from the instrumentation department...
lube oil tank level
 Posted by rajiv on 1 September, 2012 - 5:47 am
We have lube oil tank of total height 1090mm. The levels to be measured given as High=370mm, L=520, LL=550mm from tank top. Level span given is 75mm to 600mm from tank top. DP transmitter is available there as existing.

600mm is the difference between two tappings. Low tap is connected to transmitters high side and high tap (downside) to the transmitters low. Transmitter is placed 210mm below from lower tapping point.

We have changed the transmitter only. can any expert tell what should be the DP range for correspondence level?

thanks

 Posted by David on 2 September, 2012 - 8:11 am
Presumably, the LRV (4-20mA = 4mA) is at the elevation of the lower tap.

The transmitter is mounted 210mm lower than the lower tap.

To calculate the LRV, the Specific Gravity (SG) of the lube oil is needed.

LRV = 210mm (water column) * SG

The elevation of the upper tap is the likely URV.

URV = 810mm (wc) * SG

If the level span is to be 75mm less than the 600mm distance between the taps then

URV = 735mm (wc) * SG

To convert to actual physical level elevation values, the receiver device has to correct the measurement values by multiplying by 1/SG. Or just work in percent.

It isn't clear exactly where the lower tap is in relation to the tank top reference point used for the alarms.

 Posted by Sam on 3 September, 2012 - 7:30 am
Hi,

this seems to be a very simple dryleg level setup but a little more information is needed if you want us to give you a accurate explanation of how to do it. We need to know the exact positions of your tap off points on the tanks measured from the top for the vessel down or from the bottom of the vessel up.

You also do not mention anything about the temperature of the oil so we have to assume that it is the same as the ambient temperature. If it is not let us know if the oil is hotter or colder than the ambient temperature since it is possible that you might have to do a wetleg and not a dryleg on the application.

It will also be helpful to know the make and model number of the DPT that you want to use since doing a level application with a SMART transmitter could be different than working with a old electronic 4 to 20mA, or pneumatic DPT depending on the temperature of the product.

 Posted by Roy Matson on 4 September, 2012 - 7:35 pm
It's a little unclear what your measurements are but.
Transmitter is 210 mm below low tap point
So the zero is 210 mmWC x specific gravity of oil
Span is (210 + 600) mmWC x Specific gravity of oil

LL DP is (210 + 50) mmWC x SG
L DP is (210 + 80) mmWC x SG
H is (210 + 230) mmWC x SG

We are assuming that the LP side of the transmitter remains empty, if the tank is vented to atmosphere you don't even need to connect the low side.

 Posted by CSA on 4 September, 2012 - 10:37 pm
I'm unclear about the measurements that were provided, too, but thank you, Roy Matson, for putting the measurements in tabular form. That should help the original poster. I always find it best to draw the configuration and put the dimensions on it for the best "picture" of the application.

One thing I'm not unclear about, though: the L.O. Tank should have a slight negative "pressure" (vacuum) on it all times when the L.O. Mist Eliminator is running so the transmitter must be connected to the top of the tank.

 Posted by Roy Matson on 5 September, 2012 - 7:35 pm
I agree, if it's under + or - pressure it must be connected, I always like to run my tubing up above the top point about a foot then back down so if ever it gets over-filled there's no chance of filling the dry leg.

 Posted by Sam on 7 September, 2012 - 2:04 am
CSA/Roy, thank you for the input, and yes you are right, but I just think from our side we should not complain when we get questions that does not make any sense if we do not ourselves give clear understandable answers.

CSA - "connect transmitter to the top of the tank" Even I was a bit confused with this thinking that youn want to install the transmitter on top of the tank.

Rather say "connect the LP side of the transmitter to the top of the tank"

Roy -" run my tubing" - Rather say "run my tubing from the LP side of the transmitter"

Easy enough for a experienced technician to figure out what you are saying, but keep in mind there are a lot of inexperienced people out there that are trying to learn and feedback like this could confuse them more rather than helping them.

 Posted by CSA on 7 September, 2012 - 11:58 am
Sam,
It was not very clear from the original post which was the high tap and the low tap and which transmitter ports were connected to which taps.

Yes; I could have been a little clearer, and probably should have been. The high tap should be above the maximum expected oil level, and as Roy suggested it would be good to have a loop in the high tap tubing extending above the tap to prevent accidentally getting fluid in the sensing line. The purpose of this tap is to ensure the transmitter is sensing the pressure/vacuum on the top of the lube oil level inside the tank, otherwise the level reading will not be accurate.

In the cases where I have had to to implement a lube oil tank level transmitter the transmitter was physically at the level of the low tap, so there was no offset required. It's not clear to me from the original post where the transmitter is located with respect to the low tap.

Sorry for any confusion I may have caused because of confusion I was caused by the original post.

 Posted by William \(Bill\) L Mostia Jr PE on 8 September, 2012 - 1:21 pm
While this may be just a matter of the description perspective semantics, for a d/p cell type of level measurement, the high side tap of the d/p cell is connected to the bottom of the tank and the low side tap is connected to the top of the tank.

