Hello all,

I would like to get your views on measuring the lube oil level for monitoring purpose.

Irrespective of the type of oil in question, i.e. synthetic or mineral lube oil, which rate is suitable to measure the lube oil tank? the absolute level from tank top to tank bottom, or the working level (with minimum and maximum indicated) as dictated by the P&ID of the system's drawing.

By using the absolute dimension and total volume for the tank, it is almost double in litre per height of oil if i compare it with relative measurement which takes into consideration of the working height and oil volume.

your experiences and discussion are appreciated.

 

In most cases there is a reason for measuring the level of a product in a vessel and that is to control the level at a certain percentage. If your are only interested in doing that then it is normal practice to use the settings on the P&ID as an indication of where to set the zero and 100% of your level transmitter. In other words if you plan to control the level at say 60% of the total height of the vessel then you can set the zero of the transmitter at say 15%of total vessel height and 100% at say 90% of total vessel height. The transmitter will then show 0% at 15% of actual vessel height and will show 100% at 90% of actual vessel height.

This might sound strange to you since it is true what you are saying in that you will not have a accurate indication of the litters of the product still left in the tank if you do that but we are not interested in that, we are only interested in the percentage of liquid in the tank at any given time and do the control of the level accordingly.

Now it can be done differently and more accurately as well if you are interested in the actual volume or the actual litters left in the tank at any given time or at any given height but then you have to work out the total volume of the vessel using accurate inside dimensions normally found on the internal vessel drawing and also take into consideration the density of the product if you want to work out the total weight of the liquid if you want to know that. Level in percentage have a direct relationship with volume and volume can easily be converted to litres, so if you draw up a excel spread sheet and put the level in percentage in and then do a formula to calculate the volume and the litres from that it is possible to do. There are even some level transmitters that have built in tables for you that you can configure that will give you a volume or litres readout directly. You can still use the proposed zero and span markings or the total height it will not make any difference if your calculations are correct but one of the reasons we use the proposed markings at say 10 and 90% and why the design engineers are suggesting these is because of the coned ends of a lot of vessels. Level measurements is not linear in these cone shapes and in order to do a nonlinear measurement or even to do a calculation to get it accurate is a lot of hassles and difficult so it is standard practice to use the datum line on a vessel as the zero and span marking if there is no other indication of what should be used as your zero and span settings. The datum line on a vessel is normally where the done is welded to the cylindrical part of the vessel. We will never use the bottom coned end of the vessel as zero and the top part of the cone as 100% since it is important that our measurements are linear all the way through the measuring range in order to do accurate control but like I said if you are interested to know the actual litres left in the vessel at any given time or height or percentage it is definitely possible to do and can be done very accurately with a little bit of effort.

 

Sam,
thanks for your reply.

You have given an excellent point that i cannot really find in any reference that i have (P&ID, maintenance, handbook) or maybe i have not exhausted my effort. But nonetheless, i now have a better idea to pursue this topic.

If the oil type used (for synthetic or mineral) at each lube system are similar across different brands, i should expect some deviations of the sp gravity and density no? but ultimately if i correlate the working height vs the liters that i should be able to compute, they shouldn't matter. correct me if i'm wrong.

 

Something to take into consideration is that the pressure at the bottom of a column of liquid will always be the same regardless of the volume of the liquid. In other words the pressure at the bottom of a 2 meter long tube 6mm in diameter filled with sea water will be exactly the same as the pressure in the ocean 2 meters deep. For this reason we tend to want to use the formula sg x Height in mm that will give you an answer in mmH2O which can easily be converted into pressure by multiplying your answer with the gravitational acceleration of 9,81. Your answer will then be in pascals and dividing that with 1000 will give you a result in Kpa. If you use the height in meters your answer will be in mH2O and by multiplying with 9,81 will give you Kpa directly.

We are normally interested in these pressure values since we calibrate our differential pressure transmitters according to these calculated values for the min and max values but if you are going to go for volume and litres you are correct in saying that the sg of the different oils will have no influence on your calculations since at say 25% of the vessel the space occupied by the liquid will be a certain volume and that volume will be a certain amount of litres and that will not change. Certainly if you take a pressure measurement of the different types of liquid at the same height these pressure readings will change due to their difference in density but at say25% vessel height the amount pf space occupied will still be the same. That is why I said before level and volume has a direct relationship since if you know when the vessel’s level is, you can do a calculation in excel from that to give you a reading in M3 and litres.

To recap you first want to measure the actual level in the vessel using the formula sg x height to get the level in mH2O. The answer in mH2O can be used directly as your height for the volume calculation. Obviously then it is simple enough to work out the litres for that particular level. In excel it is easy to do since you don’t have to work out tables and all you need is one formula that will work out the litres for you as soon as you input the %level of the vessel.

 

Sam,

great detail you have provide. Thank you very much for the response. This is really helpful.

Have a great day.