Question

Consider the following boiler hazards:

1- High steam pressure in the boiler exceeding design limits

2- High steam temperature in the boiler

3- Low water level in the boiler

4- High water level in the boiler

5- High pressure of hot gases inside the furnace

6- Low pressure of hot gases inside the furnace

7- Flammable and toxic gases build up in flues

8- Explosive gas mixture in the furnace or flues

Consider each of the above hazards separately. For each of the hazards give the following details:

a) Suggest the suitable safety control function

b) Draw a simple block diagram of this control function using manual drawing tools or computer software (do not copy and paste the images, no freehand drawings)

c) Explain the working of the above control function in your own words

d) Explain how the above control function protects against the respective hazard

(Five marks for each hazard. Total marks for practical: 40)

Answer 1

Solution to 1: High steam pressure in the boiler exceeding design limits

a) A safety valve such as a pressure relief valve (PRV) can be used to release steam automatically when the pressure exceeds the design limits.

b)

c) A pressure relief valve for a boiler consists mainly of a spring whose stiffness is meticulously designed according to the desired pressure in the system. In some valves there is even a screw to adjust the operating pressure, which works by increasing the compression on the spring. When the pressure of the steam upstream of the valve exceeds the desired pressure (known as the lifting pressure) , the valve lifts up to release the high pressure steam. The valve remains open till the pressure inside the system falls back to the design pressure (below the lifting pressure); an closes once equilibrium is achieved. Usually, the lifting pressure is slightly higher than the design pressure to prevent the repeated rapid opening and closing of the valve and thereby getting damaged.

d)A safely valve, prevents the buildup of high pressure inside the boiler by acting as the path of least resistance, when the pressure exceeds a certain level. In absence of this control system, boiler could even explode.