Accurate Oil Storage Tank Custody Transfer

by | Feb 12, 2019 | Industry, Level, Measurement Instrumentation, Oil & Gas

Jim Cahill

Chief Blogger, Editor

For oil & gas well pads not connected to pipelines, custody transfer of the produced oil and produced water occurs via truck transport. The trucks load oil from holding tanks and require accurate measurements for the custody transfer from producer. Similarly, this is true for the produced water in their holding tanks sent off for disposal.

In a Flow Control article, API 18.2: Ensure Safety While Providing Better Level and Volume Measurement Options, Emerson’s Tom Wienke shares effective measurement technologies for this application.

The forerunner to API 18.2 is API 18.1: Measurement Procedures for Crude Oil Gathered from Small Tanks by Truck. This standard only permitted manual measurements of this custody transfer process. Tom explained the manual steps:

Step 1: The truck driver climbs to the top of the tank and opens a thief hatch. A tape measure is lowered into the tank to determine the oil level. This first measurement is called the opening gauge and establishes the baseline measurement.

Step 2: The next step is to lower in a temperature sensor to read the oil temperature, which is used to support a density calculation. A temperature reading should be taken at three depths due to stratification, with the three readings averaged.

Step 3: A collector is dropped in to capture a sample. Specific gravity can be measured using a hydrometer. Some of the sample goes into a centrifuge for the “grind out” to determine how much water and solid material is mixed in. The measurements from this sample are applied to the entire lot.

Step 4: At this point, if the truck driver is satisfied with the quality, the oil can be pumped into the truck. When the transfer is over, the driver takes a second level measurement (closing gauge) with the tape to calculate the volume transferred.

He highlighted the hazards of this manual approach:

All these steps are accomplished from the top of the tank, which presents a variety of safety concerns. Aside from the potential for the driver to fall off the tank, opening the hatch can also release a plume of hydrocarbon vapor, which could potentially include hydrogen sulfide.

The API 18.2 standard enabled the measurements to be taken with instrumentation. Since oil tanks may contain some water and water tanks contain oil, level measurement technology needs to be able to find this interface or area of emulsion. Tom highlighted two technologies that are suitable for this challenge:

…magnetostrictive and guided-wave radar (GWR). Both can provide the necessary precision to satisfy 18.2 and are unaffected by cold, rain or any other distractions to a human operator.

guided wave radarMagnetostrictive level measurement:

…use floats that move on a rod extending down to the bottom of the tank. The bottom float is weighted so it sinks in oil but floats in water, whereas the upper float always remains at the top of the oil.

Inaccuracies can occur with tar build up on the floats or with thick emulsion layers of oil and water.

Unlike magnetostrictive technology, guided wave radar technology better addresses the oil-water interface:

…where the separation is indistinct, the lower magnetostrictive float will still come to rest where the density change is sufficient to cause buoyancy, so it will indicate an interface whether one exists or not. When a GWR transmitter is used, if no distinct interface is present, it will not indicate one.

Read the article as Tom explains other important elements in accurate custody transfer and how these online measurements provide safer working conditions for the well pad operators and oil & water transporters.

Learn more in the Guided Wave Radar section on Emerson.com. You can also connect and interact with other level measurement and oil & gas industry experts in the Measurement Instrumentation and Oil & Gas groups in the Emerson Exchange 365 community.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.