Products to mitigate liquid sloshing
Erik Eenkhoorn is a PhD Student in the research group Mechanical Engineering Department, his Supervisor is Professor André de Boer from the Faculty Engineering Technology Faculty (ET)
The transport of bulk liquids is quite obscure, one never makes eye-contact with the fluid during its "cradle to grave" life-cycle. The non-transparency of mobile tanks is the prime cause of the lack of awareness in society of the liquid behaviour inside these tanks.
Logistic organizations determine the quantities of bulk liquid to transport and apply strict economic and optimization conditions. This rarely uses global supply in the chain of liquid raw materials into consumable liquid end products, often characterized by many intermediate links. This logistic economic optimization leads to a strong increase in the transport of liquid loads that only partially fill the mobile tanks in which they are transported.
The complexity and extent of undesirability of accidents results in continuously increasing tightening of laws and regulations. Unfortunately, this increase does not appear to focus on the circumstances inherent to bulk liquid transport, being the core, source, or cause of conditions leading to such accidents.
The “risks” of liquids being transported are currently related to the "danger" of the liquid, but only in terms of the "flammable", "explosive", "toxic", etc., properties of the liquid. The risks in terms of the dynamic behaviour of the liquid part load and its effects on the stability of the means of transport are not or not sufficiently recognized. For example, there is no driver training for drivers of tank trucks required before they are allowed to drive such tank truck, let alone such training courses being available in the market. The hazards of liquid dynamics become only visible in cases of accidents. Accidents with tank trucks are generally experienced as "very serious". This perception by the public at large is partly due to the sensational way in which the media tends to bring such accidents in the news.
The complexity of bulk liquid transportation as it has become was investigated and analysed. The source of the hazards or undesirable features of bulk liquid transportation leads to the conclusion that the occurrence of sloshing of liquids during transportation, is not correctly addressed.
The sloshing of liquid bulk cargoes in mobile tanks should be avoided by "securing" these cargoes, as is done and mandatory by the transportation of any other non-liquid cargo. Securing cargo in a means of transportation shall be done such that the cargo does not and cannot move relatively to the means of transportation. This is achieved by applying "load securing" which can and will transfer all the necessary forces between cargo and the means of transportation and does not provide any space for the cargo to move relatively to the means of transportation.
Baffle plates, as required by law for the transportation by road of flammable, explosive hazardous liquids, do not provide for this “enclosed” force transfer. These baffles continuously convert kinetic energy into heat through constant damping of dynamic fluid movements. Baffle plates result in unnecessary, additional, fuel consumption of tank trucks, while their contribution to avoiding keeling-over accidents is nil. The reduction of the height of the centre of gravity in the designs for new tank trucks is achieved by using tanks with an "oval" cross section. These oval tanks, other than tanks with a circular cross-section, lead to unwanted horizontal forces on the tank resulting from centrifugal liquid movements. Especially tank-trailers, connected to a towing engine-vehicle via a "king-pin", prove to be sensitive to these forces causing a considerable tilting moment on the tank truck.
Current legal obligations or considered design improvements, which aim at reducing the likelihood of accidents, were proven to be insufficient. Therefore, it was investigated whether inflatable components placed in a transportable tank partially filled with liquid could reduce this risk of accidents. These inflatable components are positioned in the tank, like any "load securing" provision, before the transportation of the liquid. When positioned inside the tank, they are "manipulated" before or during the liquid loading. The manipulation is to result in not any space being available for the liquid to use for movements, and in enabling all forces occurring during the transportation to exchange between load and means of transport and vice versa. This slosh mitigating method of operation, using an inflated component, is tested with prototype products in scale models and with full size inflated components in tank trucks.
Inflatable components fill that space in the transportable tank where there is no liquid. All air that would normally be "free" in the tank is now in the inflated component, above the liquid. This presence of such component also prevents the liquid from evaporating. When the liquid does not evaporate, there will also be no emission of such vapours from the tank, even when filling the tank with liquid. This possibility of making transportable tanks for hazardous substances, emission free, nicely meets with the rapidly tightening legislation on this subject. Avoiding contact between liquid and air may during transportation may furthermore be desirable from a fluid quality perspective.
Inflatable components are the most important part of a slosh mitigation product. These slosh- mitigating products also comprise pneumatic and electronic subsystems that are to take care of the desired operations of the inflatable component. Slosh mitigation product varieties, which do not require any changes to the tank in which they are placed are more complex than the most “simple” versions. The complexity does however offer possibilities of providing for additional functions, such as catering for the overflow protection of the tank.
The application of load securing, especially also for the transportation of bulk liquids, is strongly recommended resulting from this research. Securing loads avoids unpredictable and dangerous dynamic behaviour of liquids during transportation. The possibility of using products with an inflatable component placed in the transportable tank should be permitted by law, as well as that the law should change for that purpose, if such deems to be necessary. In addition, the legislator should reconsider the current mandatory use of baffle plates.
