A full pressure breakdown occurs across the primary seal faces in a tandem seal arrangement. The secondary or outboard seal normally operates at a pressure lower than that at the primary (inner) seal; however, it is designed to sustain the full pressure if the primary seal fails. The primary vent is routed to the plant’s flare, and the secondary vent is usually led to the atmosphere if nitrogen is used as the separation gas.
A tandem seal with intermediate labyrinth is used to preclude the leakage of process gas to the atmosphere. A clean buffer gas introduced at the intermediate labyrinth at slightly higher than the primary vent pressure creates a differential pressure that helps to keep the process gas from migrating to the secondary seal faces and helps to sweep the leakage of the seal gas in the primary vent and the leakage of the separation gas in the secondary vent (Figure 6). A tandem seal arrangement is more widely used than single and double-seal arrangements in most hydrocarbon and critical process applications.
Separation (barrier) seal
The primary functions of a barrier seal are to prevent the migration of bearing housing lube oil into the dry gas seal cavity and to avoid process gas leakage into the bearing oil. Two common types of barrier seals include labyrinth and segmented dual carbon rings. A carbon ring seal can be of the contacting or noncontacting type. A noncontacting design is preferred to avert localized heat generation and to protect the carbon ring seal against premature wear. A barrier seal should be suitable for bidirectional rotation. Figure 7 shows the arrangement of a carbon ring separation seal in a dry gas seal cartridge.
A clean and dry separation gas is continuously injected into the barrier seal to protect the migration of lube oil from the outboard bearing housing. The separation gas should be filtered to 5 microns solid particles and should be 99.98% free of the entrained liquid particles 3 microns and larger.
Either nitrogen or air is used as the separation gas. Nitrogen, being an inert gas, is recommended as the separation gas for safety reasons. Nitrogen generation equipment should be considered if nitrogen is not readily available at the job site. The majority of the flow through the secondary vent of a dry gas seal is the separation gas together with a very small amount of the seal gas. Air can potentially create an explosive environment in the dry gas seal secondary vent when it is mixed with a combustible process gas. Combustion in the secondary vent can occur if the process-gas-to-air mixture is within the explosive limits and a source of ignition exists in the secondary vent.
If air must be used as the separation gas, the dry gas seal control system should be designed to create either a lean or a rich environment in the secondary vent. This precaution is necessary to minimize the risk of formation of an explosive mixture in the secondary vent. A lean environment is formed by injecting air in the secondary vent, which results in the hydrocarbon-plus-air mixture below the mixture’s lower explosive limit (LEL).
Injecting the process gas into the secondary vent forms a rich environ- ment. It results in the hydrocarbon-plus-air mixture above the mixture’s upper explosive limit (UEL). Typically, the mixtures that have from 5 to 15 % methane (CH4) in air or from 17 to 54% CH4 in O2 are considered explosive mixtures. A fuel/air mixture with less than 5 to 17% of fuel (hence ex- cess of oxidizer) is a lean mixture and a fuel/air mixture with greater than 15 to 54% of fuel (excess fuel) becomes a rich mixture.
The dry gas secondary vent must be routed to the plant’s flare if a rich mixture exists in the vent. The amount of air or process gas required to be injected in the secondary vent is determined from an engineering study and it must evaluate all possible operating conditions, including the primary seal failure situation. Figure 8 shows a graph of hydrocarbon percentage versus molecular weight. The area above the UEL line in this graph rep- resents a rich mixture, the area below the LEL line identifies a lean mixture environment and the area between UEL and LEL lines in this graph is the region of an explosive mixture.