Systems Design: General Purge Systems Design Considerations

DOE-HDBK-1132-99

gas used depends on the chemical constituents of the vessel contents. Though air

purge is used in nonflammable mixtures, nitrogen purging of vessels is the most widely

used method in nonreactor facilities and is the primary subject of discussion in this

section.

Systems Design: General Purge Systems Design Considerations . Nitrogen

3.2.1

systems are normally delivered as packaged systems that include the liquid

nitrogen storage tank with appropriate pressure build-up and economizer circuits,

ambient vaporizers, interconnecting piping, and discharge pressure regulators to

maintain delivery at customer-required pressures. Procurement of a standard,

packaged system on a performance-based specification should be considered;

however, upgrading of individual components may be required in certain cases

where the failure of the inert gas supply to vessels causes undesired vapor

space mixtures to form.

Components Design Considerations: Storage Tanks . Liquid nitrogen storage

3.2.2

tanks normally are furnished to operate up to 250 psig saturation pressure. The

appropriate operating pressure selected should be based on the following

factors:

delivery pressure at the process vessels;

storage capacity required (i.e., the higher saturation pressure, the lower

the volume of free gas available per unit liquid volume);

length of service piping from vaporizer to use point (i.e., the longer the

piping, the higher the transport losses);

safety consideration of high-pressure storage (see ASME Boiler and

Pressure Vessel Code, Section VIII).

Pressure Buildup Coils Design Considerations . Pressure build-up coils in

3.2.3

tanks automatically maintain the station pressure through tank pressure

excursions. Materials used in the coils should be selected based on evaluation

of material strength and heat transfer capabilities.

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