Questions

3.17 Automatic Level Control Systems

1 Which one of the following statements is true of on / off control of a steam boiler ?
a) Two capacitance level probes are required
b) Boiler pressure is reduced when the pump operates
c) An electrical frequency controller is required on the pump
d) It is unable to operate effectively against varying boiler pressure conditions
2 What is the purpose of the control valve after a feedpump with a modulating boiler level control system ?
a) For isolation purposes
b) To maintain the pump outlet pressure
c) To modulate the water flow
d) To modulate the water pressure
3 What is the purpose of the water recirculation line on the outlet of a pump on a modulating boiler control arrangement ?
a) To protect against the pump overheating
b) To prevent pump cavitation
c) To regulate water flow from the pump
d) As an indication that the pump is delivering
4 Which of the following is a disadvantage of a single element level control ?
a) It requires a pump with a variable speed drive for accurate level control
b) A low steam demand can result in low water level lockout
c) A high steam demand can result in high water level lockout
d) The water level must fall for the feedwater valve to open
5 Which of the following is an advantage of two element control over single element control ?
a) A variable speed drive pump is not required
b) Steam demand has little effect on water level
c) Only one conductivity level probe is required
d) Steam flowrate can be adjusted in accordance with the prevailing water level
6 A three element water level control:
a) Is controlled by the water level probe and a steam flowmeter
b) Controls the number of pumps operating at any one time in a multi-boiler installation
c) Makes it unnecessary to re-circulate water after the feedpump
d) Caters for changes in feedwater pressure