Hydraulic leak

I have a GB42 with hydraulic bow thruster and stabilizers. The system has an inline oil cooler using sea water as a coolant. The sea water then goes into my exhaust system and is expelled out.
We recently noticed a sheen in the water coming from the exhaust. The cooler had rusted internally and fluid was leaking into the salt water discharge..
Regrettably we were taking sea water into the hydraulic system. We drained and replaced the hydraulic oil twice (15 gallons each time) There is still some emulsion of water/oil in the system, but greatly reduced. Please tell me that a little bit of water in the system will not be catastrophic. If not so, is there a easy way of purging the system, ie pumping air into the system. Alternatively do I need to pull all of the hoses? Tell me something good.
Jim

No good news, water is not a lubricant and will cause component failure. I would ditch that saltwater cooler, maybe run engine coolant through it or use a keel cooler? Air cool it with a vent outside? Saltwater inside is by definition expensive! If the hydraulic reservoir is in the engine room, it would be straightforward to add a cooler using engine coolant. Engine coolant is not corrosive like saltwater.

I don't know how else to flush the system, there may be people who can power flush it out with a cart like they use to ground power aircraft? I would not run any of those components until you are sure they are clean.

Is there a drain at the bottom of your reservoir tank? The water should settle to the bottom of the tank where is can then be drained off. You will probably have to let the system sit for a while for the water to settle out. Then run it to cycle the fluid through and observe for foam. The let it settle out and drain off. Keep repeating, assuming of course that you are actually draining off water and can see an improvement

That pump is probably a piston type pump and they for sure do not like contaminants. Any water left in the system is going to cause pitting in the piston barrels and trash the pump.

Water is in all the components and is going to require a major flushing to get it out. You won't be able to get all the water out from the reservoir.... big job. I'd consider calling a mobile hydraulic tech and look into getting some help or an opinion.

You could end up having to take some things apart and flush out components, or create an open ended loop and flush out components using a different pump vs the main pump. Possible consider by passing the reservoir to cut down on wasting so much oil....also possibly by pass the main pump, and use an electric pump to push low grade oil through all the components and collect it in a bucket. Whatever route you take, cleanliness is the key as the main pump needs super clean, contaminant free oil.

Conall

No idea about cost, but one can buy a coalescing oil/water filter.
You have to find one that passes oil and repels water. Many do the opposite.

The issue is the residual contaminated oil in the lines that is left behind when you change the oil in the reservoir. On restart, the residual contaminated oil mixes with the new oil contaminating the whole. The contamination level drops with each change, but will struggle to get to zero, and require a lot of oil changes to get to a low point.

The procedure I suggested, assuming the water separates out over 24hrs or so, will let you drain out the water without changing all the oil.

Another approach would be to drain the tank, and connect the pump intake to a pail of fresh oil. Then you would be pulling uncontaminated oil in through the system. But I think it would be tricky to direct the flow through all the devices, and manage the flow rate so you don't just suck the pail dry it two seconds. Hand cranking the pump may be required, for example.

It's probably easier to do the drain and fill thing.

Add a return filter. Something like the picture. A spin on filter will hold about an ounce of water. The longer the filter, the slower the oil goes thru, catching more water. The one in the picture is $32 on Ebay.

The problem with running the system is the salt water that's already in the pumps and those expensive stabilizers. You pretty much need to get it all out before you run the pump.

Open hoses and drain all you can.
Keep flushing. Consider an inexpensive oil for the flushing. Flushing oil should be available from a good supplier. Discuss with them what the problem is.
Do not leave seawater in the system or the cost and pain of repairs will make the oil cost and flushing work seem cheap. Residual seawater is corrosive to the pump, the motors, the valving and reservoir.
Dispense with the cooler entirely unless you run the system long and hard. The tank may be enough reservoir to act as a decent radiator.
If no then you will need to search out a cooler from someone like Sendure Mfg. But you will need to know and discuss the hydraulic pressure it must work with safely. They make coolers of bronze, not steel.

You won't be able to get the water out with a separating filter. The pump you have is operated on a pilot circuit so when a no hydraulic pressure is needed, the pump idles at 500 psi. Once pressure is needed, the pilot turns the pump on and pressure goes up to 2000 or 2500 psi. At idle pressure of 500 psi, the oil and water are being thoroughly mixed together and a separator wont' work.

The pump you have is probably four or six thousand dollars.

There's no way that system can operate without using a cooler. The oil needs to stay below 160 degrees, or the pump life will be drastically shortened. When those stabilizers are running, the pump is putting out 2500 psi and generating an enormous amount of heat. Without a cooler, at those pressures, the pump is doomed. Even when no devices are being used, the pump is still running at 500 PSI, and will heat up quickly ( I'm assuming a live PTO straight off of the main engine drives the hydraulic pump ). Either way, a cooler is essential, and I'd 100% guarantee it's in the original system design.

The cooler's in the return circuit so pressures are not an issue. The old cooler failed so for sure the filter is going to need changing as bits of the cooler are in the filter. Water's going to get trapped in the filter, so every flush will need a new filter. The cooler needs to be cupro nickel if you're using sea water.

Be careful trying to suck oil into the system via a bucket, the main hydraulic pump and no filter.

I still think you'll have to take some things apart and use an electric pump to individually flush each component and lines to make sure you have clean oil in the system. A good mobile hydraulic mechanic should be able to do that job in a day.

Conall

Not easy, for sure. We need to be aware that oil, fuel, and the air can hold water up to the saturation level, and then it will settle out.
I had a outboard lower unit leak in some water, all milky, and as an experiment I poured it into a glass and let it sit for a year. About 1/4" of water appeared at the bottom fairly quickly, but the entire solution was still saturated/milky after a year.

Sorry, my advice was off. I missed the stabilizers so the system is working hard all the time.
As Conal said you are going to have to drain and flush everything properly or suffer serious saltwater corrosion damage.
I would also call and talk to the mfr. of your system. I,ll lay odds someone else has been caught.
You will need a new cooler appropriately sized but not another steel one. Corrosion likely killed it.
Question: did the original cooler have zinc pencils? If not it should have had and if so were they changed frequently?
Working pressure must be known, system relief valve set pressure must be known or the new cooler could be blown. It must be built to handle the pressure and system spikes.