A pet hate of mine is hydraulic power units in which the pump is made ‘lift’ the oil into its intake. In other words, mounting the pump above the tank or more precisely, above minimum oil level. Like a suction strainer, it’s another ‘engineered’ barrier which prevents the pump’s chambers from filling freely and completely.
According to most manufacturers, mounting the pump above minimum oil level is an ‘approved’ mounting position for most pump designs. ‘Approved’ meaning, the manufacturer says it’s OK to do it. But ‘approved’ doesn’t mean it maximizes pump life. Because making the pump lift its oil does the opposite. This is particularly true for piston and vane pumps, which due to their design, do not cope well with vacuum-induced forces.
Pump inlet conditions also affect noise and heat load. When exposed to atmospheric pressure at room temperature, mineral hydraulic oil contains between 6 and 12 percent of dissolved air by volume. If the pressure on the oil is reduced to less than atmospheric pressure – due to a restriction in the pump intake or required lift – this air expands and becomes a higher percentage of the volume.
These expanding gas bubbles at the pump inlet collapse as the pumping chamber is exposed to system pressure (gaseous cavitation). The result is heat generation and noise. The larger the air bubble, the higher the noise level and heat generated. If the absolute pressure at the pump intake continues to fall (higher vacuum) the oil can start to change state from a liquid to a gas – a process known as vaporous cavitation.
Many years ago I remember having a conversation with a Bosch Rexroth engineer on this subject. And his words were to the effect: “The only pump inlet conditions to have are 100 percent boost”. Meaning, ideally, you want the pump inlet to be supercharged under all operating conditions.
While supercharging the pump inlet is not practical in most applications, there is virtually no excuse for not having a flooded inlet. A flooded inlet means there’s a head of oil above the pump. In other words, the pump is mounted in such a way that its intake is below minimum oil level — see inset.
In the case of industrial power units, this rules out mounting the pump on top of the tank. And in most cases, it will rule out mounting the pump inside the tank – with the electric motor mounted vertically – unless the pump is submerged to a depth where its inlet is below minimum oil level (without installing a drop tube).
Besides making the pump lift its oil, both of these mounting positions make maintenance extremely difficult. Pump inside the tank being the worst. But unfortunately, (for the owners of this equipment) mounting the pump inside the tank has almost become standard practice for electric power units, because it’s a cheap method of construction.
So if you’re a hydraulic equipment user, specify that the pump must have a flooded inlet for all your future equipment purchases. And if you manufacture hydraulic power units, do your customers a favor: ensure all your designs feature a flooded inlet. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.