William Forsthoffer

Centrifugal pumps require bearings to carry radial/axial loads. Bearing alternatives:  Anti-friction, hydrodynamic ring oil lubricated or hydrodynamic pressure lubricated. Oil lubricated (anti-friction) bearings are used in majority of process pumps to carry loads

Pressure lubricated hydrodynamic bearings are normally used for high pressure, high horsepower, high speed applications

Criteria in API Standard 610 for pressure lubrication: when product of pump rated horsepower and rated speed in rev/min is greater than 2.0 million

Pressure lubrication systems can either be integral or separate, but should include, as a minimum, an oil pump, reservoir, filter, cooler, controls and instrumentation

Ring oil lubrication may be applied to hydrodynamic journal bearings in less severe service (when dN factor is less than 300,000. A dN factor is the product of bearing, size (bore) in millimeters and the rated speed in revolutions per minute).

Mechanical seals are often used in pumps handling hazardous as well as non- hazardous liquids that must be contained within the unit.

The single seal arrangement is most widely used in process industry.

Single seal design consists of a rotary face in contact with a stationary face.

For most services, a carbon face mating against tungsten carbide is satisfactory

Seals offer the advantages of long life, low maintenance and high reliability.

In general, seals handling light specific gravity liquids at low temperature and high  vapor pressure give most problems in the field.

Materials for cold service seals must be suitable for temperatures of start-up, cool down and running ; the atmospheric side must be held above 0

F) to prevent ice formation; and there must be enough liquid at the seal surfaces

Successful operation of any seal depends largely on correctly specifying liquid conditions of vapor pressure, temperature specific gravity, etc.

API Standard 682 is an excellent resource for overall mechanical seal application guidelines.

The pressure in the seal chamber (stuffing box) must be at least 25psig above the pump suction pressure.

Articles

Bearing and mechanical seal application in pumps

BEFORE NOT AFTER: THE FIELD MONITORING PHASE

This is the third and final article in our Ensuring Critical Machinery Excellence Series - Before Not After. It is important that the steps outlined in this series are accomplished early in the process to prevent unscheduled shutdowns and failures.

After critical machinery is successfully tested (Part II in the Turbomachinery International Handbook 2017) and installed on site, the final phase is to establish an effective monitoring program.

Condition monitoring should be done on the key components within each train. If done well, the user will be able to determine a change in condition before it becomes a detrimental issue and causes an unplanned shutdown.  Additionally, the user can accurately determine a turnaround scope based on condition, not time.

The concept of Component Condition Monitoring (CCM) can be used properly to maximize user profit. Whether a pump, compressor, steam turbine, gas turbine, motor or even a gearbox, each one contains the same five major components which need to be monitored.  They are the rotor, journal bearings, thrust bearings, seals and auxiliary systems.

The most important component to monitor and usually the most overlooked is the rotor. While the perception is that vibration monitoring is looking at the condition of the rotor within the machine, that is not entirely true. While useful, vibration monitoring is the equivalent to checking one’s pulse. It does not tell the whole story and is usually the effect of something going within the rotor.

Since the entire process is in direct contact with the rotor, the only way to truly monitor its condition is by analyzing the operating data and calculating rotor efficiency. This can then be trended with flow, head, speed, and sometimes Inlet Guide Vane (IGV) position in order to tell if the machine is fouling or wearing its internal components. The key point is to trend this information continuously for critical machinery and begin to investigate an issue immediately when any change is observed.

Monitoring the condition of the journal bearings mainly consists of monitoring vibration (whether casing for anti-friction bearings or shaft vibration for hydrodynamic bearings) and Babbitt temperature. It is easy to fall into the trap of operating until yellow alarm values are hit. But it is vital to catch an increase in vibration or temperature well before it hits the alarm in order to diagnose the problem and resolve the issue before a component failure.

By using existing diagnostic systems, it is possible to set up a soft alarm for vibration and temperature (or other values like 1x vibration or phase angle change to detect unbalance) to detect an immediate change in journal bearing condition. Typically, soft alarms are set at around 30% greater than normal running values.

