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MP231
04-07-2013, 01:08 AM
As a PLC programmer I have been asked to write logic for a Grasso Compressor that does not have an oil pump. It does have a Pressure Control solenoid valve.
Can anybody briefly explain how this type of oil system works and how to start the compressor and monitor the pressure ?
I am more used to having an external oil pump which we start before the compressor and allow oil pressure to build up
Thanks

HVACRsaurus
04-07-2013, 02:07 AM
Hi MP,

Screw compressor with out oil pump is referred to as "differential fed" oil pressure.

The compressor discharge pressure (or more correctly oil seperator pressure) pushes the oil from the seperator, through the oil filter, through the header & into the oil galleries.

What needs to be measured (and controlled for safety) is oil manifold pressure (otherwise seperator pressure or discharge pressure, but manifold would be preferable) versus suction pressure. The difference between manifold pressure & suction pressure is referred to as "differential oil pressure".

Obviously warnings, alarms & safeties need to be programmed for low differential pressure, usually with a time delay.

Differential oil feed is usually only an option for single stage or high stage duty - not for booster / low stage duty.

Also, best practice is to have a minimum pressure valve fitted on oil seperator outlet (aka discharge line). Upon start up when seperator pressure is low - the minimum pressure valve remains closed until the oil seperator builds up suitable pressure, then the min press valve opens - allowing passage of discharge gas for normal operation. Sometimes referred to as a cold start valve or various other names.

Basic control sequence would be

a) At rest i.e. zero differential pressure, allow compressor start but activate oil pressure timer

Then

b) compressor starts & oil differential pressure increases to above trip value - happy days - compressor is allowed to run unless oil pressure falls below trip value for delay time

Or

c) compressor starts & oil differential pressure does not increase above trip value within delay time - fail compressor on low differential oil pressure.

Magoo
04-07-2013, 02:15 AM
With out external pre-lube or continuous oil lube pump, the system relies on seeing a differential oil pressure driven by discharge pressure driving oil flow. A start instruction will rely on a pressure differential pressure from two transducers to continue. No or low prescribed oil diff pressure equals a fault condition, time delayed by seconds. BUT some grassos have internally driven oil pumps, and inboard oil filters. Just to confuse things. Same rules apply. Hope this helps.

MP231
04-07-2013, 04:12 AM
Thanks for the reply guys - can't get a clearer explanation than that !
I also noticed from a photo of the compressor that it has 2 each load and unload solenoids. I have only ever come across one of each. Any reason for the two you know of ?

HVACRsaurus
04-07-2013, 05:41 AM
Yep, the simple explanation is "Grasso have Two"

Just the same otherwise, one PLC output for load and one for unload. Connect the two corresponding solenoids in parallel.

RANGER1
04-07-2013, 09:02 AM
A oil float on oil separator is also important with protecting compressor. Which model do you have to be completely clear with advice?

PaulZ
04-07-2013, 01:24 PM
Hi MP
Another few things to add into your program is if the compressor crash stops you have to energise the two unload solenoid valves as there is an internal spring which will push the slide to the unload position when the compressor is not running.
The oil separator should have a level switch installed as a safety so you don't run out of oil because with differential feed machines you are relying on pressure only and you could have gas pressure in the manifold and not oil pressure.
When the compressor is started you will have to load the compressor reasonably quickly to assist in getting the discharge pressure up as quickly as possible.
Regards
Paul

MP231
05-07-2013, 03:57 AM
Thanks for the Info - Lots to take into account. RANGER1 unfortunately I don't have the model number at this time but I'll defo check on the Oil Level monitoring.

Iceman717
09-07-2013, 04:17 AM
I don't like the idea of dry-starting a machine until oil pressure builds up.

RANGER1
09-07-2013, 08:50 AM
I don't like the idea of dry-starting a machine until oil pressure builds up.
Why if it has roller bearing cannot see the problem. Trick have a 15 min oil trip timer

Josip
09-07-2013, 09:29 PM
Hi, MP231 :)


As a PLC programmer I have been asked to write logic for a Grasso Compressor that does not have an oil pump. It does have a Pressure Control solenoid valve.
Can anybody briefly explain how this type of oil system works and how to start the compressor and monitor the pressure ?
I am more used to having an external oil pump which we start before the compressor and allow oil pressure to build up
Thanks

welcome to RE forums ...

I'm just curious ... is this some petrochemical plant ....

Other guys gave you a very good advices, but still you have a lot to do to wrote a good logic ... hope all transmitters are installed within compressor ...

... with external oil pump and oil pressure by-pass valve things will be easier for you ...

Best regards, Josip :)

MP231
10-07-2013, 05:08 AM
Hi Josip

No its not petro chem just a simple chiller. Yes a lot of good advice and at the end of the day the mechanical guys will ensure it is running and stopping as it should. Nice to get as much of it as possible right beforehand though. Thanks.

Iceman717
12-07-2013, 02:38 AM
Why if it has roller bearing cannot see the problem. Trick have a 15 min oil trip timer

I don't know. I guess I'm just too used to machines with external pumps and needing to see a certain oil pressure before it will start. Spinning things without oil pressure gives me the willies.

Tom

RANGER1
12-07-2013, 07:04 AM
I don't know. I guess I'm just too used to machines with external pumps and needing to see a certain oil pressure before it will start. Spinning things without oil pressure gives me the willies.

Tom
Frick who I imagine dominate the USA market have been doing it for a considerable time, seem to handle differential oil pressure feed. Rollers with clean oil are pretty forgiving. A Frick engineer mentioned a new Frick was somehow piped up incorrectly & one roller ran for several months without fresh lube before failing (believe it or not). Even recipes run for up-to 90 secs without oil pressure before they cut out on oil trip. Like with a lot of things, a little lube goes a long way!

joe-ice
12-07-2013, 05:44 PM
We used to use small bitzer screws like this,they used to have a float switch on the seperator/ resevoir,an oil flow switch on the oil line connected into the int thermister box,there was also a heater on the resevoir controlled by a stat to keep the oil temp up in downtime.There was also a solenoid on the oil line to stop the oil migrating and locking the rotors when the machine was off.