Many different sizes and types of tanks may be used to
store 2-EHN. Heat must be allowed to dissipate.
In most respects, vertical tanks are the most
practical overall solution. The low auto-ignition temperature (130°C) of 2-EHN
can lead to an air-vapour explosion in the headspace of vessels, which can
rupture, spilling the contents. Vapour-air explosions release less energy per
unit volume than those resulting from self-reaction of the liquid. Peak blast
force is a key design criterion. Vertical tanks can be fitted with a frangible
roof to minimize damage in case of a pressure blow-out. API 650 is a widely
used standard that can serves as a reference for specifying such tanks.
Vertical tanks are also easier to configure with water deluge systems.
Horizontal tanks are used for the storage of 2-EHN. However, they have no ‘roof’, so the fitting of a frangible roof is not an option. If the contents are heated above 100°C, then there is a risk of tank failing at the dished ends, thus forming a « missile » with potentially catastrophic effects. Safety principles should be strictly applied to prevent heating of the product.
A fire in the storage area will heat the storage
tanks. A fixed cooling-water deluge system to supply cooling water in event of
fire is mandatory to minimize this risk. API 650 standard indicates that a
system that delivers a cooling-water flow rate of at least 15 litre/minute/m2 of storage tank surface area can achieve effective
cooling. If this volume of water is not continuously available for cooling
purposes, additional non-insulating fire cladding of the tank walls could be
considered within the context of the overall risk assessment of the storage
Risks of tank bursting under pressure should be minimized.
The principle here is to protect the
product from heat.
A deluge system provides the best protection against
product overheating. No heating system of any kind should be installed, and
existing heating systems must be permanently disabled. When laying out the
route for new pipe work to carry 2-EHN, the designer should avoid sources of
heat and potential fire. When using existing pipe work facilities, the designer
should ensure that heated pipes are not used for 2-EHN.
As a general principle, locating 2-EHN storage tanks in an open area, away from inhabited buildings, is recommended. The site should be remote from possible fire hazards to minimize their exposure to external heat and fire impingement if fire breaks out. The extent of this separation is a local decision to be determined by the site risk assessment. For example, the NFPA 30 standard may be used to help determine the appropriate distance from other storage tanks and equipment, to maintain protection in case of fire.
Firewalls between the tanks will improve thermal protection.
If sufficient space is not available for firewalls, then non-insulating
tank-wall fire cladding may provide additional protection. Screening walls and
non-insulating fire cladding may be combines to achieve acceptable protection.
The better the protection, the longer the stored 2-EHN will endure external
heating, and the lower the risk of thermal decomposition within the storage
system. Instrumented fire and heat detection systems should be installed.
Bulk storage vessels should preferably be vented directly into the atmosphere, far from ignition sources, if local regulations permit. Conservation vents with frangible roof seams are also acceptable. For smaller equipment, standard engineering practices for design of emergency discharge should be followed. Tanks containing 2-EHN should preferably be at atmospheric pressure. The vent outlet is to be positioned in a safe area, sufficiently high and far from ignition sources.
Containment wall or bunding
To minimize the consequences of a spill and leakage
into the environment, a containment wall (bund) should surround tanks, with a
minimum capacity to handle tank contents and deluge water.
Ensure good ventilation during drumming / de-drumming. Filled drums are to be stored far from heat sources and other flammable products, and protected by firewater. Special care should be taken when opening drums, as they may be pressurized.
Materials of construction
Proper selection of construction materials for 2-EHN service is
essential to ensure the integrity of the handling system and to maintain
product quality. Although stainless steel tanks are always preferred, 2-EHN may
be stored in mild steel tanks, provided they are kept free from water bottoms
through regular maintenance.
Mild steel in general is a material that requires careful consideration
for use in a 2-EHN handling system. 2-EHN, in the presence of water bottoms,
can hydrolyze slowly to form nitric acid, which can increase the corrosion rate
of mild steel. For this reason, stainless steel is the preferred material for
storage of 2-EHN.
