EPT 07-T-07 Dry Gas Seals for Centrifugal Compressors

Version 0

Dry Gas Seals for Centrifugal Compressors

EPT 07-T-07

May 1998 Draft

Scope

This Mobil Engineering Practice Tutorial (EPT) is limited to dry running type shaft seals for centrifugal type gas compressors. These seals are commonly referred to as "Dry Gas Se als." Dry gas seals require no liquid lubrication whereas the more conventional compressor seals require a supply of seal oil.

EPT 07-T-07 Dry Gas Seals for Centrifugal Compressors May 1998 Draft

© Mobil Oil,1998 2 of 34

Table of Contents

Scope...................................................................................................................................................1

1. References..................................................................................................................................5

1.1. MEPS–Mobil Engineering Practices............................................................................5

1.2. Mobil Data Sheets ..........................................................................................................5

1.3. API–American Petroleum Institute ...............................................................................5

2. General ........................................................................................................................................5

3. Seal Application Considerations ..........................................................................................6

3.1. Operating Data ................................................................................................................6

3.2. Maximum Allowable Operating Conditions .................................................................6

3.3. Seal Conversion Justification........................................................................................7

3.4. General Contractor for Feasibility Study.....................................................................8

3.5. Selection of Dry Gas Seal Manufacturer.....................................................................8

3.6. Rotor Dynamics Analysis Study...................................................................................8

3.7. Required Remachining of Shaft and End Covers......................................................9

3.8. Proposed Control System Design ................................................................................9

3.9. Additional Requirements ................................................................................................9

4. Seal Design Considerations..................................................................................................9

4.1. O-Ring Seal Location .....................................................................................................9

4.2. Shaft-to-Sleeve Clearance ..........................................................................................11

4.3. Tolerance Rings ............................................................................................................11

4.4. Placement and Design of Anti-Rotation Devices.....................................................11

4.5. Bidirectional or Unidirectional Design........................................................................12

4.6. Seat Rings......................................................................................................................12



4.7. Shaft Lock Nut Design .................................................................................................13

4.8. Overspeed Spin Testing of Seal Ring .......................................................................13

4.9. Balancing Procedure for Rotating Sub-Assembly ...................................................13

4.10. Expected and Guaranteed Leakage Rates ..............................................................13

4.11. Impeller Side Labyrinth Seal .......................................................................................14

4.12. Start-Up Under Vacuum Conditions ..........................................................................14

4.13. Oil Separation Seal.......................................................................................................15

4.14. Supply Responsibility ...................................................................................................15

4.15. Documentation ..............................................................................................................15

5. Control System Design.........................................................................................................16

5.1. Simplicity of Design ......................................................................................................18

5.2. Definitions ......................................................................................................................18

5.3. Control Subsystems .....................................................................................................18

6. Factory Test .............................................................................................................................22

6.1. Operating Conditions ....................................................................................................22

6.2. Overspeed Spin Test....................................................................................................22

6.3. Balancing ........................................................................................................................22

6.4. Seal Testing Procedure ...............................................................................................23

6.5. Disassembly Inspection ...............................................................................................23

6.6. Compressor Testing (Mechanical and Performance) .............................................23

7. Packaging and Shipment......................................................................................................24

8. Remachining End Covers and Rotor.................................................................................24

9. Installing Dry Gas Seals ........................................................................................................24

9.1. Preparation ....................................................................................................................24

9.2. Rotor Installation ...........................................................................................................25



9.3. Seal Installation.............................................................................................................25

9.4. Non-Drive End Seal Assembly Installation ...............................................................26

9.5. Oil Separation Seal.......................................................................................................30

9.6. Thrust Bearing ...............................................................................................................30

9.7. Shaft Lock Removal .....................................................................................................31

9.8. Journal Bearing .............................................................................................................31

9.9. Drive End Seal ..............................................................................................................31

10. Commissioning and Operation ...........................................................................................31

10.1. Separation Air/Gas .......................................................................................................31

10.2. Static Leakage...............................................................................................................32

10.3. Pre-Lubrication..............................................................................................................32

10.4. Start-Up ..........................................................................................................................32

10.5. Control System Adjustment.........................................................................................32

10.6. Data Collection..............................................................................................................32

10.7. Leakage..........................................................................................................................33

10.8. Maintenance ..................................................................................................................33

11. Seal Removal...........................................................................................................................33



1. References

The following publications form a part of this EPT. Unless otherwise specified herein, use the latest edition.

1.1. MEPS–Mobil Engineering Practices

MP 15-P-04 Centrifugal Compressors

MP 15-P-09 Lube & Seal Oil Systems

MP 15-P-22 Packaged Centrifugal Compressor

1.2. Mobil Data Sheets

Mobil Data Sheets Mobil Data Sheet Home Page

T07T0701 Dry Gas Seals for Centrifugal Compressors - Dry Gas Seal Application - Customary Units

1.3. API–American Petroleum Institute

API STD 617 Centrifugal Compressors for Petroleum, Chemical, and Gas Service Industries Sixth Edition

2. General

The conversion of centrifugal compressors from lubricated type shaft seals to dry gas seals and specification of dry gas seals for a new compressor shall be in accordance with requirements of this MEP, unless superceded by more stringent local regulations.

Refer to MP 15-P-09 for additional reference.



3. Seal Application Considerations

It is recommended that a study be carried out to determine the feasibility of converting a particular gas compressor from conventional lubricated shaft seals to dry gas seals. The study shall include the following:

3.1. Operating Data

The operating conditions for the subject application shall be recorded on the Mobil Data Sheets for this MEP.

3.2. Maximum Allowable Operating Conditions

3.2.1. Operating Limits MP 15-P-04 and MP 15-P-22 recommend the following operating limits for dry gas seals:

Seal Pressure 11,031 kPag (1600 psig) (suction pressure)

Seal Temperature 149°C (300°F)

Seal Rim Speed 91 m/sec (300 ft/sec) (at outer edge of seal face)

3.2.2. Hydrocarbon Service For large size centrifugal compressors in hydrocarbon service, the sealing pressure is normally the limiting parameter for the application of dry gas seals.

