APPENDIX 4: DESIGN AND CONSTRUCTION OF TANKS

General Tank Design Aspects

In addition to the precautions for flammable liquids, the maintenance of a uniform temperature, preferably below 25°C, is important. The internal surface of the tank, including safety devices, must be smooth to avoid adhesion of condensed styrene and thus build-up of polymer. For emergency situations, facilities must be provided, both to adjust the inhibitor content and to inject air or oxygen into the pump suction and/or circulation system. The use of inert gas implies that styrene vapours can be captured for treatment or recovery, but also that with TBC as polymerisation inhibitor, in line with a minimum of 10-15 ppm in the liquid phase, the oxygen content must in theory absolutely be maintained above 4-5%-vol. (note: when stored under air, the oxygen solubility in styrene is about 50 ppm; see Table 2). In other words, if pure nitrogen or other inert gas is used, air or oxygen must be injected into the liquid.

Because at higher doses TBC can form coloured complexes, the standard inhibitor content of 10-15 wt. ppm is sufficient for normal temperatures and storage time. Stagnant product must be avoided. Blocked lines and infrequently used valves are typical situations in which the proper conditions are not met. Therefore, to achieve protection in each corner, the tank contents must be circulated frequently. Experience has shown that efficient circulation is also important to avoid a temperature gradient in the tank. Inlet, outlet and recirculation connections are required and optionally, as explained later, a swing pipe.

Tanks should be designed and constructed in accordance with an appropriate and recognized standard of good engineering practice (e.g. British Standards, German Industry Norms (DIN) and American Petroleum Institute Standards) and take into account local climate conditions.

4.1. Construction materials and tank internals

The standard construction material is carbon steel or stainless steel. Aluminum is satisfactory but will not withstand a fire. Internal structures like beams and pipes should be minimised as these provide places for condensed styrene to accumulate and polymerise. The insulation of storage tanks especially in warm and hot climates is recommended. A white reflective exterior coating will increase the effectiveness of the insulation.

Copper will combine with the organic acids and oxidation products present in the monomer. Therefore, copper and copper alloys must be avoided. The resulting impurities will colour the monomer green and will inhibit polymerisation of styrene.

The roof openings above the normal liquid level should be large in diameter and kept to a minimum number. By sloping the tanks toward the drains, horizontal tanks can be drained more completely. Floors of large vertical tanks may be tilted toward a small built-in sump with a bottom drain.

4.2. Pressure rating

The design pressure should be to API-620 or equivalent. This sets the specification for the relief system as well as minimising breathing losses during ambient temperature changes and enabling vapour conservation measures during filling. Containment of breathing losses during a temperature rise of the contents of 20°C should be considered. Preferably the vents should be connected to a vapour collection and recovery or treatment system and thus minimise hydrocarbon emission.

4.3. Fire protection

Considerations in site selection and tank spacing include proximity to other flammable material storage facilities, nearby sources of ignition, accessibility of fire fighting, and the impact of vapour cloud explosion on nearby areas. Fire monitors may be considered to provide cooling in the event of an external fire. Monitoring of oxygen levels within the vapour space inside the tank is one approach to ensure that oxygen levels remain below 8% at all times and thus an explosive atmosphere is avoided. A flame arrestor or pressure vacuum relief valve may be installed between the tank and any external opening if an explosive atmosphere within the vapour space is possible. Flame arrestors should be purged with nitrogen to prevent polymer formation. All safety equipment should undergo regular maintenance and replacement to avoid fouling/plugging by styrene polymerisation. See Table 3 for recommended inspection details. The installation of a foam chamber or sub surface injection connection is also recommended for automatic fire fighting, particularly on larger tanks.

4.4. Lining

Tank linings can help control or reduce polymer and stalactite formation, in particular in hot climates. A non-porous, non-wettable, and smooth tank lining prevents retention and attachment of the condensed (uninhibited) monomer droplets. As a result, the monomer drains quickly back into the pool of inhibited liquid monomer before polymerisation can take place. Both inorganic zinc silicate and epoxy linings can be used for storage tank surfaces as well as internal necks for safety features. The resistance characteristics of these should be checked, and information obtained from the manufacturer on their long-term performance for styrene storage. Rubber-based linings and polyamide-cured epoxies should not be used.

4.5. Uniform temperature control

Efficient mixing is important to achieve a uniform temperature. Therefore, inlet, outlet and re-circulation connections are usually located near the bottom of the tank and can be reversed so that the product is pumped from the bottom and discharged through the swing pipe below the liquid surface. If desired, cooling the content from the bottom up can be achieved via the swing pipe, i.e. styrene can be withdrawn below the liquid surface and returned to the bottom. It is stressed that return of product to the tank should always be below the liquid level, to prevent static electric charge build-up, and that a reliable siphon break should be provided in dip pipes. It goes without saying that in warm climates, a chiller will be a minimum requirement to maintain the desired storage condition.

4.6. Lines and valves

The following are recommended engineering practices for styrene:

1. Lines smaller than 25 mm in diameter should not be used,
2. A minimum of flanged connections is preferred because of leak potential,
3. Lines should not be buried because of the difficulty of checking for leakage,
4. All lines should be sloped so they can be completely drained for maintenance,
5. Newly installed lines should be pressure tested by an approved method before insulation.

Thermal expansion in blocked styrene lines exposed to the sun and without relief protection can cause high pressure, which can result in failure of gaskets, pump seals, and pump housings.
Excessive temperatures in blocked lines can also cause polymerisation. If burial is preferred, the regulatory requirements to protect soil and ground water should be observed.

