November 08, 2007

Oil Tanks - Tonnage Measurement - New Cost Effective Method

Recently Zaki had posted a write up on his blog related to this issue, whereas I commented about inaccuracy of this method & my experience. I had been doing it in my earlier days but I was never satisfied with the results due to large inaccuracies. So I thought to share it with you which will help you in taking care of these aspects.

One may not think or consider an inaccuracy of 0.5-2.5 % seriously but the view can be made clear from a simple example in case of tanker of etiher diesel or fuel oil. The tankers usually do not have more than 2 meter in height. If you dip tape & measure where an error of <1">) than you have an error of 0.5% (10mm / 2000mm which is tank height)to 1.2%.

The Method
In the conventional method a metallic dip tape is dipped from the top of the tanker or storage system, which goes in due to its weight and thus we measure the level.

The Problem
The problem arises due to three factors i.e. Density of Oil, Viscosity of Oil & Surface Tension (to some extent). As said, tape goes inside due to its weight so a buoyancy force also act on it which increases its floating nature.

Secondly due to viscosity & floating tendency (as described above) the tape goes inclined or slacks inside the tank. This goes unnoticed due to invisibility.

The Impact
The impact of this can be realized as below (Tanker example is already given above), Now we will consider an storage tank of ~5000 KL which is an usual size for FO storage & an usual height shall be ~10-15 meters. So let us consider 12 meter as general average height. If you go & measure the level in the tank which shall be somewhere ~70% of the total height than total H becomes ~8500 mm. Now in this case slack may go upto 2" (inches) which comes ~0.7%. practically it goes to as high as 1.5% however if I consider only 0.5% error than can you imagine the impact?

The impact come from your usage e.g. in a fertilizer plant the average daily consumption goes ~200-250 KL / day. Now 0.5% of it become ~1 KL / day. This will soon result in your inevntory error with in a month (30 KL difference) which is practically two tanker loads. So you might land up in great errors.

The Solution
There are two ways to solve this problem in practical life.
  1. Use some standard level gauge measurement system.

  2. There are many devices claimed to be accurate but the question is how much they are accuarte & what is the cost of accuracy you are paying for?

    Automatic gauging is the term coined for this solution but let us look at practical example & its cost.

    Equipment Cost / Tank - $3100 Average
    Installation - $500
    Monitor - $2200
    Cables etc. - $1600

    Minimum Total Cost - $7400 / Tank

    Accuracy Claimed - 0.1%
    Actually achieved - 0.3 - 0.5%
    RESULT - Practically no Gain, system remains same only adjustment frequency is changed.


  3. Use a simple manual method as described below.

  4. Based on my experience this is the best method so far I have found & is practically very low cost, in fact only ~1500$/year. Just install a simple tube of any transparent material if it is flexible than good, otherwise also, no problem e.g. Plastic tube or glass tube.

    Connect it to the bottom of the tank at any drain point or spare point. It doesn't matter where this bottom point is as long as your expected liquid level is above this point. Now there are two possibilities - 1. if you are using flexible tube & 2. If you are using straight (non flexible) tube.

    In case of flexible tube after connecting it to the bottom point (Drain should be closed) take it up to the expected level height manually. Open the bottom drain slowly & hold the tube from the maximum level; oil has risen in it. Touch it to the tank wall & straight the flexible tube portion in your reach on the tank wall. See if any drop or rise is there in the tube. Hold it straight on the wall of the tank & mark the level in the tube on tank wall using a pencil. This is your most accurate level point in the tank.

    In case of straight / nonflexible tube, it can be connected at the bottom of the same drain & top of the tube can be connected to the vapor space with a isolation valve so that vapor lock can be released whenever level is measured. It will look like a level gauge tube.

    However the disadvantage of this method is that it is not recommendable for volatile liquids e.g. Naphtha etc. rather they are not installed on big size tanks to avoid any mishap during leakages. So don't forget to install extra isolation valves on extended metal pipes required for connecting this tube. This will help in complete isolationof tube from tanks.

The Gain
In this case the error will not be more than 2-3 mm (Or say Max 5 mm) if done properly which will result in 0.05% max error. This will cause an error of 0.1 KL/Day for the same example of fertilizer plant. So the same error will occur after a year instead of a month.

Also it is very inexpensive compared to ~$7000 investment /tank but you need a person to do it which will cost ~$1500 / Year (Labour cost).

If you have nay question kindly let me know on the forum or on the comments section of this post.

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