Airport Fuel Inventory

Graueradler

Graueradler

Pattern Altitude
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Apr 11, 2005
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Russellville, AR
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Graueradler
Every month's end, when we stick the tanks, we end up about 200 gallons short on 100LL. Jet A comes out OK, usually a little positive. These are 10,000 gallon horizontal cylindrical tanks. We top off the trucks so we know how much is in them and then stick the tanks to determine how much is in them. We are using fuel finder paste on the side of the stick so we are getting a good distinct line that doesn't creep/wick. We've put security devices on the truck sumps and collected data on the truck for a month sticking it at COB and first thing the next morning without any anomaly. We bungled data collection on the storage tank looking for evaporation. We should get good data there this month (sticking it immediately after filling the truck and immediately before filling the truck the next time through out the month). Is it possible to have this much evaporation? The tank is normally less than half full so it contains a lot of air and has an unrestricted vent. It would breath a lot with day/night temperature changes. We are getting good agreement between the truck totalizer meter and billed volume. Yet everytime, we start with begining of the month inventory, adjust for volume bought and sold to determine an expected inventory and come up with an actual inventory about 200 gal less than expected. The sales meter calibration has been checked and the meter is very accurate.
 
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Graueradler said:
Every month's end, when we stick the tanks, we end up about 200 gallons short on 100LL. Jet A comes out OK, usually a little positive. These are 10,000 gallon horizontal cylindrical tanks. We top off the trucks so we know how much is in them and then stick the tanks to determine how much is in them. We are using fuel finder paste on the side of the stick so we are getting a good distinct line that doesn't creep/wick. We've put security devices on the truck sumps and collected data on the truck for a month sticking it at COB and first thing the next morning without any anomaly. We bungled data collection on the storage tank looking for evaporation. We should get good data there this month (sticking it immediately after filling the truck and immediately before filling the truck the next time through out the month). Is it possible to have this much evaporation? The tank is normally less than half full so it contains a lot of air and has an unrestricted vent. It would breath a lot with day/night temperature changes. We are getting good agreement between the truck totalizer meter and billed volume. Yet everytime, we start with begining of the month inventory, adjust for volume bought and sold to determine an expected inventory and come up with an actual inventory about 200 gal less than expected. The sales meter calibration has been checked and the meter is very accurate.
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I think that's explained in this paper:

http://www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf
 
TMetzinger said:
how many gallons do you sell per month?
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Not much. I don't have the number available at home but it is so little that this 200 gal. loss is knocking our net almost in half. Weare only buyng half tanks (~4000 gallons) because it takes us more than a month to sell it.
 
Is this an above ground storage tank, or underground? If underground are you monitoring for leaks around the tank?
 
It would be more effort, but why not stick the storage daily at COB, too, and compare the change to daily sales records for a month? Graph the results and determine the variance.

When I was helping at 1R7 on the weekends a few years ago we'd stick the storage tank every morning and evening to confirm the local motorcycle/drag racers weren't tapping into it.
 
Graueradler said:
Above Ground
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Well, that would certainly eliminate the possibility of it leaking from the tank, or at least leaking and you not noticing it. That leaves evaporation (I doubt it) or theft or somebodies fuel measuring equipment is not accurate. If you are measuring the gozinta's and the gozoutta's accurately the inventory should be pretty accurate.
 
I'm guessing expansion/contraction of the fuel due to temperature variances by measuring at differents times and temps (didn't check the attachment/links above, so if I'm being redundant, sorry).
 
That's a 2% shortage every month. Could be evaporation, which would affect avgas a lot more than jet fuel. I'm no expert, so I don't know if that's an unreasonable amount for evaporation or not, but given the "unrestricted vent," it's the first thing to come to my mind. It would be interesting to compare the summertime shortage with winter months -- warmer temps would produce greater evaporation.
 
Ron Levy said:
That's a 2% shortage every month. Could be evaporation, which would affect avgas a lot more than jet fuel. I'm no expert, so I don't know if that's an unreasonable amount for evaporation or not, but given the "unrestricted vent," it's the first thing to come to my mind. It would be interesting to compare the summertime shortage with winter months -- warmer temps would produce greater evaporation.
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http://www.conocophillips.co.uk/NR/rdonlyres/8BF99A0D-A711-4DE6-9D4A-FE02D716B1D7/0/msds6.pdf

Everything you wanted to know about 100LL, including evaporation rate.
 
gkainz said:
I'm guessing expansion/contraction of the fuel due to temperature variances by measuring at differents times and temps (didn't check the attachment/links above, so if I'm being redundant, sorry).
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The thermal expansion coefficient is quite small. Also very hard to apply as we buy net gallons (delivery corrected to a reference temperature) but dispense uncorrected volumetric gallons. To actually account for it, you would have to record fluid temperature for every sale. Actually, It should tend to decrease the loss in the summer time when the actual temperature is above the reference temperature.
 
