If you’ve had to live with antique or just plain old furniture, you know that drawers made without mechanical tracks can be a pain. Keep them straight and they work. But if you push a drawer in crooked, it can get stuck. Now, imagine the same concept on a scale 1,000 times larger, and you’ll understand what it’s like maintain a storage tank with a floating roof.
Per Skogberg examines the situation more closely in his article in the March 2018 issue of Hydrocarbon Processing, Keep a Floating Roof on an Even Keel. While there are lots of advantages of floating roofs, like old drawers, keeping them straight and moving freely can be a challenge. As he points out, more than half of the large storage tanks scattered around the world have floating roofs, so it’s a widespread concern.
Designs vary, but most floating roofs use pontoons distributed around the interior surface to provide uniform support across the entire area. As the tank is filled and emptied, the roof simply follows the liquid movement, taking advantage of the natural leveling of the contents. Seals around the edge contain hydrocarbon vapors and prevent infiltration of rainwater, while still permitting free movement up and down. Such is the case when the storage system is working properly; unfortunately, there can be problems.
When floating roofs work as designed, they’re great, but correct operation depends on multiple procedures executed correctly, particularly draining rain and snow that accumulates on top of the roof. If too much remains where it shouldn’t, the weight can get things out of whack.
If the drainage system becomes clogged and if water or snow accumulate, the additional weight can cause an imbalance with one area becoming too heavy, resulting in the roof sloping. The worst outcome is a jammed roof that is no longer able to move, and that becomes bent or distorted as product is pumped into or out of the tank. This type of failure can also lead to loss of seal, release of vapors and water entering the tank. Correcting such a situation is a major undertaking and can remove the tank from service for a considerable period of time.
As a result, when pumping liquid in or out of a tank, it’s important to make sure the roof is moving freely and staying flat. The traditional method is sending somebody to stand on the roof while it’s moving, or positioning someone where it is possible to see enough sets of level marks around the circumference to verify everything is moving as it should. Obviously, there are personnel safety and labor costs complicating these activities, but automation provides a better solution.
Given the potential complications of roof issues, it is not surprising to see many operators working to reduce the need for visual inspections, and adopting an automated solution using a system of intelligent level instruments to monitor the status of their floating roof tanks. These instruments provide the advantage of continuous surveillance and real-time verification, along with operational and safety improvements. If any deviation from normal operation occurs—such as increased or decreased buoyancy, tilting or the liquid level changing but the roof not moving—then automatic systems will issue an instant, actionable alert.
Radar level instruments are ideal for this kind of application, and there are two basic strategies commonly applied, each using sets of three radar transmitters to monitor movement of the roof. Per explains them in greater detail in the article, but one approach uses non-contact radar, such as the Rosemount 5400 series; and the other guided-wave radar, such as the Rosemount 5300 product family.
These two approaches accomplish the same thing but do it differently, so one might be better for a specific application than the other. The article helps sort through the differences to reach the ultimate goal of better continuous monitoring of roof position, within a larger picture of tank inventory measurement and management.
You can find out more about the Rosemount radar level product lines, and meet other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level or Refinery Group plus other specialty areas for suggestions and answers.