Mycom N8K oil pressure problem

Hello!
I have recently installed a new nh3 chiller
PHE flooded evaporator and PHE condenser
The unit has a mycom N8k compressor with variable speed controller and 3 stage unloader/loader

When the unit was compissoned oil pressure was good, as listed in the specs(2-3 bar) the oil pressure was adjusted to around 3 bar
But there is a strange issue, sometimes the unit goes into a low oil pressure alarm, when i run the compressor oil pressure is good and not unstable but once every other day the unit shuts off on low oil pressure. The setpoint of alarm is 1.5 bar with 30sec delay

There is almost no foaming of the oil in the sump, and i have cleaned the oil filter/strainer.

Oil temp is around 40c (oil cooler installed)
Oil level 80% of sight glass

The problem is that i am never there to observe when the problem occurs, it may run for 2 days before shutting down on oil alarm

Operating data
Superheat 2K
Evaporation temp 7°C
Condensing temp 24°c
Discharge temp 57°c
Oil type ; cappella premium 68

Hi HwT,

Are you sure there is no intermittent issue with your electrical safety control circuit? It may not be a legitimate oil pressure trip.

HwT,
Minimum speed is 900 rpm, check otherwise also expect low oil pressure.
If compressor cycles off for any reason, oil pressure switch should be at end of control circuit.

The minimum speed is set at 900 and max 1480 rpm.
There is no oil pressure switch, only pressure probe connected to PLC. If the oil pressure falls under 1.5 bar there is 30 sec delay before shutdown
I wish i could have some kind of logging of the pressure but not possible at this time...

HwT,
Does it cycle off at any stage?
Like in another recent post, can oil return introduce liquid from oil separator, or chiller?
If so inspect, adjust, or turn off.
If you can run at 900 rpm, see if you can adjust oil pressure up, to check if reserve oil pressure, as could be worn pump.

As far as I have been told this happens when the compressor is running , i will get confirmation of this.
The oil return has solenoid and a float with 10min delay from startup to get any liquid out of the oil separator.

Attachment 16011Attachment 16012Attachment 16013

Hello,

If it's not a mechanical issue in your case, there could be an other way. it happen only with VSD driving.
Prior you have to check if your wiring is comply with electomagnetism rules of wiring.

Then, we had in the past this kind of oil pressure faillure.
If you stay in front of the gauge you can't see any lack of oil pressure but it's fail.
It's a resonance of the pressure transmiter depending of the speed and teeth number in the oil pump.
Problem can be solved by adding a calibrated orifice in the transmiter (here is an AKS3000).
Other non official solution, adding pipe length to the transmiter connection.

tried without succes: Nozzle, capacitor, PLC filtration,....

as long as there is gas in the transmiter connection, no faillure. it begins later....or not.

Attachment 16019Attachment 16020Attachment 16021

second try to insert in line

Thanks for the reply cricri, as far as i understand, installing the orifice in the pressure transmitter solved the issue? How small is the hole drilled?
We use aks 33 Attachment 16030

Yes it solved the issue.
No thread in an AKS33. but It works better than AKS3000 (less disturbances, we tried that also)
From memory we drilled a 0.5mm hole in axial position (thicker).
It's now an accessory from factory called Snubber. It's a smaller hole but thinner wall.

Investigate also for mechanical issue or log the pressure as we did.

In addition,
You can use the M/Fem 3/8 snubber from Danfoss 060G0251.

Attachment 16035

A picture of it.

Screened cable is pretty standard for transmitters & transducers.
Also earthing of power supply loop.

Hello! thanks for the help in the thread!
The problems seems to be solved, we found one cable from the VSD was not shielded, i assume this cased the issue because there has not been any problems since we replaced the cable..