Dry Gas Seal System Control

The control system should be kept as simple as possible to maintain reliability. For tandem seals, the system control for the seal gas supply has traditionally been a differential pressure control, using a direct-operated control. The differential pressure usually was set at 10 to 25 psid. The flow using this control is generally much higher than needed. The differential pressure control can be modified to measure the pressure drop across an orifice and the control converted to a volume control. A velocity across the inner labyrinth of 5 fps would be considered sufficient to prevent back diffusion. This arrangement does have the advantage of reducing the seal gas requirement. For a practical design, the velocity should be increased to at least 10 to 15 fps to allow for wear. Many of the compressor manufacturers will recommend a higher value for more margin.

For the double-opposed seal, normally an inert gas is injected between the two opposed seals. This gas is pressure controlled to maintain a differential pressure higher than the process side pressure. The supply to this seal is critical because a failure will permit the differential pressure across the outer seal to reverse, which will result in a seal failure. This sea! arrangement usually incorporates a buffer to the process side of the seal to keep any dirty process gas away from the dry gas seal. The leakage to process from the gas seal is too low to keep any dirt from reaching the seal. However, there is somewhat of a problem. If the buffer is differ ential pressure controlled, this differential pressure must be added to the process pressure which in turn sets the injection pressure. For this reason a volume control is a better choice because the differential pressure involved is not as great.

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 ¡s referred to as separation gas. One reason for choosing the carbon ring style barrier 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.

For the tandem arrangement gas seal, a primary seal vent must be provided to vent the leakage across the process side seal. This vent may be lo flare or other suitable gas disposal point. The back pressure under normal conditions should be kept to a low value. A small amount of back pressure is recommended to keep a positive differential across the secondary seal. Leakage measurement may be provided in the vent line to provide health monitoring of the primary seal. Unfortunately, the rotameter, which would be the obvious choice, should not be used because of its lack of reliability. If an orifice or needle valve is used to set the back pressure to the seal vent, pressure upstream of the restriction can be measured for a relative flow measurement. This type of reading does provide trend data that may be used to judge the seal's performance.

The balance of the controls consists of the required pressure switches and/or transmitters to provide monitoring of the system and alarm and shutdown functions for the critical buffers. Filter differential normally is also monitored and alarmed on high filter differential pressure.

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