Thrust bearings are straight forward to monitor as shaft displacement and Babbitt temperature values are set up in the Distributed Control System (DCS) and in diagnostic systems. Just like journal bearings, though, these values need to be monitored for change. When a parameter is in alarm, it is generally too late to save that component and a shutdown many be imminent.

One parameter often missed is balance line differential pressure (DP), which helps determine the condition of the balance drum labyrinth seal. As the balance line DP increases, this indicates more leakage, and therefore more thrust towards suction and higher bearing pad temperatures on the suction side thrust assembly (usually marked as the active pads).

If not monitored and the displacement and temperature increase, it is likely that the thrust bearing will have to be replaced. Yet these values will only start again. If the balance drum labyrinth is worn, wait until the next planned outage when the compressor can be opened to replace the labyrinth.

The main parameter to be monitoring for seals is leakage. Investigate if leakage rates increase more than 10%. By monitoring the seal support system, you can generally observe why leakage rates rise.

Auxiliary systems are often not monitored adequately as many of their components have to be monitored locally since their instrumentation is not brought into the DCS. Therefore, compile a list of items and parameters to check as operators do their rounds.  The most important parameters to check in all auxiliary systems are control valve positions.

Control valves in these systems are basically flow meters. Any control valve that holds a steady position has maintained the same flow (because pressures upstream and downstream do not change significantly). Likewise, an increase in valve position indicates increased flow to the component it supports. This tells the user many things about the system and the component.

CCM is useful as long as the correct parameters are captured and trended. Trending requires that data gathering begins when component condition is good to establish a baseline. All components should be trended on the same time basis in order to identify issues at their earliest stage. For example, a change in compressor efficiency will result in higher vibration if the reason for efficiency drop is fouling. The better component monitoring is continuously performed, the more efficient you will be in solving issues before they become a serious problem.

Michael S. Forsthoffer is President of Forsthoffer Associates Inc. (FAI). FAI has been providing machinery consulting to the global oil, gas and chemical industry since 1990.

William E. Forsthoffer is CEO of FAI. Further details on best practices pertaining to this article can be provided free of charge. For more information visit www.forsthofferassociates.com or contact michael@forsthofferassociates.com

ENSURING CRITICAL MACHINERY EXCELLENCE, PART III

BEFORE NOT AFTER: THE DESIGN AND TESTING PHASE

MICHAEL S. FORSTHOFFER & WILLIAM E. FORSTHOFFER

After critical machinery is properly selected during the purchase phase (Turbomachinery International, July/Aug 2016), the next phase concerns optimum design and testing. All critical machinery is custom designed and must be properly checked during the engineering, manufacturing and testing phases to assure maximum safety, reliability and on-stream time. This phase begins with a vendor coordination meeting (VCM). The objective is to confirm that the scope of supply and design of each individual component are in accordance with the purchase order. To assure an effective VCM, the meeting must be held in the vendor’s works with an agenda that covers all aspects of machinery design and notes the vendor engineers that are required to attend from each department.

The EPC contractor machinery specialist is responsible for the agenda, but it must be reviewed by the end user and sent to the machinery vendor a minimum of two weeks prior to the meeting. It is important that this meeting be conducted during the early design phase (approximately six weeks after order placement). Any delay beyond that exposes the user to scope, design omissions and schedule delays. Be sure also that the end user is represented by an experienced machinery specialist, operator and maintenance engineer as a minimum. It is vital that detailed meeting minutes be kept by all parties, reviewed and confirmed with action points, responsible parties and due dates at the end of each day of the meeting. Depending on the amount of machinery involved, meeting duration can be two to five days and possibly longer.

Depending on the risk class, machinery complexity and component experience, design audit meetings may be required. The need for them would have been determined during the purchase phase of the machinery based on component proven experience (Figure). A design audit is conducted by either the EPC contractor, end user or a 3rd party consultant. Regardless of who conducts the audit, the leader must have previous design experience with the type of the machinery concerned. An agenda should be sent to the vendor a minimum of one month in advance to assure the vendor is prepared and has the proper personnel involved. Design audits, like the coordination meeting, must be conducted early in the design phase so that any modifications will not affect the manufacturing schedule. Engineering design audits as well as manufacturing audits may be conducted. If a manufacturing audit is conducted, the lead must have prior manufacturing experience.

