The shaft end seals of high-speed rotating centrifugal compressor units mainly include four categories: labyrinth seals, float ring seals, mechanical seals and dry air seals. Among them, the dry gas seal, also known as the dry operation gas seal, uses the clean gas compressed by itself for sealing. It is an advanced sealing form at the shaft end of the turbine compressor and belongs to non-contact sealing. It has good sealing performance, long service life, and low operation and maintenance costs. Currently, this sealing form is the preferred choice for large-scale turbine compressor units.

During the operation of the unit, the dry gas seal often gets damaged, which seriously affects the high-load stable operation of the ammonia synthesis plant. The annual production loss and maintenance costs caused by this amount to tens of millions. Now let's analyze the causes of the damage based on relevant case studies, so as to solve this problem.

Introduction to Case Equipment

The compressor of this unit is a centrifugal compressor driven by a steam turbine. The steam turbine is a dual-output shaft condensing steam turbine and is connected to the compressor through a diaphragm coupling. The entire set of units is equipped with an auxiliary lubrication and control oil system, a steam and condensate system, and a dry gas sealing system.

The compressor is of a three-cylinder and four-stage structure, that is, three cylinders and four stages of compression, namely the low, medium and high-pressure cylinders and the first, second, third and circulation stages. The first and second stages of compression correspond to the low and medium pressure cylinders respectively, while the third and circulation stages of compression are in the high-pressure cylinder. The high-pressure cylinder, medium-pressure cylinder and low-pressure cylinder are respectively arranged on both sides of the steam turbine. At the outlet of each cylinder block, inter-section coolers are respectively set up to cool the outlet gas to ensure the compression efficiency. After the coolers, the gas is returned to the inlet of each section through the anti-surge circuit.

The shaft end seals of each cylinder block of the compressor adopt dry gas sealing devices. Each cylinder block is equipped with a dry gas sealing control panel station. The flow rate, pressure and other parameters of the sealing gas at each level of the dry gas seal, as well as the cleanliness of the sealing gas, are all controlled by the panel station. The parameters are shown in the following table.

Compressor design and operation parameters

Process Introduction

The main process of this unit is that the steam turbine simultaneously drives the three cylinders of the centrifugal compressor. The process gas is compressed through the low-pressure cylinder of the compressor and then cooled by the inter-stage cooler before entering the inlet of the second stage of the medium-pressure cylinder of the compressor. A section of anti-surge circuit is set between the outlet and inlet of the low-pressure cylinder of the compressor. After compression in the medium-pressure cylinder of the compressor, it passes through an inter-stage cooler for cooling and then enters the three-stage inlet of the high-pressure cylinder of the compressor. A two-stage anti-surge circuit is set between the outlet and inlet of the medium-pressure cylinder of the compressor. The high-pressure cylinder of the compressor compresses and increases the pressure. After being mixed with the circulating gas from the air supply port, it continues to be compressed. The compressed high-pressure process gas is discharged from the high-pressure cylinder of the compressor. Two anti-surge circuits are respectively set between the outlet of the high-pressure cylinder of the compressor and the three-stage inlet and the air replenishment port of the circulation section.

A small branch is led out from the outlet of each cylinder block as the first-level sealing gas for the dry gas seal of each section. When the unit is started up, the first-stage sealing gas adopts medium-pressure nitrogen at 1.8 MPa. During normal operation, the medium-pressure nitrogen is switched to the outlet process gas. The second-stage buffer gas and isolation gas adopt low-pressure nitrogen at 0.4 MPa.

Hydrogen at 1.70MPa from the pressure swing adsorption unit and nitrogen at 1.85MPa from the air separation unit are proportioned at the inlet of one section of the compressor. After being mixed in a ratio of approximately 3:1, they enter the compressor. A manual shut-off valve is installed on the hydrogen pipeline, and a DN150 regulating valve is set on the nitrogen pipeline to adjust the amount of nitrogen entering the system. During normal operation, the hydrogen shut-off valve is fully open, and the hydrogen-nitrogen ratio is regulated by adjusting the amount of nitrogen.

The structure and working principle of dry gas seals

Dry gas seal is a new type of non-contact seal that developed in the late 1960s by fundamentally improving mechanical seals on the basis of gas dynamic pressure bearings. Mainly by adding a dynamic pressure groove on the moving ring of the mechanical seal, the non-contact operation of the sealing end face is achieved.

The John Crane Company of the United Kingdom was the first to apply dry gas seals to the gas conveying equipment of offshore platforms in the late 1970s and achieved success. Dry gas seals initially emerged to address the sealing issue at the shaft end of high-speed centrifugal compressors. Due to their non-contact operation, they are particularly suitable for use as shaft end seals in high-speed and high-pressure equipment.

MEI SONG

Dry gas sealing structure

Generally speaking, a typical dry gas seal structure consists of components such as a stationary ring, a moving ring (also known as a rotating ring), an O-ring, a spring, and a spring seat (cavity). The structure is shown as above.

The static ring is located in the spring seat, and the dynamic ring is fixed on the rotor. Under the sealed no-load condition, the stationary ring fits with the moving ring under the action of the spring. The mating surface is flat, smooth and clean. There is a series of helical grooves on the mating surface of the moving ring. As the rotor rotates, the gas is pumped inward to the root of the helical grooves. The grooved area outside the root is called the sealed dam. The sealed dam exerts resistance on the gas flow and increases the gas film pressure.

The pressure between the mating surfaces of the moving ring and the stationary ring separates the surface of the stationary ring from that of the moving ring, maintaining a very small gap, which is generally about 3μm. When the closing pressure generated by the gas pressure and the spring force is equal to the opening pressure of the gas film, a stable equilibrium gap is established, and the dry gas seal enters the normal operating state. The structure is shown in Figures 1 and 2.

MEI SONG

The helical groove structure on the end face of the moving ring

Problems that occurred during operation

Since the original start-up, the dry gas seal at the drive end of the medium-pressure cylinder (i.e., the end near the steam turbine) has suffered multiple damages, seriously affecting the normal operation of the ammonia synthesis unit. When the dry gas seal is damaged, the pressure and flow rate of the leaked gas from its first-level seal will almost simultaneously reach the interlock value, causing the unit to interlock and stop. From the situation of multiple damages to the dry gas seal, the following patterns exist:

one

The operation of dry gas seals mainly involves the control of pressure and flow rate. During the start-up process, the first-stage sealing gas is switched from medium-pressure nitrogen to process gas. The flow rate and pressure control of the dry gas seal at the drive end of the medium-pressure cylinder are carried out in accordance with the process indicators, and the switching of the first-stage sealing gas is also strictly carried out in accordance with the operating procedures. The daily operation of the dry gas seals of the three cylinder blocks is no different, but it is always the dry gas seal at the drive end of the medium-pressure cylinder that gets damaged

2.

The damaged dry gas seals all showed normal in the static tests before startup. When the first-level sealing gas is introduced in the parking state, the pressure and flow rate of the first-level leaked gas are both zero, indicating that the dry gas seal has a good sealing effect

3.

From the disassembly and inspection of the dry gas seal, it can be seen that all are first-level seals damaged. The extent of each damage is different. Some have severely worn end faces of the moving ring and the static ring, while others have cracked end faces of either the moving ring or the static ring

4.

Dry gas seal damage all occurs during the start-up process, and it usually happens within a certain period of time when hydrogen and nitrogen are introduced after passing the critical speed

Conclusion

The above is a general overview of this case. In the next article, we will discuss the causes of the damage and the solutions. If you have different opinions, please feel free to leave a comment or send us a private message for discussion.

Source: Internet

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