3.2.3. High Pressure Gas Injection Service In general, dry gas seals are suitable for all compressors in gas processing, gas gathering and gas transmission services. The only E&P service for which dry gas seals are currently marginal is high pressure gas injection service where a sealing pressure of approximately 17,500 kPag (2500 psig) is encountered. For high pressure applications, request that the seal manufacturer supply installation lists and references for similar design dry gas seals operating at elevated pressures.

3.2.4. Carbon Rings and Metallic Seat Rings Dry gas seals employ pairs of carbon rings and metallic seat rings. Refer to Figures 1 and 2. Seal capsules are available with one, two or three sealing faces. Capsules with two seal faces are called "tandem" seals and are the



most popular arrangement for centrifugal compressors in gas processing service. In a tandem seal, the first seal carries the whole load and the second seal is a safety backup.

Figure 1: Seal Face Arrangement

Figure 2: Seal Swirl Pattern

3.3. Seal Conversion Justification

Factors to be considered in justifying the conversion from conventional lubricated type shaft seals to dry gas seals are:

1. Process contamination

2. Safety considerations

3. Environmental pollution considerations

4. Seal oil consumption

5. Reliability



6. Maintainability

7. Operability

8. Operating costs

3.4. General Contractor for Feasibility Study

It is recommended that the compressor manufacturer be selected to perform the feasibility study and to receive the purchase order if the results of the study are positive. The compressor manufacturer knows all the critical dimensions and data concerning the compressor and is the best qualified to perform the feasibility study.

A possible disadvantage of this arrangement is that it is sometimes difficult to communicate with the seal vendor when the communications pass through the compressor manufacturer. Advise the compressor manufacturer that some technical communications may occur directly between Mobil and the seal manufacturer.

3.5. Selection of Dry Gas Seal Manufacturer

The following companies are currently in the business of manufacturing dry gas seals. John Crane Co. is the leader in the industry and probably has more seals in operation than all the others combined. John Crane's prices appear to be very high, but more competition from other vendors shall hopefully reduce the price of dry gas seals in the future.

1. John Crane Co., Morton Grove, Illinois and Slough, England

2. Kaydon Seal Co., Baltimore, Maryland

3. Stein Seal Co., Kulpsville, Pennsylvania

4. EG&G, Sealol Division, Cranston, Rhode Island

5. Pacific Co., Dortmund, Germany

6. Burgmann Co., Wolfratshausen, Germany

3.6. Rotor Dynamics Analysis Study

Require that the compressor manufacturer perform a rotor dynamics analysis with both wet and dry seals. If the rotor is marginally unstable with wet seals, there may be a problem operating with dry seals. Lubricated seals (bushing type) at high operating pressures sometimes act as secondary bearings to help stabilize the shaft. Dry gas seals give little or no lateral support to the shaft. Therefore, always require that a rotor dynamics analysis be performed by the compressor manufacturer.



3.7. Required Remachining of Shaft and End Covers

3.7.1. Bushing Type Seal If the wet seal is a bushing type, the cavity shall be rather small and it may be necessary to enlarge the cavity to receive the dry seal capsule. Enlarging the cavity shall require remachining the end cover or purchasing new end covers, either of which can significantly affect the economics of converting to dry gas seals.

3.7.2. Mechanical Type Seal Remachining may not be required if the wet seal is a mechanical type seal (ISO-Carbon). Because the seal cavity is rather large, it is probable that the new dry gas seal can be designed to fit the existing cavity.

3.8. Proposed Control System Design

The feasibility study shall include the proposed control system to support the dry gas seals. Include manufacturer's names and model numbers of the major items, set points, etc.

3.9. Additional Requirements

The feasibility study shall also include the following:

1. Full size layout drawings showing the proposed dry gas seal installed in the seal cavity

2. Full size machining drawings for remachining the shaft and the seal cavity, if required

3. Total cost and delivery for all new materials

4. Estimated downtime for retrofit

4. Seal Design Considerations

The following Sections concern the design of the dry gas seal assemblies.

4.1. O-Ring Seal Location

It is recommended that the O-ring seal, Figure 3, Item 3, between the shaft and the seal assembly be placed in the shoulder at the inboard end of the seal assembly. This



placement of the O-ring in the end shoulder is preferred over placing the O-ring in the bore of the seal where it can be cut during installation.

Figure 3: Seal Cross-Section View

4.1.1. O-Rings in the Bore If the O-ring are placed in the bore, it is recommended that the shaft have steps in its diameter so the internal O-ring is not required to slide a long distance over a constant diameter. O-rings in the bore shall be located in a groove in the shaft rather than a groove in the sleeve. The O-ring can fit tightly in an external groove as opposed to being loose in an internal groove where it may fall out and be cut during installation.



4.2. Shaft-to-Sleeve Clearance

4.2.1. Eccentricity Reduction The clearance between shaft and sleeve, Figure 3, Item 6, shall be kept to a minimum to reduce eccentricity that can cause unbalance due to the relatively heavy weight of the tungsten carbide ring.

4.2.2. Allowable Tolerances There shall be sufficient clearance between the shaft and sleeve to ensure that assembly shall be possible in the field under unfavorable conditions. The seal capsule drawing shall show allowable tolerances for the shaft outside diameter and sleeve inside diameter.

4.3. Tolerance Rings

Some seal manufacturers install tolerance rings (Figure 3, Item 7) in the inside of the seal sleeve to keep the seal concentric with the shaft. The tolerance ring is a strip of corrugated stainless steel metal.

• If the tolerance ring is too loose, it cannot maintain concentricity.

• If the tolerance ring is too tight, installing the seal shall be difficult. Request that the vendor show the minimum diameter of the tolerance ring corrugations and the amount of interference or crush (percentage).

