Storage vessels and their safety aspects

Storage Vessels and their Safety Aspects:

Storage vessels are used to contain chemicals in bulk. Storage tanks to contain 10 tonnes to 10000  tonnes material are constructed.  Oil refineries, petrochemical plants or fertilizer plants have even bigger tanks also. If such tank leaks, breaks or caught in fire, a great hazard is possible. Bhopal tragedy was also arose from a storage tank of methyl isocyanate. When a fire, explosion or gas leak takes place from a bulk quantity, tremendous efforts are required to control the situation and save loss to people, property and environment. Therefore safety of storage tanks need; highest attention.

The main types of storage are :

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1.    Liquid at atmospheric pressure and temperature (i.e. ambient condition).

2.    Liquefied gas under pressure and atmosphere temperature.

3. Gas under pressure and 

4. Refrigerated storage of liquids at low temperature (e.g. ammonia).

Safety Design and Maintenance:

The properties of the material to be stored; proper selection of the material of construction, metal thickness, design, nozzles, connections, pipe lines, fittings, valves, pumps, lining, coating, jacketing, insulation, cladding etc. and colour coding are necessary. 

Dyke or Bund:

This is required to restrict the spread of the leaking material, to safely contain within its periphery and to restrict the surface area of the leaking material in order to reduce its evaporation and for ease of fire fighting or other emergency control activity. The contained material can be safely disposed of or if it is burning, it can be extinguished easily by reaching up to dyke wall.

By partition walls in dyke, reactive chemicals or chemicals of different flash points can be kept segregated. Dyke is always required if the liquid is stored at the atmospheric or low pressure and temperature. Refrigerated chemical when leaks, it remains liquid till it reaches its boiling point. Therefore dyke is required for it. e.g. refrigerated ammonia. While opening dyke discharge valve  of a flammable material, spark should be avoided otherwise fire can travel inside the dyke and catch the tank.

Vent Pipe:

While filling a tank air has to come out. A vent pipe at the top is required for this purpose. It is also useful to depressurise the vessel before opening it. Its diameter should be sufficient for easy escape of the air. Vent pipe may have a bend or roof to prevent outside thing (rain, bird etc.) going inside. Vent pipe should have wire mesh, flame arrester or breather valve if any flammable content is in the tank. They should be kept clean to avoid choking.

Overflow Pipe:

For the overfilling safety a safe overflow pipe is required. Its diameter should be slightly more than the inlet pipe diameter so that the overflow will be maintained if inlet flow is continued. If the diameter of the overflow pipe is smaller than that of the inlet pipe, continuing overflow may reach up to  the top (may enter the vent) and damage the tank if the pressure is excessive. Connection of the ovetorflow pipe should be 2 to 3 inch below the roof joint so that the material will come out (overflow) leaving some air space under the roof and the tank will never be subjected to the filling pressure. The overflow pipe should be extended unto bottom to disallow any free fall of the material and to discharge it in the dyke or any container safely.

Level Indicator:

The quantity (measure) of the material in the tank should be externally visible so that its excessive (unsafe) filling can be avoided, minimum low level can be maintained and an order for next requirement can be placed.

Pressure Relief Devices:

If pressure inside a tank rises due to any reason, it may burst the tank from its weakest part or cause leakage from where it is possible. The content thus coming out is a material loss and in addition, it may create fire, explosion or toxic hazard. Therefore to avoid such situation a pressure relief device is necessary.

Safety-relief valves can be used either as a safety valve or a relief valve, depending on the application. They are used for gas, vapour and liquids.

Difference between Safety relief valves and Pressure relief valves :

Pressure Relief Valve - is the term used to describe relief device on a liquid filled vessel. For such a valve the opening is proportional to increase in the vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually.

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Pressure Safety Valve - is the term used to describe relief device on a compressible fluid or gas filled vessel. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.

Hence Safety valves are used in gas systems, and relief valves are used in liquid systems.

Safety valves open fully once activated, while relief valves open gradually.

A safety valve is a control mechanism that releases the pressure when it crosses a set threshold and then resets back to the original position. e.g. the valve on a pressure cooker or autoclave where you want to keep the contents at/under a specified pressure.

A relief valve is a relief mechanism to instantly relieve the pressure, that is triggered in extreme circumstances when the pressure has suddenly exceeded or is approaching failure criteria for the system itself, when materials approach their yield limit and parts can start breaking. e.g. relief valve on a fire engine when the pressure in the hoses exceeds a limit or in a refinery or chemical plant when there is a fire or system failure.

In oil service, the difference is that relief valves are set at a certain pressure and are set into closed systems designed to relieve pressure out of a closed pipe system. So basically where these were equipped we never had to worry about trapping line systems or worry about thermal expansion because the pressure relief valves would blow off pressure to tankage as needed. 

Fusible plug is a fitting filled with an alloy that melts at a predetermined temperature (not pressure) and gives way to the material to come out. Fusible plugs are used in boilers, domestic pressure cookers and compressed gas cylinders to prevent violent bursting. They are used for gas, vapour and liquids of high temperature.

Fire or Explosion Relief is provided by making the seam between the shell and the roof of the tank deliberately weak so that it may rupture first and the shell stays intact.

Vacuum breaker is also a type of pressure relief device and works like a spring loaded safety or relief valve but in the reversed direction. The outside atmospheric air pressure being higher than that inside the vessel, opens it to break the vacuum. This is required when the vacuum may break increase to collapse the vessel. This device is used for air only.

Flame proof Fittings:

Electric fittings like motors, starters, switches, lamps, tubes etc., near the storage tanks containing flammable materials should be of approved flameproof types and conforming to Indian Standards. Flameproof electric fitting is most essential if the material is having flash point less than 23 °C or if it is to be heated up to or above its boiling point, and also in case of solvent distillation. 

Earthing & Bonding:

Tanks and piping containing flammable substance should have double earthings of appropriate type. Earthing pit should be recharged periodically for easy flow of the current. Resistance should be measured (below 10 K-ohm) and recorded. Pipe joints/flanges should have copper bonding to maintain electric continuity. Flange-guard is useful to divert the leakage downwards.

While loading or unloading flammable liquids or gases, the vehicle (road or rail tanker) should also be earthed during such operation.