Flashcards in Objective 6: Chemical Feed Systems Deck (22)
What is the current status of using Shot & Pot Feeders?
Chemicals are injected over a relatively short period of time using a shot feeder assembly, usually in the form of a closed pot attached to the feedwater line, wherein the chemical is transported into the boiler by exposure to the feedwater.
How are chemicals added in a Closed Bypass Feeder?
The chemicals, in solid briquette form, are placed inside the pot and the feedwater flow is directed through the pot, dissolving the chemical and carrying it to the boiler. The rate of injection can be crudely controlled using a regulating valve on the feedwater line which restricts flow, forcing more flow through the pot as desired.
How are chemicals added in a Closed Gravity Feeder?
Liquid chemical is poured into the pot and the the regulating outlet valve will determine how fast the chemicals feed, by gravity, into the feedwater line.
How is chemical fed into boilers in a large majority of industrial boilers?
Continuous feed system
Continuous Chemical Feed System
Chemicals are fed to the boiler system at the most efficient injection points and at a steady and continuous rate.
What are the advantages of Chemical Continuous Feed?
1. Greater flexibility in controlling residuals and less chance of losing positive residuals, thus less chance of scale and corrosion
2. Reduction in large swings in residuals and boiler solids, making blowdown control much easier and improving overall efficiencies
3. Safer handling of chemicals
What are Day Tanks made of?
Stainless steel, rubber lined, or special plastic or fiberglass materials
How are Day Tanks used in Chemical Continuous Feed Systems?
1. The chemicals (liquid or powder) are added to their respective day tanks by the operator
2. The remainder of the tank is filled with water
3. A mixer in each tank is used to ensure a complete and consistent mixture
4. Each tank has one or two dedicated pumps, complete with pressure gauges, isolating valves, and check valves
5. Normally one batch of chemical is mixed per day in each tank to maintain a constant concentration
What is a disadvantage of using Day Tanks for Continuous Chemical Feed Systems?
The difficulty in maintaining a consistent concentration in each tank and confusion raised amongst different operators
How can confusion be reduced among different operators operating Day Tank Continuous Chemical Feed Systems?
Using a table of tank level vs. chemical addition. The higher the level in the tank, the less chemical would be added.
Which Continuous Feed System is better?
Pump Tank Continuous Feed System
Each chemical is pumped to its required location in pure form from pump tanks.
Describe the construction of Pump Tanks
Each chemical is kept in a stainless steel pump tank from which the pump takes direct suction. A gauge glass on the pump tank allows the operator to ensure there is always a level in the tank and to initiate refilling the tank, when necessary, from a supply tank. Each tank has at least two pumps.
How is residual control improved in Pump Tank Continuous Feed Systems?
The operator need only adjust the pump flow without also being concerned about chemical concentration.
What are the 2 most common designs of Chemical Feed Pumps?
1. Reciprocating, Plunger-type
2. Electronic Impulse-type
Which of the 2 most common designs of Chemical Feed Pumps allows for fine control of chemical flow?
Describe the construction of Plunger Pumps
Has one or more reciprocating plungers, driven by an electric motor, through an adjustable control box with a graduated scale, with the ability to change the length of the pump stroke.
How does one increase the flow from a Chemical Pump
Increase the stroke
At what pressures are plunger pumps useful?
All pressures, especially when injecting chemical directly into a boiler
Electronic Impulse Pumps
Use a moving diaphragm to push the chemical into the feed line