Safety of Pressure Vessels and Pressure System

 

Pressure Vessels

A pressure vessel consists mainly of a series of sheets of metal suitably shaped and welded together. They are designed to contain stored energy above atmospheric pressure. Pressure vessels, typically steam boilers and air receivers, are inherently potentially dangerous items of plant and there is a long and tragic history of accidents associated with them.

-Typically injuries relate from:

The impact from the blast of an explosion or release of compressed liquid or gas.

Impact form parts of equipment that fail or any flying debris.

Contact with the released liquid or gas, such as steam, and fire resulting from the escape of flammable liquids or gases; and

Oxygen depletion or enrichment.

-The typical causes of pressure system failure are:

• Poor equipment and or system design.
• Poor maintenance of equipment.
• An unsafe system of work.
• Operator error, poor training, and supervision.
• Poor installation: and
• Inadequate repairs or modification.

Steam Boilers

The purpose of a steam boiler is to produce steam under pressure from raw materials (fuel, air and water).

The potential heat is made available through combustion, and this is transmitted to and stored by water vapor.

Installations and fittings

To ensure maximum safety of operations, the following installations and fittings are recommended for all pressure vessels, in particular steam boilers:

Water Gauges

This is a gauge that shows the water level within the steam boiler and will provide information that the water level is too low leading to failure. Water gauges should be situated so that the operator can see the minimum working level. Tubular gauges should be protected.

Safety Valves

To prevent overpressure the safety valve should be designed to be capable of discharging the total peak evaporation of the boiler. They should be so positioned or arranged to discharge safely and to prevent tampering with settings.

Pressure Gauge

The maximum permissible working pressure should be clearly marked on the gauge.

Blow Down Valve

A blowdown valve has three important functions: the removal of settled boiler water solids (salts etc) content; ejection of sludge and solids precipitated from the boiler water which settles at the bottom of the boiler; and emptying the boiler prior to inspection

Stop Valve (Steam)

Located between the boiler and steam pipe or outlet, used to control the flow of steam from the boiler.

Feed Check Valve

Is essentially a non-return valve to prevent water escaping from the boiler should the pressure in the feed pipe be less than that in the boiler.

Anti-Priming Pipe

Priming is the phenomenon whereby water is carried over from one part of the boiler into another part, the action is mainly siphonic in nature. To prevent the loss of water which would result, an anti-priming pipe is fitted to break this siphonic action.

Automatic Controls

The majority of steam boilers are now automatically controlled, and the most common water level and firing controls are float operated controls situated outside the boiler.

Standards for automatic controls are:

• To assist the boiler attendant who constantly supervises the boiler; and
• Intended to replace continuous supervision with occasional supervision.
• The minimum recommended requirements for automatic controls for boilers not continuously supervised are:
• Automatic water level controls-to effectively maintain predetermined water levels.
• Automatic firing controls-should control the supply of fuel to the burners and shut off the supply in the event of:
• Flame/pilot light failure.
• Failure to ignite fuel.
• Safety valve pressure is reached.
• The water level falls.
• Failure of induced draught fans or flue damper; and
• Independent overriding control-should cut off the fuel supply.

Electrical failure to safety-faults in circuits should cause fuel and air supply to shut off. Positive means requiring manual resetting should also be provided if there were to be a failure of the electrical supply.

 

Hazards Associated with Boiler Operation-

Overheating caused by Low Water Level

The main causes are:

Lack of testing and maintenance controls; and

Inadequate standards of controls.

Over-Pressurizing Caused by Pressures in Excess of Vessel Capacity

The main causes are:

Failure of pressure relief devices.

Operator error; and

Inadequate design and lack of information on safe working pressure.

Corrosion

The effects of vessel corrosion are:

Wastage (loss of metal thickness and strength).

Grooving (a form of mechanical corrosion, due to expansion and contraction and accelerated by a buildup of solids).

Distortion (a situation where excessive scale is allowed to buildup on surfaces);

Soot, leading t sulphuric acid when moist.

These conditions can result in explosion or failure of the system.