accumulator with an isolation bladder api 614 centrifugal compressor

[Nicolas13]

For an old process centrifugal compressor equiped with an isolation bladder inside the accumulator (API614 Figure 2A-14; 50 liter), the oil pressure does not correspond to the height of the liquid (4.5m). About 150mb is missing. The bladder is causing this anomaly; the existing bladder material is nitrile and thickness 3mm, shore hardness is 80. The old bladder material was neoprene, thickness is unknown.
Question: what should be the bladder characteristics: thickness and shore hardness?

Thanks Nicolas

 

[georgeverghese]

Okay, so that corresponds to a density difference of 340kg/m3, which is too much a variation in density from cold to hot conditions for lube oil. So something is wrong. When was the last time the pressure instrument was calibrated?
Otherwise, could your lube oil be contaminated with some light condensate which it may have picked up from contact with process gas - what sort of shaft seals do you have - is the lube oil separated from the process gas by some inert gas barrier at the shaft seal?

 

[Nicolas13]

Thanks for your reply:
The pressure instruments are new, two are installed with the same measures, one for each seal. The seal are liquid film type with cylindrical bushing. No inert gas is used.Contaminated oil is evacuated from the oil system thru oil drainers.
If you refer to API614 figure 2A-14, the gas reference pressure is 22 bar and the oil suppling the seal should be at 22,4 bar. The difference of 0,4 bar is the hydrostatic height, control by the oil level in the upper vessel, this is operating well on site. But I don't find the 0,4 bar between the oil and gas. All oil and gas pipings are connected very close, and no differential pressure can come from the piping arrangement. That's why I suspect the bladder is the problem, it seems two thick for me.

 

[georgeverghese]

If there is no separation gas, then most likely this oil is contaminated with much lower density dissolved condensate picked up from the process gas. Since this condensate is dissolved in the oil, it wont separate at the drainers.
In modern installations, lube oil does not come into contact with process gas at the shaft seals since an inert gas barrier separates these 2 - this is modern safety practice. Lube oil tank vents have caught fire in the past where the older configurations were used.