Oil separation

[marianom]

Hi Everybody. Please receive a big hello from Argentina, South America.

I need some help with a chiller from sabroe (2SAB 163 and 1 SAB128).
the evaporator is a PHE from Alfa Laval MK15 with 70 cassettes. The performance of the PHE is reduced by oil on the evaporator. We are purging an average of 1.5lts of oil per day every week.
To do this we have to stop the chiller for a minimum of 5 hours to let the oil set down.
The recirculation pipe for liquid ammonia is 4in, and as collecting pot it has a T leg of the same 4in diameter.
My impression is that this is to small to separate the oil from the ammonia. Recirculation of ammonia is 3500 kg/h according to PHE calculation.
Suction temperature is -7°C it should be -3°C, discharge 13 bar
Does anybody knows a safe separation velocity for a liquid ammonia to oil separator?
Oil is PAO 68 from york

thank you

[Andy]

Hi Everybody. Please receive a big hello from Argentina, South America.

I need some help with a chiller from sabroe (2SAB 163 and 1 SAB128).
the evaporator is a PHE from Alfa Laval MK15 with 70 cassettes. The performance of the PHE is reduced by oil on the evaporator. We are purging an average of 1.5lts of oil per day every week.
To do this we have to stop the chiller for a minimum of 5 hours to let the oil set down.
The recirculation pipe for liquid ammonia is 4in, and as collecting pot it has a T leg of the same 4in diameter.
My impression is that this is to small to separate the oil from the ammonia. Recirculation of ammonia is 3500 kg/h according to PHE calculation.
Suction temperature is -7°C it should be -3°C, discharge 13 bar
Does anybody knows a safe separation velocity for a liquid ammonia to oil separator?
Oil is PAO 68 from york

thank you

Hi Marianom:)

seems you are experiencing a common fault with the SAB 163 Units (high oil carryover).

There is probably a fault in the refrigeration system, check for the following.

1/ liquid carryover (low suction superheats and low discharge temperatures)
2/ Compressor running for long periods of time at part load on the slide.
3/lower than normal condensing temperatures (increased volume flow through the oil separator, beyond normal design)


If you can post a log of the following we can probably tell you what is wrong.

Saturated suction pressure (in deg c)
suction temperature

Saturated discharge pressure (in deg c)
discharge temperature

liquid temperature off condenser
pressure at condenser outlet (not necessary but helpfull)

oil temperature

Chiller fluid in
Chiller fluid out

Looking forward to you post :)

Kind Regards Andy:)

[Andy]

Hi Everybody. Please receive a big hello from Argentina, South America.

I need some help with a chiller from sabroe (2SAB 163 and 1 SAB128).
the evaporator is a PHE from Alfa Laval MK15 with 70 cassettes. The performance of the PHE is reduced by oil on the evaporator. We are purging an average of 1.5lts of oil per day every week.
To do this we have to stop the chiller for a minimum of 5 hours to let the oil set down.
The recirculation pipe for liquid ammonia is 4in, and as collecting pot it has a T leg of the same 4in diameter.
My impression is that this is to small to separate the oil from the ammonia. Recirculation of ammonia is 3500 kg/h according to PHE calculation.
Suction temperature is -7°C it should be -3°C, discharge 13 bar
Does anybody knows a safe separation velocity for a liquid ammonia to oil separator?
Oil is PAO 68 from york

thank you

Anyway why are you not returning the oil automatically, and filtering it back into the compressors.

Normally Sabroe on their chillers have a pot that can be hot gased to return the oil to the compressors.

Kind Regards Andy:)

[marianom]

Andy:
We do have a return system, with the pot you describe. the problem is that the oil does not settle down while any compressor is running. So we have to stop the refrigeration plant to collect the oil and pump it back to the compresors,now manually because the compressors are off.

[marianom]

Thank you for your advice andy!!
I need to go to the compressor in order to read these values.
The main oil separator is a standard coaleser filter from Sabroe.
the oil carry fordward is 25 ppm for these coalesers and calculating this figure on oil is very aproximately the 1.5 liter/day we are pumping out of oil.
So it is possible that the oil separation pot on the liquid recirculation pipe of the evaporator to be to small?

