TANK HEATING SYSTEMS IN PORT TERMINALS USING THERMAL OIL
Pirobloc designs and supplies highly efficient thermal fluid systems for heating storage tanks in ports. We have extensive experience of the design, installation, commissioning and maintenance of thermal oil systems and boilers for heating storage tanks in port facilities.
A heating system is needed at port terminals to ensure fuel and raw materials stored in tanks remain in the liquid state. The main fluids found in these tanks are:
- Heavy oil
As these fluids are in a solid or semi-solid state at room temperature, it is imperative to use a heating system while they are inside the storage tanks. The tanks should be heated even if they are adequately insulated.
All these products must be above a certain temperature to remain in a liquid state so they can be handled properly. Therefore, all port facilities need a thermal oil system whose reliability is absolutely guaranteed, since these fluids must be prevented from ever solidifying inside the tanks.
Heating the tanks with a thermal oil system is the safest and most efficient solution for these facilities, as it offers numerous advantages over the other available technology, steam heating.
This heating process requires keeping the product not only at a suitable temperature when it reaches the terminal, but having it slightly higher. This is because, in most cases, the product temperature is as low as possible when transporting to the terminal to reduce costs. Therefore, the storage temperature should be increased slightly so the product is not too viscous in the tanks; this would result in transfer pumps needing to consume a lot of energy to distribute the product.
Failure to store these products at the right temperature can cause problems such as an inability to transfer them from the tank to another terminal; or they become solid inside, as can happen with asphalt or bitumen.
HEATING STORAGE TANKS
As mentioned above, heating storage tanks at port terminals is mandatory. Most hydrocarbons, asphalts, bitumens and heavy fuel oils require temperatures above ambient for proper handling. These are highly viscous products at room temperature, and handling them in these conditions is impossible, whether for internal transport operations within the terminal or for external distribution.
The heating system generally adopted within these tanks is a grille or coil through which oil circulates. This is the simplest system. This coil can be arranged at different heights to achieve better heating uniformity.
The size of these coils must be chosen appropriately to transfer all the power from the thermal oil boiler to the tank. Thus, the corresponding engineering calculations must be made to determine sufficient heating surface area, diameter and length for the pipes. This geometry will have its own load loss or Delta P, which is crucial for the proper dimensioning of thermal oil pumps and the whole hydraulic circuit.
The mass to be heated and the required temperature rise must be known for proper sizing of the heating coils and for determining the boiler power.
Ensuring the reliability of the heating system is fundamental in this type of application, as it is absolutely essential to prevent a large mass of product from solidifying inside the tank. Therefore, it is common to install 2 sets of boilers and 3 pumps in each facility. This way, there is always 1 spare boiler and 2 spare pumps.
Some facilities also have a suction heater in addition to the heating coil inside the tank to reduce the viscosity in the suction zone. It is an open mouth exchanger where the thermal oil tube bundle is in direct contact with the fluid to be transported.
The system can be designed for simultaneous or alternate operation of both boilers. Any of the 3 pumps can work with any boiler using a manual valve system according to the scheme shown below:
Types of heating
There are two basic types of heating: direct and indirect. As the name implies, direct heating means the product contained in the tank is in direct contact with the heat source; while indirect heating involves an intermediate fluid that transports energy from the point where it is produced (the heater) to the tank, where it is then transferred to the product stored in the tank.
Direct heating is carried out using electrical heaters inserted into the storage tank. Generally, 3 or 4 groups of heaters are used, placed at different points on the bottom of the tank.
It is an efficient heating system, as there is no loss of energy between the source (the heater) and the product being heated.
Any problem that occurs with one of the heaters can be quickly resolved by replacing it with a new one. If the tank is empty, this is a very quick operation.
However, the system requires a lot of electrical energy in the tank area so consumption is very high. Also, the heaters installed must be for classified areas.
The high energy consumption of a direct heating system makes this solution not economically viable in most cases. These types of systems may be suitable for tanks with a reduced capacity, or when the tanks are located in areas where it is excessively complex to install an indirect heating system.
