The Essential Guide to Dry Gas Seals: Structure and Function

I. Introduction

Dry gas sealing is a new type of non-contact shaft seal developed based on gas-lubricated bearings in the late 1960s, with spiral groove seals being the most typical example.

After years of research, the John Crane Company in the United States was the first to introduce dry gas sealing products for industrial use.

Practice has shown that dry gas seals offer many advantages over conventional mechanical contact seals. They are mainly used in pipelines, offshore platforms, refineries and petrochemical industry, suitable for any gas transmission system.

Since dry gas seals are non-contact seals not limited by PV value, they are especially suitable for large centrifugal compressors under high speed and high pressure conditions. The advent of dry gas seals represents a revolutionary advancement in sealing technology, solving the challenges of gas sealing without the limitations of seal lubricating oil.

Furthermore, the gas control systems required are much simpler than the oil systems of film seals.

Furthermore, the emergence of dry gas seals has changed traditional sealing concepts, organically integrating dry gas seal technology with barrier sealing principles.

The new concept of “using gas as a sealant” replaces the traditional concept of “liquid sealing, gas or liquid”, guaranteeing zero leakage of any sealing medium. This makes dry gas seals widely applicable in the field of pump shaft seals.

The following table compares the leak rates of compressor dry gas seals to other common seals:

Type of seal		parameter	Leakage rate (Nm 3 /min)
Gas Lubrication Seal	Dry Gas Seal	Groove depth 5 µm	0.025
	Carbon Ring Seal	Four groups, 10mm wide with a gap of 0.05mm	0.37
	Labyrinth seal	Number of teeth 15	1.82
Oil Film Seal			Seal oil leak quantity
			End of media (L/min)	Atmospheric end (L/min)
	Floating Ring Seal	2 groups, each 20mm wide, with 0.05mm gap	0.12	0.6
	mechanical seal	Oil film thickness 1 µm	0.0012	0.0017

Test conditions for the experimental unit: shaft diameter of 140 mm, speed of 5,000 rpm, process gas pressure of 0.6 MPa and sealing oil (gas) pressure of 0.75 MPa.

Compared to conventional contact mechanical seals, dry gas seals offer the following main advantages:

1. Elimination of the seal oil system and the additional energy load required to operate it.
2. Significant reduction in unplanned maintenance costs and production downtime.
3. Prevention of contamination of process gases by oil.
4. Minimum seal gas leak.
5. Low maintenance costs and good economic practicality.
6. Low energy consumption for sealing operation.
7. Long seal life and reliable operation.

II. Working principle of dry gas seals

Compared to other mechanical seals, dry gas seals are fundamentally similar in structure. The main difference is that a dry gas seal's O-ring has evenly distributed shallow grooves. These grooves allow the seal to operate in a non-contact state, generating a fluid dynamic pressure effect during rotation, separating the sealing surfaces.

The groove shapes on the sealing end face of dry gas seals are mainly categorized into unidirectional and bidirectional types.

Unidirectional grooves are most commonly used in today's compressor units. They can only be used in units with unidirectional rotation, generating opening force in the desired direction; if reversed, the negative opening force may damage the seal.

However, compared to bidirectional grooves, they can generate greater opening forces and gas film stiffness, offering greater stability and more reliable prevention of end contact, and can thus be used at very low speeds and under significant vibration.

Bidirectional grooves are also common. This type of groove has no directional requirements, and is suitable for forward and reverse rotations without damaging the seal. Its application range is wider than unidirectional grooves, but its stability and resistance to interference are inferior.

Through repeated experiments and comparative studies on various types of dry gas seal grooves, it has been confirmed that the helical groove design offers the highest gas film stiffness with minimal leakage, achieving the best leakage rate. Below is a detailed introduction to this type of groove.

The diagram below illustrates a typical dry gas seal with helical grooves on the sealing surface, less than 10 micrometers deep. When the seal operates, the sealed gas is drawn tangentially into the helical grooves, moving radially from the outer diameter toward the center (i.e., the low pressure side), prevented by the seal dam from flowing toward the side. low pressure.

The gas is compressed as it moves along the variable transverse shape of the helical grooves, creating a localized area of ​​high pressure at the root of the groove, separating the end faces by a few micrometers to form a film of gas of some thickness.