4.4. Placement and Design of Anti-Rotation Devices

4.4.1. Keys/Pins An anti-rotation key (Figure 3, Item 8) or pin is normally installed between the shaft and the sleeve to prevent sleeve rotation on the shaft in case of a failure. Pins are preferred in tight fitting holes over keys.

• Verify that the location of the pin or key shall not allow the key to cut an O-ring during insta llation.

• The key is usually blind during installation, making the installation difficult. Request that a window or slot be cut in the installation plate so that the key can be observed engaging in the keyway during installation.

• Recommend that the keyway be twice as wide as the key to facilitate seal installation. This looseness in the key or pin fit should cause no problems because the tolerance rings are tight enough to carry the normal torque and prevent any reversing oscillations.



4.5. Bidirectional or Unidirectional Design

4.5.1. Unidirectional Seals Unidirectional seals are the most popular, but different seals are required for the drive end and non-drive end of the compressor. This increases the inventory for spare parts and there is always the danger that the wrong direction seal may be installed.

• To avoid mistakes of installing the seal on the wrong end, the seals shall be made physically different. This can be accomplished with pins, keys, different diameters, etc.

• The item that is different in seals of opposite rotation is the swirl pattern on the seat ring. Refer to Figure 2. It may be possible to have a seat ring with opposite swirl patterns on both faces. Then the seat ring could be reversed in an emergency to reverse the rotation. This will reduce inventory of spare parts.

4.5.2. Bidirectional Seals Bidirectional type seals are being offered by more vendors and have the advantage that one seal can be used as a spare for either end and there is no danger of installing a seal in the wrong end. However, the performance of bidirectional seals is said to be less reliable and tolerant than unidirectional seals. On easy applications, bidirectional seal performance may be acceptable.

4.6. Seat Rings

4.6.1. Material Tungsten carbide is the most popular material for seat rings, although silicone carbide has a better heat dissipation rate.

• Both tungsten carbide and silicone carbide are very brittle.

• Some seal manufacturers are considering using ductile metal with a hard surface coating.

• Other manufacturers are considering composite materials with carbon fibers, etc.

4.6.2. Shroud

The seat ring (tungsten carbide, etc.) shall be surrounded by a shroud made from ductile steel. The purpose of the shroud is to contain the pieces of the tungsten carbide ring if it shatters in service.



4.6.3. Drive Pins A pin or key is required to drive the tungsten carbide ring. In the past, keys were used between the sleeve and the inside bore of the tungsten carbide ring. This was the worst design because the pin hole caused a stress riser in the bore of the ring.

The next design was to use pins on the back side (face) of the carbide ring. This was better, but the latest development, and the best, is to drive the tungsten carbide from its outer surface with pins or flats between the inside of the shroud and the outside of the tungsten carbide.

4.7. Shaft Lock Nut Design

• The shaft lock nuts (Figure 3, Item 4) shall have left- and right-hand threads if the shaft is so designed.

• It is recommended that the shaft nuts have small cap screws or set screws in the face of the nut to prevent the nuts from loosening in service. The set screws in the shaft nuts are preferred to nylon plugs previously used by some manufacturers. The problem with the nylon plugs is that if the nylon plugs are too long, a false sense of the nut being tight may occur. Also, the retaining power of the nylon plugs is doubtful.

4.8. Overspeed Spin Testing of Seal Ring

An overspeed spin test shall be performed bringing the seal ring to a speed of 1.22 times the maximum continuous speed. This speed shall generate stresses of 1.50 times the normal stress levels. The overspeed spin test shall be performed after the seal ring is finish-machined.

4.9. Balancing Procedure for Rotating Sub-Assembly

1. Balance the rotating components and rotating sub-assembly per API STD 617 as a minimum. Verify that the balancing test shall be performed with a half key installed in the bore of the seal, if so equipped.

2. After the balancing is completed, remove the seal from the balancing mandrel, index it 180 degrees and make a final verification that the balance has not changed.

4.10. Expected and Guaranteed Leakage Rates

4.10.1. Leakage Rate for a Typical Dry Gas Seal The expected leakage rate for an assembly can be estimated by the following:



Metric Units:

( )

+

000,10N

203D

1.0P0198.0

Where L = standard liters per minute

P = sealing pressure, kPag

D = seal nominal diameter, mm

N = operating speed, RPM

Customary Units:

( )

+

000,10N

0.8D1.0P0045.0

Where L = standard cubic feet per minute

P = sealing pressure, psig

D = seal nominal diameter, inch

N = operating speed, RPM

4.10.2. Guaranteed Leakage Rate The guaranteed leakage rate shall be no more than twice the expected leakage rate.

4.11. Impeller Side Labyrinth Seal

Seal gas from the control system shall be injected between the impeller side labyrinth (Figure 3, Item 1) and the seal capsule. Should the labyrinth have an O-ring on its outside diameter or shall it be a tight fit in the cavity? The labyrinth shall have an anti-rotation pin or key, and puller holes for removing the labyrinth.

Consider using abradable type labyrinths to reduce the amount of seal gas consumption.

4.12. Start-Up Under Vacuum Conditions

Some compressors or processes shall be evacuated prior to start-up. For these applications, the seal gas stream shall be pressurized to maintain a positive pressure between the labyrinth seal and the primary dry seal face. In these situations, an abradable seal type labyrinth seal is recommended to reduce the amount of seal gas leakage into the evacuated compressor casing.



4.13. Oil Separation Seal

The purpose of the oil separation (Figure 3, Item 5) or barrier seal is to prevent lube oil from migrating down the shaft and into the dry gas seal. The oil separation seal can be either a common labyrinth type seal or a seal consisting of two carbon rings. The double carbon ring type seal is much superior and preferred to the common labyrinth seal.

4.13.1. Carbon Ring Type Seals The advantage of the carbon ring type seal over the common labyrinth seal is that the carbon ring type seal consumes less separation gas. The carbon ring type seal offers a better barrier against gas leaking into the bearing cavity and prevents the migration of lube oil down the shaft into the dry gas seal.