As mentioned above, indirect heating is carried out by an energy transport circuit, where the heat-carrying fluid flows from the heat source (the heater) to the heat consumer (the tanks); here, the energy is transferred through coils or exchangers and then back to the heater. During this process, no heat is added or removed between the system’s boundary walls, except for losses to the environment, which are minimized by efficient insulation of the pipe network.
The heat transfer fluid must possess specific properties to be able to carry out its function of transporting energy efficiently and at a reasonable cost.
Some of these necessary properties are:
- Good heat transfer capacity
- Good thermal stability, so it can be used for long periods while still being functionally stable.
- Low viscosity throughout the entire working range, particularly in starting conditions, to prevent high energy consumption.
- Low melting point, so work can be safely stopped for extended periods of time.
- Low corrosive action on the components in the system.
- Technically adequate to handle the specific process requirements.
- Low toxicity and not harmful to the environment, so it can be easily disposed of once exhausted.
- Reasonable purchasing and maintenance costs.
- Low risk for personnel and machinery, ensuring safety and avoiding high costs if there are leaks.
The most widely used heat transfer fluids are steam and so-called thermal fluids.
Although no heat transfer fluid can meet all the above conditions, thermal oil or fluids are more efficient than other heat transfer fluids, such as steam.
Thermal fluids are by far the best means of heat transfer for heating tanks at port terminals due to their high technical performance, their ability to work at high temperatures and low pressures, their high level of precision in determining the final product temperature, their versatility and their flexibility.
A key advantage of facilities using thermal fluid as a transfer medium, in contrast to steam, is the absence of corrosion, both in the heater and in the facilities, and thus the absence of leaks. This allows for continuous, stable production and simple maintenance.
Heating with thermal fluids
The simplest and most common heating system consists of a grille or a coil inside the tank with circulating oil (Figure 1). This coil can occasionally be placed on multiple levels, particularly in high-rise tanks, to minimize the product’s natural convection effect, which could lead to temperature gradients.
(1) Flanges to connect the thermal fluid to the tank. (2) Suction heater. (3) Internal coil. (4), (5) General thermal oil circuit. (6) Thermal oil to the coil. (7) Thermal oil to the suction heater.
Also, there is usually a suction heater (Figure 2) inside the tank, independent of the heating coil.
Figure 2. Suction heater
These heaters warm the products in the suction area of the storage tanks, to reduce fluid viscosity in this area and keep it in a liquid state; thus, it can be pumped properly when necessary, without excessive energy consumption in the transfer pump.
They are essentially open-mouth exchangers at the end of the casing located inside the tank, allowing a large proportion of the product to pass into the tube bundle through which the thermal fluid circulates (Figure 3).
The fluid that circulates through the primary circuit (the tube bundle) is the thermal fluid, while the product located inside the tank circulates through the secondary circuit (the casing).
The equipment is connected by a main flange on the side of the tank, at the bottom. The heated product to be transported exits through a flange installed in the housing on the outside of the tank.
The thermal fluid heating system is generally designed with two thermal oil boilers, given the importance of ensuring continued heating in this type of application. The aim is to prevent a huge mass of product from ever becoming highly viscous or solidifying inside the tank. This risk is very high with products such as asphalt or bitumen.
One of the thermal fluid boilers will be in standby mode, as a single boiler can produce 100% of the necessary heat. Occasionally, the 2 boilers can work at the same time, as the capacity range of one boiler is designed to produce 60-70% of the heat if required. In both cases, the 2 boilers continue to operate so, if one fails, a start-up operation will not be necessary.
The circuit also contains 3 recirculation pumps that can operate with either of the 2 boilers through a manual valve system. There are 2 spare pumps should a single boiler be given responsibility for all production, and 1 spare if both boilers are chosen to work (Figure 4).