A dip pipe should extend to the bottom of the tank and be provided with a pinch hole (siphon break). The electrical continuity across connections should be checked to ensure grounding. Lines constructed of carbon steel are acceptable, but copper, bronze, or plastic should not be used. Large lines are usually joined by welded flanges, for smaller lines threaded joints are satisfactory.

Styrene can dissolve pipe dope and thus cause product coloration and contamination. To avoid this, the pipe threads can be wrapped with PTFE [poly (per)tetrafluoroethylene] tape.

As with all transport equipment, caution should be taken with the choice of valve. All valves should be protected against climatic extremes, shock and mechanical stress. For rail tankers left standing in the sun, resultant high temperatures can promote polymerisation of styrene. A fusible airline or equally effective safety shutoff valve should be installed to close off lines in case of excessive external heat.

Emergency block valves for isolation of equipment by remote activation may be considered on storage tanks, unloading stations and long pipelines holding large amounts of styrene. Plug cocks and ball valves, lined with fluoro-elastomer, give excellent service with stainless steel balls. Gate valves are less satisfactory, but usually cheaper, and may be used. Lubricated plug cocks and non-lubricated ball-type valves are satisfactory. The piping system should use either ball valves or plug valves. However both types of valve can trap styrene that may polymerise, seize the valve and render it inoperative. Thus all valves should be regularly inspected according to the recommendation in Table 3. Gate valves may be considered for applications with infrequent use. The piping system should use plug valves rather than ball valves for all manually and infrequently operated valves. Ball valves may trap product that may polymerise, seize the ball and thus make it inoperative. To limit emissions from packed glands, bellow valves may be considered. Drain line valves should be provided with a cap or end-flange. Valves must be protected against freezing, heat shock, or mechanical stress.

4.7. Pumps

Most pumps are suitable for styrene, except those having copper, bronze, or plastic parts. Centrifugal pumps with enclosed impellers and mechanical seals are the most widely used, but displacement pumps and rotary pumps are satisfactory. A canned or magnetically driven pump may be considered to comply with tight fugitive emission regulations. All metal should be properly grounded to avoid static accumulation. Because deadheading will cause heat build up and lead to polymerisation, a minimum flow line should be installed.
The pump and circulation system should allow injection of instrument air, TBC stock solution, to inject foam to smother a fire, and a diluent, e.g. toluene or ethyl benzene, to reduce the viscosity of the tank content during a runaway polymerisation. In addition to mechanical reliability aspects, environmental performance parameters should be considered in the selection process for seals. For mechanical seal components it is recommended to specify the stationary face (e.g. tungsten carbide) and rotating face (e.g. carbon). Tandem seals with barrier fluid are recommended.

4.8. Tank pit

A containment system is a basic requirement for styrene storage to:

1. Limit the spreading of a product spill and thus limit the flammability and environmental hazard,
2. Control run-off during fire fighting,
3. Provide a barrier against soil and groundwater contamination,
4. Allow spilled product to be effectively covered with foam.

An under storage leak detection system is recommended. For containment the storage size, configuration and hydraulic load due to sprinkler water and rainwater and whether foam will be the standard medium/practice for fire extinguishing should be considered. To contain the styrene in such a situation, a siphon or under-flow weir may be considered.

4.9. Loading and unloading facilities

A remote impoundment for flammables could be considered.

4.10. Drainage

Installation design should allow for recirculation and complete drainage, and subsequently flushing with inert gas. This is especially important when styrene is stored for extended periods or when equipment is used intermittently.

4.11. Accessories

Gaskets and O-rings

For flanged connections at ambient conditions it is recommended to use gaskets of PTFE or graphite with a reinforced rim inside. O-rings in styrene service should be made from a co-polymer of fluorinated ethylene and fluorinated propylene. Rubber or other styrene soluble materials and PTFE encapsulated O-rings should be avoided. For gland sealing, graphite (optionally PTFE) is the material of choice.

Filters

Since small amounts of foreign matter may enter a storage tank from various sources, a filter in the transfer piping between tank and processing equipment is recommended. A replaceable cartridge or filter is recommended. Suitable materials are polyester and nylon. Especially for sealless pumps, an inline basket strainer is recommended. The strainer will have adjacent isolation valves and connections for blowing it clear, for safe removal and cleaning.

Flexible transfer hoses

To avoid product contamination, the most important aspects for transfer facilities are easy cleaning and product compatibility. Therefore lightweight aluminum pipe and swing joints, with seals of fluoro-elastomer are preferred.

Composite hoses, which are static-conductive, and flexible metal hoses (woven metal type) are satisfactory, but they require more care and attention to keep them properly clean and to prevent damage or breakage during use.

Hoses from fluoro-olefin elastomers give the best service and are the most widely used of the composite hoses.

Neoprene elastomer synthetic hoses do not have sufficient resistance to aromatics and are therefore not recommended for styrene service.

Multi-layered polypropylene and PTFE hoses are flexible and chemically resistant, and are therefore recommended materials.

Because styrene can attack the interior of a hose, it should never be allowed to stand in any of the composite hoses. All residual styrene should be drained, and all intermittently used transfer lines should be thoroughly cleaned to prevent formation of polymer and other undesirable reaction products. All transfer lines and hose connections should be properly grounded to prevent build up of static electricity. Regular preventive maintenance is recommended for flexible transfer lines.

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Patrick Detournay
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