demister oil separator

[faryuki]

Hello sir,

I have margarine plant refrigeration system and this is an old plant. This system is use screw compressor unit with vertical oil separator and water cooled oil cooler.

We suspect that the oil separator is dirty due to oil carry over, but the oil separator design using demister type which cannot replace.

Does anybody have experience how to cleaning the oil separator with demister type?

Thank you

rgds
Faryuki

[US Iceman]

How much oil do you think is being carried over into the system?

How much are you draining out on a regular basis?

Does the separator have coalescing elements, or just the demister pads?

Has the suction pressure increased higher than the compressor was originally designed?

Is the discharge pressure being operated lower than the compressor was originally designed?

All or a combination of these will affect oil carryover, besides high velocities through the demister if it is plugged up.

[faryuki]

How much oil do you think is being carried over into the system?

How much are you draining out on a regular basis?

Does the separator have coalescing elements, or just the demister pads?

Has the suction pressure increased higher than the compressor was originally designed?

Is the discharge pressure being operated lower than the compressor was originally designed?

All or a combination of these will affect oil carryover, besides high velocities through the demister if it is plugged up.

* oil carry over is around five litres/ day if we drain at the evaporator.
* the separator only demister pads and cannot replace because no flange at oil separator.
* the suction pressure is maintain at 0.8 ~ 1.5 Kgf/cm2. design TE is -20 degC
* discharge pressure maintain at 13 ~ 14 kgf/cm2. design Tc is 38 degC

Now we thinking about install second stage oil separator with coalescer type with flange for maintanance.

Need your advise and your experience for this case.

Many thanks.

rgds
Faryuki

[US Iceman]

Did your problem start to occur suddenly, or seem to become worse over time?

[TXiceman]

Simplest solution is to add a secondary separator down stream and use a coalescing element in it. Done this several times to help an old plant.
Ken

[gwapa]

I would like to know if the commpresor is using mineral or syntetic oil?

I had an similar experience wirh a srew compressor. It used some coalecents element to filter the ammonia.

For any reason one of those elementes moves from its base and the ammonia passed thought without filtration.

We get some sckematic drawing from the srew provider and instal a manhole in the separator and reinstaled the filter.
Regards

[TXiceman]

Cutting on the oil separator which should be a coded vessel will require the vessel to recoded or at least a repair stamp.

Mesh type demisters usually do not "go bad", but sometimes shift and allow oil to bypass.

IF you have recently change type or brand of oil, this could cause an excessive carry over. Mixing oils is a no-no in my book. Too often I have seen the oil carry over sky-rocket after a change or addition of a different oil. Manufactures or blenders of oil, generally use different additive packages and sometimes the different additives will not mix and cause excessive oil foaming which leads to oil carry over.

Ken

[gwapa]

Ken
You are right . The vessel was made under european Code. So we made the modification following the Code Procedure

The mineral oil es itself a mixture of diferents oil coming out of destilation tower. This mixture is disigned to lubricated the compressor at its working temperature. However in the compression process the temperatur could reach at 100 °C. That make that the lighter part of the oil mixture travels and pass the filters as a gas. Only the tiny oil drops are stoped in the demister and some few in the coalesente filter.

The sintetic oil does not have this trouble.

[TXiceman]

This does not sound like a closed loop refrigeration cycle, but an open loop gas compression unit. To stop all of the vapor state oil you have a couple of options.

In order to get more droplet oil, you can use an after cooler to condense the oil vapors, but keep the temperature high enough that you will not start to condense your compressed vapors. You will still ahve some vapor state oil. A secondary oil separator can collect the droplet oil.

Another option is to use a carbon bed absorption unit. The bed will absorb the oil vapors, but it needs to be regenerated periodically. You can shut down and regenerate or you can install parallel carbon beds and switch to the other while one is under going regeneration. Regeneration can take on different forms, but will usually involve a heated gas or steam to drive off the oil. Them you have to deal with cleaning up the regenerated waste oil or just dump it.

The carbon bed absorber is used on a lot of oil injected screw gas compressors that have to have fairly oil free discharges.

Most synthetic oils have a pretty high vapor pressure so they do not vaporize as much oil at higher temperatures.

Ken

[gwapa]

TXiceman
We made some test and we observed that cooling dawn the ammonia we purge less oil in the evaporators .So we disight an oil separator as a flash intercooler. All the ammonia gases overheated coming out from all compresores bubble into ammonia liquid at condensing state . That make the gas flumes and spray condens. The separator has inside a demister . The ammonia gas coming out the second separator then flow to the evap condenser.

I vote to use syntetic oil

Thaks for your advise
Regars