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GlueBall Date : 29th November 2002 21:20 Typical Fuel pumps run at 115VAC, 3 phase, and can draw up to about 3.45KW (3450 W). …….The question obviously is: Why did the Hydro-Aire pumps reach 1100 degrees without an automatic thermal cutout?
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Thanks for the info that the pumps are 3-phase and that the circuit can supply up to 3,750 W.
From this info I assume/infer that when a pump runs dry, it:
(a) does NOT overspeed or otherwise run away, because its synchronous,
(b) ought to draw less current than when pumping, maybe an order of magnitude less power, and
(c) at this low power it could run progressively hotter because of lack of cooling, but fairly slowly, so that all parts of the pump would tend to warm up together.
From AD 022451 I see that:
(a) “… The cause of this overheating is believed to be friction between the pump parts; however, the specific cause of the friction is unknown at this time. …..”
(b) “……. The pump was found with the thermal fuses of the electrical motor winding open …..”, so that in at least one case the thermal fuse worked.
(c) “….. In addition, the aluminum bearing housing adjacent to the back side of the end plate had melted. ….”. Aluminum alloys melt about 1100ºF, so that seems to be good evidence of the temperature.
(d) “……. According to the pump manufacturer, bluing of the steel and/or Stellite materials is evidence of temperatures in excess of 1,100 degrees Fahrenheit. …….. Adjacent areas of the shaft were blackened. ……”. This suggests that most of the shaft did NOT reach this high temperature.
A back-of-the-envelope calculation suggests that if the pump were running dry, and if:
(a) enough friction developed to draw 3,750 W from the power supply, and
(b) the mass of the bearing, adjacent housing, and adjacent part of the shaft was about a pound, and that most of the electrical energy goes into this mass of metal, then that mass could reach 1,100ºF in about one minute (and without tripping the circuit breaker). Perhaps only a few ozs of metal would actually melt.
I wonder sometimes whether there is a little too much trust in the power of liquids to cool.
It is well known to the designers of steam boilers and boiling-water nuclear reactors that once a metal surface in water reaches about 50ºC over the boiling point of the water (I‘m working from memory), a continuous layer of steam is formed which effectively thermally insulates the surface, and the temperature may then jump several hundred degrees before another thermally stable state is reached.
I could imagine this happening to a binding shaft /bearing combination, particularly if only a limited amount of fuel were in contact with restricted flow. A test would have to be done to see if this could happen in practice. If there were no air it might even be safe, though there might be carbon deposits which might cause trouble later.
A nominally dry pump could still have pockets or valleys full of fuel, which could boil off and splash around onto a hot shaft/bearing.
I don’t understand the references to the “end plates of the priming and vapor section” and “scoring was found on the inducer housing of the pump”. Are these two different locations? Is the end plate a (the?) bearing?
Kriskross: A Question: Are the “pretty coloured pictures“ available anywhere for us to look at?
I don’t have any answers. Just questions.
Here is an extract from one of the AD’s:
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…… override/jettison pump ...... Since scoring was found on the inducer housing of the pump, the pump was disassembled, at which time evidence of severe overheating of the priming and vapor pump section was found. The Stellite (cobalt/chrome/nickel/iron alloy) end plates of the priming and vapor section of the pump were blued and cracked from thermal stress, and the steel pump shaft in that same area was also blued. According to the pump manufacturer, bluing of the steel and/or Stellite materials is evidence of temperatures in excess of 1,100 degrees Fahrenheit. It is believed that such temperatures could only be reached during dry running of the pump. ......... Adjacent areas of the shaft were blackened. In addition, the aluminum bearing housing adjacent to the back side of the end plate had melted.
The other pump was an override/jettison pump ........... The pump was found with the thermal fuses of the electrical motor winding open, which indicates an overheat condition in the motor section. Such overheating may be due to the pump rotor dragging or locking. Evidence of severe overheating of the priming and vapor pump section was found. The end plates of the priming and vapor section of the pump were blued and cracked from thermal stress, and the pump shaft in that same area was blued.
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