very small. If the Orifice class is too small, the engine may surge when reducing RPM. If the Orifice class is too big, the engine may not make takeoff power. If the torque motor or the EEC/BOV Controller fails, the BOV will continue to operate but will be very slow. This is because the torque motor goes to a fixed neutral position, altering the ability of the torque motor to modify the air going to the BOV. The modified P3 pressure will eventually overcome the P2.8 pressure acting on the BOV and allow the BOV to close to a point where engine performance is regained. Care should be taken when advancing or reducing the throttle. If you’re experiencing an intermittent surge while reducing RPM, try inspecting and cleaning the orifice first. The orifice is small and it won’t take much to block the opening enough to cause a problem. To discuss this topic or other topics with your Pratt & Whitney engine, call me at 269.969.8486.
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