Objective 4: Blowdown Flashcards
What is an important objective in the operation of most boilers?
Efficient control of boiler blowdown
Why is efficient boiler blowdown important?
Heat loss results in a reduction in operating efficiency
Simple Blowdown System
Blowdown that is totally dependent on the operator, both continuous and manual.
Is any heat recovered from simple blowdown?
No
Explain the process of Simple Blowdown
Manual and continuous blowdown lines discharge into a blowoff tank. The pressure of the water drops to almost atmospheric in the tank and part of the water flashes into steam. The steam vents to atmosphere while the water drains to sewer.
What determines how much throttling an operator manually applies to Continuous Blowdown in a Simple Blowdown System?
Conductivity tests
What is problematic about an operator manually adjusting the continuous blowdown valve in response to conductivity tests?
Conductivity limits can be overshot, esp. if feedwater conditions change and boiler solids fluctuate. Therefore it is very easy to have excess blowdown which is inefficient.
What is a miscellaneous problem with wide fluctuations in blowdown rates?
Erratic solid levels in the boiler result in erratic residuals of phosphate, sulphate, etc. which leads to confusion over the amounts of chemicals that should be added on a daily basis.
Heat Recovery Blowdown System
Blowdown system where heat is recovered from the blowdown water and its flash steam
Explain blowdown through a Heat Recovery Blowdown System
Manually controlled continuous blowdown is directed into a flash tank. The flashed steam is removed from the top and sent back to the deaerator, thus reducing the amount of make-up steam that is req’d from the steam system into the deaerator. The blowdown water is drained, via a level control, through a heat exchanger and then to a sewer. Condensate, returning from the deaerator, passes through the heat exchanger where it absorbs heat from the blowdown water, thus cooling the blowdown considerably.
Is Manual Blowdown also directed through a flash tank in a Heat Recovery Blowdown System?
Usually no, as it is short-duration and high-flow, so it is usually directed to a normal blowoff tank.
Automatic Blowdown System
Adjustment of Continuous Blowdown is automatic, resulting in a very steady concentration of dissolved solids, even as feedwater quality changes during the day.
Which Blowdown System is the most efficient.
Automatic Blowdown System, when combined with heat recovery.
How does Automatic Blowdown work?
Continuous Blowdown Line is fitted with a modulating control valve. A sample of boiler water is continuously drawn off, upstream of the control valve, and is sent through a sample cooler. It then enters a measuring cell where the conductivity of the water is continuously detected and the result transmitted to a conductivity controller. The desired conductivity as determined by the operator and input as the setpoint which the controller adheres to.
