Can hydraulic EDP in B737 overheat?

[mirkoni]

Hello all,

On the hyraulic panel there is an OVERHEAT indication above electric motor driven pump. There is no such indication above mechanical EDP. In some textbooks there is a mention of case drain as a way to measure internal heating of the mechanical EDP.
Is it right to assume that mechanical EDP is much less likely to overheat, or will practically not overheat at all, thus there is no overheat indication?

Thanks

[vapilot2004]

For both the Classics and NGs, the electric motors on the EMDPs are high current devices and create a considerable amount of heat when operating, particularly under high load. Continued operation can lead to premature motor failure and high system fluid temps, but the condition is typically temporary.

In contrast, the EDPs are purely mechanical devices with comparatively (to the EMDP) small heat rise and no control over their engagement (only bypass/depressurization valves are fitted) and as such, overheat warnings were not installed.

You are correct in assuming an overheat condition is more likely for the EMDP. I am unaware of a temperature switch located in the EDP case drain line.

Notes:
The 37's bigger brothers all have EICAS indications and advisory/caution warnings for all main hydraulic pump circuit temperatures.

Some EMDPs are equipped with internal temp switches that will cause the motor power supply relay to trip off until the motor temp (at the windings) drops below a preset value.

The overheat warning lights are a MEL item for both the Classic and NG, provided the LOW PRESS warning system is operational.

[mirkoni]

vapilot2004,

Thank you very much for the answer, it all seems logical. In the meantime, I found something in the 737 FCOM and similar explanation in youtube video on hydraulic system for PMDG 737NGX (https://www.youtube.com/watch?v=Nk32bZjg9rY at 14:22) If I understood you well, elder brothers with EICAS/ECAM have both EDP and EMDP overheat information.

[vapilot2004]

No worries, Mirkoni.

On all Boeing wide bodies and 757, engine driven and electric pumps have temperature probes fitted. Meanwhile, like the original '37 design, the engine driven pumps are still run from the gearbox, and use the same arrangement with no drive disconnect, just a pressure relief bypass when the pump is "switched off".

Another contrast to the 737 is the use of demand pumps on the wide bodies. On the 737, the EMDP's run as long as they are switched on (and not overtemp), while on the big brothers, some of the electric pumps only run when needed or during certain phases of flight.

[torghabe]

Jumbo has limit quantity of fuel in main tanks for continuous (more 15 min) operation, and does't matter what is it - EDP, Demand pump or AUX. Not only that pumps can produce heat, but only fuel can remove heat from hyd. liquid.

[A and C]

The EDP on the 737 As said above has the option to turn it off, or rather offload the pump and leave only the flow through the case drain line to cool the pump.

The final protection from pump failure is the spline drive between the engine wheelcase and the pump is designed to shear should the pump seize.