[Andy]

Thank you for your advice andy!!
I need to go to the compressor in order to read these values.
The main oil separator is a standard coaleser filter from Sabroe.
the oil carry fordward is 25 ppm for these coalesers and calculating this figure on oil is very aproximately the 1.5 liter/day we are pumping out of oil.
So it is possible that the oil separation pot on the liquid recirculation pipe of the evaporator to be to small?
Yes it is too small, but that is not the problem, 1.5 litres a day seems high even for 3 compressors.
What about automatic oil return, why drain when a well designed oil return system will return the oil to the compressors without wasting it:)

Kind Regards Andy:)

[Andy]

Andy:
We do have a return system, with the pot you describe. the problem is that the oil does not settle down while any compressor is running. So we have to stop the refrigeration plant to collect the oil and pump it back to the compresors,now manually because the compressors are off.

Yes saying this your oil pot is two small:) but better check your oil return system operation, there maybe a fault there.
Oil rectification systems return oil and liquid refrigerant, boiling off the refrigerant and returning the oil. Movement in the system should not effect the rectification of oil, the oil will still go to the bottom. Check the system operation, I am sure thre is a problem with the rectification or it's sequencing.

Kind Regards Andy:)

[Camille]

is there a chance that you could sketch for
your system, compressors with the discharge oil separators, chillers etc, so I could comments on your oil problems

Regards
Camille

[NoNickName]

Had similar problem.
Changed the oil with POE 32, and problem is now solved.
If you want to stick with POE 68, then use ICI which is less miscible than Castrol and similar.
Remember that 68cSt is the kinematic viscosity of oil at 40°C.
The same oil at -10°C becomes a molasse with a viscosity of over 1500cSt.

[marianom]

This are the condition of the main compressor working at 100%

Suction:

pressure:……………2.7 bar equi -3.6°C
Temperature:…... – 1.8 °C.
superheating temp:….1.8 °C.

Discharge:

Pressure:……………10.7 bar equiv 30.3°C
Temperature:……...64.1 °C.
superheating temp:....34.2 °C.

Glycol:

Temperature out:…….. 0.5 °C
Temp in: ....................4°C

Oil Pao 68:

Pressure:…………….6.5 bar.
Diferencial:………...0.2 bar.
Temperature:……...39.9 °C

Compressor capacity:................100 %.

Motor Current:…..224 A (50HZ 380 v)

[US Iceman]

HI marianom,

Have you checked the oil return line from the separator(s) back to the compressor(s)?

Is the oil returning through these lines?

[marianom]

Nonickname:
The Oil we use is PAO 68, would you please clarify what POE means?

Regarding the viscosity of the oil at low temperature, I agree with you that at low temperature the viscosity high, but i don´t know what to install to separate the oil before without a big investment.

[The MG Pony]

Poly Ester Oil = POE, why that order no idea but technicly it aut to be PEO

[marianom]

Yes Iceman, We did.
the problen is not the oil return system, is that the oil is not setling down on the separation pot, while the compressors are running. There is a level indicator for oil on the pot, and is only activated if when the compressor is off (we know it works, because when there is a lot of oil on the evaporators activates the level whith the compresor running)

[marianom]

Yes Iceman, the oil returns,the problem is that when the oil pot on the evaporator fills up, the system returns that oil to the compressor, but if inmediately you stop the compressor and let it rest for 4 to 5 hous you may purge up more than 30 liters from a PHE

[Andy]

This are the condition of the main compressor working at 100%

Suction:

pressure:……………2.7 bar equi -3.6°C
Temperature:…... – 1.8 °C.
superheating temp:….1.8 °C.

Discharge:

Pressure:……………10.7 bar equiv 30.3°C
Temperature:……...64.1 °C.
superheating temp:....34.2 °C.

