Blowdown control systems

Other Discharge Sources Including Process Water Treatment, Air Pollution Control Systems and Compressor Blowdown... 

All these methods will require careful monitoring initially to set up and determine the correct rate of blowdown once the plant is operating. In order to take the necessary sample from the boiler the boiler(s) should be fitted with a sample cooler. To automate the continuous blowdown a conductivity-controlled system may be installed. Here a controller continuously compares the boiler water electrical conductivity with a value set in the controller. Depending on whether this is above or below the set rate, it will automatically adjust the blowdown flow rate. 

The ISOM operators were troubleshooting operations when they received, via radio, the first notification that the blowdown dram was overflowing. In response operators used the computerized control system to shnt the flow of fuel to the heater, while the other operators left the satellite control room and ran to redirect traffic away from the blowdown drum [15] (see Fig. 4.6). 

Fission product noble gases entering the water-steam circuit in the event of a tube leak are completely volatilized and transported with the steam to the main condenser where they are extracted and released via the off-gas stack. This release is monitored by a continuously operating detector device located in the condenser off-gas line. Non-volatile fission and activation products which are transported over the leak to the water-steam circuit remain completely in the water phase of the steam generator by the action of the blowdown purification system their activity concentration is kept at a level which is controlled by the injection rate on the one hand and by the purification rate on the other. Because of the very low vapor pressures of these elements and their chemical compounds (dissolved ions or insoluble oxides/hydroxides), their transport to the steam under the prevailing conditions (270 °C, 7 MPa) is only possible by droplet entrainment. This means that partitioning between liquid and steam phases is proportional to the steam moisture content, which is usually well below 0.1%. 

PC 8 Steam generator overfill due to control system failure and combined primary and secondary blowdown... 

The isolation valves between the reactor coolant system and the chemical and volume control system are active valves that are designed, qualified, inspected and tested for the isolation requirements. The isolation valves between the reactor coolant system and chemical and volume control system are designed and qualified for design conditions that include closing against blowdown flow with full system differential pressure. These valves are qualified for adverse seismic and environmental conditions. 

Special design requirements, such as pressure relief device requirements, provisions for eductor tube installations, or compressed gas valves used in fire control systems which have high blowdown flow requirements, may dictate greater diameters. 

The next step is to apply a number of loss control credit factors such as process control (emergency power, cooling, explosion control, emergency shutdown, computer control, inert gas, operating procedures, reactive chemical reviews), material isolation (remote control valves, blowdown, drainage, interlocks) and fire protection (leak detection, buried tanks, fire water supply, sprinkler systems, water curtains, foam, cable protection). The credit factors are combined and appHed to the fire and explosion index value to result in a net index. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

Offshore oil platforms are highly automated, requiring little direct operator input to maintain production. In the event of a serious abnormality such as a fire or a gas escape, the control room worker is required to make decisions as to whether to depressurize one or more systems and which systems to blowdown. Other workers have the facility to depressurize some systems at a local control room. 

The human factors audit was part of a hazard analysis which was used to recommend the degree of automation required in blowdown situations. The results of the human factors audit were mainly in terms of major errors which could affect blowdown success likelihood, and causal factors such as procedures, training, control room design, team communications, and aspects of hardware equipment. The major emphasis of the study was on improving the human interaction with the blowdown system, whether manual or automatic. Two specific platform scenarios were investigated. One was a significant gas release in the molecular sieve module (MSM) on a relatively new platform, and the other a release in the separator module (SM) on an older generation platform. 

The major finding of the study was that the manual blowdown philosophy, particularly with respect to gas situations, was not clearly defined. This was most apparent in the offshore attitudes and perceptions regarding the importance of blowdown as a safety system. No decision criteria specifying when blowdown should or should not be activated were provided for the support of control room staff. Blowdown was essentially left to the discretion of the workers. Consequently, the offshore interpretation of this vagueness and ambivalence amounted to a perceived low priority of blowdown. It was concluded that this percephon would probably lead to a significant delay in blowdown or possibly the omission of blowdown when it was actually required. 

Blowdown on a boiler is mandatory. On small boilers, the required operation of the main blowdown valve may be sufficient to control the quality of water within the boiler. On medium and large plants, additional systems are employed. 

The third and most automatic system is the conductivity-controlled blowdown. This constantly measures the level of solids in the water and instigates an automatic variable blowdown on a continuous or intermittent basis. 

One of several different types of BW blowdown systems that automatically controls the frequency and duration of the BD period. Some systems provide continuous BD. 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

Once calculations are completed on a depressurization system it will become readily apparent high volumes of gases will be flowing through the header to a flare. In some cases the practicality of simultaneously depressurizing all of the process equipment and vessels will be difficult to accomplish. In these cases a sequential blowdown of the vessels should be considered. Providing for the "worst" vessels first or controlling the system to blowdown the area most affected first are desirable options. 

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