The carbon ring seal can be either a split ring design with a garter spring around the outside of the carbon segments or it can be a one piece carbon ring that fits snugly over the shaft or shaft sleeve.

4.13.2. Segmented Carbon Ring-Type Barrier Seals Segmented-type carbon rings are proved in aircraft jet engine service and are ideally suited for service as an oil separation seal.

Install two rings side-by-side and inject buffer gas (nitrogen) between the two rings. Supply pressure to be 35–70 kPag (5–10 psig). Buffer gas (nitrogen) consumption is only 50–100 standard l/min (2–4 SCFM) per compressor end.

4.14. Supply Responsibility

Obtain agreement as to which vendor shall supply the miscellaneous components such as inner labyrinth, retaining bolts, oil separation seal, shaft lock nut, drive key, anti-rotation pins, control system, etc. The bills of material shall include all required hardware and indicate which vendor shall supply each item.

4.15. Documentation

• The seal manufacturer shall supply a cross sectional drawing of the seal assembly and detailed installation and operating instructions.

• The compressor manufacturer shall supply:

− Drawings showing how the seal assembly and associated parts such as labyrinth seal, barrier seal, etc. fit into the seal cavities at both ends of the compressor

− Detailed installation instructions and complete bills of material



− All special tools and fixtures that are required to install or remove the dry gas seals

− Drawings showing the proper installation of the tools

− Written instructions explaining the use of the tools

• Request a drawing showing all of the following:

− Flange connections for the previous seal oil connections

− Connections to be plugged

− New services for the remaining ports.

− Flange or pipe size and pressure ratings

• Request a recommended spare parts list with prices.

4.15.1. Payment upon Receipt of Documentation Specify in the purchase order that 15 percent of the total payment shall be withheld until the required documentation is received by Mobil. List the documentation as a separate item on the purchase order.

5. Control System Design

The primary purpose of the control system for dry gas seals is to purge the dry gas seals with clean, dry, filtered gas which shall prevent contamination of the seal faces by dirty gas.

The control system also monitors the seal performance and warns of any changes from normal operating parameters. Refer to Figure 4 for a schematic of a basic control system. The numbered references refer to the numbered Sections of this document.



Figure 4: Control System Schematic



5.1. Simplicity of Design

The design of the control system shall be as simple as possible. One of the main advantages of dry gas seals is to increase the reliability of the compressor by eliminating the complex seal oil system. Keep the control system as simple as possible.

5.2. Definitions

• Seal Gas is introduced between the impeller labyrinth and the dry gas seal capsule. The main requirements of the seal gas are that it is clean and dry in order to keep the seal faces clean.

• Separation Gas is introduced between the dry gas seal and the bearings to prevent migration of oil into the seals and to prevent process gas leakage to atmosphere.

5.3. Control Subsystems

The dry gas seal control system is composed of the Seal Gas Subsystem and Separation Gas Subsystem.

5.3.1. Seal Gas Subsystem

5.3.1.1. Seal Gas Source

• Decide whether compressor discharge gas shall be the source for the seal gas or whether some other (possibly cleaner) source shall be used for the seal gas. Enter information on the Mobil Data Sheets for this MEP. Verify that the seal gas temperature is within the temperature limits set by the seal manufacturer.

• An alternate source of seal gas shall be employed when the compressor is shutdown. This shall prevent the introduction of foreign material into the seal during down time and during the time the compressor is being started and pressurized.

• If an alternate seal gas supply is to be used during downtime, provide a double check valve arrangement. As a result, the compressor discharge pressure shall automatically come into service when the compressor discharge pressure becomes hig her than the alternate supply.

5.3.1.2. Controlling Seal Gas Flow

• In the past, the most popular method of controlling seal gas flow was to use a pressure control valve to set the seal gas pressure at 70–100 kPag (10–15 psig) above compressor gas reference pressure. However, variations in the labyrinth clearance caused large variations in seal gas flow rates. Recycling large quantities of seal gas is inefficient and is not



required. It is only necessary to have a small positive flow of seal gas across the labyrinth to prevent contamination of the dry gas seal.

• The more efficient way to control the seal gas flow is to install small diameter flow orifices in the lines before the seal gas enters the compressor. Typical diameter for these orifices is approximately 0.125 in. This allows the pressure control valve to work against known orifice opening (0.125 in) rather than working against a labyrinth seal, which continually wears and changes through the life of the compressor. These two orifices also tend to equalize the seal gas flow to both ends of the compressor, which can sometimes be a problem on low-pressure compressors. This design was first seen on Solar Inc. compressors during 1997.

• The recommended method is to install a pressure control valve reference 15–20 psi above balance piston reference pressure and to install small (approximately 0.125 in) orifices downstream of the control valve. See Figure 4.

5.3.1.3. Filters for Seal Gas

The seal gas filters shall have a 5-micron nominal rating. Use two filters with isolating valves so that one filter element can be replaced while the other is in service. The bowl of the filters shall be equipped with bleed valves vented to a safe area.

Install a differential pressure indicator and alarm switch on the filters, such as the indicating switch manufactured by ITT-Barton.

5.3.1.3.1. Coalescing Type Filters

1. The filters shall be the coalescing type to capture any liquids. These coalescing type filters normally flow from inside to outside, which is the reverse of most oil filters.

2. Locate the coalescing filters downstream of the pressure-cut control valve to catch any liquids formed in the pressure reduction.

3. Perform flash calculations. If the calculations indicate that condensation shall occur in the filters, then drain traps shall be added to the bottoms of the filter housings.

5.3.1.4. Low Pressure Switch

Install a differential pressure switch to sound an alarm if the seal gas pressure falls below suction pressure.

5.3.1.5. Tubing and Piping

Stainless steel tubing or pipe shall be used throughout the seal control system, but as a minimum, downstream of the filters.



