ArcticFox Posted October 3, 2009 Share Posted October 3, 2009 Can anyone tell me why we leave these on all the time? I was looking at the hydraulic schematic and with the switches off it close the supply valve and the pump output valve (same valves that are closed when you pull the fire handle). Now then, with those valves closed and the engine still turning the pump would still be producing pressure, but the only place for it to go is back through the return line, and because of the runaround circuit the pump would continue to be fed hydraulic fluid from the return side. My thought is that it would prevent the pump from cooling - and would eventually overheat as the hydraulic fluids capacity to remove heat is reduced because it will basically be recycling the same fluid. Is that why the switches always remain on? Someone asked me and I do not want to give out a wrong answer. I read the -1 and it does not mention either in the before starting engines checklist, or in the description of the system why the switches should be switched to on. Thought there would be a note/warning/caution or something, but haven't found it. Quote Link to comment Share on other sites More sharing options...
NATOPS1 Posted October 3, 2009 Share Posted October 3, 2009 (edited) Good thought provoking question. A few reasons to leave the switches in the ON position, but probably not the only… 1. When left in the ON position there is NO pressure increase due to thermal expansion when the engine is not running. (there is a pressure relief but that’s later…) 2. As for the engine running and “pressure” being put out by the pump remember the pump is a pressure regulated, variable displacement, flow sensitive type pump. Meaning it only “pumps” when the pressure output is less than 3000 psi. 3. Normally with the pump ON there is a “load” on the pump at all times (small as it may be) (flight controls moving) so the pump is continuously varying its displacement to maintain 3000 psi. 4. If the pump is turned OFF the output pressure would increase to 3000 psi due to the fact the pressure outlet is isolated from the system components (no load). LATER.... This will cause the pump to "stop" displacing fluid. The only “flow” would be through the pump to lubricate the pump and any thermal expansion would bleed to the return side of the runaround system. The thermal relief is set at 20psi. There would be NO flow on the pressure output side. The pump output is not supplied to the inlet. 5. Also with the switches in the OFF position both the Pressure and Supply valves are closed. This would keep the solenoid “pressure side” valve always energized and eventually it would fail. Edited October 3, 2009 by NATOPS1 Quote Link to comment Share on other sites More sharing options...
Steve1300 Posted October 4, 2009 Share Posted October 4, 2009 Good thought provoking question. A few reasons to leave the switches in the ON position, but probably not the only… 1. When left in the ON position there is NO pressure increase due to thermal expansion when the engine is not running. (there is a pressure relief but that’s later…) 2. As for the engine running and “pressure†being put out by the pump remember the pump is a pressure regulated, variable displacement, flow sensitive type pump. Meaning it only “pumps†when the pressure output is less than 3000 psi. 3. Normally with the pump ON there is a “load†on the pump at all times (small as it may be) (flight controls moving) so the pump is continuously varying its displacement to maintain 3000 psi. 4. If the pump is turned OFF the output pressure would increase to 3000 psi due to the fact the pressure outlet is isolated from the system components (no load). LATER.... This will cause the pump to "stop" displacing fluid. The only “flow†would be through the pump to lubricate the pump and any thermal expansion would bleed to the return side of the runaround system. The thermal relief is set at 20psi. There would be NO flow on the pressure output side. The pump output is not supplied to the inlet. 5. Also with the switches in the OFF position both the Pressure and Supply valves are closed. This would keep the solenoid “pressure side†valve always energized and eventually it would fail. Wow, very complete anwer, Natops. The only thing that I can think to add is that there is no runaround circuit on many Herks now. The system was removed with the change to a different type of pump. The Lockheed Service News goes into detail about these other two brands of pumps, and even with one of them, there are two different designs. I'd recommend some reading for those of you who deal with replacing pumps. Certain one require the runaround circuit be removed if the pump is installed, and with others - it does not matter. Quote Link to comment Share on other sites More sharing options...
US Herk Posted October 4, 2009 Share Posted October 4, 2009 The only thing that I can think to add is that there is no runaround circuit on many Herks now. The system was removed with the change to a different type of pump. The Lockheed Service News goes into detail about these other two brands of pumps, and even with one of them, there are two different designs. I'd recommend some reading for those of you who deal with replacing pumps. Certain one require the runaround circuit be removed if the pump is installed, and with others - it does not matter. My understanding is the plumbing is still there, just capped off - reason being - in the field, you may get the old style pump and require it.... Quote Link to comment Share on other sites More sharing options...
Dan Wilson Posted October 18, 2009 Share Posted October 18, 2009 My understanding is the plumbing is still there, just capped off - reason being - in the field, you may get the old style pump and require it.... Well sometimes it there and sometimes its not, you really dont know until your downstream with an abex and have to change it and get a stratopower or even a vickers to replace it with. Dan Quote Link to comment Share on other sites More sharing options...
NATOPS1 Posted October 19, 2009 Share Posted October 19, 2009 Yea did not want to cloud the subject....just give a usable answer.... Quote Link to comment Share on other sites More sharing options...
AMPTestFE Posted October 20, 2009 Share Posted October 20, 2009 Look in the Engine Shutdown checklist, Pilot's column for the applicable Caution. Quote Link to comment Share on other sites More sharing options...
ArcticFox Posted October 22, 2009 Author Share Posted October 22, 2009 Yea, I finally found that - and it makes sense , just one of those "wonder why" moments. Thanks for all the good replies. Quote Link to comment Share on other sites More sharing options...
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