Steam temperature is one of the most challenging control loops in a power plant boiler because it is highly nonlinear and has a long dead time and time lag. Adding to the challenge, steam temperature is affected by boiler load, rate of change of boiler load, air flow rate, the combination of burners in service, and the amount of soot on the boiler tubes. After separation from the boiler water in the drum, the steam is superheated to improve the thermal efficiency of the boiler-turbine unit. Modern boilers raise the steam temperature to around F C , which approaches the creep slow deformation point of the steel making up the superheater tubing. Steam temperatures above this level, even for brief periods of time, can shorten the usable life of the boiler. Keeping steam temperature constant is also important for minimizing thermal stresses on the boiler and turbine.
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Page Content. This is achieved through a controlled injection of water into the superheated steam. A secondary attemperator called final stage is often placed after the inter stage temperature transmitter in order to prevent thermal damages to the steam turbine during start-up. The final stage attemperator ensures that the steam temperature upstream the turbine does not rise too fast. The DAM-B is a high performing ring style attemperator with a welded flow profiling liner for superior evaporation and performance.
It complies with all existing standards and is always pressure tested on both the steam side as well as the water side. The DAM-B desuperheater is installed in the steam pipe with a number of water atomizing nozzles attached. The size of the nozzles, their number and insertion length may differ depending on steam desuperheating needs and steam pipe diameter.
The DAM-B is able to handle severe thermal cycling The nozzles receive water from a common spray water pipe encircling the steam pipe, supplying water evenly across the nozzles Spray water flow is controlled via an external spray water valve connected to the DAM water connection piece. This valve is regulated via a temperature control system that uses the downstream steam temperature to determine the amount of spray water needed.
The placement of the temperature transmitter and the installation of the steam pipe are of critical importance for achieving accurate steam temperature regulation.
A flow profiling liner is welded to the inside of the DAM body to improve system turndown and to protect the steam pipe against thermal shock and erosion in the downstream pipe. The atomising spray nozzle is housed inside a nozzle holder inserted into the pipe outlet. Water is routed through the pipe leg and the nozzle chamber before being supplied to the spray nozzle. The nozzle itself has a spring loaded plug which extends as the pressure in nozzle holder increases.
The amount of water being injected by each nozzle is determined by a number of factors, including the diameter of the nozzle body opening, adjustment of the spring, and the pressure differential between the steam pipeline and the water pipeline. The cooling water enters the inner nozzle chamber through a number of water channels. Water is rotated around the nozzle plug thanks to the special arrangement of the water channels. The plug and the seat are designed to create maximum water velocity at the nozzle edge point.
The high velocity of the water when it leaves the nozzle guarantees fine atomisation, quickly evaporating the spray water. In order to maintain a specific opening water pressure inside the inner nozzle chamber, the nozzle plug is preloaded by a spring. The force required to open the nozzle is set by the adjustment nut.
As the nozzles spray perpendicular to the steam flow, the high relative velocity of water to steam creates an efficient secondary level of atomisation. Alternatively, nozzles and springs can be made available in inconel, suitable for high temperature applications and conditions without water injection.
The example below shows two attemperators 3 and 4 installed in a desalination plant, controlling the steam turbine input temperature. More attemperators can be used for lower pressure stages if reheating is performed. As the DAM-B attemperators are mechanically atomising, they do not require high pressure atomising steam to vaporise the water droplets. Steam that can instead be used for the desalination plant.
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DAM-B: Steam Attemperator
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