Project Success After the coordination meeting and any design audits are completed, timely vendor document and drawing issue and review by the EPC contractor and end user assure proper design, minimizing additional costs and schedule delays. Document and drawing issue dates and maximum allowable review times must be established during the VCM. Project success depends on strict adherence to this schedule. If the purchasing phase and VCM/design audits were conducted thoroughly, the documents and drawings should take little time to review as many of these details were already discussed.

As a concerned end user, it is your responsibility to ensure that from the beginning of the project, EPC and vendor personnel are experienced and that teamwork is emphasized. This is achieved by early review of CV’s and personal interviews. On occasion, certain individuals may not be acceptable based on their experience level and interpersonal skill characteristics. As an additional tip: during review of documents and drawings, watch out for vendor and EPC “cut and paste” documents and drawings. It is in their interest to minimize engineering time. As a result, frequent use of previous project material may be utilized and this may not be in strict accordance with your project design requirements. Therefore, always follow up on the next issue to assure that your comments have been incorporated.

After a period of approximately 10 to 14 months depending on machinery content and vendor workload, the critical machinery is ready for the factory acceptance test (FAT). The importance of properly conducting and witnessing this test cannot be overemphasized. FAT requirements are part of the purchase specification and should be noted on the appropriate data sheet. Specific details should be established during the VCM. Require that the FAT agenda be submitted for review a minimum of two months prior to the FAT Test Date. A review of the FAT agenda by the EPC and the end user, and a timely response to all comments/questions by the vendor is the first step to ensuring that the FAT will be conducted with minimal misunderstandings and delays.

The next step is for the EPC and end user to require a pre-test meeting the day before the FAT. A pre- test meeting agenda is issued by the EPC to the vendor at least two weeks prior to the FAT. This pre-test meeting confirms the test agenda and discusses any other items the EPC or user want to check during the test not listed in the test agenda. Vendors are generally flexible in allowing other checks as long as they are discussed in the pre-test meeting and can be justified. The last step prior to the FAT is for the EPC and end user to establish a FAT team of experienced machinery, operations and maintenance personnel who will be responsible for field installation and start up.

During the FAT, the main objective is to confirm that the design and manufacturing of the equipment is acceptable for field operations. The performance test should be carried out at 80% or more of the actual operating speed. This allows for proper mechanical checks as well under load tests, which are important especially if the mechanical test is to be performed under vacuum conditions. It is useful during the performance test to have a member of the test team calculate performance based on operating data from vendor tests. While vendor calculations will most likely be correct, this gives the vendor a chance to verify that everything is correct as well as providing the team with instant test curves and knowledge of how to trend performance in the field. The mechanical test should confirm that mechanical components are sound at operating speed. This includes confirming vibration and axial displacement are within acceptable tolerances, and dry gas seal leakages are minimal.

Note that it should be stated during the pre-test meeting that you would like to open the dry gas seal secondary drain port (make sure this port breaks through the area of the seal at the bottom dead center of the cavity) if you have a tandem seal. There should be zero oil leaking through this port, indicating that installation and assembly were properly performed.

One final key check to be conducted during the FAT (for barrel compressors) is to utilize the job-specific bundle removal tooling when changing the rotor out to the next one for the other mechanical test. This confirms that the tooling provided with the equipment will properly work in the field. A senior maintenance representative from the end user should be present during this check to ensure he or she is aware of the proper procedure to use this tooling. Note: this tooling should not be used in the field for at least five years.

Once all of these tests are finished, all documents are to be signed by the proper parties and a report provided by the vendor in a timely manner. The vendor will generally provide rough hand-drawn curves to the team directly after the test with the proper drawn curves in the final report.

