Emerson Process Management

Deaerator Level Control Valve (DALC)

Application Discussion
AD103
June 15, 2003

In order to remove corrosive gases entrained in boiler feedwater an open feedwater heater called a deaerator is incorporated into the power cycle. The deaerator is designed to heat the incoming feedwater to the saturation point in order to reduce the solubility of any entrained gases. These are mainly oxygen, carbon dioxide and ammonia that become very corrosive at elevated temperatures. Since main steam temperatures can reach as high as 1050 degrees, it is necessary to remove these gases to protect piping and associated equipment.

Figure 1 shows the cross section of a typical deaerator. In this design, the incoming feedwater is pumped from the condensate pumps through the low pressure heaters. The water is sprayed in a fine mist into the area that contains the steam drawn from the high pressure heater drain and extraction steam from the cold reheat section. This divides the water droplets increasing the overall surface area therefore increasing the overall heat transfer. The water is then passed over a set of trays that separates the water into thin sheets from which the gas can easily escape. The gases are then vented out of the deaerator to the atmosphere.

Figure 1: Cross Section of Deaerator

The deaerator is located as high as possible in a plant to provide the suction for the boiler feedpumps. This can be as high as 60 feet in combined cycle plants and 120 feet in conventional fossil fuel plants. By placing the deaerator as high possible the need for booster pumps in conjunction with the boiler feedpumps is eliminated.

In order to ensure mixing in the deaerator and the elimination of corrosive gases the level of incoming feedwater and steam must be properly controlled. The steam is controlled through the heater drain valves and the extraction steam valves. The water level is controlled by what is commonly referred to as the Deaerator Level Control Valve or DALC valve. Location of the valve varies by plant design, but is most commonly found in the lower levels of the plant. Figure 2 shows a common layout including the condensate pumps and LP heaters.

Figure 2: Process Flow Diagram Including DALC Valve and Associated Equipment

This is a very demanding application since the valve service conditions vary with plant load. During initial startup, the condensate pumps are operating at high pressures and minimal flows with minimal downstream pressure since the deaerator has not built up any pressure. This causes nearly a full pressure drop to be taken across the DALC valve. As the unit load increases, the condensate pumps can’t maintain the same pressure, therefore the inlet pressure to the valve decreases. At this point, the pressure in the deaerator has built up causing a minimal pressure drop across the valve. These varying conditions require a valve that possesses a large amount of turndown.

To address this application, the initial conditions must be tackled first. As the inlet pressures to the valve can vary between 200 – 600 psig and the valve is required to withstand nearly a full pressure drop, the occurrence of cavitation is possible. Cavitation, the formation and subsequent collapse of vapor bubbles in liquid flow streams, is a major source of damage in control valves and adjacent piping.

As the unit load increases, the outlet pressures increases to anywhere between 50 and 300 psig. During this mode of operation, cavitation is not occurring and a large amount of flow is required.

Fisher recommends the use of a characterized Cavitrol III trim in the appropriate valve body to address this application. The Cavitrol trim utilizes a staged pressure drop to eliminate the formation and subsequent vibration and damage associated with cavitation. Cavitation protection is provided to address operation when the unit first comes on line where low flow rates are accompanied by high pressure drops. As the pressure drop through the valve decreases and flow demand increases, the characterized trim transitions from an anti-cavitation mode to a flow control mode. This is accomplished by small holes in the lower area of the valve trim and larger holes in the upper portions of the valve trim for unrestricted flow capacity.

As with any severe service application, tight shutoff is recommended. This protects the valve from damage when the valve is closed and the condensate pumps are operating. Tight shutoff also helps to preserve treated water during this mode of operation. Any leakage will cause a change in the deaerator level. High deaerator levels typically result in treated water being dumped to a waste water system thus wasting money on costly chemical treatment.

 

 

Product Bulletin: Fisher Cavitrol III One-, Two-, and Three-Stage Trims

 

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