Under this thickness of gas film, the opening force generated by the action of the gas film balances with the closing force generated by the spring and medium forces, allowing the seal to operate without contact. The gas film formed between the sealing surfaces of the dry gas seal has a certain positive rigidity, ensuring the stability of the seal's operation. To obtain the required fluid-dynamic pressure effect, the dynamic pressure grooves must be located on the high-pressure side.

The diagram above shows the forces acting on a helical groove dry gas seal, illustrating how the rigidity of the gas film ensures stable seal operation. Under normal conditions, the closing force of the seal is equal to the opening force.

When external disturbances (e.g. process or operational fluctuations) occur, leading to a decrease in gas film thickness, the viscous shear force of the gas increases, increasing the fluid dynamic pressure effect generated by the helical grooves, thereby increasing the gas film pressure and opening force maintain the balance of forces and restore the seal to its original gap; conversely, if the seal is disturbed and the gas film thickness increases, the dynamic pressure effect generated by the helical grooves weakens, reducing gas film pressure and opening force, allowing the seal to return to its original gap.

Therefore, as long as it is within the design range, when external disturbances are eliminated, the seal can always return to its designed working clearance, which means that the dry gas seal has a self-adjusting function that ensures stable operation and reliable.

The main indicator of seal stability is the stiffness of the generated gas film, which is the ratio of the change in gas film strength to the change in gas film thickness. The higher the rigidity of the gas film, the stronger the interference resistance of the seal and the more stable its operation.

III. Typical dry gas seal structures

There are different general structural forms of dry gas seals, suitable for various working conditions. In practice, dry gas seals used in centrifugal compressors mainly include the following four structures:

1. Single face seal

Single-sided sealing is mainly used for non-hazardous gases, i.e. situations where a small leakage of the medium gas into the atmosphere is permitted. The gas used for sealing is the process gas itself. This type is commonly used in domestically imported units, such as carbon dioxide compressors.

2. Tandem Seal

The tandem dry gas seal is a sealing structure with high operational reliability, normally applied where a small leakage of the medium gas into the atmosphere is allowed. It is widely used in the introduced units of petrochemical companies.

A tandem dry gas seal can be considered as two or more sets of dry gas seals connected in the same direction, end to end. Similar to the single-face structure, the sealing gas is the process gas itself. Typically, a two-stage structure is used where the first stage (primary seal) supports the full load and the other stage serves as a backup seal without bearing pressure drop.

The process gas leaked from the primary seal is introduced into a burner for combustion. A very small amount of unburned process gas leaks through the secondary seal and is safely vented.

Should the primary seal fail, the secondary seal acts as an auxiliary safety seal, preventing massive leaks of the process medium into the atmosphere.

3. Tandem Fence with Intermediate Labyrinth

When leakage of the process medium to the atmosphere is not permitted, nor is the leakage of buffer gas into the process medium, an intermediate labyrinth seal can be added between the two stages of a tandem structure.

This structure is used for flammable, explosive and dangerous gases, preventing external leakage. Examples include H2 compressors, high H2S natural gas compressors, ethylene, propylene and ammonia compressors.

In addition to the process gas, this structure also requires an additional nitrogen gas route as sealing gas for the secondary seal. The process gas leaked from the primary seal is entirely introduced into a flare for combustion by nitrogen gas.

All gases leaked into the atmosphere through the secondary seal are nitrogen. Should the primary seal fail, the secondary seal also serves as an auxiliary safety seal. This structure is relatively complex, but due to its greater reliability, it has become the standard configuration in shaft seals of medium and high pressure centrifugal compressors.

4. Double Sided Seal

A double-sided seal is equivalent to two single-sided seals arranged opposite each other, sometimes sharing a rotating ring. It is suitable for conditions without flare systems, where a small seal gas leak is permitted mid-process. The introduction of nitrogen gas between the two sets of seals forms a reliable locking seal system.

The nitrogen gas pressure is controlled to always maintain a level slightly higher than the process gas pressure (0.2-0.3 MPa), ensuring that the direction of gas leakage is always towards the process medium and the atmosphere , thus preventing process gas leakage. to the atmosphere. The double-sided sealing structure is mainly used for low-pressure toxic, flammable and explosive gases.

4. Dry Gas Seal Design Overview

Dry gas seals operate with non-contact faces during operation, but brief contact occurs during the startup and shutdown phases, necessitating the use of wear-resistant materials for the contact surfaces.

