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There are many instruments and systems available that can provide the combination of measurements that will allow an accurate quantity assessment and, hence, good inventory management. But before a particular instrument is chosen and a system installed into your plant, factors affecting application requirements should be considered. Varec can help you to understand the implications of each factor and will assist you with the selection of the most appropriate measurement system for your needs.
Many installation requirements are based around the specific tank type and accuracy required and can be broken down into the following categories:
- Tank types
- Product stored
Tank Types
There is a wide range of storage tanks found throughout the oil & gas industry. Depending on the tank type or mounting options, a particular tank gauge type or measurement solution may be more suitable. Generally, tanks are situated above ground and have fixed cone roofs. The height will vary from a few meters to 30 meters tall. Some of the other tank features you may find include:
- Internal or external floating roofs
- Horizontal and vertical bullets above and below ground
- Spheres
- Refrigerated or heated tanks
The tank shape itself can also cause errors. A vertical cylindrical tank suffers more deformation and errors than a spherical type tank. The considerations mentioned below indicate that tank gauging for bulk storage tanks demands qualified and experienced engineering to achieve and guarantee highly accurate tank volume and mass measurement.
Tank Capacity Table (Strapping table)
The error in the tank capacity table is caused by the errors made with the proving equipment. Over time, the capacity of the tank will also change, so most companies recalibrate the tank every five to ten years.
Tank Corrections
The tank corrections in the Tank Capacity Table are not always 100% accurate. The advantage is that these errors are generally reproducible and appear on every measurement.
Tank Bottom Movements
The tank bottom always moves in a different way when the tank is filled. Therefore, at the bottom of each tank there is a "no go" zone (no measuring allowed), for instance, when the roof is not floating, but resting on its supports.
Tank Shell Thermal Expansion
Due to temperature changes, the tank characteristics and capacity may change. A tank with a stored product of 200°C has a different capacity and will deform more than at ambient temperatures, however, quite good predictions and corrections can be made.
Roof Weight Changes
With floating roof tanks, the actual roof weight also changes. With heavy winds the roof starts to oscillate, whatever the size or weight of the roof. Also, the rubber seals at the side cause friction, so the roof does not move equally with the product. In cold climates there can be a large amount of snow on the roof, which also causes substantial changes in the roof weight. Every time the roof weight or position changes, the product level will change also.
Datum Plate Movements
Due to the tank deformation, the datum plate will move. This datum plate is a physical plate welded in the tank, which represents the official zero level in the tank. Measurement below this level is not permitted. It may not even be possible to get product out of the tanks, due to the mounting position of the inlet and outlet pipes.
Gauge Installation
Right above the datum plate in a tank is the only official measuring point in the tank to measure the correct level. However, in some cases it is not possible to mount the level gauge at this particular point, so the gauge is mounted at another place. A correction is then made for the tilt of the tank. In some areas the ground is quite weak and the tank will move slightly over the years. If a new tank calibration (TCT) is made, the error is eliminated.
Stability of the Gauging Platform
Not all mounting positions are always very stable. For instance, the roof of a fixed roof tank is only a few millimeters thick. Walking on such a roof will cause roof movements resulting in errors in the measurement. In some cases, this will cause permanent changes in the position of the gauge in relation to the product level.
Product Stored
Due to the wide range of products stored, from unprocessed crude to refined white products, a single technology is not suitable for every application. Storing particular products will often bring other factors into the tank gauging choice. Certain products may require refrigeration or heating; the product may be highly viscous or gaseous. As the tank gauge penetrates the tank, it must be not only be able to measure the product effectively, but also be able to withstand these additional environmental conditions.
Operational Requirements
Operational requirements are specific to your business, how you work and can be broken down into the following categories:
- System integration
- Inventory control or custody transfer
- Product movements and operations
- Maintenance
- Accounting and reconciliation
- Environmental compliance
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