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Tycho
15-09-2014, 08:02 PM
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?

RANGER1
15-09-2014, 09:12 PM
Tyco,
Not sure how fine your strainer is, but if using fine oil pump strainer have found they can look clean but have to be washed & blown out with compressed air, to get rid of the fine stuff.
If they worked before you should clean & check again.

RANGER1
15-09-2014, 09:20 PM
So the whole oil charge is brown like muddy water?
So there is a water cooled oil cooler on each compressor, might leak in when servicing compressor,
or if water pressure higher than package pressure.

PaulZ
16-09-2014, 02:21 AM
Hi Tycho
Probably a silly question but have you had the oil tested, dirty brown oil doesn't sound too good? Could there be a water leak in the oil cooler?
What type of oil is it, mineral / synthetic?
What type of oil pump is used?
Regards
Paul

Tycho
26-09-2014, 05:33 PM
Tyco,
Not sure how fine your strainer is, but if using fine oil pump strainer have found they can look clean but have to be washed & blown out with compressed air, to get rid of the fine stuff.
If they worked before you should clean & check again.

The filter is 250my, it was clean as the day it left the factory however I cleaned it with brake cleaner and compressed air and it didn't change anything.

Tycho
26-09-2014, 05:37 PM
So the whole oil charge is brown like muddy water?
So there is a water cooled oil cooler on each compressor, might leak in when servicing compressor,
or if water pressure higher than package pressure.

The system is around a year old and has not been "opened" in that time.
water pressure on the oil coolers is 1 bar.

on our control system we log all pressures and temperatures 6 months, and according to the log the pressure on the discharge, oil separator and oil filter has never been below 4-5 bar.

Tycho
26-09-2014, 06:27 PM
Hi Tycho
Probably a silly question but have you had the oil tested, dirty brown oil doesn't sound too good? Could there be a water leak in the oil cooler?
What type of oil is it, mineral / synthetic?
What type of oil pump is used?
Regards
Paul

Not a silly question at all :)

the on board engineers are supposed to take oil samples every six months, but for some reason (on all ships), they never take oil samples from the refrigeration compressors.
Last time I was on board and noticed the dirty oil and managed to get it to run without the suction strainer I told them that they need an oil change and that i had had to remove the filters to make it run.
I also asked them if they had added any oil and they had added about 20-40 liters on each compressor (they are supposed to enter it in the engine room log when they add oil to the freezing system, but they never do, yay for that).
The captain was straining on the moorings to get back out fishing while I was there so I didn't get to take a "clean" oil sample for myself, but I'm going to take one next time, before we change the oil so we can find out what has happened.
I did take a sample of ammonia to check if there was any moisture, and there was nothing.

The oil is Mineral I believe, Mobil Gargoyle Arctic 300.

The oil pump is a Maag NP 70/70 pump.

When this first started going downhill I thought it might be the oil pumps or the internal bypass valve in the pumps or the labyrinth seals in the compressor (Howden WRV compressor breaking down after a year, if I live to see the day), but after seeing the oil and how it works after removing the filter (and that we haven't heard from the ship in 3 weeks (means it works :)), I have concluded that "something" has happened to the oil that has deteriorated it to a point where the small pressure drop in the suction filter of the oil pump causes the oil to foam and cause cavitation in the pump suction.


on a side note, the ship had a R-22 system before and for a while (3-4 months) they still had the oil cans for the R-22 on board. After I saw the dirty oil I checked and saw that the cans were gone, and I asked them if they might have used the old oil by accident.
The answer was no of course, but when I asked where and when they had moved them off the ship the answer was a bit elusive :)



I'm hoping the solution will be a simple oil and filter change (the filter difference has been ok, but we changed them 2 months ago just to be sure).

posted here to see if anyone might see a mechanical failure somewhere that I had missed :)

But it seems we are on the same page :)