Ron Levy said:
That's a 2% shortage every month. Could be evaporation, which would affect avgas a lot more than jet fuel. I'm no expert, so I don't know if that's an unreasonable amount for evaporation or not, but given the "unrestricted vent," it's the first thing to come to my mind. It would be interesting to compare the summertime shortage with winter months -- warmer temps would produce greater evaporation.
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I'll be checking out the Tango Whiskey link but the equations in the reference provided by Lance indicate that the loss is a fuction of absolute temperature so the difference between summer and winter may not be as great as one would think.

I've only been involved in this for about 6 months and we have been refining record keeping and methods so I don't have a good database to trend from. For example, an early discovery was that the stick had been in use so long that an inch and a quarter was worn from the bottom. The finish was also gone so the fuel line wicked up the stick so fast that, even correcting for the length, it was very difficult to be confident in an accurate reading. WE now have a new stick and are using "fuel finder" paste on the side that produces a clear stable line. As the tanks are horizpntal cylinders, that missing length was a significant but not constant error.

We have a quality control visit from the fuel supplier next week so I plan on seeing how much I can learn from him.

The tanks are relatively secure. Chain link fencing with barbed wire at the top. The trucks were the weak point from a security standpoint. All you had to do was stick a bucket under the drain and open a sumping valve. That has been corrected with physical security devices.

I'm also wondering if there could be a construction dimension error with the storage tank so that the depth/volume chart is a little off. I really don't think so, just trying to ID and explore all logical possibilities.
 
Graueradler said:
I'm also wondering if there could be a construction dimension error with the storage tank so that the depth/volume chart is a little off. I really don't think so, just trying to ID and explore all logical possibilities.
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Doesn't seem like that could result in a cumulative loss of 200 gallons per month, assuming the tank was filled more than once during the period over which the loss was recorded. It seems to me that since tanks are sometimes fitted with evaporation control devices, evaporative losses must be significant when a tank is well vented and nothing is done to eliminate the loss.
 
lancefisher said:
Doesn't seem like that could result in a cumulative loss of 200 gallons per month, assuming the tank was filled more than once during the period over which the loss was recorded. It seems to me that since tanks are sometimes fitted with evaporation control devices, evaporative losses must be significant when a tank is well vented and nothing is done to eliminate the loss.
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The last three end of month readings have each been at about 12" increasing level intervals. If the volume table understated the volumes required to make those level changes, you would have a shortage indicated each month. The table would have to be farther off than I think is possible to make that much of an error though. I thinking evaporation is more likely, especially since we are holding the level low thereby increasing the air volume. The tank probably averages at least 7000 gallons of air volume above the liquid.
 
You can verify the volume measurement table if you can get an empty tank and do a base-line fill, measuring (with your stick) every 'x' gallons. Might be worth seeing if you can do that the next time your delivery comes in--fill your truck to empty the tank, and validate your tables.

Secondly, are you 'sticking' the tank from the exact same spot in the opening each time? As with airplane tanks, if the tank isn't exactly level and you don't stick it in the same spot every time, you'll get varying measurements from your calibrated stick.

Finally, I'd start sticking the tank every night at the end of the day, and compare it with last night's ending gallons + deposits - withdrawals, just like you'd balance your checkbook.

With 200 gallons difference for month, that's over 6.5 gallons per day. That doesn't make sense. Seems way too much to me. If that were the case, even with the smaller surface area and smaller vents, I'd imagine most of us would come back to the plane for our next flight with tanks much lower in level than when we left... and I've not seen that behavior (unless I've overfilled and expansion has physically dumped liquid fuel on the ground).
 
Hmmmmn. Any Drag Racers in your town? 200 gallons per month....is just about right.
 
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Damn! You caught me. ;)
 
Mike Schneider said:
I assume you are sticking the 10,000 gallon 100LL tank before & after your supplier makes a delivery and comparing the amount delivered to what the stick says was delivered. Do the measurements compare favorably?
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Right
 
I don't have the capacity table at home but using Mike's chart, to go 1 ft. from 45% to 50% (fat middle of the horizontal tank) would take 1270 gallons. 200 gallons is 16% of that. No way is the capacity table that far off. It boils down to evaporation, theft, or an elusive logic error. I've beat my brains on it so long that I can't believe it is a logic error.
 
Maybe I'm a pessimist, but I suspect internal shrink.

Someone who's familiar with your sticking procedures. 200 Gallons would probably fly my Cherokee for a few weeks with no probs.
 
Are you sure there aren't any leaks? Maybe rust on the bottom of the tank?
 
