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Control systems Desuperheaters
Other considerations (continued) Cooling water supply Typical cooling water supply options are as follows: - Boiler feedwater (BFW) (taken from the pressure side of the boiler feedpump). - Demineralized water. - De-ionized water. - Condensate. Town’s water or process water may also be used, but depending on hardness, salts may be deposited on the inside of downstream pipework and the face of valve seats and plugs. Cooling water quality The quality of the injected water is important. The TDS (Total Dissolved Solids) of the injection water should be as low as possible since all these solids will come out of solution and be deposited on the faces of valves and could block up the small orifices in the desuperheater nozzles. Cooling water temperature Generally, the hotter the better. This is because hot droplets need to absorb less heat to reach their flash temperature than cold ones. Hence, hot droplets will evaporate more quickly, producing a more efficient desuperheating process. Using hot water also has the additional advantage that smaller amounts of water will fall onto the inside walls of the pipework. Because of the benefits of using hot water, it is logical to insulate the water supply pipes to minimize heat loss. Cooling water pressure and flowrate In order to inject the cooling water, its pressure at the desuperheater nozzle must be equal to or greater than the operating steam pressure in the pipe. The requirement varies from one type of desuperheater to another, but typical minimum values are: - Spray type desuperheater steam pressure +8 psi (+ 0.55 bar) - Venturi type desuperheater steam pressure +1.5 psi (+ 0.1 bar) - Steam atomizing type desuperheaterequal to steam pressure For the spray and Venturi type desuperheaters, the highest water inlet pressure required will be at the highest cooling water flowrate. It should be noted that the water flowrate is a function of the square of the pressure difference between cooling water and the steam. So if the water flowrate is to be increased by a factor of 4 for example, then the pressure difference must increase by a factor of 42 = 16. This is the reason why it is important not to over-specify the turndown as high cooling water pressures are quickly reached (especially with spray type desuperheaters). If an independent or booster pump is used, a recirculation loop will be required to ensure that there is always flow through the pump. Cooling water control valve A pressure drop will be required over the water control valve. We have already said that ideally the water should be as hot as possible so care is needed to ensure that flashing conditions do not exist across the control valve. Superheated steam pressure control It is desirable that a constant steam supply pressure be maintained. The temperature of the steam after the desuperheater controls the amount of water added. The higher the temperature, the more the control valve will open and the greater the amount of water that is added. Usually the target is to reduce the steam temperature to within a small margin of saturation temperature. In virtually all applications the upstream pressure will be controlled and constant, however, if the superheated steam supply pressure is increased, the saturation temperature will also increase. The set value on the Controller will not change, and an excessive amount of water will be added as the control system tries to achieve the set temperature. This would result in very wet saturated steam with its resulting problems. Control In this document we have frequently used the term 'turndown' to describe the performance of the different types of desuperheater. However, as far as an installation is concerned, it should be remembered that the desuperheater is only one element of a desuperheating station. Obviously, if the controls that are fitted have lower turndown than the desuperheater, then the turndown of the desuperheater station will be reduced. For example, in a particular pressure reducing/desuperheating station, the rangability of the cooling water valve may not be as high as the desuperheater. In this case it will be the rangability of the water control valve that limits the turndown of the desuperheating station. Separator station In applications where there must be no moisture present in the resulting steam (such as prior to a turbine for example) it is recommended that a separator is installed downstream of the desuperheater. This will protect downstream pipework and equipment from the effects of moisture in the event of a control system failure or abnormal operating conditions, for example at start-up. The separator must be located after the temperature sensor thereby giving the water droplets as much time as possible to evaporate.
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TI-P475-06 CTLS Issue 4
Page 6 of 12
Desuperheater Online Program Sizing Guidance
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