Materials for the friction couples in dry gas seals typically include materials with low coefficients of thermal expansion, high modulus of elasticity, tensile strength, thermal conductivity and hardness, such as SiC or cemented carbide for the hard face, and impregnated graphite or SiC for soft face. Dynamic grooves are generally machined into the surface of the dynamic ring.

Because the structure of dry gas seals is not significantly different from that of conventional mechanical seals, the design of dry gas seals mainly focuses on the parameters of the groove shapes on the seal faces. The theoretical basis of dry gas seals is based on the principles of spiral groove thrust bearings, adhering to the Reynolds equation and Navier-Stokes equations.

Our company employs the finite element method for numerical calculations, with proprietary software developed in-house to calculate the pressure distribution of the gas film on the spiral grooved sealing surface, further determining the load capacity, gas film stiffness and the gas leakage rate of dry gas. stamp.

The stability and reliability of dry gas seal operation depend on the rigidity of the gas film on the sealing surface. The impact of process parameters and spiral groove structural parameters on sealing performance is mainly reflected in their effect on gas film stiffness; the greater the rigidity, the better the stability of the seal.

In addition to considering the stiffness of the gas film, our company also focuses on the leakage rate of the seal, aiming for the highest possible stiffness/leakage ratio. This means that the seal has high rigidity and low leak rates. Only dry gas seals with maximum stiffness-to-leakage ratio and significant gas film stiffness can guarantee optimal, stable and long-term operation.

The structural parameters of spiral grooves that affect the stiffness of the gas film include the depth of the groove, the spiral angle, the number of grooves, the ratio of the groove width to the weir width, and the ratio of the groove length to the length of the dam, requiring optimization through specialized software. Process parameters that affect gas film stiffness include:

1. Buffer gas viscosity: The viscosity of the buffer gas significantly impacts the stiffness of the gas film; Higher viscosity results in stronger hydrodynamic effects and greater stiffness.
2. Sealing gas temperature: Gas viscosity varies with temperature; higher temperatures result in higher viscosities and greater rigidity of the gas film.
3. Sealing speed: Higher speeds improve hydrodynamic effects, increasing the rigidity of the gas film. Ideally, without considering the impacts of seal machining and installation accuracy, higher speeds improve dry gas seal stability without being limited by the PV value of the mechanical seal, making dry gas seals particularly suitable for high speed applications .
4. Seal face diameter: At the same speed, a larger seal diameter results in higher linear velocity and greater gas film stiffness.
5. Buffer gas pressure: Buffer gas pressure has minimal impact on gas film stiffness; generally, higher pressures slightly increase stiffness.

V. Dry gas seal control system

To ensure the reliability of dry gas seal operations, each assembly is equipped with a corresponding monitoring and control system. This system keeps the seal operating in its ideal design state. If the seal fails, the system quickly triggers an alarm, allowing maintenance personnel to resolve the issue immediately.

Here we will introduce a typical tandem dry gas sealing system.

The schematic diagram below illustrates the system. Under normal conditions, a flow of gas is taken from the unit's outlet, passing through two stages of filtration (with 3μm precision), resulting in a dry and clean gas. This gas serves as a buffer for the dry gas seal, entering the seal chamber.

The pressure is controlled to be slightly above the reference process gas pressure during normal operations (typically 50KPa), preventing impurities such as dust and oil condensed in the unrefined process gas from entering the seal face, which could adversely affect the dry gas seal performance. The system employs a differential pressure transmitter to measure the pressure difference between the buffer gas and the reference gas.

The signal controls a pneumatic diaphragm regulating valve located at the buffer gas inlet, adjusting the inlet pressure to maintain a constant differential pressure with the reference gas. Most of the buffer gas entering the seal chamber returns to the process gas through a labyrinth seal.

A small portion leaks through the first stage dry gas seal, known as a first stage gas leak. Most of this is safely burned in a flare.

A stable gas film, essential for optimal long-term operation, can only form under the correct pressure differential. The system achieves this by installing a butterfly valve on the first stage leak gas outlet, adjusting the opening of the valve to generate the appropriate backpressure. This valve also serves to limit leaks if the first stage seal fails.

Furthermore, nitrogen gas is introduced as an insulating gas through a filter and a pressure reducing valve in a subsequent labyrinth seal. Its pressure is slightly higher than the bearing housing oil pressure (usually atmospheric pressure), creating a reliable locking seal system.

