Control system of dry gas seal - Classification and requirements of supply gas

Under the condition that the dry gas seal is correctly designed according to the operating conditions, its operation mainly depends on the normal matching between the two ends of the seal, that is, the dynamic ring and the static ring. If the gas entering the seal chamber is mixed with small impurities, it will seriously affect the reliability and life of the whole dry gas seal system. Therefore, a seal gas filter unit is provided to ensure its safe operation. The amount of seal leakage indicates whether the dry gas seal operates normally. Therefore, a set of leakage monitoring unit is needed to monitor its leakage to determine whether the dry gas seal is operating normally. When the leakage of dry gas seal exceeds a certain value, the monitoring system considers that the seal has failed, and the system sends an alarm signal, or even interlocks and shuts down, to ensure the safe operation of the unit. In addition, in order to prevent the dry gas seal from being affected by the lubricating oil, it is also necessary to match the seal gas isolation unit to prevent the bearing lubricating oil from entering the dry gas seal system. To sum up, the control system of dry gas seal (also called "support system") is composed of three parts: seal gas filter unit, dry gas seal leakage control unit and seal isolation gas unit. It controls and monitors the gas in the sealing system from the aspects of flow, pressure, temperature and cleanliness. However, the specific structure of dry gas seal control system is different due to the different seal structure layout, seal medium type, pressure and temperature.

The supply gas of dry gas seal is mainly divided into seal gas, buffer gas and isolation gas.

1. Sealing gas

The seal gas generally refers to the gas on the main seal face. During normal operation, ensure the supply of seal gas during unit operation. The interruption of sealing gas will lead to dry grinding of sealing surface, and the seal will burn out in a short time; In addition, if the seal gas contains impurities (liquid and solid), it will also damage the sealing surface and affect the operation of the whole unit. Therefore, continuous and clean seal gas supply is crucial for the operation of the whole sealing device.

The selection of sealing gas should be considered from both safety and economy. In general, when transporting carbon monoxide, nitrogen, hydrogen, air and other gases, the compressor outlet process gas is used as the sealing gas. When transporting petroleum gas or the gas contains more hydrocarbons, nitrogen is used as the seal gas, and nitrogen or air is used as the dry gas seal for pump and kettle. However, sometimes in consideration of economy or operation safety, the specific situation also changes. The process gas at the outlet of the compressor is used as the dry gas seal of the seal gas, and the auxiliary seal gas (such as nitrogen or air) shall be used in case of startup, shutdown or failure. Before starting, the auxiliary seal gas shall be introduced first. When the unit runs to the operating speed range or the compressor outlet process gas reaches more than 1.4 times of the inlet pressure, the compressor outlet process gas shall be switched to the main seal process. Before the unit stops, the compressor outlet gas shall be switched back to the auxiliary seal gas to ensure the normal operation of the seal.

During operation, the key of dry gas seal is to ensure that a certain thickness and stiffness of gas film is generated between the moving and stationary rings. The formation of this gas film completely depends on the groove type on the moving ring, and the depth of these grooves is shallow, only 3~8um small impurities or oil stains can fill the groove type; In addition, if the gas contains liquid substances such as water, the liquid vaporization will destroy the gas film when rotating at high speed, which will also lead to wear of the contact between the moving and stationary rings. Therefore, the seal gas containing solid impurities and liquid is the enemy of dry gas sealing. The gas supply of the seal gas system is continuous and reliable, and the gas shall not contain solid impurities or liquid. The dry gas seal is clean and dry from a literal point of view.
The seal gas filter unit is the core of the dry gas seal control system. The seal gas enters the dry gas seal through the filter unit to ensure that the sealing surface is not damaged by particles and impurities, so as to ensure the long-term operation of the dry gas seal. The differential pressure alarm is set before and after the filter. When the differential pressure exceeds a certain value, switch the filter, clean or replace the filter element at the same time to ensure the quality of the filtered gas.

2. Buffer gas

Buffer gas (also called front gas) is actually a gas separated from seal gas before entering dry gas seal. It enters the buffer chamber through the flow limiting orifice plate, and then enters the machine after labyrinth sealing. Its function is to prevent the untreated process gas in the machine from spreading outward to pollute the seal face, thus affecting the normal operation of the seal.

3. Isolation gas

In order to prevent the bearing lubricating oil from entering the dry gas seal and polluting the sealing surface, a set of isolation gas system is provided for it. Isolation gas refers to the gas introduced between the dry gas seal face and the bearing to isolate the dry gas seal and the bearing lubricating oil. Nitrogen is generally used as isolation gas. The isolation gas is started before the lubricating oil system is put into use, and then closed after the lubricating oil system is stopped. Ensure that the lubricating oil will not enter the sealing contact surface to ensure the safety of the dry gas sealing system. If the isolation gas contains solid or liquid, it can be treated in the same way as the sealing gas.