Mike Schneider said:
Graueradler, what are the external dimensions of your tank? What is your best guess as to what the wall thickness is (1/4")? Does it appear to be a perfect cylinder (flat ends)? Would you take a picture of the capacity table and post it. Someone will figure this out.
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I can't really confirm dimensions until tomorrow afternoon after I've been to the airport but here is data from a spreadsheet I was using at home. It shows 96 inside dia by 324 inches length. It is a flat ended horizontal straight cylinder. The spread sheet is from an equation someone posted on the board for me some months ago. The one I am using at the airport agrees exactly with the printed table supplied with the tank. This must be the same spreadsheet. There are two identical tanks (Jet-A and AvGas) with identical tank specific tables. We don't have a problem with Jet A .

To the question of leaking - there is rust inside the tank, I can feel it with the end of the stick, but it is on a concrete pad. We would be seeing blue dye if it was leaking. We aren't seeing any blue dye.

Diameter Length (inches)
96 324

Measured Height Gallons
1 18.3
2 51.5
3 94.3
4 144.7
5 201.6
6 264.2
7 331.8
8 404.1
9 480.6
10 561.0
11 645.0
12 732.5
13 823.1
14 916.7
15 1013.1
16 1112.2
17 1213.8
18 1317.8
19 1424.0
20 1532.3
21 1642.7
22 1754.9
23 1869.0
24 1984.8
25 2102.2
26 2221.1
27 2341.5
28 2463.3
29 2586.3
30 2710.5
31 2835.9
32 2962.4
33 3089.8
34 3218.1
35 3347.4
36 3477.4
37 3608.1
38 3739.5
39 3871.5
40 4004.0
41 4137.0
42 4270.4
43 4404.1
44 4538.2
45 4672.5
46 4806.9
47 4941.5
48 5076.2
49 5210.8
50 5345.4
51 5479.8
52 5614.1
53 5748.2
54 5881.9
55 6015.4
56 6148.3
57 6280.9
58 6412.8
59 6544.2
60 6675.0
61 6805.0
62 6934.2
63 7062.5
64 7190.0
65 7316.4
66 7441.8
67 7566.0
68 7689.1
69 7810.8
70 7931.2
71 8050.1
72 8167.5
73 8283.3
74 8397.4
75 8509.7
76 8620.0
77 8728.4
78 8834.6
79 8938.5
80 9040.1
81 9139.2
82 9235.6
83 9329.2
84 9419.9
85 9507.3
86 9591.3
87 9671.7
88 9748.2
89 9820.5
90 9888.1
91 9950.7
92 10007.6
93 10058.0
94 10100.8
95 10134.1
96 10152.3
 
Graueradler said:
To the question of leaking - there is rust inside the tank, I can feel it with the end of the stick, but it is on a concrete pad. We would be seeing blue dye if it was leaking. We aren't seeing any blue dye.
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Is it resting on the pad? If the concrete was cracked, would it be able to go through the pad to the ground undetected? Or is it on legs above the pad, in which case you should see the resulting drips and blue stains?
 
cwyckham said:
Is it resting on the pad? If the concrete was cracked, would it be able to go through the pad to the ground undetected? Or is it on legs above the pad, in which case you should see the resulting drips and blue stains?
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It is on supports holding it above the concrete.

I asked the Exxon inspector today about evaporation and he said that no way could we be loosing that amount through evaporation.

After that, I got sucked into a problem with pilot controlled lighting for the runway and VASI and had no more time for fuel.

Good thing the control box failed now. It turns out that the manufacturer is terminating support for this model at the end of the year.
 
You know, Bobby, you may end up having to drain the storage tank and calibrate a new dipstick....:(
 
Steve said:
You know, Bobby, you may end up having to drain the storage tank and calibrate a new dipstick....:(
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I seem to recall our old farm fuel tanks having a separate stand pipe with sightglass fastened to a drain bung, readily showing the contents of the tank at a glance... I would be surprised if you couldn't find something similar for your tank and eliminate the imprecision of the dipstick...

found a bunch of 'em with google liquid level gauge
http://www.seetru.com/
http://www.johnernst.com/level_gauges_index.html
http://flow-control.globalspec.com/Industrial-Directory/liquid_level_gauge
 
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Found a major part of the problem today. Did a line by line review of the month end inventory spread sheet. Found two problems. Avgas unaccounted for went from 213 gallons to 97 gallons (while Jet A went from about 30 gallons excess to about 70 gallons excess). I had accounted for fuel pumped through the truck meter but not into airplane fuel tanks (for the tractor, the tug, etc) and used it in the comparison between pumped fuel and billed fuel but forgot to reduce expected inventory by that amount. Also had missed one line in a summation of "waste fuel" (sumping, filter changes, etc.) The range of the sum function was one line short and that line had data on it.
 
Awe Crap!!! I hate it when it's a software problem, not a hardware problem! :)
 
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