Despite clear industry guidelines published over a decade ago and revised in 2015, level measurement experts Hycontrol still regularly encounter asphalt plants with insufficient safety protocols for preventing spills of hot bitumen.
Introduction – Bitumen Storage in the UK
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Bitumen is pressure-delivered hot (up to 230°C) and must be stored at a constant high temperature (ranging from 125-190°C depending on the grade and type of product). As a result, spillages as a result of overfilling these tanks have a high potential for causing serious injury. Whilst the risks associated with the storage of hot bitumen have been known for decades, it is only comparatively recently that clear guidelines for handling this dangerous and difficult substance have been published. The RBA first published concise guidelines for ensuring bitumen storage safety in October 2004, responding to year-on-year increases in injuries and incidents at sites across the UK (peaking at 82 spills in 2003). The document, entitled ‘Guidance for Safe Bitumen Tank Management’, laid out issues resulting in spills:
- Ignorance of tank dimensions and capacity;
- Unsuitable and unreliable measurement instrumentation;
- Poor site communication.
The guide outlined the technology required to overcome these issues, the need for changes in staff attitude and training, and the need for clear communication. In the decade since this key document was issued there has been a sharp decline in the number of spillage events. The recent Eurobitume memorandum has served to reinforce and underline the ongoing importance of the original RBA document.
However, Hycontrol have clear evidence that there are still a significant minority of sites making do with unreliable bitumen level control equipment, poor tank content information and outdated safety systems. These factors present a highly dangerous environment, potentially putting plant personnel and drivers at risk of receiving life-changing or even fatal injuries.
David Wadsworth, Hycontrol’s UK Sales Manager, is clear on the dangers: “Frankly it is remarkable that, eleven years on from the RBA’s definitive statement on bitumen level monitoring and safety, we are still seeing asphalt facilities that are making do with totally outdated technology to monitor a potentially lethal substance. Plumb-bob and ‘cat-and-mouse’ pulley-based systems are still common, despite their propensity to jam or wear out. All too often these devices are also expected to double as a High Level Alarm (HLA), or else another inappropriate technology like a float switch is used to fulfil this function. This technology being used is barely more advanced than the float in a domestic toilet. Again, devices like this are prone to wearing out and becoming blocked up with sticky bitumen – and when they do fail what will then stop the tank from overspilling?”
“What’s also clear from our site visits is that the lack of clearly-displayed tank contents information continues to be a major safety issue,” continues Wadsworth. “Site operators are now far better educated regarding capacity, knowing that there are parts of the tank that are unavailable to fill and the need for a defined safety margin at the top of the tank. But danger arises if a tank is not empty and the level measuring equipment is inaccurate. In this situation operators may have a false idea of how much bitumen they can safely receive. Meanwhile tanker drivers will know the quantity they are carrying but risk being misinformed of the available capacity in the vessel they are blowing into!”
Capacity
It is believed that prior to the RBA’s original guidelines at least 90% of bitumen accidents were attributable to the problem of drivers and staff not understanding ‘true’ available capacity. There will always be space at the bottom of the tank below the draw-off point which will never be drained, and must be discounted from its stated capacity. It is also not possible to fill the tank above the overfill pipe. Exceeding that level would result in a spill, but this unavailable space is still included in the vessel’s nominal capacity.
Consider the following example of a bitumen tank with a stated capacity of 100 tonnes:
| | | |
|---|---|---|
| Nominal Tank Capacity (NTC) | 105.8m3 | 97.3 tonnes |
| Unavailable Tank Capacity (Top and Bottom) (UTC) | 5.3m3 + 1.0m3 = 6.3m3 | 5.8 tonnes |
| Available Tank Capacity (ATC=NTC-UTC) | 99.5m3 | 91.5 tonnes |
| Safe Working Tank Capacity (SWTC=ATC x 0.9) | 89.6m3 | 82.4 tonnes |
Therefore of the 100 tonne capacity, only 82.4 tonnes can safely be stored. It is little wonder then that misunderstandings were commonplace.
Whilst this is now largely understood by site operators and workers, any benefit it may bring is negated if the level measurement and alarm equipment being used is unsuitable or out-dated. The high-temperature, viscous characteristics of bitumen can easily cause poorly-selected equipment to fail or operate erratically. Also, if it is not maintained then even the most advanced technology will prove ineffective. These issues must also be addressed in order to achieve a satisfactory solution.
Accurate Level Measurement
The guidelines state that not only should all bitumen tanks be fitted with a reliable contents gauge that can trigger a HLA at 90% available tank capacity, but they also require a separate High High Level Alarm (HHLA) that is completely independent of the contents gauge and which will trigger at 92.5% available capacity. Only by doing this can site managers and staff begin to be truly confident of their ability to accurately judge tank contents and to avoid overfilling. The illustration to the right shows a typical ‘good’ tank set-up.