5.3.1.6. Flow Orifice in Primary Leakage

Install a relatively small flow orifice in the primary leakage lines. The purpose of this orifice is to hold a slight load against the secondary seal face to keep the seal cool and functional. Another advantage of the flow orifice is to elevate the leakage pressure above any pressure spikes that may occur in the flare header.

5.3.1.7. Primary Leakage Pressure Switch

• Install a pressure switch upstream of the needle valve in the primary leakage lines. Set this switch to sound an alarm if the leakage increases above normal levels.

• Experienced operators of dry gas seals prefer to have an alarm only (no shutdown) and to continue operating on the secondary seal until the problem can be investigated and the compressor shutdown manually, if required.

5.3.1.8. Leakage Measurements

Measuring the leakage from dry gas seals is difficult because the leakage rates are very low, in the order of 1 to 25 to 50 standard l/min (2 SCFM) per seal. Rotometers could be suitable for this service, but some process plants prohibit rotometers because of safety concern about the glass or plastic windows. Also, rotometers do not give reliable service for long-term operation because they typically become dirty or fouled and give erratic readings.

• It is not necessary to precisely measure the leakage on a continual basis. Pressure indicators on the leakage line upstream of the needle valve give a general indication of the leakage rate.

• Once per month, or more frequently as required, a more precise measurement of the leakage can be obtained with a set of portable, calibrated rotometers. Use a set of approximately three rotometers with different flow ranges. Connect the rotometers with a flexible line (Swagelok). The readings from these calibrated rotometers can then be trended for preventive maintenance.

5.3.1.9. Leakage Disposal Options

• Connect the primary vent to a collection header so that the leakage gas can be saved by recompressing.

• The small amount of leakage gas can be vented locally to the atmosphere through a separate vent pipe, if safety and environmental regulations permit such venting.

• Connect the leakage to a flare gas header. However, one problem with the flare header connection is that pressure spikes in the flare header may reflect back onto the seal control system and give false alarms. To avoid



problems with pressure spikes in the flare header, install a check valve and relief valve to vent to the atmosphere.

• On compressors with very low suction pressure and/or high flare header pressures, it may be advantageous to equalize the seal cavity pressures at some intermediate pressure above suction pressure.

5.3.2. Separation Gas Subsystem

5.3.2.1. Separation Gas Source

Inert gas such as nitrogen is preferred for separation service. If nitrogen is not available, a supply can be generated with one of the small membrane units that are readily available from several manufacturers. Low-grade nitrogen with up to 6 percent oxygen is acceptable for this service.

Many pipeline compressors in North America use compressed air for separation services because a source of nitrogen is not available in remote locations.

5.3.2.2. Separation Gas Pressure Control

• The separation gas supply can be controlled by a simple pressure regulator to hold approximately 20–35 kPag (3–5 psig) pressure against the oil separation seal or labyrinth. The consumption of separation gas is normally very low (150 standard l/min [5 SCFM] per end), especially if carbon ring-type barrier seals are used.

• Optional pressure indicator and Pressure Switch Low (PSL) switch can be added to sound an alarm if the separation gas supply pressure fails.

• Design the system so that the separation air/gas shall continue to flow after shutdown to prevent post-lube or pre-lube oil from leaking into the dry seal assemblies.

5.3.2.3. Secondary Leakage

The slight leakage from the secondary seal shall be piped to the atmosphere. This leakage stream is primarily separation gas (nitrogen) from the oil separation seal. Do not install any flow restrictions in the secondary leakage path, because flow restrictors shall prevent free venting and may force process gas leakage into the bearing housing.

• It is not necessary to measure the secondary leakage on a continual basis because the secondary leakage is primarily separation gas (nitrogen).

• The size of the secondary vent line shall be large tubing or pipe at least as large as the size of the secondary vent port on the compressor end cover.



5.3.2.4. Oil Drain Valves

Provide a manual drain valve from the low point in the chamber between the dry gas seal and the oil separation seal. Periodically opening this valve shall indicate whether lube oil is leaking past the separation seal into the dry gas seal cavity.

6. Factory Test

6.1. Operating Conditions

The following shall be agreed on before the start of the factory test.

Table 1: Operating Conditions

Rated Operating Conditions, Suction (Ps): Discharge (Pd):

Operating Speed N Rated: N Max. Continuous = x1.05:

Anticipated Seal Pressure:

Primary Leakage, Expected: Guaranteed:

Secondary Leakage, Expected: Guaranteed:

Serial No. Drawing No. Direction of Rotation

Drive End

NDE

6.2. Overspeed Spin Test

Conduct an overspeed spin test of the tungsten carbide ring to a speed of 1.225 times the maximum continuous speed. This shall verify that the brittle tungsten carbide material is free of flaws that may cause the ring to fail during normal operation.

6.3. Balancing

• Dynamically balance each rotating component separately, then assemble all components and balance as an assembly per API STD 617.

• Verify that a half key is installed prior to balancing. Double-check the procedure by removing the seal from the balancing mandrel, indexing it 180 degrees, and checking the balance a second time.



6.4. Seal Testing Procedure

Each dry gas seal assembly shall be tested statically and dynamically as follows:

• Static leakage, primary seal at 25, 50, 75 and 100 percent Pd

− Breakaway torque at 25, 50, 75 and 100 percent Pd

− Static leakage, second seal at 25, 50, 75 and 100 percent Pd

− Breakaway torque at 25, 50, 75 and 100 percent Pd

• Primary leakage at N rated and 50, 75, 100 and 125 percent Ps

− Secondary leakage at N rated and 50, 75, 100 and 125 percent Ps

• Primary leakage at N maximum continuous and 100 percent Ps

− Primary leakage after 60 minute run at N rated and 100 Ps

− Secondary leakage after 15 minute run at N rated and 100 Ps

− Simulate emergency shutdown with rapid depressurization

− Repeat static leakage tests on primary and secondary

6.5. Disassembly Inspection

Disassemble the seal assembly and inspect for abnormal contact patterns, rubs, scratches, etc.

