Boiler Safety Valve

Boiler Safety Valves:

One of the most critical automatic safety devices in a steam system is the safety valve. It protects lives, equipment and property from potentially dangerous levels of temperature and forces caused by excessive steam system pressure.
The main purpose for a safety valve is to prevent the pressure in a system to exceed certification pressure.
Above certification pressure no one can guaranty the systems safety - and especially for a steam system with a very hot gas with a huge amount of latent heat the consequence with a failure can be dramatically.

Design:

The size of a safety valve depends primarily on the maximum boiler output and the operation pressure of the system. The safety valve must as minimum have the evacuation capacity of all the vapor the boiler can produce running at full power at the working (or certification) pressure.

1. For a higher pressure the steam is compressed and requires less volume and the size of the valve can be reduced
2. for a lower pressure the steam is expanded and requires more volume and the size of the valve is increased
3. Although the principal elements of a conventional safety valve are similar, the design details can vary considerably. 
4. The DIN style valves (commonly used throughout Europe) tend to use a simpler construction with a fixed skirt (or hood) arrangement whereas the ASME style valves have a more complex design that includes one or two adjustable blow down rings. 
5. The position of these rings can be used to fine-tune the over pressure and blow down values of the valve.
6. For a given orifice area, there may be a number of different inlet and outlet connection sizes, as well as body dimensions such as centreline to face dimensions. 
7. Furthermore, many competing products, particularly of European origin have differing dimensions and capacities for the same nominal size.

Safety Valve Operation:

1. When the inlet static pressure rises above the set pressure of the safety valve, the disc will begin to lift off its seat.
2. However, as soon as the spring starts to compress, the spring force will increase.
3. This means that the pressure would have to continue to rise before any further lift can occur, and for there to be any significant flow through the valve.
4. The additional pressure rise required before the safety valve will discharge at its rated capacity is called the over pressure.
5. The allowable over pressure depends on the standards being followed and the particular application.
6. For compressible fluids, this is normally between 3% and 10%, and for liquids between 10% and 25%.
7. In order to achieve full opening from this small over pressure, the disc arrangement has to be specially designed to provide rapid opening.
8. This is usually done by placing a shroud, skirt or hood around the disc.
9. The volume contained within this shroud is known as the control or huddling chamber.
10. As lift begins and fluid enters the chamber, a larger area of the shroud is exposed to the fluid pressure.

Safety Valves Regulation:

1. Each boiler (including exhaust gas boiler) and steam generator is to be fitted with at least one safety valve and where the water-heating surface is more than 46.5 m2 (500 ft2), two or more safety valves are to be provided. The valves are to be of equal size as far as practicable and their aggregate relieving capacity is not to be less than the evaporating capacity of the boiler under maximum operating conditions.
2. In no case,

- The inlet diameter of any safety valve for propulsion boiler and superheaters used to generate steam for main propulsion and other machinery to be less than 38 mm (1.5 in.) nor more than 102 mm (4 in.).

- For auxiliary boilers and exhaust gas economizers, the inlet diameter of the safety valve must not be less than 19 mm (3/4 in.) nor more than 102 mm (4 in.).

3. In all cases, the safety-valve relieving capacity is to be determined on the basis of the boiler heating surface and water-wall heating surface along with the fuel-burning equipment.

SAFETY VALVE   - PRESSURE ACCUMULATION TEST:

1. Safety valves are to be set under steam and tested with pressure accumulation tests in the presence of the Surveyor.
2. The boiler pressure is not to rise more than 6% above the maximum allowable working pressure when the steam stop valve is closed under full firing condition for a duration of 15 minutes for fire tube boilers and 7 minutes for water tube boilers.
3.During this test, no more feed water is to be supplied than that necessary to maintain a safe working water level.
4. The popping point of each safety valve is not to be more than 3% above its set pressure.

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References: 

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