The production of liquid petroleum products like gasoline, gasoil, naphtha, etc. has been increasing over the years with the number of motorised vehicles worldwide. The petroleum products produced in refineries need to be transported to customers or other refineries for further processes in a safe, fast and cheap way. The easiest and cheapest way of transporting these products is by pipeline, but generally this option is only available for nearby facilities. The next options would be rail or sea, where possible. A flexible mode of transportation is by road tankers, but only limited amounts of product can be transported in one tanker, because of its storage capacity.
The product quality and quantity also play significant roles in transportation. Not all refineries have their own transportation vehicles and so outsource their transportation activities. The products have to be transported in a safe and proper way to ensure that they do not become affected by environmental factors. Another important aspect is the loss of containment during transportation, so-called transport-loss. Therefore, the transportation company has to ensure that the transportation vehicles are suitable for the transported products.
Liquid petroleum products belong to the group of dangerous goods and the transport of these goods by road is regulated by the ‘European Agreement Concerning the International Carriage of Dangerous Goods by Road’ (ADR). In the ADR, one can find a list of all dangerous goods and the exact description for the safe way to transport these goods. According to the ADR, liquid petroleum products have to be transported by road in road tankers (tank cars) or liquid bulk containers which are suitable for the special products. For example, light straight run naphtha (LSRN), a volatile petroleum product with high vapour pressure and low specific gravity (or high API), has to be transported in road tankers classified as L4BN or L4BH. The description of these character combinations are given in Table 1.
Tüpras (Turkish Petroleum Refineries Corp.), consists of four petroleum refineries in different locations (Izmit, Izmir, Kirikkale, and Batman), which have a logistic network between them. A part of this network consists of transportation by road. The transportation routes between the Izmit, Kirikkale and Batman refineries are shown in Figure 1.
Figure 1. Transportation routes for four refineries.
The most common products transported between these refineries are gasoil and naphtha, but the most critical one is LSRN because of its properties. The vapour pressure of the LSRN produced in Tüpras’ refineries is around 100 kPa. So this product has to be transported by road tankers with the classification L4BN or L4BH. However, it could be faced with product loss problems during transportation, especially in the summer season. Due to this, it was decided to analyse these losses first by developing a method to determine or predict them.
Method for determining losses
Although it is very difficult to determine the losses resulting from the transportation of petroleum products, a new method can be enhanced to specify the source of the problem regarding relief valves (RVs). There should be a RV or pressure safety valve (PSV) on road tankers to protect overpressure of the vehicle. The pressure of this equipment should be determined according to the properties of the fluid carried. If the fluid is very light and volatile, it will have a low specific gravity and distillation cuts; therefore the pressure of safety valves should be higher to protect against sudden vaporisation.
By means of a simulation model, the temperature, which is the value at the opening pressure of RVs, can be determined. After calculating the temperature value, control of road tankers can be carried out according to exposure to that given temperature. If the temperature is within the reasonable range based on that season or month, it can be inferred that these losses are caused by opening the RV, and the set pressure of the RV is quite low to carry these products.
To use the simulation model, distillation curve and specific gravity of the petroleum product that is being transported would be necessary. The filling volume of tankers and RV set pressure has to be known. After gathering all necessary information, the fluid stream which leads to product loss is composed in simulation software. At this stage, it can be assumed that the rest of the volume after filling the fluid in the tanker is air. Consequently, an additional air stream with the residual volume is created and mixed with the fluid one, which is composed depending on the distillation curve and specific gravity. In this mixed stream, RV set pressure and maximum temperature that can be seen in the interval of a product loss problem are defined and the vapour fraction of this stream can be analysed. If vapour fraction is bigger than 0, it can be concluded that the RV has opened and some of the products have been released to the atmosphere. This analysis can also be done in a different way. If the amount of loss is known in tankers, this value is entered into the vapour fraction part of the software and the temperature value at the RV set pressure can be calculated. After this calculation, the possibility of hitting this temperature in a particular season or month can be controlled.
A sample schematic view of simulation model and assumptions used for simulation are shown in Figure 2.
Figure 2. Schematic view of simulation model.
At Tüpras, this method has been applied since 2017 to determine the problems resulting from RV set pressure. During the transportation from Batman to the Izmit and Kirikkale refineries especially, product losses are seen frequently during summer due to high ambient temperatures occurring in Batman. In addition to high ambient temperature, low RV pressure is another reason for product loss problems. Besides, it is very important to take the ambient temperature considering the assumptions seen in Figure 2. With this method, road tankers with low RV set pressure can be determined and necessary actions can be listed regarding transportation processes and equipment.
The most important point to take into consideration is that all the analyses based on these simulation models are conducted for stationary tanks or tankers. The effect of exposure to constant or changing ambient temperature or transportation time of the tankers in simulation models cannot be determined since these models are carried out in steady state systems.
Product loss problems during transportation are inevitable if the products are volatile (with low specific gravity and high vapour pressure) and not transported in a way which is not appropriate to ADR.
To evaluate the product losses, a simulation method can be used to take into consideration the RV set pressure of the container, the distillation curve of the product, the volume of the products (hydrocarbon and air) in the container, and the exposed temperature. After completing the simulation model, it can be analysed whether this temperature or pressure is reasonable in terms of product loss or not.
Further, it can be decided which type of container will be used for safe transportation of the specific products. This method has been applied since 2017 and some precautions have been taken in order to solve this problem by changing relief set pressure values.
Written by Alparslan Yildiz, Gözde Dönmez Yavuz, and Seçil Uzun, Tüpras.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/17032021/cut-your-losses/