This ensures that lubricating oil from the bearing housing does not enter the dry gas seal and prevents residual process gas from contaminating the lubricating oil in the bearing area.

A portion of the insulation gas enters the bearing housing, while the remainder mixes with the small amount of unburned process gas from the first stage leak gas, known as second stage leak gas. This can be safely released into the atmosphere as an environmentally harmless gas.

The primary method of determining whether the seal is working properly is to monitor the first stage gas leak. If an anomaly occurs, the pressure and flow of the first stage dry gas seal will increase significantly.

If it reaches a predetermined high alarm value, a pressure transmitter sends a signal to the control room, triggering an alarm signal. This alerts operators to verify that the control system pressure is within the design range.

When the amount of gas leakage reaches an extremely high alarm value, it indicates that the dry gas seal has failed, causing the system to shut down to prevent damage to the equipment.

SAW. Dry Gas Seal Installation Precautions

Dry gas seals are highly precise components that require special attention during installation, disassembly and use. The following precautions are typically recommended:

1. Non-specialist manufacturers should not disassemble seals due to their complex assembly relationships, high cleaning requirements, specialized assembly tools, and need for precise dynamic balancing.
2. Transport, installation and disassembly require the use of positioning plates.
3. The relative position of the cavity and the shaft requires high precision; confirm the relevant dimensions in advance and adjust with shims if necessary.
4. During installation, maintain the concentricity of the rotor and casing by securing the rotor in place.
5. Typically, start with installing the end of the thrust plate to ensure accurate positioning of the seal on the other end.
6. Thoroughly clean the seal chamber and all inlet and outlet pipes, ensuring a higher standard of cleanliness than oil pipes.
7. Do not use grease for lubrication; use silicone grease instead.
8. After installing the seal on the unit and removing the positioning plates, ensure that the axial displacement of the rotor does not exceed 2 mm.

VII. Maintenance during dry gas seal operation

Dry gas seals, designed for a wide range of applications, typically require no maintenance under normal conditions.

However, it is essential to monitor seal leaks daily. An increase in leakage may indicate potential seal failure and attention should be paid to the following aspects:

1. Helical groove dry gas seals are designed for unidirectional rotation, therefore reverse rotation should be avoided. Additionally, operating at low speeds, below 5 meters per second, for long periods can damage the seal.
2. Ensure a stable flow of sealing gas. Maintaining a constant and uninterrupted seal gas flow is crucial to the normal operation of dry gas seals.
3. Avoid operating the seal under negative pressure. Negative pressure in double-face seals can significantly increase leakage under static conditions and damage the sealing faces under dynamic conditions. For tandem seals, this can lead to contamination from unfiltered process gas, causing rapid seal failure.
4. Monitor changes in seal leakage. Leakage variations directly reflect the operational status of the dry gas seal. Factors such as fluctuations in process gas, shaft movement, oscillations and changes in pressure, temperature and speed can affect leakage. Normal operation is indicated by stable leak rates; an upward trend suggests seal malfunction.
5. When the filter differential pressure reaches the alarm value, change the filters and replace the filter element immediately.
6. When starting the unit, wait until the dry gas seal control system barrier gas has built up sufficient pressure before starting the oil lubrication system.
7. When turning off the unit, wait until the unit has come to a complete stop and for more than 10 minutes after the oil system has stopped before turning off the dry gas seal control system.

VIII. Required conditions for dry gas seal retrofit applications

After extensive research and testing, dry gas seals have been widely adopted in industrial applications. Modern industry's increasing demands for energy efficiency, consumption reduction and environmental protection have made reliability, minimal leakage, longevity and stable operation of shaft seals in centrifugal compressors, which transport large volumes of hazardous gases, a necessity.

Compared to conventional mechanical contact seals, dry gas seals offer incomparable advantages: longer service life, no process medium leakage and lower maintenance costs. These benefits are in line with the objectives pursued by various types of shaft seals.

Dry gas seals can be successfully adapted and applied to centrifugal compressors, centrifugal pumps, reactors and other equipment, provided that the following two conditions are met:

1. The basic requirement for the operation of dry gas seals is the availability of an on-site gas source. The gas source can be a process gas or an environmentally friendly inert gas, such as nitrogen, coming from within the plant or from a dedicated nitrogen generator.
2. The shaft seal installation location must have sufficient axial and radial space and appropriate openings.