The next important step is to choose suitable technologies for the level gauge and the HHLA. Whatever is chosen will need to withstand exposure to an extremely nasty, high-temperature, sticky product that coats everything it comes into contact with or that is exposed to its carbonising vapours. There are many measuring principles marketed as being suitable for use on bitumen but in reality few can continue to operate when coated – most fail or give false readings and alarms.
Having been consulted by the RBA throughout the compiling of their guidelines and with over thirty years of experience as a level measurement innovator, Hycontrol are better placed than most to point out shortcomings in bitumen level safety at sites they visit. Whilst the original guidelines list TDR, radar and hydrostatics as suitable technologies, Hycontrol (who have conducted extensive field tests and include all three technologies in their product range) will only ever offer TDR for bitumen level measurement and the High Level Alarm. Experience has shown that the bitumen tank environment will inevitably coat a radar transmitter, leading to loss of function – a problem which, as we will see, does not affect TDR. Hydrostatic devices will function whilst submersed in bitumen, but to provide accurate level measurement they must be calibrated to the Specific Gravity of the liquid. The temperature of bitumen fluctuates in a tank and the SG of bitumen varies with temperature, meaning Hydrostatics cannot reliably be used to trigger the HLA due to the potential variance of the actual level.
TDR technology, originally used to detect breaks in subsea communication cables, uses pulses of low power microwaves sent along the conducting probes. In a typical bitumen application, the stainless steel probes of the TDR sensor are fitted to flanges at the top of the tanks and extend down into the product. The probes allow a very narrow measuring profile up to a maximum range of 24 metres, particularly useful for measurements in tanks which have internal structural supports.
At the point where the microwaves meet the air-bitumen interface, they are reflected by the bitumen back along the probe. The measured time between emission and reception back at the sensor head is proportional to the distance. This information is then converted to a 4-20 milliamp signal which is fed to the level display.
The advantage of this technology, enabling it to overcome the inherent bitumen coating issue, is that microwaves travel virtually at the speed of light and pass over any coating on the cable with negligible effect. Periodic cleaning is recommended to prevent the coating becoming too thick, but performance is unaffected by changes in pressure, temperature or viscosity, making TDR an effective solution for hot bitumen.
An Independent Alarm
Eurobitume UK recommendations state the High High Level Alarm switch must be independent of the continuous level monitoring gauge. It also needs to be equally resilient to the build-up of product on the sensing element. Float switches and similar technologies will inevitably fail due to product coating and maintenance is an expensive issue – indeed, Eurobitume guidelines no longer recommend them as a suitable technology.
Hycontrol’s extensive site experience has shown that RF Admittance probes best suit the HHLA requirement as build-up on the sensor does not impede its function. A protective insulation electrode between the conventional main electrode and the grounding sleeve allows the electronics to differentiate between coating and the actual product level. Again, whilst periodic cleaning is still recommended, utilisation of the correct equipment will have the added benefit of a vastly reduced maintenance requirement. Admittance probes have no moving parts and so won’t wear out like old-fashioned mechanical devices.
Clear Displays and Clear Understanding
Lastly the Eurobitume guidance states the need for visible, unambiguous display information for both drivers and site staff.
Panels should have very clear indication of the alarms and their status, along with flashing beacons and loud sirens to alert users when an alarm level is reached. This is vital to ensure tank integrity is maintained. Panels can also include displays of contents, ullage and tank temperature. For unmistakable clarity, a large, multi-LED traffic light is can be used to indicate safe ullage for a full load (green), triggering of the HLA (amber) and triggering of the HHLA (red). Using this simple, universally-understood system removes any ambiguity. Clear, easily-readable safety signage is also essential.
The correct equipment is the first part of the bitumen safety puzzle. Equipment must then be correctly installed and commissioned. Regular maintenance is necessary to keep instrumentation clean and functional, which is emphasised in the guidance. Equipment calibration must be checked and corrected to ensure that the accuracy of level readings is maintained.
Conclusion
Maximum bitumen safety is an achievable end, but it is a multi-faceted issue requiring:
- Accurate and reliable contents measurement, with unambiguous presentation of salient information.
- Correct calculation of ullage using tank contents information to determine whether there is sufficient room in the tank to allow a new load to be delivered without overfilling.
- Effective communication between plant personnel and bitumen tank drivers, together with strict adherence to operating procedures; participation and commitment during the delivery and offloading process.
- Provision of suitable safeguards to warn of impending overfills should something go wrong.
In summary, Hycontrol’s David Wadsworth says: “Despite the progress made over the last decade in educating site managers, H&S officers and staff about tank capacities and the risks associated with bitumen storage, the suitability, reliability and accuracy of the measuring equipment they are using is sometimes questionable at best. Only through a combination of appropriate, well-maintained monitoring equipment and alert, informed site staff can we be confident in the safety of bitumen storage.”
