open - это обычная кольцевая линзовая (lens ring) прокладка gasket, она открывает проходное сечения трубопровода
blind - это глухая (сплошная) линзовая прокладка, она закрывает проход в трубопроводе.
Их иногда выпускают парой в виде "очков", тогда трубопровод можно открыть кольцевой прокладкой или (поворотом очков) закрыть/перекрыть глухой прокладкой


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Note added at 1 час (2021-12-03 15:39:06 GMT)
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Вот описание таких "очков" с 2 прокладками https://www.spetech.com.pl/en/products/static-gaskets/lens-g...
Spectacle Blind Lens gaskets are used in purpose to separate pipe systems or to connect with each other. Set of two Lenses of certain size (where one of the gasket is blind and the other is a ring) is connected by flat section of steel. Such Spectacle-Blind pair is applied in pipelines which regularly need to be separated from other installations.
When pipeline is in "working conditions" Spectacle Blind Lens is mounted in the „open" position so that flow through the pipe is possible. "Maintenance condition" on a pipeline section is the reason to change the spectacle to the „close" position. In such case, after removal of bolts, the Spectacle Blind is rotated and after re-installing the bolts, pipe is blocked and no flow is possible.