6.6. Compressor Testing (Mechanical and Performance)

The seal manufacturer shall be notified of the procedure for testing the compressor at the compressor factory. It is customary to pull a vacuum on the compressor casing for the mechanical run test. During this vacuum run, a positive pressure shall be maintained against the primary seal by supplying seal gas at a faster rate than it is leaking past the labyrinth seal into the compressor casing. Abradable type labyrinth seals can help reduce this seal gas leakage rate.

During the mechanical and performance tests, take steps to verify that only clean gas is used for seal gas and that the seal faces are not back-pressurized by accidentally imposing a higher pressure on the downstream side.



7. Packaging and Shipment

• All O-rings in external grooves shall be replaced with new O-rings. Package the seal assembly in a hermetically sealed plastic bag with desiccant.

• Each dry gas seal shall be shipped with a spare set of external O-rings, silicone type O-ring lubricant, extra tolerance rings and super glue for the tolerance rings.

• Package the seal in a custom built wooden shipping container that includes a block of foam rubber with a cavity to receive the seal assembly. The shipping container shall have two hinges and a lock hasp to facilitate inspection.

8. Remachining End Covers and Rotor

• On some retrofits, it shall be necessary to remachine the end cover and/or the shaft. This remachining is to be carried out in accordance with the detailed instructions and drawings from the compressor manufacturer.

• Mark a "K" on both ends of the shaft to indicate the orientation of the seal drive keys.

• If possible, check-fit the dry gas seal on the shaft and in the end covers before the rotor is installed.

9. Installing Dry Gas Seals

Verify that the latest revision of the seal assembly cross-section drawings are available at the compressor site. Check the complete bill of materials to verify that all required parts, fasteners, O-rings and special tools are available.

9.1. Preparation

• Verify that the outside surfaces of the shaft are smooth and free of burrs and deep scratches. Verify that all required chamfers are present to prevent cutting the O-rings. Measure the outside diameters of the shaft and compare with design dimensions.

• Check the anti-rotation key or keyway for correct location, height, etc. Remove any burrs or sharp corners that may hinder installation. Verify that a "K" is marked on the end of the shaft to show the orientation of the key. (This mark shall be beneficial when it is necessary to change the seal assembly at some time in the future.)

• Verify that the shaft lock nut fits the threads on both ends of the shaft.



• Verify that internal diameters and dimensions for all seal cavities are in accordance with design tolerance.

• Verify that the inside surface of the cavity is smooth and free of burrs, deep scratches and sharp edges that may cut the O-rings. Verify that all required chamfers and radii are present.

9.1.1. Cleaning

• Remove any seal oil from internal passages with mineral spirits and/or hot steam and blow dry with compressed air.

• With compressed air, blow all cuttings and dirt from the cavity and from the drilled passages in the end cover.

9.1.2. Check-Fit Components

• Check-fit the seal cartridge on the shaft to verify that it shall fit. The internal tolerance strips may make installation difficult. Do not use hammers or knockers to install the seal as blows may crack the carbons.

• Check-fit the seal assembly in the cavity of the compressor end cover. Check fit the internal labyrinth seal. Verify that the anti rotation pin in the labyrinth is installed. Verify that the labyrinth shall go all the way "home," and shall not prevent the complete installation of the seal assembly.

9.2. Rotor Installation

• Install the rotor in the compressor and install the end covers. Install the seal drive key or keyway at the top position to facilitate seal installation.

• Space the rotor in the center of the stationary components.

9.3. Seal Installat ion

9.3.1. Thrust End Seal

• It is recommended that the thrust end seal be installed first. Install a shaft locking fixture on the opposite end of the shaft to lock its position axially.

• Measure the distance from the shoulder on the shaft to the flange face in the cavity to verify that the shaft is spaced the same for both ends and that the rotor is still in the center of the stationary components. If it is necessary to shift the rotor, verify that the axial float capability specified on the outline drawing is not exceeded. Grind the spacer washers (Figure 3, Item 2) to the correct thickness, if so equipped.



• Once the shaft is correctly spaced axially, lock the shaft in place with the fixture on the drive end of the shaft.

• Connect the seal gas and buffer gas connections to the end cover and blow the lines clean. Clean the inside surface of the cavity and the end of the shaft.

9.3.2. Labyrinth Seal Installation

• Clean the labyrinth seal and install a new O-ring with O-ring lubricant. Install the labyrinth over the end of the shaft and center the shaft in the housing with a shaft jack. Center the shaft within 0.1 mm (.005 in). Use a test rod of the proper length to center the shaft, which is much faster than using micrometers.

• Install the internal labyrinth seal. Verify that the anti-rotation pin or hole is at the correct orientation. Take frequent readings to verify that the seal is going in squarely and is not being cocked. When the labyrinth seal is fully installed, take depth micrometer readings to verify that it is installed to the correct depth in accordance with the outline drawing.

9.4. Non-Drive End Seal Assembly Installation

Select the correct seal for the non-drive end. The seal assembly shall be marked "non-drive end." Verify that the direction of rotation is correct. Record the part number of the seal and its serial number.

9.4.1. O-Ring Exercising The seal capsule shall be exercised prior to installation to minimize O-ring hang-up which possibly could cause high initial leakage. This exercising can be accomplished as follows:

1. Place the seal cartridge on a clean table with the installation plate facing up.

2. Loosen the inner row of socket head cap screws two complete revolutions.

3. Pull up on the installation plate until uniform contact is achieved with the socket head cap screws.

4. Push down on the installation plate and release/pull up five or six times.

5. Tighten the inner row of socket head cap screws. Loosen the outer row of socket head cap screws two complete revolutions.

