Skip to main content

Dry Gas Seal Failure Modes

BY BHUSHAN NIKAM.

Invented in the mid-20th century and typically equipped in process gas centrifugal, dry gas screw compressors and expanders, dry gas seals (DGS) are the preferred gas lubricated dry seal solutions available on the market. They have become the standard for new machines.


DGS are robust, simple, consume less power, and are more efficient in reducing leakage than their predecessor. Various configurations such as tandem with and without an intermediate labyrinth (Figure 1), single (Figure 2), and double (Figure 3) are available & shall be selected based on process requirements. In this article, we discuss the various DGS failure modes and how they should be addressed: 


PRESSURIZED HOLD/STANDBY

Pressurized hold, also called settle-out condition, occurs when the compressor remains at a standstill, but the casing is pressurized. If an alternate process gas lacks sufficient pressure and flow, process gas enters the seal cavity through the process labyrinth and contaminates the primary seal. This causes seal damage when the compressor is restarted. Minimum ambient site temperature also must be considered as the seal will be at the same temperature during standstill conditions, which will cause the process gas to condense and deposit on seal face grooves.

To avoid this kind of failure, the seal gas must be supplied with the required pressure even during a blackout. An alternate supply of seal gas should be considered when gas is not available from the compressor discharge. But it should not change the composition of the process gas. A seal gas booster should be considered when alternate gas is unavailable.

START-UP OR COMMISSIONING

The cause of the majority of DGS failures is contamination. This happens mostly during commissioning by not following OEM recommendations and best practices. Seal gas panel components including piping are properly cleaned and flushed with air, and end connections are blinded and dispatched to the site. However, site situations are always different. The piping upstream of the console must also be cleaned thoroughly including interconnecting piping between the console and the compressor. Corrosion inhibitors must be removed. The seal gas supply temperature dew point margin must be higher than or equal to the recommended value as per API. Failure to do any of the above will lead to contamination followed by degradation of the lift-off effect, friction between the static and rotating faces, parts deformation, O-ring extrusion, heat generation causing thermal shock on the rotating seat, and eventually failure of the rotating and or static rings.

NORMAL OPERATION

Although a DGS is less susceptible to failure during continuous normal operation, it may happen due to upset conditions leading to contaminated seal gas supply or condensate formation as a result of pressure drop across conditioning equipment. The flow velocity requirement across the process labyrinth varies depending on the process gas, usually 5 m/s. High velocity must be considered for some processes. If available pressure is not enough, consider installing a seal gas booster which will keep pressure at the seal cavity higher than on the process side. Ensure that a properly sized coalescing seal gas filter is installed which will filter out particles above 3μm. The gap between rotating faces is 3-5μm (a human hair is 70μm). Additional requirements, as per the recent API 692 code, should be considered as necessary.

SEPARATION SEAL FAILURE

A separation seal, also known as a barrier or tertiary seal, is located in between the DGS and the bearing box. Its purpose is to avoid lube oil ingress from the bearing to DGS side during normal operation and minimize process gas flow to the bearing side in the event of DGS failure. Flow consumption is much less than the secondary side. But depending on the type of seal applied, enough flow is necessary to avoid oil ingress to the DGS side. Nitrogen is typically used but dry air can also be supplied if the process allows it and does not create an explosive mixture. On the other side, high flow is not desirable as it may over-pressurize the lube oil reservoir. The vent line must be checked regularly and any oil traces should be drained and rectified.

REVERSE PRESSURIZATION

Reverse pressure occurs when downstream pressure is higher than the upstream supply pressure. If specified, a seal should be designed for reverse differential pressure as recommended by API. This must be confirmed by the DGS vendor as well. During reverse pressurization, contaminated gas or liquid droplets can travel from the flare vent line back to seal faces resulting in O-ring dislodging, loss of performance, and subsequent risk of seal damage. A differential pressure control valve with PDIT can be applied to avoid these issues. If not, necessary arrangements should be implemented so as not to reverse pressurize the DGS. Confirm flare line minimum, normal, and maximum pressure with the customer. Ensure DGS leakage gas does not create an explosive mixture with other hazardous gases in the flare.

RAPID DEPRESSURIZATION

The compressor casing may be depressurized after shutdown in case of over-haul, emergency shutdown, planned maintenance, or as per process requirements. DGS O-ring material must be chosen based on the depressurization rate. Consult with seal vendors if the decompression rate is different a standard application. If special considerations aren’t given to the selection of O-rings, they can be subject to explosive decompression due to rapid depressurization. Additionally, the decompression rate must be selected right at the basic design stage. Special attention must be given to avoid the Joule Thompson effect based on gas composition. This can lead to condensation of the gas and the process side may be exposed to Minimum Design Metal Temperature (MDMT). If material is not selected according to MDMT, the subject material may fail.

CONDENSATE OR LIQUID FORMATION

Gases and air have dew points which vary based on pressure, temperature, and type of gas (Figure 4). The gas used as a seal gas from the compressor discharge undergoes reduction in pressure and temperature which causes condensation. Similarly, when the dew point temperature is achieved, condensate forms. Eventually, droplets travel through the rotating and stationary seal faces where they will create a blistering effect resulting in failure of the seals faces.