Glycol:

Temperature out:…….. 0.5 °C
Temp in: ....................4°C

Oil Pao 68:

Pressure:…………….6.5 bar.
Diferencial:………...0.2 bar.
Temperature:……...39.9 °C

Compressor capacity:................100 %.

Motor Current:…..224 A (50HZ 380 v)



All appears well, the only thing I see is low suction superhaet, but not real low for a flooded chiler.

If on site I would try the following.

1/ lower the liquid level in the surge vessel above the PHE:)

2/ Increase the condensing pressure:)


But not both at the same time. Your compressor amps seem low for a Sab163, but I suppose the condensing pressure would account for that.

With ammonia DO NOT use POE oil, ammonia carries water around with it POE oil absorbs water:(

You could also try taking the oil/refrigerant mixture for the oil rectifer off at a slightly higher point. But try rasing the condensing pressure;)

Kind Regards Andy:)

[NoNickName]

Poly Ester Oil = POE, why that order no idea but technicly it aut to be PEO

Because it correctly spells polyolester.

[marianom]

Thank you Andy.

we have tried the first, and the heat exchanged drops dramatically with just a reduction of 1 inch from the recomended level on operation.
the second, happends naturally due to the condensing temperature depends on the fouling of the condenser river side (CIP washed every 4 weeks)
We can not see any significant difference.
We are looking to introduce a modification on the installation to collect the oil.
do you recomend a second coalecent filter or a oil to liquid separator on high pressure side?

[Andy]

Thank you Andy.

we have tried the first, and the heat exchanged drops dramatically with just a reduction of 1 inch from the recomended level on operation.
the second, happends naturally due to the condensing temperature depends on the fouling of the condenser river side (CIP washed every 4 weeks)
We can not see any significant difference.
We are looking to introduce a modification on the installation to collect the oil.
do you recomend a second coalecent filter or a oil to liquid separator on high pressure side?

The coalecent filter will work well, if you can afford it. Dominic Hunter is a common manufacturer that I would recommend. If your Sabroe was a Grasso this coalecent element would be in the top of the oil separator as standard. :)

Oil wash systems, I have no experience with, but what little I know is(or have heard) is they can be problematic to set up, and expensive to maintain:)

One common Coalecent separator will be enough, with a level switch to open a valve into each compressor on lower than normal oil level:)

Kind Regards Andy:)

[Josip]

Hi, marianom :)


HI marianom,

Have you checked the oil return line from the separator(s) back to the compressor(s)?

Is the oil returning through these lines?

This is a good question and point where your problem starts. You must check return of oil into compressor from oil separator i.e. keep your oil within compressor and not allowed oil to escape to condenser and come back to your evaporator (PHE) reducing capacity.


Yes Iceman, We did.
the problen is not the oil return system, is that the oil is not setling down on the separation pot, while the compressors are running. There is a level indicator for oil on the pot, and is only activated if when the compressor is off (we know it works, because when there is a lot of oil on the evaporators activates the level whith the compresor running)

I think this is ok. Why? Because I think that oil pot is there just for that reason, to empty the oil after long stanstill (time needed to set the oil down to oil pot) to assure normal work. Your level switch can be activated only when your compressor is off, thus without any turbulence flow through PHE. (The same is with manual air purging, you must stop the plant for couple of hours to separate air from ammonia and purge the air).

I do not know but I think that you have two oil return systems. One from oil separator and another one from PHE.

Your problem is with the first one. Return of oil from oil separator to the chamber with shaft seal. There you have (I think) one unit with small regulating valve and sight glass (see picture). Inside of that unit under plug you have also a small filter which must be checked.

Hope this will help ;)

BTW, how old is your plant and when you faced those problem?

Best regards,

Josip :)

[US Iceman]

Yes Iceman, the oil returns, the problem is that when the oil pot on the evaporator fills up, the system returns that oil to the compressor, but if immediately you stop the compressor and let it rest for 4 to 5 hours you may purge up more than 30 liters from a PHE


This would seem to imply more oil is getting into the low side of the system than the oil recovery can handle.

That would indicate the coalescing element is leaking somehow (elements not seated properly in separator, or element itself is bad).

