Steam Boiler: Superheater

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Requirement Design of Superheater

The requirement design of superheater will be better to establish before manufacturing process will be performed. The design superheater should provide uniform distribution of steam and temperature at all loads, and provides thermal expansion of headers, tubes, spacers and supports, and should be accessible for cleaning, examination and removal of elements.

The designer of steam boiler should arrange the superheater tube banks as conveniently as possible to perform maintenance and periodic inspection. The requirement should describe what programs are being undertaken, if any, to resolve problems of gas side corrosion and erosion of superheater tube materials at elevated temperatures.

Horizontal tube sections of superheater should be arranged with sufficient clear spacing between tubes in the gas flow at least equal to tube diameter and such as to avoid slag formation. Tubes should be arranged in parallel rows. Horizontal tube banks in the convective pass may have minimum baffles installed for the prevention of acoustical standing wave formation.

The wall thickness for superheater tubes, at least, should have a corrosion and/or erosion allowance of not less than 0.5 mm. All superheater tube joints including connection to headers should be of welded construction in accordance with ASME BPV Section l.

Superheater tubes should be arranged and terminated in outlet headers in a manner which will yield through mixing of the steam, thereby resulting in the same temperature at each of the main steam outlets. The superheater arrangements should be provided so that by the arrangement of the connecting tubes and by the mixing points a uniform flow admission and sufficient temperature balance are insured.

Tube connections should be shop-welded stub ends on headers. Stub ends should be prepared for field welding of superheater tubes. Any welding of ferritic to austenitic stainless metals should be done in work shop.

Spacers and hangers for superheater tubes should be designed to provide for appropriate sliding expansion, and should be of material suitable for temperature encountered. All headers should be external to the gas stream and should be provided with welded inspection hole covers as required for inspection. The header in the gas stream should be shield to avoid header damage caused by the flue gas. Inlet and outlet headers and/or lead pipes should have thermo wells with thermocouples and connections for pressure gauge, lest gauge and transmitter should be furnished. The design superheater must be equipped also with desuperheater and safety valve to control pressure and temperature.

Based on description above, requirement design of superheater is a must to be performed prior to begin manufacturing process to get high performance and efficiency.

Safety Valve in Superheater and Reheater

Superheater and reheater in steam boiler must be equipped with safety valve respectively. Safety valve in superheater is installed at main steam line piping between superheater outlet and the first main stop valve. Safety valve is one of appurtenances in steam boiler which is used to release pressure if the working pressure exceeds maximum allowable working pressure.

Designer steam boiler should consider the size, relieving or discharge capacity, set pressure and number of safety valve. These parameters depend on the presence of valve between steam boiler and safety valve in superheater and the pressure drop upstream of safety valve.

Installation of safety valve in reheater must be applied minimum one safety valve. Determining discharge or relieving capacity of safety valve in reheater is different with safety valve in boiler. It must be calculated separately. Safety valve in reheater can be located between reheater outlet and the first main stop valve.

If the temperature of steam flow exceeds 450 F, the safety valve should be equipped with casing with a spindle, bonnet, body and the base. These materials are fabricated from alloy steel or steel, or other material which has high heat resistance. The connection of safety valve can be flanges to make easy for maintenance or welded connection.

Figure 1: Safety Valve in Superheater

Classification of Superheater in Steam Boiler

Superheater is a device consists of tubes which get the heat from the product of combustion to add additional heat into steam, so the temperature of the steam passing through the tubes raise higher than its saturation temperature. The superheater can help steam boiler increase its capacity because help turbine generator get its required steam both in terms pressure and temperature.

Classification of superheater can be classified based on design of flow:

1. Parallel Flow Superheater

A superheater which has direction of steam flow in superheater same with the direction of flue gas flow

2. Counter Flow Superheater

A superheater which direction of steam flow in superheater opposite with the direction of flue gas flow

3. Mix Flow Superheater

A superheater which direction of steam flow in superheater same and opposite with the direction of flue gas flow

Classification of superheater can be classified based on heat transfer:

1. Convection Superheater

The convection superheater is located somewhere in the flue gas flow, where it absorbs heat transfer by convection.

2. Radiant Superheater

Radiant superheater is located in or near furnace of steam boiler and receives heat transfer from combustion process by radiation.

Classification of superheater can be classified based on arrangement:

1. Vertical Superheater

The superheatar are arranged vertically. This type has is easy to suspend and has free moving to expand. But the disadvantage, it is not drainable.

2. Horizontal Superheater

The superheatar are arranged horizontally. This type has is more difficult to suspend and should give special support to give moving for expand. But the advantage, it is drainable.

Design Spray Desuperheater of Steam Boiler

In steam boiler, desuperheater can be known as attemperator which has function reduce and control temperature of superheated steam. Superheated steam is derived from saturated steam which be heated again through superheater. Temperature of superheated steam must suitable for turbine generator. If temperature of superheated steam is overheated, desuperheater with type spray will sprays amount of water from steam boiler feedwater pump into steam flow to reduce its temperature.

Amount of water will be injected into superheated steam must have high purity, if not it can leave some troubles like deposit on the superheater tubes and can cause erosion on turbine blade. Design spray desuperheater type must include thermal sleve in pipe line desuperheater, to avoid thermal shock by water droplets which are sprayed through the nozzle strike the hot surface of pipe desuperheater.

Fig. 1: Design Spray Desuperheater of Steam Boiler

Design Superheater of Steam Boiler

Superheater is a device for producing superheated steam, typically a group of tubes exposed to heat through which steam is passed. Its purpose is to add additional heat to the steam. Thus a superheater is employed in the steam boiler to add additional energy into steam and raise its temperature. The function of superheater are increase the capacity of the steam boiler, eliminates erosion of the steam turbine, reduces steam consumption of the steam turbine.

Superheater is the tube banks that attain the highest temperatures in a steam boiler and consequently require the greatest care in the design, fabrication, and construction to ensure that the permissible metal temperatures are never exceeded. The final sections of superheater must be placed in the highest gas temperatures, which calls for adopting the most appropriate high-temperature alloy for the tubing from considerations of metal temperatures, fouling due to ash compounds and corrosion due to salts in ash.

The most important aspects of the design of superheater are: uniform distribution of steam and gas across all the sections to minimize unbalance of flows, optimally high steam velocity in all the tubes to keep the metal temperatures as low as possible, and minimum steam pressure losses. Superheater pressure drop is normally limited to ∼8% of outlet pressure to reduce the pumping load.

The superheater is usually drainable along a single loop of an inverted U. As the required

superheat increases or a larger control range is desired, a double inverted U-type superheater may have to be adopted, making the heater nondrainable. The superheater in a package steam boiler is largely convective. The degree of control range is also not as large as in field-erected steam boilers. Occasionally, a loop is pushed into the furnace space to achieve higher temperature.