Both!
Purpose of having the tank higher is to increase the head pressure on the inlet. Same reasons why suction should be large, straight, without a filter, and strainer oversized if one is used. When pressure drops too low at the pump inlet the fluid can cavitate in small local bubbles. Cavitation is a bit of fluid basically 'boiling' by turning to vapor at low temperature and low pressure. When the bubble gets compressed through the pump, the vapor bubble collapses and the shock wave causes an impact on the metal surfaces of the pump parts. Very much like detonation in an engine. If you tap on a metal surface with a molecule sized hammer long enough, eventually a piece of metal fatigues and flakes off. Enough molecules come off and eventually the pump has pit marks. An engine piston gets the sandpaper looking marks from detonation. A pump part will show the same microscopic pits due to cavitation. Usually it doesn't matter in noticeable performance. By the time the pump fails and is removed from service, the other damages are so great the cav marks are not noticeable.
Cavitation is not the same as sucking air, but the higher suction can also draw air into bad joints. Doesn't seem like much, but a few inches of mercury vacuum is about what most pumps can tolerate and a couple feet of tank height helps a lot.
More important for mobile equipment because it runs outside in colder weather, and typically is gas engine drive at higher rpm. There is much less time for the tooth to fill as the pump turns, so has to have more head pressure to push the oil in. The obvious solution there is reduce speed until the oil is warmer and flows easier. Note: cavitation is NOT noticeable in flow output. It occurs long before the flow is reduced. If the oil is really thick enough for things to work slow, that is obvious. Cavitation doesn't hurt at cold start if it only happens for seconds or minutes at slow startup.
Industrial stuff is typically on top of tank for convenience in layout, less floor space, easier to work on, less mess when changing pumps. Also usually runs 1800 rpm electric motor speed, and more constant temperatures inside a building. Some use the L shaped mount where the pump/motor is beside the tank, but takes more floor space, which is expensive space in most factories.
It is good practice, but NOT required by any means. How many mobile and industrial pumps are mounted above tank level? Most of them...
It does make the 'priming' issue more important though. A positive displacement pump will prime. It is not like a centrifugal water pump that simply runs forever spinning in air if the housing is not primed. So actually 'priming' is not the main issue, it is lack of lubrication during startup. In most cases the oil left in there from manufacturing or testing is enough to lube it for a few seconds. Maybe this machine had oil suction leaks and lost its 'prime' over many years of storage and started dry. Maybe it is just coincidence and the pump just failed.
I really stress putting oil in the pump inlet and outlet for that lubrication on startup, and filling the case of piston pumps or hydrostatic pumps/motors. They won't fail immediately, just shortens the life a lot and somewhere down the road the mechanic wonders why pumps are failing in a year instead of 5 years. Of course, a year of mobile equipment life may be 25 years of logsplitter life so many of these concepts don't matter that much in real life in this board.
Anyway, sounds like pump is toast, can be changed in half hour for less than $200 and life is good. Way better than changing some 700 pound, $25,000 hydrostatic pump, Course that is not 'personal money'....
k
Purpose of having the tank higher is to increase the head pressure on the inlet. Same reasons why suction should be large, straight, without a filter, and strainer oversized if one is used. When pressure drops too low at the pump inlet the fluid can cavitate in small local bubbles. Cavitation is a bit of fluid basically 'boiling' by turning to vapor at low temperature and low pressure. When the bubble gets compressed through the pump, the vapor bubble collapses and the shock wave causes an impact on the metal surfaces of the pump parts. Very much like detonation in an engine. If you tap on a metal surface with a molecule sized hammer long enough, eventually a piece of metal fatigues and flakes off. Enough molecules come off and eventually the pump has pit marks. An engine piston gets the sandpaper looking marks from detonation. A pump part will show the same microscopic pits due to cavitation. Usually it doesn't matter in noticeable performance. By the time the pump fails and is removed from service, the other damages are so great the cav marks are not noticeable.
Cavitation is not the same as sucking air, but the higher suction can also draw air into bad joints. Doesn't seem like much, but a few inches of mercury vacuum is about what most pumps can tolerate and a couple feet of tank height helps a lot.
More important for mobile equipment because it runs outside in colder weather, and typically is gas engine drive at higher rpm. There is much less time for the tooth to fill as the pump turns, so has to have more head pressure to push the oil in. The obvious solution there is reduce speed until the oil is warmer and flows easier. Note: cavitation is NOT noticeable in flow output. It occurs long before the flow is reduced. If the oil is really thick enough for things to work slow, that is obvious. Cavitation doesn't hurt at cold start if it only happens for seconds or minutes at slow startup.
Industrial stuff is typically on top of tank for convenience in layout, less floor space, easier to work on, less mess when changing pumps. Also usually runs 1800 rpm electric motor speed, and more constant temperatures inside a building. Some use the L shaped mount where the pump/motor is beside the tank, but takes more floor space, which is expensive space in most factories.
It is good practice, but NOT required by any means. How many mobile and industrial pumps are mounted above tank level? Most of them...
It does make the 'priming' issue more important though. A positive displacement pump will prime. It is not like a centrifugal water pump that simply runs forever spinning in air if the housing is not primed. So actually 'priming' is not the main issue, it is lack of lubrication during startup. In most cases the oil left in there from manufacturing or testing is enough to lube it for a few seconds. Maybe this machine had oil suction leaks and lost its 'prime' over many years of storage and started dry. Maybe it is just coincidence and the pump just failed.
I really stress putting oil in the pump inlet and outlet for that lubrication on startup, and filling the case of piston pumps or hydrostatic pumps/motors. They won't fail immediately, just shortens the life a lot and somewhere down the road the mechanic wonders why pumps are failing in a year instead of 5 years. Of course, a year of mobile equipment life may be 25 years of logsplitter life so many of these concepts don't matter that much in real life in this board.
Anyway, sounds like pump is toast, can be changed in half hour for less than $200 and life is good. Way better than changing some 700 pound, $25,000 hydrostatic pump, Course that is not 'personal money'....
k