Nitrogen barrier seals

In black oil service. I ve seen asphalt plant operators wait a few minutes after a pump is shut down before they block-in the external seal flush flow. Of course, for autostart pumps, leaving the external seal flush closed is not an acceptable option. But, in the above example, maintain the autostart option is costing 2 million a year in degraded product yields. I suppose that s the justification for Nitrogen Barrier Seals, as described below. 

Application of Nitrogen Barrier Seals Using Double Mechanical Seals... 

A dry or nitrogen barrier seal is typically used as the outer seal on a double mechanical seal (see Fig. 37.1). Double mechanical seals are used when the process fluid is environmentally objectionable or above its auto-ignition temperature. The nitrogen barrier seal replaces the reservoir fluid providing seal flush lubrication to the outer seal in the more conventional double mechanical seal. 

As dry nitrogen barrier seals are about twice as expensive as a conventional seal, it s rather difficult to see the justification of the... 

If the pump runs backward, most (not all) nitrogen barrier seals will be ruined. 

Contamination from Bearing Lubrication Oii. A barrier seal is required on the outboard side of the dry gas seal, between the gas seal and the compressor bearing. The primary function of the barrier seal, typically buffered with air or nitrogen, is to prohibit the flow of bearing lubrication oil into the gas seal. Contamination of the dry gas seal from lube oil can occur when the barrier seal fails to function as intended. 

The dry gas seal has one of two types of barrier seal (seal between the bearing and dry gas seal) either a labyrinth or single or double carbon rings. Normally the seal system includes provision to supply buffer gas to the barrier seal, also known as a separation seal. The gas to this seal is referred to as separation gas. One reason for choosing the carbon ring style bander seal is to keep the separation gas usage to a minimum. The gas is normally nitrogen. The basic control is by a direct-operated pres sure control valve. 

Recent developments and adaptations of high-speed seal technology have brought gas barrier seals into mixer applications. With a gas such as nitrogen as the barrier fluid, process contamination is virtnally eliminated, and a pressure monitor can be used to detect leaks. The problem is that gas is a poor lubricant for the seal surfaces. However, by machining swirls or pockets in the inside surface of the seal faces, gas pressure can be raised locally to lift the seal surfaces apart. 

Polymeric seals are used for many electrical applications including oil retention, nitrogen or sulfur hexafluoride gas pressure barrier, moisture exclusion, and lubricant retention. Mechanical seals and O-rings are typically specified based on polymer type (i.e., silicone, buna, Viton) and durometiy. The latter is a measure of the elastic resilience of a seal. The base polymer is chosen on the basis of compatibility with the selected contact material. Catalogs from seal suppliers are... 

The barrier performance of rigid containers was measured on both a second modified MOCON instrument and by a head space technique in which nitrogen purged and sealed containers were stored in an oxygen environment for periods of time. Containers were generally measured at 23°C with 100% relative humidity (RH) inside and either 50 or 75 RH outside. Container oxygen transmission rates (OTR) are expressed as cm30 /container-day-atm. 

After cutting the pipes, the primary vessel was isolated. Afterwards the reactor block was sealed (fig. 2.1). This work was achieved in 1991. At the present time the primary vessel contains approximatively 170 kg of primary sodium on the form of oxide deposits and "puddles of sodium trapped. First and second barriers are maintained in nitrogen atmosphere at an overpressure of a few millibars. 

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