Hydraulics At Work
How To Avoid 'Dry Starting' Hydraulic Pumps

How To Avoid 'Dry Starting' Hydraulic Pumps

One of our members wrote me with this question:

"I was subcontracted to remove a 30 GPM, 4000 PSI rated hydraulic pump from one plastic injection press and install into a similar hydraulic system for the same company.

The hydraulic pump and prime mover are both beside and below (approx. 12") the bottom of the reservoir; a "flooded inlet" design. AND, except for the ball valve at the tank port, there are no other restrictions.

The customer insisted that I fill the pump's case; I did so, but assured him that except for the hydraulic reservoir being empty, the pump would always have an adequate oil level. I know to be safe, there's no harm in filling the case of a hydraulic pump, but if a pre-fill is forgotten, would it damage the pump?"


This is a fair question and worthy of detailed discussion here. Because like almost all other questions relating to hydraulics, the short answer is: "it depends".

And 'it depends' because there are always a number of possible variables to consider. In the above situation, we are considering a piston-type hydraulic pump with a 'flooded inlet'. A flooded inlet means there is a head of hydraulic oil above the pump's inlet. In other words, the hydraulic pump is mounted below minimum oil level. And in a perfect world, this would be the ONLY position you would find hydraulic pumps mounted in!

Now, when you have a flooded inlet, there are essentially two things you want to make sure of before you spin the pump. First, that the pipe or hose between the tank and the pump's inlet is purged of air. And second, if the pump has a 'case' that it is indeed full of oil.

Gear and vane-type hydraulic pumps don't have a 'case' as such, so once you have purged the intake line of air, you are good to go. Oh, and you can't purge the air from the intake line without opening the isolation valve (if fitted) first - which is kind of reassuring.

But if it's a hydraulic piston pump you're dealing with, the physics are a little different and there's another possible variation to consider: Some hydraulic piston pumps have a 'flooded housing' (case). This means the pump's inlet port is connected to, or is common with the housing or case. In other words, if the pump has a flooded housing, flooding the inlet, floods the housing/case as well - in theory at least.

Hydraulic piston pumps with a flooded housing are easy to pick, because the generally don't have a case drain line. They don't need one because the case is common to the inlet, and therefore, the hydraulic tank.

But this doesn't mean flooded housing piston pumps don't require special attention during pre-start. Imagine an installation where there is say 3 feet or 1 meter of head above the hydraulic pump, and the intake line is 4" or 100 mm in diameter and 1 meter long. The hydraulic pump's housing has a volume of about a gallon or 4 liters.

Now imagine you have just changed out this flooded housing pump, so the intake line and the pump's housing will both be full of air. When you open the intake line isolation valve, oil floods the inlet and the housing of the pump. But the air in the inlet line and case has nowhere to go - except to be compressed into the housing of the pump (remember, there is no case drain line to allow this air to purge back to the hydraulic tank).

The solution is simple enough, but often overlooked. After the isolation valve is opened, the uppermost plug in the hydraulic pump's case should be carefully 'cracked' to allow trapped air to escape from the housing. This ensures none of the pump's internal parts are 'dry' upon start-up.

Hydraulic piston pumps which do NOT have a flooded housing can still flood their own case - assuming they have a flooded inlet of course. But it happens SLOWLY. This is because the only connection between the pump's inlet and its case are via internal clearances - mainly between the pistons and their bores.

So in this scenario, with the hydraulic pump installed and the inlet flooded, oil leaks slowly past half the total number of pistons - the other half will be communicating with the pressure port. And as the case fills, air is displaced back to tank via the case drain line.

As you can imagine, it can take several hours or longer for the pump's case to be completely filled in this way. So in a situation where the hydraulic pump is changed out and the machine is immediately re-started, it IS essential that the pump's case be filled manually with clean hydraulic oil through its uppermost case drain port to avoid damage.

If this all sounds way too complicated, I can simplify it for you: Before re-starting a hydraulic piston pump, crack loose the uppermost connection on its housing or case. If oil comes out, the housing is full and you can check this step off. If oil does not come out, you need to make it so that it does.

So to answer this member's question: Yes, the customer was correct in insisting the pump's case be filled with oil - or at least confirming that is was full. Skipping this step can be a very costly mistake. 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.

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