Finding an oil / water interface level

09/03/2010


Dear Control Engineering: I was reading the article on differential pressure sensors, and I was wondering if I could use that density calculating feature to determine the interface point between oil and water in the same tank?

Let’s think about how that feature works and see how to solve your problem. As the article pointed out, if you have two taps on the side of a tank some known distance apart, you can calculate the density of the contents, regardless of the depth, as long as both taps are submerged. This works because density is mass per unit of volume and the pressure device is giving you, for all practical purposes, the weight of the volume of liquid between the two taps. It takes some math to get the final answer, but nothing all that complicated. Of course this assumes you have only one homogenous product in the tank. Two products with different densities that don’t mix will complicate things.

If you install one of these density measuring setups near the bottom of the tank, it will give you the average density of the liquid at that level, but it won’t necessarily help you find the interface layer. It can tell you if the interface layer is above, below, or even in between the taps, but no more detail than that. This suggests two possible solutions:

First, if you don’t mind the idea of drilling lots of holes in the tank and mounting multiple sensors, you will be able to determine the interface, at least to the extent that it falls between two given sensors. It’s sort of like trying to determine the level in a tank by using multiple point level sensors. The number of measuring points will determine your precision.

Second, if the liquid level in the tank doesn’t change much, you could spread out the two taps. As long as both taps are submerged and the interface level is between them, you can determine where it is because the reading is actually an average density for the volume that is between the taps. As long as you know the density of both components, you can calculate the interface point relative to the two taps.

If you need a more precise measurement, the simplest approach would be to use a technology that can find the interface level, such as a magnetostrictive device that uses two floats on the same shaft. The lower float is designed to sink in oil but float on water. The upper float will float on the surface of the oil. This will give you a very precise measurement of both levels.

If you’re determined to use the density measuring feature but can’t really use the two suggestions mentioned earlier, a hybrid approach could work. You will need some sort of top-down level sensor that will give you the volume in the tank. This could be radar, ultrasonic, single-float magnetostrictive, or a few others. The DP density sensor will give you the average density for the total tank contents, or at least the contents above the taps. If you know the total volume in the tank and have the density reading from the bottom to give you a true average for the whole tank, you can calculate the proportion of oil and water, assuming you know the actual density of both.

Ultimately determining the answer depends on analyzing what information you have and filling in the blanks. The combination of factors will give you the answer.

Peter Welander, pwelander@cfemedia.com

Visit the Control Engineering Process Control Channel.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Improving flowmeter calibration; Selecting flowmeters for natural gas; Case study: Streamlining assembly systems using PC-based control; CLPM: Improving process efficiency, throughput
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel systems
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me