Piping and Valves cont'd - doe-std-1128-98

DOE-STD-1128-98

inside the facility. The most successful backflow-prevention device is the

deliberate separation of lines.

Every pipe that enters or exits a process cell or contaminated area from

or to occupied areas should be equipped with block valves.

Process-piping systems carrying radioactive liquids should be designed

to eliminate traps wherever possible and to permit flushing and draining

except for those with loop seals. Floor drains should have the capability

to be sealed.

Reduction in the size of pipelines in contaminated process-piping

systems should be made with eccentric reducers installed flat-side down

to avoid the formation of traps. Eccentric reducers are necessary only for

horizontal pipe runs.

Changes in the direction of process piping should be made with long-

radius elbows or bends. Long-radius bends should be used, where

practicable, except in lines that transport solids, where blinded tees or

laterals have been proven to prevent erosion. Blinded tees will encourage

solids buildup. The number of bends should be minimized and pipe

diameter should be increased.

If gaskets are required in process piping or associated hardware, the

selected gasket material should not deform or degrade and leak when in

service. Teflon should be avoided for most applications but, if needed, its

use will require implementation of a most rigorous inspection routine to

ensure recognition of degradation and replacement prior to failure.

Except for shielding walls, pipe sleeves should generally be provided

when piping passes through masonry or concrete walls, floors, and roofs.

The sleeves should be sloped to drain toward the controlled area. The

space between the pipe and the sleeve should be packed and sealed. If the

sleeve is to be sealed, then additional provisions should be made for

draining the annulus.

If underground piping for transporting radioactive or hazardous materials

is required, it should be installed inside another pipe or tunnel that

provides a second barrier to the soil. Provisions to detect a failure in the

primary piping (leak detection) should be provided. An effective solution

may be to install a double-walled pipe with an annular space that can be

sampled at intervals not exceeding 300 feet. The underground piping

should also have cathodic protection.

All valves that are not functionally required to be in contact with

contaminated liquids should be located in nonradiation areas (e.g., steam,

air, water) in accordance with IAEA Safety Series No. 30 (IAEA, 1981).

Process valves should not be located at low points in the piping except in

cases where it is necessary in order to properly drain the piping when

needed.

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