 Posted by RK on 8 September, 2012 - 10:50 pm
You can calibrate the transmitter to 136 mmWC (assuming 0.8 as sp gravity of the oil). As the tank will be under slight vacuum as CSA has told, and assuming oil vapours accumulation on LP side tube will not happen, you can zero suppress the transmitter by correcting the zero pot (this will not change the span considerably). Else you can fill oil in the LP tubing through a tee and ball valve on the LP tubing top, till the oil falls back into the tank. This will eliminate the zero suppression (or elevation??)

 Posted by Sam on 10 September, 2012 - 5:11 am
Ok this is getting very confusing. I will try and bring everything together.

If we assume the lube oil sg is 0,85 and,if we assume that the bottom tapoff point on the tank is exactly in line with the bottom of the tank and, if we assume that the top tapoff point is exactly in line with the top of the vessel and, if we assume that the application is at ambient temperature and if we assume that the transmitter that will be used is a SMART transmitter we can answer the question other wise we are only speculating so I will do a quick generic writeup for a dry leg application for the above assumptions.

In a dry leg application the LP leg of the transmitter is ALWAYS connected to the bottom of the vessel, regardless of what type of DPT you are working with.

Should you work with a old electronic 4 to 20mA transmitter and you want to do a wetleg on the application you will connect the HP leg to the top and the LP leg to the bottom. Other than that the HP leg is always at the bottom even when you want to do a wetleg with a SMART DPT.

Looping the LP leg high above the tank before bringing it in and connecting it to the top of the tank could be benificial in low pressure applications where there is a risk of over filling at times.

In high pressure applications the looping of the dryleg is pointless since the pressure inside the vessel is so high that it will push the liquid inside the vessel up the dryleg within seconds should a upset ocur.

Finally working on a closed pressurized vessel is no different than working on a vessel with a vacuum in. The calibration procedure stays the same. In short, disregard the pressure or vacuum inside the vessel and just do a normal calibration as if the application is open to atmosphere. Applications where there is pressure or vacuum confuses a lot of technicians and I have seen where some tried to use the internal pressure as zero and then add the calibration on top of that. No, forget about the pressure or vacuum and work with the application as if it is at normal atmospheric pressure. The reason for this is that the pressure inside the vessel at the top and bottom tapoff points are exactly the same and eliminate each other giving you a zero defferential when the tank is empty even if there is 100Bar inside the vessel.So if you now fill the tank the transmitter will only read the head of the liquid.

So in the above application you will use the distance from the transmitter (installed below the tank) to the bottom tapoff point (if this is to be zero%)times the sg as your LRV and the distance from the tansmitter to the top tapoff point times the sg as your URV (if this is to be the 100% mark).

If the tapoff point is not your zero and 100% marks use the distances from the transmitter to the zero and 100% marks on the vessel and times these distances with the sg of the product.

It is readly as simple as this for all level applications. Even wet legs are easy to do these days since we have SMART tranmitters and capililiry level applications is just as simple to do. Again forget about the pressures and vacuums inside your vessels and you will find that levels all of a sudden seems much simpler to do since that is where the confusion lays for most and not with how to do the level application.

Good luck

 Posted by Roy Matson on 10 September, 2012 - 11:30 am
Sam,
You are correct in saying we should be more precise, but since it's up to the poster to question anything he's not sure about.

Just to nit pick.

You stated the SG at 0.85 and ambient this is unlikely. The oil in the line from bottom tap to transmitter will be at ambient but usually in a lube tank the oil is quite hot.

I always make a loop in the LP sensing line and it seems to work well. even if the vessel is over-pressured the fluid is not able to enter because the pressure in the sensing line is the same as the tank right up until the level reaches the top tap, it may get forced up but as soon as the level / pressure drops it will run back out.

I think your summing up the situation was very good and may cause a few to consider all the possibilities before just rushing in.

Cheers
Roy

 Posted by Sam on 11 September, 2012 - 6:44 am
On a general note about this looping of the LP legs, something I have encountered before is that sometimes a technician will do this since he feels unsure how to setup a wetleg for the application and by looping up the LP leg he is hoping that the condensate that forms inside the LP leg will just run back into the tank.

His way of thinking is also that since the transmitter is so far away from the tapoff point, no condensation will form on the LP leg going down to the transmitter and condensation will only form in the part of the LP leg closest to the hot liquid, in other words the part of the loop directly going up from the tank and the LP leg will therefore stay dry.

At first glance I disregarded this as nonsense and that condensate will form every where inside the LP leg but thinking about it later, the air or gas in the loop going down will be much cooler than in the tubing directly connected to the tank. Could it be that this can work as an alternative to doing a wetleg? I personally don't think so since this specific application did work for a day or so and then started giving problems again since, the way I saw it, some condensation did form in the LP leg, it just condensated slower due to the cooler temperature in that side of the leg than on the side connected to the tank directly but will eventually form.

If anyone else have an opinion or similar experience on this LP leg looping, let us hear about it.

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