So...

We have an installation with 3 identical compressor units with howden WRV163 compressors on them, watercooled oil coolers and a 3 way mixing valve, NH3.

they were commissioned a year and a half ago and worked like a charm up until 3 months ago.

The customer started getting problems, loosing the oil pressure or shut down on high oil temperature.

I've been on site three times, first time to fix it, second time to find out what was happening and third time trying to find out just what in the H was going on.

First visit, I changed the oil filters and the 3-way thermostat on 2 of the units, serviced the oil pump on 1 unit that was the problem child, that didn't work, so I switched the oil pump between two units and still one unit remained the problem child.
Did some adjustments on the oil injection valve (from the oil manifold and into the screws to help cool the discharge), noticed that looking sternly at the valve was the difference between maintaining oil pressure or cutting out on high discharge temp. but managed to get it running at 100% with an oil pressure of 3.2 bar and a discharge temp of 82C (cut out is 2.4 bar and 85C).
moving the spindle on the oil injection valve about 2mm could take the oil pressure from 3 bar to 6 bar!

Second visit, I had to adjust the oil injection valve on all three compressors to try and stabilize them, and again managed to keep them running.

Third visit, at my whits end, noticed that if I opened consumers (platefreezers) to fast (was trying to work like a fisherman operating the plant), the discharge pressure rose so quick that it caught up with the oil pressure from the pump and thereby lost the oil pressure difference. the condenser pump is variable speed, so I made the regulator faster so that the condenser pump could keep up with the "abuse"

Now I am left with the problem of high discharge temperature. Also the nagging feeling of something not being right with the oil injection valve since looking at it the wrong way makes the difference between low oil pressure or high discharge temp is grinding at the back of my mind.

Usually, when commissioning a unit like this, you start with the oil injection valve barely open, then you use the bypassvalve on the oil pump to adjust the lubricating pressure, and as you adjust the oil injection valve to add more oil to keep the discharge temp down you follow with the bypassvalve to maintain the oil pressure.

On these units I had the bypass line closed and I still couldn't maintain oil pressure.

After much ado, I had three guys from the office on the line while I was running back and forth making adjustments and reading them the values.
One of them (former service technician and my mentor), kept asking about the suction strainer before the oil pump, It's clean I said, it's clean, I've had it out 2 times already.
Then he said "unless anyone has any objections, I want you to remove the strainer from one unit and run without it"

No objections, I sighed and removed the strainer...

This time, as opposed to the other two-three times I had inspected the strainer and changed the oil filter I noticed that the oil was very dirty, it looked like brown muddy water.

I didn't really get my hopes up that this would work, but after I started the system without the strainer in the oil pump suction, I could again adjust the oil injection valve 1/2 turn and the change in oil pressure would only change 0.2-0.3 bar and I could again use it to regulate the discharge temp.

Obviously something has happened to the oil, as far as I know they haven't had an oil change since the system was commissioned a year and a half ago, in my mind, nor should they have had to... Normally on systems like this, they just add some oil every 2-3 months to replenish whatever has been carried over, and they send in oil samples to be analyzed once every 2 months.

in the end, all I am left with is that the oil has degraded so much that they get cavitation in the oil pump suction when the strainer is in... anyone else seen something similar?