Problems with dry gas seals in centrifugal compressors

If the pressure of the sealing gas is higher than that of the pre-pressure gas, reverse pressure will be applied to the seal, which can easily lead to damage to the main seal

The main function of the primary seal gas is to prevent dirty gases inside the compressor from contaminating the primary seal surface. As the compressor operates at high speeds, this gas is delivered into the primary seal chamber through the spiral grooves on the primary seal surface, thereby creating a rigid air film between the seal surfaces that serves to lubricate and cool them. The vast majority of this gas enters the machine through the labyrinth at the compressor’s shaft end; only a small amount of gas – ≤16 Nm3/h at 20 kPa and 50°C – enters the outside at a higher point for discharge through the primary seal face. The main function of the secondary seal gas is to prevent a small amount of medium gas that leaks from the primary seal surface from entering the secondary seal surface, thereby ensuring the safe and reliable operation of the secondary seal. Most of this gas, along with the small amount of medium gas that leaks from the primary seal surface, is vented through the high point outside the primary seal chamber; only a small amount, ≤1.0 Nm3/h, enters the high point of the secondary seal’s venting chamber through the secondary seal surface. The design choice for dry gas seals depends primarily on factors such as the gas composition, gas pressure, process conditions, and safety requirements. In practical applications, dry gas seals come in three main configurations: single-face, tandem, and double-face. The series dry gas seal is the most commonly used design configuration. A series dry gas seal may consist of two or more stages of such seals connected end to end in the same direction; each stage shares part of the load. Typically, a two-stage structure is employed, with the first stage (the primary seal) bearing the entire load, while the second stage serves as a backup seal that does not experience any pressure drop. An inert gas is introduced between the backup seal and the primary seal to create a barrier seal, ensuring that the sealing medium does not leak out into the atmosphere at all. In high-pressure applications, a three-stage series seal is required, with the first two stages sharing the total load, while the third stage serves as a backup and blocking seal.

1. To ensure that both sealing gases enter simultaneously and to protect the sealing surfaces, if the seals fail, the press becomes unusable; gas leakage occurs, and neither safety nor environmental protection can be guaranteed. The differential pressure may be high or low – the reasons could be: the seals are truly damaged resulting in significant leakage, 2) the instruments are faulty and provide inaccurate readings, 3) one of the sealing gas pathways is blocked! Another factor is the pressure difference between the feed air and the balance pipe; both are intended to protect the compressor seal

In my understanding, the main function of the pressure difference is, first, to prevent oil and gas from entering the bearings, as a measure to avoid contamination； Secondly, use primary, secondary, and tertiary buffer gases to discharge the combustible gases to safe areas, taking explosion prevention into account. Currently, imported centrifugal natural gas compressors generally have only two stages of buffer gas. Another requirement regarding the differential pressure is that the primary isolation gas for the dry gas seal must be filtered. After a differential pressure is established (during unit operation as well as when the unit is shut down under pressure), the less pure process gas needs to be pushed back into the compressor, in order to prevent liquid and solid contaminants from entering the gas film gap. Generally, the pressure difference of the primary vent is checked; if it is too high, there is usually a problem. The specific situation needs to be analyzed, as there are many possible causes.