Michael S. Forsthoffer is President of Forsthoffer Associates Inc. (FAI). FAI has been providing machinery consulting to the global oil, gas and chemical industry since 1990. William E. Forsthoffer is CEO of FAI. Further details on best practices pertaining to this article can be provided free of charge. For more information visit www.forsthofferassociates.com or contact michael@forsthofferassociates.com

ENSURING CRITICAL MACHINERY EXCELLENCE, PART TWO

BEFORE NOT AFTER THE DESIGN AND TESTING PHASE

After critical machinery is properly selected during the purchase phase (Turbomachinery International, Sept/Oct 2016, p.41), the next phase concerns optimum design and testing. All critical machinery is custom designed and must be properly checked during the engineering, manufacturing and testing phases to assure maximum safety, reliability and on-stream time. This phase begins with a vendor coordination meeting (VCM). The objective is to confirm that the scope of supply and design of each individual component are in accordance with the purchase order.

To assure an effective VCM, the meeting must be held in the vendor’s works with an agenda that covers all aspects of machinery design and notes the vendor engineers that are required to attend from each department. The EPC contractor machinery specialist is responsible for the agenda, but it must be reviewed by the end user and sent to the machinery vendor a minimum of two weeks prior to the meeting. It is important that this meeting be conducted during the early design phase (approximately six weeks after order placement).

Any delay beyond that exposes the user to scope, design omissions and schedule delays. Be sure also that the end user is represented by an experienced machinery specialist, operator and maintenance engineer as a minimum. It is vital that detailed meeting minutes be kept by all parties, reviewed and confirmed with action points, responsible parties and due dates at the end of each day of the meeting. Depending on the amount of machinery involved, meeting duration can be two to five days and possibly longer.

Depending on the risk class, machinery complexity and component experience, design audit meetings may be required. The need for them would have been determined during the purchase phase of the machinery based on component proven experience. A design audit is conducted by either the EPC contractor, end user or a 3rd party consultant. Regardless of who conducts the audit, the leader must have previous design experience with the type of the machinery concerned. An agenda should be sent to the vendor a minimum of one month in advance to assure the vendor is prepared and has the proper personnel involved.

Design audits, like the coordination meeting, must be conducted early in the design phase so that any modifications will not affect the manufacturing schedule. Engineering design audits as well as manufacturing audits may be conducted. If a manufacturing audit is conducted, the lead must have prior manufacturing experience. After the coordination meeting and any design audits are completed, timely vendor document and drawing issue and review by the EPC contractor and end user assure proper design, minimizing additional costs and schedule delays.

Document and drawing issue dates and maximum allowable review times must be established during the VCM. Project success depends on strict adherence to this schedule. If the purchasing phase and VCM/design audits were conducted thoroughly, the documents and drawings should take little time to review as many of these details were already discussed. As a concerned end user, it is your responsibility to ensure that from the beginning of the project, EPC and vendor personnel are experienced and that teamwork is emphasized. This is achieved by early review of CV’s and personal interviews.

On occasion, certain individuals may not be acceptable based on their experience level and interpersonal skill characteristics. As an additional tip: during review of documents and drawings, watch out for vendor and EPC “cut and paste” documents and drawings. It is in their interest to minimize engineering time. As a result, frequent use of previous project material may be utilized and this may not be in strict accordance with your project design requirements. Therefore, always follow up on the next issue to assure that your comments have been incorporated.

After a period of about 10 to 14 months depending on machinery content and vendor workload, the critical machinery is ready for the factory acceptance test (FAT). The importance of properly conducting and witnessing this test cannot be overemphasized. FAT requirements are part of the purchase specification and should be noted on the appropriate data sheet. Specific details should be established during the VCM. Require that the FAT agenda be submitted for review a minimum of two months prior to the FAT Test Date. A review of the

FAT agenda by the EPC and the end user, and a timely response to all comments/questions by the vendor is the first step to ensuring that the FAT will be conducted with minimal misunderstandings and delays. The next step is for the EPC and end user to require a pre-test meeting the day before the FAT. A pre- test meeting agenda is issued by the EPC to the vendor at least two weeks prior to the FAT. This pre-test meeting confirms the test agenda and discusses any other items the EPC or user want to check during the test not listed in the test agenda.

