Floating roof monitoring using radar technology — Part 2
Automatic floating roof monitoring can provide certainty that floating roof tanks are working as intended.
The world’s first floating roof storage tank was built in 1923 and it is estimated that today, more than half of all storage tanks are of this type. Safety, economy and effectiveness were the driving forces behind the innovation back in the day, and the key reasons for using floating roofs have remained the same ever since. However, as we saw in Part 1 of this article, there are a number of risks associated with floating roofs that need to be mitigated.
One method of risk mitigation is regular physical inspection, but there are various reasons why this may not be performed regularly. Alternatively, an automatic floating roof monitoring (AFRM) system means operators can be certain that their floating roof tanks are working as intended.
As described previously, there are two main ways to automatically measure roof tilt: with non-contacting radar and with guided wave radar.
AFRM with non-contacting radar
In this case the gauges are attached to the tank shell at the top of the tank or, if it is an internal floating roof tank, to the external tank roof. Typically, three non-contacting radars are mounted 120° apart around the edge of the tank. In the case of an external floating roof tank, illustrated in Figure 1, the gauges are usually mounted on a swivel arm to create clearance from the tank wall and at the same time allow personnel to reach the gauge for service purposes. A reflector bed is placed on the tank roof below each radar gauge. This ensures accurate measurement without being affected by any protruding objects on the roof surface.
Principle of tilt detection
The non-contacting radar gauges measure the distance down to the floating roof. The tilt of the roof is then tracked by comparing the distances d1, d2, and d3 as shown in Figure 1.
Distance measurement with non-contacting radar gauges is a technology that has proven itself to be very reliable. It is suitable for both external and internal floating roof tanks, and for any tank size.
Alarm limits and system accuracy
There are many factors that influence the monitoring system’s measurement accuracy. They need to be taken into account when configuring the alarm limits for each tank. These factors include, for example:
- Tank wall and roof movements — large storage tanks always have some movement of roof and walls due to sun and shade causing heat expansion or contraction.
- Weather conditions — strong wind and rain can cause the swivel arm, the floating roof and the tank wall to move or lean.
- Tank fill level — the tank walls will bulge as the tank is filled, which causes movement of gauges mounted to the wall.
- Inherent instability of a floating roof — a floating roof is never 100% level. Some small degree of tilt will always be present even in perfect conditions.
This means that a general figure for what constitutes excessive tilt cannot be provided — this will simply vary too much between different sites and even different tanks. The best way to determine suitable alarm limits is to keep track of roof movements after installation and then set the limit based on the typical tilt that occurs for each individual tank during normal operation.
With a non-contacting radar installation, data transmission from the tank to the control room can be done with both wired and wireless communication depending on what is most suitable.
The non-contacting radar solution can also track the buoyancy of the roof — ie, if the roof is floating higher or lower than normal. This requires that a separate automatic tank gauge is installed for ordinary level gauging, measuring the product level through a still-pipe (Figure 2). By comparing the product level from the tank gauge (d4) to the roof distance from the tilt gauges (d1,2,3) it can be determined if the roof is floating higher or lower than normal.
The non-contacting system could also be configured to work as an overfill prevention sensor. In tanks where it is not possible to install an overfill sensor that measures the liquid level directly, the tilt gauges could be set up to trigger an overfill alarm if the floating roof should rise above the maximum allowed height.
AFRM with guided wave radar
In the case of guided wave radar, the radar gauges are installed directly on the top side of the floating roof. Three or more wireless and battery-powered guided wave radar transmitters are installed in nozzles spaced evenly around the roof perimeter. The guided wave radar transmitters have rigid probes that penetrate through the roof and into the liquid below (Figure 3).
Using wireless devices enables installation without the need for flexible wiring that can cope with the movement of the roof. A wireless repeater mounted at the top of the tank ensures that when the roof is at a low position the radar transmitters can still send uninterrupted data back to the control room despite the devices being below the upper edge of the tank shell.
Principle of tilt detection
The guided wave radar measures the ullage — the amount of free space above the liquid surface. If the roof starts to tilt, the radar transmitters will register a deviation of distances d1, d2, and d3 as shown in Figure 4.
Floating roof monitoring with guided wave radar is suitable for external floating roof tanks of any size.
Alarm limits and system accuracy
Many of the factors that influence accuracy of the non-contacting solution apply also in this case. The system has a natural variation that depends on conditions independent of the floating roof itself, such as sun, shade, wind and fill level.
Determining performance of the system and setting an alarm level should thus be done according to the same principle as the non-contacting solution: first monitor the tilt during normal operation and then set alarm limits based on what is found to be ordinary behaviour of each floating roof.
If a wireless network already exists at the site, integration of an automatic floating roof monitoring system with wireless guided wave radars is usually quick and painless. If not, typically only a few repeaters and a gateway need to be installed for reliable transfer of data to the control room.
The guided wave radar solution can also measure roof buoyancy. Because the gauges are installed directly on the floating roof and measure the free space (ullage), a guided wave radar system does not need a reference tank gauge for this task.
Additional solutions for AFRM
Roof tilt is not the only variable that is worth monitoring. Additional sensors can provide added value and more information about roof status. For example, external floating roof tanks often have rainwater gathering on the roof. The water must be drained off, but if the drain system is blocked, water can pool on the roof and lead to corrosion (Figure 5). If there is enough water it can even destabilise the roof. Adding a level switch to the drain sump is a simple way to know if the water level in the sump rises and spills out onto the roof. For this use, wireless instruments provide quick installation, and potentially easy integration with the roof level monitoring system.
The ability to detect hydrocarbons on the roof and in the drain sump is yet another feature that provides valuable insight into the health of the floating roof. If hydrocarbons are present, it could mean that the drain pipe or the deck is leaking. Whichever the case, quick action is necessary to limit emissions, prevent product loss and contamination, and prevent escalation of the problem. A wireless hydrocarbon sensor provides simple installation on top of the floating roof and will trip the alarm quickly and reliably.
Both level switches and hydrocarbon detectors are available in battery-powered versions with wireless communication. This provides great synergy with the wireless guided wave radar solution for monitoring roof tilt, as a wireless network with repeaters and gateways will always be installed.
The operator interface that is used for the floating roof monitoring system should perform all the monitoring and calculation of tilt automatically, and alert the operator if there is a deviation. Ideally the following alarm functionality should be available:
- Roof tilting
- Roof floating higher than normal
- Roof floating lower than normal
- High liquid level in roof drain sump
- Hydrocarbons in roof drain sump/on roof
Non-contacting versus GWR comparison
Which type of installation is preferred varies site by site or even tank by tank. For example, if wired tank gauging infrastructure is already in place, a non-contacting radar installation may be the most suitable. Or if industrial wireless networks are already used in the tank farm, installation will most likely be quicker with the guided wave radar alternative. Either way, the system is flexible and cross-compatible — mixing installation types at the same site is fully supported. When considering which kind of installation is most suitable, the comparison in Table 1 can be used as a guide.
AFRM is yet another way in which technology increases safety and efficiency in tank farms. Instead of relying on manual inspections that are time-consuming, expensive and potentially unsafe, automatic monitoring provides peace of mind through real-time, around-the-clock verification that the roof is operating as normal.
The key benefit of using radar is that it provides proactive monitoring. Where switches, liquid detectors and video surveillance are reactive and will only raise the alarm once product is already on the deck, radar-based systems immediately provide early warnings if the roof deviates from normal behaviour. This allows for taking preventive measures: sending out operators for detailed inspection and then planning repairs as part of an ordinary maintenance schedule, thus avoiding more serious failures of the floating roof.
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