Rundown tank restriction orifice in lube oil system of rotating equipment 2

[Engineer9182]

Dear all,
In run down tank of lube oil system of compressors there are two restriction orifices as you can see in drawing attached. Problem ( in our particular situation ) is in start up condition when filling time of run down tank when HV1303 is fully open, time duration of filling tank is 40 minutes as this time does not satisfy operators and they need to decrease it so they can start compressor immediately after power loss when tank gets empty after coast down time. my question is if we can increase bore diameter of FO1303 to decrease time by filling tank quickly as possible? what are constraints regarding this matter? Is there any calculations that are required to calculate this timing based on our needs? what is purpose of FO 1303 while there is another one FO1302, I mean does not it enough to have only FO1302 and why we need to have FO 1303 after gate valve? Any answer would be appreciated. Thanks to all.

 

[JJPellin]

The systems we have can be filled using the bypass valve in 5 minutes or less. Our systems do not have an orifice in that bypass line. The only reason I can see for that orifice would be to avoid overflowing the rundown tank through the atmospheric vent. You should be able to enlarge that orifice. Size the orifice to fill the rundown tank in five minutes.

Johnny Pellin

 

[JJPellin]

If the lube oil pumps were centrifugal, that orifice could protect against running the oil pumps to end of curve.

Johnny Pellin

 

[JJPellin]

The orifice could be there to avoid dropping oil pressure low enough to trigger alarm, auto-start of auxiliary oil pump or low-low oil pressure trip.

Johnny Pellin

 

[georgeverghese]

Why is there no high level trip valve on the fill line?

 

[Engineer9182]

Thanks for your comments Mr.Pellin. But I did not get what you mean by overflowing in rundown tank? Normally rundown tank should be filled in a way that FG1304 show flow inside it so we can understand that oil is flowing through rundown tank because if oil does not flow through FG it means maybe there is a chance that rundown tank be clogged by sediment in oil and does function properly in case of emergency. Second you said size it so it fills tank in 5 minutes. If you mean by test and trial or by computations or any software that could size this RO based on time. lastly What process data we need to size this RO for best flow or dp. what is important here is DP as you mentioned right? not flow.

 

[JJPellin]

The rundown tank overflows through the sight glass in normal operation. But, if the oil flow was more than that line could pass, you could overfill to the atmospheric vent and leak external. Note the capacity of your oil pump. Perform an orifice calculation to size the orifice to stay within the capacity of one oil pump, if that is what you want. Size it to fill the rundown take in a particular amount of time, if that is what you want. The pressure differential is somewhat fixed. Size the orifice for the flow rate you want.

Johnny Pellin

 

[JJPellin]

This is only referring to filling the rundown tank. The compressor is not running. I would want to fill the tank quickly, even if that requires running both oil pumps. I have four compressors with this type of rundown tank. None of them have an orifice in that fill line. I prefer no orifice at all.

Johnny Pellin

 

[Matchess]

Hi

Be careful, it is easy to overfill a run down tank as is is really difficult to size the rundown tank overflow line.
In this line you have gravity flow (the line is probably not 100% filled with oil, so you have a mix between air and gas.
This flow will be probably not stable if the oil return line is not properly sized.

What is the routing of the overflow line ? what is the line diameter compared to the supply line ?
What is the rundown tank design pressure ? are there a breather installed on it ?