Vendors are generally flexible in allowing other checks as long as they are discussed in the pre-test meeting and can be justified. The last step prior to the FAT is for the EPC and end user to establish a FAT team of experienced machinery, operations and maintenance personnel who will be responsible for field installation and start up. Design and manufacturing During the FAT, the main objective is to confirm that the design and manufacturing of the equipment is acceptable for field operations. The performance test should be carried out at 80% or more of the actual operating speed. This allows for proper mechanical checks as well under load tests, which are important especially if the mechanical test is to be performed under vacuum conditions.

It is useful during the performance test to have a member of the test team calculate performance based on operating data from vendor tests. While vendor calculations will most likely be correct, this gives the vendor a chance to verify that everything is correct as well as providing the team with instant test curves and knowledge of how to trend performance in the field. The mechanical test should confirm that mechanical components are sound at operating speed. This includes confirming vibration and axial displacement are within acceptable tolerances, and dry gas seal leakages are minimal.

Note that it should be stated during the pre-test meeting that you would like to open the dry gas seal secondary drain port (make sure this port breaks through the area of the seal at the bottom dead center of the cavity) if you have a tandem seal. There should be zero oil leaking through this port, indicating that installation and assembly were properly performed. One final key check to be conducted during the FAT (for barrel compressors) is to utilize the job-specific bundle removal tooling when changing the rotor out to the next one for the other mechanical test.

This confirms that the tooling provided with the equipment will properly work in the field. A senior maintenance representative from the end user should be present during this check to ensure he or she is aware of the proper procedure to use this tooling. Note: this tooling should not be used in the field for at least five years. Once all of these tests are finished, all documents are to be signed by the proper parties and a report provided by the vendor in a timely manner. The vendor will generally provide rough hand-drawn curves to the team directly after the test with the proper drawn curves in the final report.

ENSURING CRITICAL MACHINERY EXCELLENCE

BEFORE NOT AFTER:  THE DESIGN AND TESTING PHASE

By Michael S. Forsthoffer & William E. Forsthoffer

 Figure: Suggested design audit activity[/caption]

After critical machinery is properly selected during the purchase phase (Turbomachinery International, July/Aug 2016), the next phase concerns optimum design and testing. All critical machinery is custom designed and must be properly checked during the engineering, manufacturing and testing phases to assure maximum safety, reliability and on-stream time.

This phase begins with a vendor coordination meeting (VCM). The objective is to confirm that the scope of supply and design of each individual component are in accordance with the purchase order. To assure an effective VCM, the meeting must be held in the vendor’s works with an agenda that covers all aspects of machinery design and notes the vendor engineers that are required to attend from each department.

Depending on the risk class, machinery complexity and component experience, design audit meetings may be required. The need for them would have been determined during the purchase phase of the machinery based on component proven experience (Figure).

A design audit is conducted by either the EPC contractor, end user or a 3rd party consultant. Regardless of who conducts the audit, the leader must have previous design experience with the type of the machinery concerned. An agenda should be sent to the vendor a minimum of one month in advance to assure the vendor is prepared and has the proper personnel involved. Design audits, like the coordination meeting, must be conducted early in the design phase so that any modifications will not affect the manufacturing schedule. Engineering design audits as well as manufacturing audits may be conducted. If a manufacturing audit is conducted, the lead must have prior manufacturing experience.

After the coordination meeting and any design audits are completed, timely vendor document and drawing issue and review by the EPC contractor and end user assure proper design, minimizing additional costs and schedule delays. Document and drawing issue dates and maximum allowable review times must be established during the VCM.  Project success depends on strict adherence to this schedule. If the purchasing phase and VCM/design audits were conducted thoroughly, the documents and drawings should take little time to review as many of these details were already discussed.

As a concerned end user, it is your responsibility to ensure that from the beginning of the project, EPC and vendor personnel are experienced and that teamwork is emphasized. This is achieved by early review of CV’s and personal interviews. On occasion, certain individuals may not be acceptable based on their experience level and interpersonal skill characteristics.

