Dry Gas Seals (DGS) task

Within the gas lubricated shaft seals, dry gas seals (DGS) are the most common type of seals used in the Oil & Gas industry. The Dry Gas Seal outperforms other types of seals for demanding applications and are specified by standards like the API 692. Its main task is to prevent uncontrolled escape of process gas from a compressor to the atmosphere and to avoid contamination to, or from the lubrication system.

Dry Gas sealing systems are used for axial, centrifugal, rotary screw compressors and expanders throughout the industry.

The dry gas seal has a rotating mating ring and a stationary primary ring. When the mating ring rotates, the grooves in the mating ring create a dynamic pressure, which forms a running gap between the 2 faces. This running gap receives the clean seal gas, which acts as a barrier between the atmospheric gas/air and the process gas. The gap varies in a range of only a few microns,and therefore requires a high quality of the supplied gas.

Using a clean seal gas is essential to avoid the damaging of seal faces, which can lead to contamination and loss of process gas.

Filtration with BOLLFILTER

BOLLFILTER seal gas filters and separators provide clean seal gas by filtering dirt and moisture from the gas supplied to the Dry Gas Seal. Particles and aerosols are removed with an efficiency of up to 99,99% down to particle and droplet sizes of 0,1 µm.

The Dry Gas Supply Skid is the driver of the quality of gas delivered to the Dry Gas Seal through the regulation of pressure, temperature, flow, and gas cleanliness.

Dry Gas Seal systems are typically differentiated by 3 classifications depending on the application – Primary, intermediate, and secondary supply lines.

Primary seal gas supply line

This is commonly the first stage starting from the process side of the compressor. It uses process gas which is conditioned to be able to be handled by the DGS. Depending on the contamination levels in the process gas, different arrangements for the conditioning might be required. When high loads of aerosols, moisture or particles are expected, a pre-separator becomes indispensable.

The pre-separator is commonly a cyclone, buffer plate or demisting separator, or in some cases a combination of these.

The primary seal gas filter is commonly a coalescing filter unless it is guaranteed that the gas is 100% free of moisture or liquid contaminants. This might be the case for example in some cryogenic applications.
In general, if the knockout rates exceed 50 % of the coalescing capacity of the element, it is suggested to install a mechanical separator ahead of the filter assembly.

Intermediate treated gas supply line

The intermediate seal gas supply line is commonly only present in high pressure arrangements or when handling harmful gas. It presents an intermediate stage between the primary and the secondary seal gas for added safety. Commonly used non harmful gases in this application are air or nitrogen.

Secondary treated gas supply line

The secondary seal gas supply line is present in most applications within the Oil & Gas industry. It serves to prevent contamination into the DGS, as well as providing the “outer barrier” of the gas seal to the environment, ensuring that any leakage from the primary or intermediate seal gas is vented to the flare. Like the intermediate supply, non-hazardous gases such as air or nitrogen are the most commonly used.