Problem with hydraulic unit

[uacar]

Dear Sir,

We have a hydraulic unit. This unit operating for 2 years and controlled diverter damper.Our problem is about hydraulic oil. The hydraulic oil changed three times. Firstly we used to oil quality BP ENERGOL HLP- HM 46. Then changed the hydraulic oil as SHELL TELLUS 46. The oil deteriorated short times and oil colour changed to dark and limped. The oil smelt rather bad like a burned oil. Every conditions is good. We don't have high temperature alarm. Generally oil temperature is at 40ºC. The pressure of hydraulic accumulator controlled as a periodic. The oil analyzed every 2 months in SHELL Laboratuvary. Result of analyze is good, wear elements into limits, no water in oil , but Zn value overtime decreased.

We changed new oil todays, there is still bad smelling in tank.
Working pressure : 180 bar (PSHH)

What is the reason? What we must do?

Please guide me.

Alper

 

[automatic2]

Couple of things could be wrong. Firstly, how does your system dump pressure, are you running a pressure compenstated pump or are you open center, or?. My guess is your pressure compensated pump is not set up properly and it is generating excessive heat.

 

[boo1]

The four major causes of hydraulic oil deterioration are: Particulate Contamination, Moisture Contamination, Acid Build-up, and Oxidation

I would suspect oxidation. Rapid oxidation occurs when hydraulic oil is "cooked" in an overheated situation normally brought about by inadequate cooling or an improperly functioning system. Cooked oil is a dirty brown color and machine parts look "varnished.

The problem is most likely due to thermal stressing of the oil, as opposed to straight oxidation. Improperly functioning system can causing the oil to over heat. Small localized hot spot in a a valve of motor cook the oil, but a high temperature would never detected as a rise in bulk oil temperature. Your first indication of a problem is the change in the oil’s color.

I suspect the smell is residue from an incomplete system flush. Remember to drain and flush the entire system and clean or replace any filter screens. Also you should check with your lube representative or supplier for recommendations and compatibility. Did you check the oils Viscosity and PH?

 

[uacar]

Dear automatic2,

Thanks for your response. This unit controlled diverter with two cylinder at the exhaust of gas turbine.There is two piston pumps (VICKERS PVQ 10). You mentioned dump pressure , its open center. As well have got a high temperature control switches in the unit.

 

[uacar]

Dear boo1,

Thanks for your response.Yes we checked every analyze the oils Viscosity and TAN (Total Acid Number)but not pH. The viscosity decrease to 33 from 46 later 4 months.But TAN values around 0,30(into limits 0,1 - 2,0).
I checked every equipment one by one I did not found any hot spot when this pump is running. Maybe it can be very small point in a valve of motor cook the oil ?? Can I find a reason of problem change the valves(in order)?

 

[automatic2]

Are you possibly in a position to consider a glycol. You'll reduce a combustionable situation yet increase heat capacity. You'll likely need a seal change out to silicone or such.

 

[abdul]

Uacar,

You have neglected an important point. You say that the circulating oil temperature is only 40 degC. However, the oil trapped in the hydraulic cylinder located at the gas turbine exhaust area, is being cooked at very high temperatures, because when the hydraulic piston is not being stroked, the oil in the cylinder is trapped inside, and not being renewed. Result, the high ambient temperatures around the actuator are cooking the oil.
Solution:

1- Either install high temperature synthetic hydraulic oils, which are compatible with your hydraulic seals.

2 Or, install a small bleed line which would allow the trapped oil in the cylider to return to the tank. This allows new, cool oil to enter the cylinder. The redesign should be reviewed by the manufacturer.

 

[uacar]

Dear abdul,

Have you ever to encounter like our problem? You are correct, but we thought all oil to be discharged,when the oil changing.Every open of diverter damper, to stayed into cylinders??
There is a high temperature in gas turbine exhaust approximately 550degC. Furthermore I measured temperature in cylinders and around thats, the temperature is non-high.

 

[abdul]

Uacar,

Standard hydraulic mineral oils are only designed to handle a maximum of 120 degC. Any greater than this, and the oil will severly breakdown and oxidize. That is why you need to purchase synthetic hydraulic oils, because they can accept up to 250 degC, with no deterioration.