As an additional tip: during review of documents and drawings, watch out for vendor and EPC “cut and paste” documents and drawings. It is in their interest to minimize engineering time. As a result, frequent use of previous project material may be utilized and this may not be in strict accordance with your project design requirements. Therefore, always follow up on the next issue to assure that your comments have been incorporated.

After a period of approximately 10 to 14 months depending on machinery content and vendor workload, the critical machinery is ready for the factory acceptance test (FAT).  The importance of properly conducting and witnessing this test cannot be overemphasized. FAT requirements are part of the purchase specification and should be noted on the appropriate data sheet. Specific details should be established during the VCM. Require that the FAT agenda be submitted for review a minimum of two months prior to the FAT Test Date. A review of the FAT agenda by the EPC and the end user, and a timely response to all comments/questions by the vendor is the first step to ensuring that the FAT will be conducted with minimal misunderstandings and delays.

During the FAT, the main objective is to confirm that the design and manufacturing of the equipment is acceptable for field operations. The performance test should be carried out at 80% or more of the actual operating speed. This allows for proper mechanical checks as well under load tests, which are important especially if the mechanical test is to be performed under vacuum conditions.

Authors: Michael S. Forsthoffer is President of Forsthoffer Associates Inc. (FAI). FAI has been providing machinery consulting to the global oil, gas and chemical industry since 1990.

The function of a gas seal system is naturally the same as a liquid seal system. The function of a fluid seal system, remembering that a fluid can be a liquid or a gas, is to continuously supply clean fluid to each specified seal interface point at the required differential pressure, temperature and flow rate. Therefore, one would expect the design of a gas seal and a liquid seal to be very similar, which, in fact, they are. Then why are their systems so different?

A gas seal system is simplified in compressor applications over a liquid seal system, merely because the gas seal utilizes the process fluid. This is exactly the same case for a pump. By using the process fluid and not a liquid, one can eliminate the need for a liquid supply system and the need for a contaminated liquid (sour oil) drain system.

The following major components are eliminated:

Referring back to the function definition of the gas seal system, all requirements are met. ‘Continuously supplying fluid’ is achieved by utilizing the discharge pressure of the compressor. The requirements for ‘specified differential pressure, temperature and flow rate’ are met by the design of the seal itself, which can accommodate high differential pressures, high temperatures, and is sized to maintain a flow rate that will remove frictional heat necessary to maintain seal reliability. The only requirement not met is that of supplying a clean dry fluid, and this requirement is met by using a dual system coalescing filter.

When one considers all the advantages, the next question to ask is, ‘Okay, what are the disadvantages?’ Naturally, there are disadvantages. However, proper design of the gas seal system can minimize and eliminate many of these. Do not forget that the requirements for any system mandate proper specification, design, manufacture, operation and maintenance. One can never eliminate these requirements in any critical equipment system.

There are disadvantages to a gas seal system, which are not insurmountable but must be considered in the design of such a system. These considerations are as follows:

Positive prevention of toxic gas leaks to atmosphere

Possible oil ingestion from the lube system

‘O’ ring (secondary and components) design and maintenance

If all the above considerations are incorporated in the design of a gas seal system, its reliability has the potential to exceed that of a liquid seal system and the operating costs can be reduced.

One must consider that the relative reliability of gas and liquid seal systems is a function of proper specification, design and other factors. A properly designed liquid seal system that is operated and maintained can achieve reliabilities comparable to a gas seal system. Also, when one considers the operating costs of the two systems, various factors must be considered.

While the loss of costly seal oil is positively eliminated, with a gas seal system (assuming oil ingestion from the lube system does not occur) the loss of process gas, while minimal, can be expensive. It is argued that the loss of process gas from a liquid seal system through drainer and degassing tank vents is also significant. While this may be true in many cases, a properly specified, designed and operated liquid seal system can minimize process gas leakage such that it is equal or even less than that of a gas seal.

There is no question that gas seal systems contain far fewer components and are easier to maintain than liquid seal systems. These systems will be used extensively in the years ahead. Therefore, existing liquid seal systems which cannot be justified for retrofit, or cannot be retrofitted easily, can be modified to minimize outward gas leakage and optimize safety and reliability.