Galvanized steel, copper and copper bearing alloys are unsuitable for any 2-EHN service. Special care should be taken when selecting such items as pumps and valves, to ensure that no copper alloys (e.g. brass or bronze) are used in bearings or other internal components that may come in contact with the product.
Elastomers and gaskets
2-EHN is an excellent solvent which can degrade the
performance of some seals and gaskets. Careful selection is therefore
Piping / Lines / Hoses
Use non-insulated mild steel or stainless steel (SS) piping.
Any steam or electrical tracing must be physically
Wherever possible, dedicated lines for 2-EHN are to be
preferred to avoid safety or environmental problems.
Experimental fire testing, run by Shell, of gantry-type pipe work filled with an additive containing approximately 70% of 2-EHN, showed that a pressure relief valve set at 10 bar was sufficient to relieve pressure caused by the self-heating accelerated decomposition of 2-EHN. The pressure relief valve should discharge to a safe location.
SS full-bore ball valves are preferable. Traditional ball, gate and butterfly valves may also be used. SS, cast iron and cast steel are all suitable materials. Copper, Zinc and its alloys, aluminium and most plastics are inadequate or incompatible materials.
Product sampling is a potential source of personnel exposure to 2-EHN. Design and procedures should be developed to minimize personnel and environment exposure to 2-EHN.
Product handling: loading, unloading, pumping
Product handling is a potential source of personnel exposure to 2-EHN. Design and procedures should be developed to minimize personnel and environment exposure to 2-EHN.
Use a dedicated loading arm. Control static electricity. If a multi-compartment tank wagon is loaded, ensure 2-EHN is not shipped adjacent to heated cargoes. Ensure proper electrical grounding and electrical continuity in all facilities.
Use a dedicated hose. Control static electricity. Avoid manifolds to prevent accidental ingress of 2-EHN into heated lines.
2-EHN is a self-reactive substance. In the absence of air, it can decompose in the bulk liquid phase if heated. This occurs most commonly under pressure in a blocked or dead-headed pump, or other sealed systems, and can lead to violent bursting of the equipment. The principle here is to use equipment that does not have the potential to heat the product. Pumped transfer of 2-EHN should always be done under controlled conditions, and all transfer valves must be opened before pumps are started. Do not pump 2-EHN against a closed outlet: this may heat the product within the pump, depending on the type of the pump.Pneumatically powered diaphragm pumps provide an inherently safe and reliable means of pumping 2-EHN.Centrifugal pumps are not as inherently safe as pneumatic diaphragm pumps for 2-EHN service. In any case, extra control systems are required.Gear pumps and other positive displacement systems can heat 2-EHN rapidly when the pump outlet is closed or blocked. Consequently, their use is not recommended.
Safety pump trip controls and instrumentation must be fitted with:
A temperature trip set to stop the pump at a product temperature of 50-60°C. In exceptional circumstances, specific operations may require a higher temperature trip set up to a maximum of 75°C, subject to detailed expert risk assessment in conjunction with your supplier.
A pressure switch to stop the pump if the outlet is blocked or closed; and/or
A low flow switch to stop the pump if the outlet is blocked or closed.
To prevent the risk of heating the product, closed
circuit pumping must be avoided, including that through pressure relief valves
when the pump outlet is closed or blocked.
The majority of incidents involving 2-EHN have been due to overheating of pumps, resulting in bursting of the equipment. Therefore, careful consideration of the siting of pumps needs to be made, and the consequences of pump failure fully assessed.
Tank to tank transfer
Tank to tank transfer of 2-EHN can be safely carried
out with nitrogen or air padding, while using a pneumatic driven pumping system
or pumps as described above.
Inadequate cleaning of equipment or pipe work
introduces a risk of environmental contamination and a higher potential for
decomposition of 2-EHN residues.
A specific procedure should be developed by skilled
personnel that describes health and environmental hazards, as well as
temperature limits, to ensure that cleaning operations are conducted in an
effective and safe manner.
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