In another post you mentioned this;

the oil carry forward is 25 ppm for these coalescers and calculating this figure on oil is very approximately the 1.5 liter/day we are pumping out of oil.

If the oil carryover is expected to be 25 ppm (or 1.5 liter/day... I didn't check this, I'm only using your numbers) for the separator, then it sounds like the oil drip leg (the T you mentioned in the liquid line to the PHE) would have a similar volume (4" pipe). This was the factory design, is this correct?

Another question, which I'm not sure of the answer is... Is the oil separator designed to operate at 10 bar discharge pressure? If not, the velocity through the separator can be too high and cause oil carryover also, even if the elements are new and seated properly.

The more I think about this it seems to an oil separator problem, not oil recovery.

[Andy]

This would seem to imply more oil is getting into the low side of the system than the oil recovery can handle.

That would indicate the coalescing element is leaking somehow (elements not seated properly in separator, or element itself is bad).

In another post you mentioned this;


If the oil carryover is expected to be 25 ppm (or 1.5 liter/day... I didn't check this, I'm only using your numbers) for the separator, then it sounds like the oil drip leg (the T you mentioned in the liquid line to the PHE) would have a similar volume (4" pipe). This was the factory design, is this correct?

Another question, which I'm not sure of the answer is... Is the oil separator designed to operate at 10 bar discharge pressure? If not, the velocity through the separator can be too high and cause oil carryover also, even if the elements are new and seated properly.

The more I think about this it seems to an oil separator problem, not oil recovery.

Iceman to my knowledge this separator has not got a coalecent filter.

The manual takes about a fine element, but not coalecent:D

I may be wrong but I assumed that these units were not fitted with coalecent filters, mainly through experience, they all use oil at a higher rate than units with coalecent secondaries:(

This filter may simply be that a fine mesh filter.

Kind Regards Andy:)

[US Iceman]

Hi Andy,

You are right. The manual Josip posted does describe a "fine filter element". In the drawings it is installed in the separator in a similar manner as a coalescing element.

The demister pads normally do a very good job of removing a large volume of the liquids (oil in this case), so it would appear the "fine filter element" is being used as a final separation method.

I guess I would like to see some pictures or technical descriptions of the "fine filter element". As you say, the oil carryover rate would be higher than a separator with coalescing elements.



Normally, it is superfluous to remove or replace the fine separator element, but at an increasing oil consumption it is possible to inspect the filter or extract it...





Your suggestion about raising the discharge pressure would help to see if the oil carryover rate decreases.

[marianom]

Josip:
we review these valve adjustment, to adjust it according to the manual long time ago. Always as the manual say, told an old Master.
It seems the filter element ( if not a coalecer) is not giving us good results

the plant is 4 years old. I get in touch with it last year. they told me this problem is from the start up.
The York Service are very reticent to give us good support on these because the are afraid of claim or what ever, but this is the case, the support from them is really bad.

[marianom]

We have replaced the filter element, twice (a part from York) but up to now could get no technichal description of it.
one of them was damage because a faulty check valve imploding it (back flow)
The second time was a suggestion of the York service to improve oil recovery, but note no difference after the change.

Best regards
Mariano

[marianom]

It is clear that this oil pot is on the low pressure side , no?

[Andy]

It is clear that this oil pot is on the low pressure side , no?

Hi Marianom:)

Yes the pot is on the low side, but will be pressurised with hot gas to push the oil back to the compressors:)

Kind Regards Andy:)

[marianom]

On the attached file, you can see a drawing of the installation on the PHE side. There are four PHE, the two on the right are the evaporators. (on red the oil pot we were talking about)

You may see also that there is no oil separation on the high pressure side. There is no liquid receiver, liquid condensates on the plate heat exchanger and is fed to the low pressure side trough a level control located on the outlet of the condenser PHE.

The 2 on the left are part of the condensation system. (we use river water to condensate, but trough an intermediate PHE) to avoid the risk of river water contamination.
Also the condenser PHE have the refrigeration oil PHE.

[vicky_pillai]

Hai Everybody