Obviously, the project involves many thermal and hydraulic calculations to properly measure the coils, exchangers and their exchange surface area, as well as the network of thermal fluid pipes; this is of vital importance to ensure that the energy transported to each tank flows properly.
The main heating can also be done by heat exchangers on the outside of the tank, as if they were direct heaters. In this case, the product is sucked in by the suction heater and circulates through these exchangers connected to the tank with flanges. A valve system allows the latter exchanger to supply the product directly for transport, if it is in adequate conditions, or to an external exchanger to take it to the tank, where the heating process continues until the required moment. (Figure 5).
The fluid that circulates through the primary circuit (the tube bundle) is thermal oil, while the product inside the tank circulates through the secondary circuit (the casing).
As commented previously, direct combustion boilers are especially suitable for very specific operations, while indirect combustion boilers have very diverse and varied fields of application.
A direct combustion boiler is designed for a specific operation, with high technical sophistication, so a substantial change in product or operation specifications can render the equipment ineffective for the new process.
Meanwhile, indirect combustion boilers are much more flexible, as they use an intermediate heat transfer fluid. The fact that the heat is supplied by independent equipment located outside the tanks is an advantage, since variations in processes or facilities will not greatly affect the heating equipment.
|Concept||Direct Fired Heater||Indirect Fired Heater|
|Fuels||Liquids and gases||Liquids and gases|
|Powers||Very high (>6000 kW up to 100000 kW)||Medium/high (<8000 kW)|
|Needs for space||High||Normal|
|Combustion control||Very high||Normal|
|Financial cost||Very high||Moderate|
The absence of corrosion or leaks in the heating circuit and boiler means that indirect heating is highly efficient and highly reliable. In addition, this system requires a low working pressure, which corresponds exclusively to what is necessary to overcome the load loss in the hydraulic circuit, which involves high levels of safety and flexibility, as well as great precision in temperature control, independently of the configuration adopted (coil or external exchanger).
The heating of storage tanks in port terminals using a thermal fluid system is one that offers indisputable advantages.
(1) Suction heater. (2) Thermal fluid to the suction heater. (3) Thermal fluid to the heat exchanger. (4) Heat exchanger. (5) Pipes from the suction heater to the heat exchanger. (6) Product return to the tank. (7) Product to supply. PO1 pump.
Conceptually, a suction heater is a heat exchanger where the primary fluid is thermal oil and the secondary is the product to be maintained at a certain temperature.
Our suction heaters are supplied with all the nozzles for the entry and exit of the thermal fluid and the product to be kept in a liquid state. Support for large devices can be supplied by anchoring to the base of the tank.
As previously mentioned, suction heaters are used to heat products inside storage tanks, especially when these products are solid or semi-solid at low temperatures. To pump and transport them properly, their viscosity must be reduced and fluidity increased by heating with suction heaters.
The heat transfer fluid we propose is thermal oil heated by a thermal oil boiler.
The most common applications of this technology are for heating tanks of asphalt, bitumen, heavy fuel oil as well as other products.
Suction heater technical features:
• Power range: 0.1 to 10,000 kW
• Maximum allowable pressure: 20 bar.
• Test pressure: 30 bar.
• Maximum working pressure: 15 bar.
• Operating temperature: up to 340°C.
• Design temperature: 350 ° C
• Design code: ASME VIII Div. 1, EN 13445, AD2000
Suction heater operation
The suction heater is attached to the side of the tank by a main flange. It is usually located at the bottom and near to the product suction zone. Thermal fluid enters in the final part of the inside of the tank, which is open to facilitate the entry of the thermal oil used to heat the product inside the tank. A flange placed on the outside of the tank is the hot product outlet, which is in a suitable state to be transferred.
The heater design is an open housing U-tube. It is a fork-type tubular beam that is easy to remove and clean. It is made of ASTM A 106 Gr B carbon steel.
The heating surface and power are suitably selected to meet the process requirements. The variables to consider for its configuration are product type, viscosity, tank volume and product flow rate.