6. Carefully rotate the seal assembly upside-down with the installation plate on the bottom.

7. Push down on the rotor (sleeve) and release/pull up five or six times.



8. Carefully rotate the seal assembly so that the installation plate is on the top.

9. Tighten the outer row of cap screws.

9.4.2. Tolerance Rings Inspect the tolerance rings on the inside of the seal to verify that they are securely positioned in their grooves. Some tolerance strips are spot welded at each end, and some are secured with Super Glue, in accordance with the following procedure:

1. Remove the old tolerance ring and all traces of glue or spot-welding from the groove in the sleeve bore.

2. Carefully cut the new tolerance strip between waves to slightly longer than required.

3. Curl the tolerance ring to provide the best possible fit in the groove before glu ing.

4. Trim ends between waves to leave a gap of approximately 5 mm (0.2 in) between ends.

5. Chamfer both ends of the tolerance strip.

6. Clean both ends of the tolerance strip and the groove in the vicinity of the "T" (balance) mark with solvent that is supplied with the repair kit.

7. Position the tolerance strip so that the gap aligns with the "T" on the sleeve.

8. Glue one end of the strip in position by applying the activator portion of the glue to the five waves at one end of the strip. Use the minimum amount of glue.

9. Apply the hardener portion to the groove in the vicinity of the "T" mark (balance mark).

10. Hold the end of the strip in place until the glue dries, which takes approximately 60 seconds.

11. Glue the other end of the strip following the same procedure. Insure that the strip is tight in the groove during the gluing of the second end with no looping in the tolerance strip. Apply even pressure to the strip to hold it in the groove while the glue is drying.

9.4.3. Preparation • Check the key or keyway inside the sleeve to verify correct width and

radial dimension from centerline.



• Measure the thickness of the seal flange with a micrometer at two diagonally opposite locations and record these readings for reference later in this procedure.

• Check-fit the shaft lock nut to the end of the shaft sleeve. Verify that the threads in the nut are the correct direction (left hand or right hand). Verify that the small socket head cap screws in the face of the nut are below the surface and that the nut-mating surface is smooth and free of burrs.

• Install new O-rings on the outside of the seal assembly and lightly lubricate with silicone grease.

• Loosen the installation plate cap screws on the inner diameter one revolution to allow for slight radial movement.

• Clean the end of the shaft and cover the bearing journal and vibration probe areas with a teflon sleeve or wrap with masking tape. This covering shall protect these surfaces from damage while the seal is being installed.

• Check the orientation of the keyway in the shaft and verify that it corresponds with the orientation of the key in the seal sleeve. With a fine-line felt tip pin, mark two parallel lines from the sides of the keyway to the end of the shaft. These lines shall help to align the key with the keyway.

• Lubricate the ins ide of the cavity, the outside of the shaft and the inside of the seal sleeve with Rocol anti-seize spray or equivalent.

9.4.4. Install O-Ring and Guide Rods

• Install the O-ring and O-ring lubricant on the shaft if so equipped.

• Install long guide rods (1 m [3 ft]) at the 10 and 2 o'clock positions to help support the weight of the seal assembly as it is being slid across the steps in the shaft.

9.4.5. Engaging Seal Assembly • Lift the seal assembly with the overhead crane attached to eye bolts in

the seal housing. Set the seal over the end of the shaft and onto the two long guide rods.

• Support the seal assembly by lifting on the ends of the two guide rods. Disconnect and remove the overhead crane. Slide the seal assembly up the shaft until it is 5 cm (2 in) away from engaging with the cavity.

• Verify that all the O-rings are in place, especially any small O-rings that connect to side, radial drilled ports.



• Verify that the key and keyway between the shaft and seal are in alignment. The key shall be between the two parallel lines that were marked on the shaft.

• Install a shaft jack and center the shaft in the housing within 0.1 mm (.005 in). Use a short rod of the proper length to center the shaft, which is much faster than using micrometers.

9.4.6. Seal Installation 1. Push the seal into place by hand until it engages with the housing.

2. Install four all-thread screws to pull the seal into the cavity. Do not use hammers or knockers to install the seal.

3. Install nuts on the four jack screws and tighten hand-tight. Take four readings (top, bottom, left and right) with a depth micrometer or dial caliper to verify that the seal face is parallel with the end cover. If the seal face is not parallel with the cavity, tighten one or two of the jack screw nuts to bring the seal into alignment.

4. Tighten all four nuts by one-half turn in sequence to pull the seal evenly into the cavity without cocking. Take readings frequently to verify that the seal is going in squarely.

5. Observe the key and keyway and verify that they are engaging properly as the seal is pulled into position. If not, it shall be necessary to remove the seal and reorient the sleeve slightly.

9.4.7. Verify Seal Position

• Pull the seal into the cavity until it bottoms metal-to-metal.

• Verify that it is all the way "home" by extending a depth micrometer through two of the diagonally opposite jack screw holes in the flange of the seal.

• The reading of the depth micrometer shall equal the thickness of the seal flange.

− Compare these readings with the flange thickness readings taken previously.

− These readings shall verify that the seal housing is fully installed in the cavity.

9.4.8. Installation Plate Removal Remove the installation plates. Place the installation plates in the seal shipping container and keep for possibly returning the seal to the factory at some time in the future.



9.4.9. Cap Screws • Install the cap screws that hold the seal flange against the shoulder in the

cavity.

• Cross-torque the screws in increments to the torque recommended by the compressor manufacturer.

• Verify that the axial distance from the seal flange to the shoulder on the shaft sleeve is correct in accordance with the drawings.

9.4.10. Shaft Lock Nut Installation

• Check the shaft lock nut to verify that the threads are clean and the set screws are retracted.

• Clean the threads on the shaft and lubricate. Install the O-ring on the shaft if so equipped. Verify that the nut-mating surface on the end of the sleeve is smooth and free of burrs.

• Install the shaft lock nut and tighten it with a special wrench.

• Verify that the nut is tight against the mating face and that there is no gap between the nut and end of sleeve. Tighten the locking set screws.

9.5. Oil Separation Seal

Remove any burrs or scratches from the oil separation seal. If it is a "wind-back" type design, verify that the correct rotation has been selected. Install the new O-ring and lubricate with O-ring lubricant.

• Check the nitrogen supply port to verify that it lines-up with the oil separation seal.

