An essential requirement for the optimum performance and service life of a hydraulic pump is that its pumping chambers fill freely and completely during intake. So if getting maximum pump life is your primary concern (and it should be), then anything that makes the free and complete filling of the pump's chambers more difficult to achieve should be avoided.
Suction strainers and most other forms of inlet filtration are a common culprit. With rare exceptions, a suction strainer has no place in a properly designed and properly maintained hydraulic system.
But when you take a position against the majority, there will be many who disagree with you. And so I regularly hear from folks who feel they need to explain to me why their hydraulic system is different and why they have no alternative but to use this pump-killing device.
I prefer not to debate the point with people who have convinced themselves of the merits of suction strainers or, in some cases, use them as a substitute for proper design. I refer them to the pump manufacturer's recommendation instead. Here is an excerpt from a Rexroth Hydraulics manual1 that dates back to 1979:
"The advantages of suction filtration are strongly outweighed by the disadvantage of the pressure drop created by the element. … Any benefit the suction filter offers by keeping contamination out of the pump is offset by the possibility of damaging the pump because of cavitation. …
"Another major disadvantage of the suction strainer is that it is located inside the oil reservoir, which makes it inconvenient to service. It is for this reason that many suction strainers in hydraulic systems go unserviced until they starve the pump and cause cavitation damage. ... Due to these disadvantages … filtration at the inlet of the pump is specifically not recommended."
It speaks volumes about the hydraulics industry that this 30-year-old advice from a leading hydraulic pump manufacturer is still widely ignored today. But a suction strainer isn't the only "engineered in" barrier to the free and complete filling of the pump. Another is mounting the pump above the tank or, more precisely, above minimum oil level. In other words, making the pump "lift" the oil into its intake.
Approved vs. Maximum Life
According to most manufacturers, mounting the pump above minimum oil level is an approved mounting position for many pump designs. "Approved" means that the manufacturer says it's OK to do it. But approved does not mean it maximizes pump service life. That is 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 restriction in the pump intake or required lift - this air expands and becomes a greater 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 greater 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 - known as vaporous cavitation.
For these reasons, the perfect pump inlet condition is 100 percent boost. This means that, ideally, you want the pump inlet to be supercharged under all operating conditions.
The Need for a Flooded Inlet
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 (Figure 1).
In the case of industrial power units, this rules out mounting the pump on top of the tank (Figure 2). And in most cases, it will rule out mounting the pump inside the tank - with the electric motor mounted vertically (Figure 3) — unless the pump is submerged to a depth where its inlet port is below minimum oil level (without the need to install a drop tube on the intake).
Besides making the pump lift its oil, both of these mounting positions (figures 2 and 3) make maintenance extremely difficult - with having the 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 is a cheap and easy method of construction.
If you've been reading my column in Machinery Lubrication for a while, you're probably already familiar with the problems associated with suction strainers. But unlike a suction strainer, which is easily removed, pump mounting position is not easy to change. So, what can be done about it?
Well, if you're a hydraulic equipment user, specify that the pump must have a flooded inlet for all of your future equipment purchases. And if you design or manufacture hydraulic power units, do your customers (and your machine's reliability) a favor: ensure all your hydraulic power units feature a flooded inlet.
1) Frankenfield, T.C. 1979, "Using Industrial Hydraulics", Hydraulics & Pneumatics Magazine, p. 10-31.