Cleaning heater convection tubes
Fired heater tubes are difficult to clean, but a sequence of operations can remove unwanted material from the tubes’ external surfaces and raise the heater’s efficiency.
BYUNGMOON JEONG and SEOKRYONG JEON
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Almost every refinery and petrochemical company in the world suffers from lower efficiency than the design condition after long, continuous operation. There are many attempts to increase the turnaround cycle these days, and the continuous operation period is getting longer, with 3-5 years being common. The mechanical integrity of equipment is important for long periods of continuous operation, but it is equally important to maintain the efficiency of the plant’s design.
The fired heater is a typical item of equipment that becomes less efficient as time goes by. One of the main reasons for a decrease in the efficiency of a heater is that foreign substances build up between the tubes, little by little over the long term. Because the gap between fins is narrow, foreign substances are likely to stick in a structural fashion.
In particular, the burning of liquid fuels is more likely to produce incomplete combustion than gas fuels, resulting in much higher levels of unburned hydrocarbons and ash. Ash and unburned hydrocarbons deposited in the heater’s convection tube disturb effective heat transfer between hot flue gas and the convection tube surface (see Figure 1). Subsequently, this makes the heater’s efficiency lower than the design condition.
Fired heaters are among the most fuel consuming items of equipment in refineries and petrochemical plants. If their efficiency is low, higher fuel costs cannot be avoided. In addition, other risk factors can arise, such as overheating of the tube by increasing the load of the burner to maintain output.
Generally speaking, radiant tubes absorb more heat (60~70% of total fired heat duty) than convection tubes (30~40%) in heaters. Therefore, a radiant tube may be more important than a convection tube in terms of heat absorption. However, a radiant tube can be easily accessed and cleaned by workers using scaffolding during a turnaround. Therefore it is relatively easy to maintain the designed heat transfer from the outside surface of the tube. But this excludes heat transfer inside the tube. Radiant tubes are exposed to relatively high temperatures, creating coke in the tubes, which leads to low heat transfer efficiency.
The structure of convection tubes makes them difficult to access for cleaning, but it is important to clean the convection tube effectively to improve the heater’s efficiency. Normally, heaters consuming liquid fuel should be fitted with soot blowers which should be operated periodically to prevent foreign substances from accumulating between the tubes. Even so, if the equipment operates for a long time (usually 3-5 years), these substances will nonetheless accumulate.
In addition, environmental regulations make it necessary to reduce NOx, hence selective non-catalytic reduction is often installed in fired heaters. This equipment is installed as a nozzle between the radiant section and the convection section, and the chemical sprayed through the nozzle flows directly to the convection tube. Such chemicals make foreign substances in the convection tube become more firmly fixed. As a result, this makes tube cleaning even more difficult.
Even if foreign substances accumulated during operation are unavoidable, it is necessary to find a way to completely clean them from the tube by opening up the equipment during a maintenance period.
More than 100 fired heaters are in operation in our plant; the largest among them was selected to apply a new cleaning method. This heater consumes $60 million worth of fuel annually (heat duty of 179 MM kcal/h). It is a CCR heater for the aromatics plant in which four heaters share one convection section (see Figure 2).
Previously, only simple cleaning was carried out by spraying chemicals onto the surface of the tubes, followed by cleaning with spray water. This was not very effective; in particular, when the tube consists of several lines the tube in the middle was almost impossible to clean.
In addition, the space between convection tube bundles is too narrow (minimum 396mm) to provide approach space for cleaning workers, and the length of a convection tube (23m, 75ft) makes it difficult for workers to access them.
Following long-term operation, more than 50 years, a project to increase capacity has been carried out, but this meant that the fired heater was becoming short of heating capacity. To overcome these difficulties, we tried to apply two types of cleaning equipment. The first of these was a portable tube cleaning machine (see Figure 3). This machine is specially designed for cleaning convection tubes and is commercially available. It moves along a rail and cleans the tube by orienting in four directions (forward/backward/right/left). It can be operated with a maximum pressure of 1000 bar and is designed to physically separate foreign substances from the tube by means of a high pressure water spray. The machine has several small nozzles and each nozzle has a particular spray angle to enable it to clean large surfaces.
A simulation test was conducted to identify problems and to check the machine’s effectiveness at cleaning before application in the field. Scaffolding and temporary tubes were used to make a structure similar to a convection tube (see Figure 4).
The tube fin of the fired heater is sufficiently thick to avoid the risk of deformation or damage caused by high pressure water.
The water jet machine cleaned the tubes well. High pressure water spraying can reach between the tubes, so the lower tube also had a cleaning effect. However, it seems to be difficult for the water jet to have more than three rows of cleaning effects past the tube. Observing the surface of the cleaned tube, the scale was completely removed and looked almost like a new tube.
In addition, the machine is only about 300mm high, so it can move in a narrow space (see Figure 5). Thus, the operator can do the cleaning by remote control without having to enter the narrow space between tube bundles.
Because the convection tubes to be cleaned were over 23m long, this made it impossible to install all of the rails at once, so some sections were cleaned first, then the rails were moved to clean the rest because of the limited length of the rails.
The second machine selected is a hand jet cleaner, which can spray water jets at 2700 bar (see Figure 6). This machine can be operated by cleaning workers and cleans the surface of a tube by inserting it between tubes (see Figure 7). Since the worker must perform the work by hand with very high pressure water, individual skill is called for. This machine can compensate for the poor cleaning effect of the portable tube cleaner for the tube in the middle of the bundle.
The tip of the hand jet cleaner is designed to rotate while water is injected at high pressure, so the tubes can be cleaned effectively. The total nozzle length should be long enough to reach the centre of a tube bundle to enable all of the tubes to be cleaned.
The cleaning sequence was designed to maximise the cleaning effect and to perform the operation efficiently. The key to this cleaning strategy is to clean the inside of the bundle after removing large debris at the outset. When using portable and hand jet cleaners, low concentrations of chemical should be used first, followed by higher concentrations. The cleaning method and procedure are summarised in Table 1.
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