Heater tubes

In the refinery the salts deposit in the tubes of exchangers and reduce heat transfer, while in heater tubes, hot spots are created favoring coke formation. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

In the last chapter we said that one of the requirements of a high-temperature material - in a turbine blade, or a super-heater tube, for example - was that it should resist attack by gases at high temperatures and, in particular, that it should resist oxidation. Turbine blades do oxidise in service, and react with H2S, SO2 and other combustion products. Excessive attack of this sort is obviously undesirable in such a highly stressed component. Which materials best resist oxidation, and how can the resistance to gas attack be improved ... 

Antimony-based passivation was introduced by Phillips Petroleum in 1976 to passivate nickel compounds in the FCC feed. Antimony is injected into the fresh feed, usually with the help of a carrier such as light cycle oil. If there are feed preheaters in the unit, antimony should be injected downstream of the preheater to avoid thermal decomposition of the antimony solution in the heater tubes. 

The exit gas section contains the air heater tube bundles, flue gas dampers, and the various emission control systems such as dust collectors, electrostatic precipitators, and gas scrubbers. 

To some extent, fly ash neutralizes sulfur gases, but with high dust-burdened flue gases, the sulfur gases simply tend to make the ash very sticky, and it forms sulfiirated ash deposits on the air heater tubes and plugs the path of the gas stream. 

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

When overheated, hydrocarbons tend to breakdown, leaving carbon residues (coke). This coke builds up on the inside of the heater tubes, slowing the transfer of heat from the tube walls to the product by restricting the flow of product and acting as an insulator. As the control system attempts to maintain the process outlet temperature at the setpoint, the fuel valves will open and the tubes subjected to an increased heat load. With the diminished ability of this heat to be transferred to the process fluid, the temperature of the tubing will increase. 

Isolation or emergency shutdown (ESD) valves should be installed to stop fuel flow and the process feed flow into the heater in the event of heater tube rupture. These valves can be automatically actuated by controls or safety interlocks or can be manually operated remotely. Remote actuation can be from a control room console or in the field field actuation stations should be located at least 50 ft (15 m) from the heater. It is also common to provide a manual block valve, located at least 50 ft (15 m) from the heater, on each of the fuel and process feed lines. These should be accessible to operators in the event of an incident involving the heater. 

More specifically, typical operating conditions are a flow rate of 4 pounds per hour, a fuel temperature from the heat exchanger of 400°F, a filter temperature of 500°F, and a fuel pressure of 150 pounds per square inch gage. Fuel stability is then determined by operating until a pressure drop of 25 inches of mercury is obtained across the filter or until 300 minutes have elapsed. No heater tube appearance requirements have been designated... 

For corrosion and safety reasons, the condensate recovered from these sources is best not returned to the deaerator for use as boiler feedwater. However, depending on the contaminant, the condensate may be reused for a number of services. Our favorite reuse of such contaminated condensate is as a replacement for velocity steam in the heater-tube passes of a fired furnace. 

Some of the heat is radiated directly to the heater tubes. 

In most heaters, the majority of the heat of combustion is radiated to the refractory walls. The glowing refractory walls then reradiate the heat to the heater tubes. 

A typical firebox temperature is 1500°F. Thus, the heater tubes can reach 1300°F on loss of the process flow, even though the fuel flow has been immediately stopped. Tubes with a low chrome content may bend and distort as a result of such overheating. Even at 1000°F, residual liquid left in the tubes when flow is lost may thermally degrade to a carbonaceous solid or heavy polymer that fouls the interior of the tubes. 

One way to combat this problem is with steam. As soon as the flow is interrupted, high-pressure purge steam is automatically opened into the heater tube inlets. The steam blows the residual liquid out of the tubes, and also helps remove heat from the tubes. 

A typical process heater tube diameter is 4 to 10 in. Tube thickness is usually between V4 and V2 in. Heater tubes are often constructed out of chrome steel. A high chrome content is 13 percent. The chrome content increases the heat resistance of the tube. A tube with a 11 to 13 percent chrome content can normally withstand a skin temperature of up to 1300 to 1350°F. A low-chrome-content tube of perhaps 3 percent may be limited to 1200°F tube metal temperature. Naturally, the pressure, thickness, and diameter of the tube all affect its maximum skin temperature limitations. 

When a heater tube fails, the process fluid spills out into the firebox. Let s assume that the process fluid is a combustible liquid. Will the leaking process fluid burn The answer is, mostly not. There is probably not enough excess oxygen in the firebox to support a substantial amount of additional combustion. 

Figure 21.3 Heater tube cross section close to failure.

Dry-point deposits. We sometimes see that a certain heater tube will glow a light red and fail for no apparent reason. The particular tube position in the firebox seems to be far more subject to failure than its neighbors. The tube is not located in an area of flame impingement, and the tubes upstream and downstream are a nice dark red (see Table 21.1). 

In the process (Figure 9-8), the feedstock and recycle hydrogen gas are heated to reactor temperature in separate heaters. A small portion of the recycle gas stream and the required amount of additive are routed through the oil heater to prevent coking in the heater tubes. The outlet streams from both heaters are fed to the bottom of the reactor. 

Investigators determined that during a hectic day of operations, the chemical process operator erred. On this afternoon, he inadvertently tried to startup the heater with the burner firing and the heater tubes isolated from the circulating pump by closed blocked valves. Shortly after firing the heater, the lead operator checked the flame pattern but observed nothing out of the ordinary. 

This was a complex case, requiring a full and detailed investigation by members of technical, engineering, operations, and process safety groups. Investigators made numerous interviews and detailed observations. Inspectors found a 6-inch (150 mm) long and 4-inch (100 mm) wide hole on a ballooned section of a heater tube. Normally the tube had a 6.6 inch (168 mm) outside diameter, but it had swollen to about 8.0 inches (200 mm) in diameter at a point about 2.5 ft. (0.75 m) above the heater floor. 

The entire 8-inch (20 cm) piping system was insulated and steam traced except for the heater tubes within the heater. Operations assumed the heat transfer fluid froze in the four heater tube passes. Each pass was a bare 4-inch (10 cm) diameter heater tube with 5 bends and the equivalent of 72 ft. (22 m) of straight pipe. The foreman and the operations team discussed the situation and decided to light and maintain a small fire on the burner to slowly thaw the material in the heater tubes. This method had been successful for a startup several weeks earlier. 

The heater tubes had a carbon build-up inside the tubes which restricted flow to some passes. There was some external thinning of tubes in the higher heat flux zones. 

After the hydrogen is purified, it is ready for use in a refinery hydrogenation process. In an ammonia plant, the hydrogen-nitrogen mix is sent to an ammonia converter (Figure 4.7), which requires a start-up heater. Since the material in the heater will be exposed to hydrogen only for a short period, the time dependent curves in API 941 should be consulted when selecting an alloy for the heater tubes. 

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