• Install the oil separation seal and retain with cap screws.

• Activate the nitrogen flow to verify that nitrogen flows from the gap around the shaft. Also, verify that the secondary vent is open to the atmosphere during this nitrogen testing so as not to accidentally back-pressure the seal faces.

9.6. Thrust Bearing

Install the thrust bearing assembly.

• Be careful not to shift the rotor axially when the thrust bearings are installed.

• If necessary, adjust the thrust bearing shims and spacers so that the shaft can keep its current axial position as defined by the locking fixture at the drive end.

• Take readings on the end of the shaft to verify that the rotor does not shift axially when the shaft locking fixture is removed.



9.7. Shaft Lock Removal

• Remove the shaft locking fixture from the drive end.

• Verify that the shaft has not moved axially after the locking fixture was removed.

9.8. Journal Bearing

Install the journal bearing in accordance with instructions from the compressor manufacturer.

9.9. Drive End Seal

Install the dry gas seal at the drive end of the compressor using the same procedure. Note that the direction of rotation shall be reversed of the seal assembly, the shaft lock nut and the windback seal.

10. Commissioning and Operation

• Install the drive coupling to prevent accidental rotation of the rotor as the compressor is being pressurized.

• Blow the seal gas and separation gas lines free of foreign material before connecting them to the compressor.

• Start introducing seal gas into the dry gas seals with an alternate source of seal gas. This shall be accomplished before any attempt is made to pressurize the compressor, because pressurizing the compressor casing may introduce foreign material into the seal faces unless the seal gas is activated first.

10.1. Separation Air/Gas

1. With the seal gas supply activated, start to introduce separation air/gas (nitrogen) into the oil separation seals.

2. It is important that the nitrogen be at a lower pressure than the seal gas to avoid back pressuring the seals. (Back pressuring the seals can possibly cause them to become cocked or wedged and fail to seal properly.)

3. Nitrogen flow shall prevent lube oil from migrating into the dry gas seals. The nitrogen flow shall be activated before any attempt is made to pre-lubricate the compressor.



10.2. Static Leakage

• Close the suction and discharge valves (if available) and slowly pressurize the compressor casing to suction pressure.

− The static leakage shall be less than 10 percent of the normal dynamic leakage rate. For example, for a seal with a dynamic leakage of approximately 28 l/min (1.0 SCFM), the static leakage shall be only 2.8 l/min (0.1 SCFM).

− It shall be necessary to connect a portable rotometer with low flow capability to measure the static leakage.

• If the static leakage is excessive, bump the seal supply pressure and/or the leakage flows in an effort to break the seal free so that it shall seal properly. The seal is probably leaking because the seal faces became cocked during installation.

• If the seal will not hold a static pressure with minimal leakage, it shall be necessary to remove the seal cartridge and take corrective action.

10.3. Pre-Lubrication

With the seal gas and nitrogen separation gas flowing, start to pre-lube the compressor. Verify that the lube oil flow is adequate and normal. Open a drain valve from the bottom of the cavity between the dry gas seal and the oil separation seal and verify that no oil is present in this chamber.

10.4. Start-Up

Start the compressor in accordance with normal and safe practices. As the compressor discharge pressure increases, the supply from the compressor discharge shall normally take over and replace the supply from the start-up alternate seal gas supply.

10.5. Control System Adjustment

• Monitor the seal gas flow to both ends and adjust the needle valves to balance the flows to both ends.

• Adjust the primary leakage needle valves to take 80 percent of the pressure drop on the primary seal and 20 percent on the secondary seal.

10.6. Data Collection

During the first 24 hours of operation, read and record the following information:

1. RPM, suction pressure and temperature, discharge pressure and temperature. Seal gas pressure before and after control valve.



2. Filter differential pressure, filter in service, A or B.

3. Primary leakage and pressure for both ends. Take the flow readings with a calibrated, portable rotometer.

4. Nitrogen pressure to separation seals. Oil leakage: Yes or no, drive end or non-drive end.

10.7. Leakage

If lube oil is leaking past the separation seal, investigate and stop the oil leak.

After 24 hours of operation, test the operation of the secondary seals by closing the primary leakage needle valves and imposing the full seal pressure against the secondary seals. Measure the secondary leakage with a portable rotometer. (Realize that most of the secondary leakage is actually nitrogen from the separation seals.)

• Continue taking the readings hourly for the next two days.

• Continue taking the readings once per shift for the next two weeks.

• Continue taking the readings once per day for the next two months.

10.8. Maintenance

• Take primary leakage readings once per month with a portable set of calibrated rotometers. To verify that it is still sealing properly, close the primary leakage block valve and place the full load against the secondary seal. After the secondary seals are tested, return to normal seal operation.

• When the compressor is shut down, it is recommended that the seal gas continue to flow. This shall prevent contamination of the seal faces by any foreign material that may occur as the compressor is blown down and pressurized.

• It is recommended that the separation air/gas (nitrogen) continue to flow when the compressor is shut down so that no lube oil from pre-lube or post-lube shall leak into the dry gas seal assemblies.

11. Seal Removal

1. Position the seal drive key on the bottom position to line up with the "K" on the end of the shaft. If the key is loose, it shall fall downward and not catch the shaft when the seal is removed.

2. Remove the shaft lock nut after relaxing the set screws.

3. Install the installation plate. Leave the inner row of cap screws loose by one turn to allow slight radial movement.



4. Install four jack screws through the seal flange and jack against the shoulder of the seal cavity. Turn each jack screw one-half turn in sequence to withdraw the seal evenly and prevent cocking.

5. When the seal cartridge is removed, tighten all screws in the installation plates.

6. Place the seal in a clean plastic bag and place in the special shipping container provided with the new seal. Do not place any other loose items in the container which may damage the seal during transportation.

EPT 07-T-07 Dry Gas Seals for Centrifugal Compressors

Documents

Transcript of EPT 07-T-07 Dry Gas Seals for Centrifugal Compressors