In an earlier post, Avoiding Dangerous Conditions through Tank Floating Roof Level Monitoring, we highlighted how technology can assist in providing early warning for abnormal situations such as roof tilt in tank farms.
I received an email question:
…so when the roof moves, the radar moves with it. How do you measure the level? If I have a 10-meter tank, do I need to use a 10-meter GWR [guided wave radar]?
I turned to Emerson’s Mikael Helmer on the Rosemount Tank Gauging team for his thoughts. He responded referencing two approaches—Solution A and Solution B:
…these two solutions are primarily an addition to a standard level gauging solution. It means, the tank would have a normal level gauging system, like our Raptor 5900S with an Array antenna, measuring the level in the tank hand-gauge or stand-pipe.
…if no pipe is available, the Solution A, could be used as a Level gauging system as well, but instead of measuring the Liquid Level of the product, we measure the roof, and then compensate for the roof thickness in order for the Level gauges to preset the liquid level measurement.
In addition to this, Solution A, with its three radars at three different positions around the top rim of the floating roof tank, it can also detect a difference in level reading between the tree gauges. This will indicate a potential inclination of the roof or that the roof is stuck or not moving smoothly up and down with the liquid. The root-causes for this can be several different reasons as discussed in the presentation. Our system can be setup to report such a deviation between the three radar gauges.
Solution A:
- Tracks the roof in three different positions, with reference to the main still-pipe radar gauge
- Standard wired solution, but mounted on special arm
- Measuring on a target bed on top of the roof
- Each device measures the distance to the roof, instead of the stored product
- Comparison between the three measurements provide input on how the roof floats
- Configuration done via standard TankMaster Inventory system
- In addition, it can be used as an Overfill Prevention system
- Configurable alarms indicate when an action is required
Mikael continued:
Solution B uses GWR technology. This solution is also realized by using three guided wave radar (GWR) devices mounted at three different positions on the tank, but with the main difference that they are mounted on the tank roof itself. As you suggest, since it is following the roof movement, it does not measure the overall liquid in the tank.
Instead, it is used only to measure the roof inclination and whether it seems stuck or not. The GWR probe is only about 1.5 m (5-6 ft) long, and definitely not longer than the floating roof support legs. The measurement measures how deep the roof itself floats into the liquid. In a smooth moving roof, this measurement should be more or less steady and stable. But if, for example, a large amount of water has gathered on one side of the roof, that side will float more heavily and the “level” reading of the nearest GWR will go up. On the opposite side, the nearest GWR will show a decreasing “level” reading. Again, this difference between the three radars can be monitored to keep track on how well the floating roof floats.
Solution B—WirelessHART level transmitters deployed at three nozzles on the floating roof:
- Level instrument is mounted directly on the roof, on for example inspection/sample hatches
- Each device measures how deep the roof floats
- Comparison between the three measurements provide input on how the roof floats
- Configurable alarms indicate when an action is required