The solution is to follow the rule of chemistry and perform a dew point analysis. The designer should check gas properties from the compressor datasheet and reconfirm these with the client as smaller changes in composition can affect the dew point. Plot the dew point line for possible cases, calculate the dew point, and check its margin from the supplied temperature. The delta T should be equal to or higher than that recommended in API. If the margin is less, gas conditioning will be necessary to keep the gas dry as it passes through the seal gas system. Only the most likely failure scenarios are addressed here. But DGS failure can be caused by various factors depending on the site situation.

■ Bhushan Nikam is a Project Engineer for a major turbomachinery equipment manufacturing organization. He holds a bachelor’s degree in Mechanical Engineering from the University of Pune, IN. He can be reached at nikambb007@gmail.com.

Source:  Turbomachinery Magazine

Comments

Popular posts from this blog

Maintenance 4.0 Implementation Handbook (pdf)

WHAT IS MAINTENANCE 4.0? Industry 4.0 is a name given to the current trend of automation and data exchange in industrial technologies. It includes the Industrial Internet of things (IIoT), wireless sensors, cloud computing, artificial intelligence (AI) and machine learning. Industry 4.0 is commonly referred to as the fourth industrial revolution. Maintenance 4.0 is a machine-assisted digital version of all the things we have been doing for the past forty years as humans to ensure our assets deliver value for our organization. Maintenance 4.0 includes a holistic view of sources of data, ways to connect, ways to collect, ways to analyze and recommended actions to take in order to ensure asset function (reliability) and value (asset management) are digitally assisted. For example, traditional Maintenance 1.0 includes sending highly-trained specialists to collect machinery vibration analysis readings on pumps, motors and gearboxes. Maintenance 4.0 includes a wireless vibration sensor conne

27 steps of the Gearbox Repair and rebuilding

 27 steps of the Gearbox Repair and rebuilding: Step 1 Cleaning exterior of Gearbox and identification. Step 2 Remove all bolts from the gearbox. Step 3 Disassembly for Gearbox preliminary evaluation of the condition and repair required Step 4 Mag inspect Gearbox. Step 5 check all Gears. Step 6 Customer communication of health of the Gearbox. Step 7 Parts to be repaired or, reverse engineered parts where needed required for Gearbox rebuild. Step 8 Failure analysis during complete disassembly and evaluation of the component wear and damage. Step 9 Cleaning all internal components and housing. Step 10 Check all bearings diameters in house. Step 11 Check all shafts Step 12 inspect all Gears. Step 13 Set up check line bore of the gearbox. Step 14 Repair and rebuild Gears back to O.E.M Step 15 Replacing all bearings seals and gaskets Step 16 Repair and rebuild all shafts again to O.E.M Step 17 Realigning all gears shafts and bearings back to O.E.M Step

Thermal growth: how to identify, quantify and deal with its effects on turbomachinery

Thermal growth, as used in the field of machinery alignment, is machine frame expansion resulting from heat generation. The generation of heat, of course, is caused by operational processes and forces. Materials subjected to temperature changes from heat generation will expand by precise amounts defined by their material properties. In turbomachinery, thermal growth results from the temperature differences occurring between the at-rest and running conditions. Generally speaking, the greater the temperature difference, the greater the thermal growth. The magnitude of the growth can be calculated from three variables: ∆ T (temperature difference) C   (coefficient of thermal expansion) L    (distance between shaft centerline and machine supports) When machinery begins to generate heat, the temperature difference between at-rest and running conditions will cause thermal expansion of the machine frame, thereby bringing about the movement of the shaft centerlines. This can produce changes in

John Crane's Type 28 Dry Gas Seals: How Does It Work?

How Does It Work? Highest Pressure Non-Contacting, Dry-Running Gas Seal Type 28 compressor dry-running gas seals have been the industry standard since the early 1980s for gas-handling turbomachinery. Supported by John Crane's patented design features, these seals are non-contacting in operation. During dynamic operation, the mating ring/seat and primary ring/face maintain a sealing gap of approximately 0.0002 in./5 microns, thereby eliminating wear. These seals eliminate seal oil contamination and reduce maintenance costs and downtime. John Crane's highly engineered Type 28 series gas seals incorporate patented spiral-groove technology, which provides the most efficient method for lifting and maintaining separation of seal faces during dynamic operation. Grooves on one side of the seal face direct gas inward toward a non-grooved portion of the face. The gas flowing across the face generates a pressure that maintains a minute gap between the faces, optimizing flui

Technical questions with answers on gas turbines

By NTS. What is a gas turbine? A gas turbine is an engine that converts the energy from a flow of gas into mechanical energy. How does a gas turbine work? Gas turbines work on the Brayton cycle, which involves compressing air, mixing it with fuel, and igniting the mixture to create a high-temperature, high-pressure gas. This gas expands through a turbine, which generates mechanical energy that can be used to power a variety of machines and equipment. What are the different types of gas turbines? There are three main types of gas turbines: aeroderivative , industrial, and heavy-duty. Aeroderivative gas turbines are used in aviation and small-scale power generation. Industrial gas turbines are used in power generation and other industrial applications. Heavy-duty gas turbines are typically used in large power plants. What are the main components of a gas turbine? The main components of a gas turbine include the compressor, combustion chamb