Liquid vs. gas sealing system

Dry gas seals were introduced in the late 1960s but did not gain wide acceptance until 20 years later as the ultimate solution to compressor seal oil system problems. The dry gas seal was marketed as a ‘simple’ solution to seal oil system reliability issues, but in reality, when one considers all the possible process and operational related issues, its complexity rivals that of the oil systems.

There is no doubt, however, that a properly specified, selected and designed dry gas seal system will positively prevent the ingress of oil into the process system and deliver high degrees of safety and reliability. To ensure optimum safety and reliability of dry gas seal systems, end users must be proactive in the project phase or during seal system modifications to specify operating parameters.

The main parameters fall into the following categories:

All possible operating, start-up and upset conditions on the seal data sheet

Required system design details by incorporating all site, company and industry lessons learned into the project or revamp specification

A detailed (P&ID) and data sheet to quoting machinery vendors that will completely specify system and component design

Allowing the EP&C (contractor) and/or machinery vendor to design the dry gas seal system will expose the plant to safety and reliability issues that cannot be known by other parties.

Following the guidelines completely in this best practice and requiring compliance with all specified details will ensure a safe and trouble-free system of the highest reliability.

Dry gas seal (DGS) systems have been used for the past two decades, and are specified by many end users as the seal of choice for most compressor applications. One would therefore think that seal and system designs are well-known and proven. However, experience shows that failures are still quite common. For instance, in 2007, FAI dealt with nearly 50 DGS failures.

These failures have raised several questions. In reviewing DGS failures experienced in 2006 and the previous years, the conclusion was drawn that in the majority of cases, the root cause is that the seal and system configuration were not designed to handle all the actual site operating conditions, including start-up, shut-down and upsets that should and could have been anticipated.

The end user has the most complete knowledge of the process and plant operating procedures. Therefore, he or she needs to be proactive in terms of project DGS requirements, and specify the type of seal and system most suited to the plant and application, based on his or her knowledge and experience. Seal and compressor vendor input and experience are obviously required, but neglecting to evaluate the proposed system in detail against all operating modes subjects the user to the risk of unacceptable downtime and revenue losses, particularly in the ‘mega plants’ being built today.

The reliability of critical equipment is dependent on the reliability of each component in every auxiliary system connected with the critical equipment unit. The easiest way is to eliminate the auxiliary systems. The gas seal as used in compressor applications affords the opportunity to achieve these objectives. However, the gas seal is still part of a system and the entire gas seal system must be properly specified, designed, maintained and operated to achieve the objectives of optimum safety and reliability of the critical equipment.

Failure to consider specific plant operating conditions and seal system lessons learned has resulted in dry gas seal systems of low MTBF (less than 12 months) and large revenue losses.

The following examples highlight omitted details in dry gas seal specifications that have resulted in seal MTBFs of less than 12 months

Failure to identify the actual gas properties (sour gas, gas composition)

Failure to identify saturated seal gas conditions at start-up, upset or operating conditions

Failure to properly specify maximum flare header pressure

Failure to define the actual dew point of supplied nitrogen for intermediate and separation gas

Failure to prohibit the use of orifices in the secondary vent resulting in seal pressure reversals

Failure to specify oil sampling devices in the secondary seal vent port (sight glasses, valves or automatic drainers) leading to secondary seal oil contamination and eventual failure.

This best practice has been used since the late 1990s to specify dry gas seal system requirements during projects and for field modifications. This approach has resulted in dry gas seal systems of the highest safety levels and reliability (seal MTBFs greater than 90 months).

How to prevent revenue losses in dry gas seal systems

All flush plan 52s (un-pressurized tandem seals) must be continuously vented to flare or safe point to ensure that the seal pot is not contaminated 

with the inner seal process fluid release through the outer seal. A laminated warning sign is recommended at each application of this kind. 

Un-pressurized dual seals (formerly called ‘tandem’) require an open path to flare or high point safe location to remove all seal fluid vapors (usually flammable and/or toxic) from the seal pot and direct them to a safe location.

A valve closure and/or restriction in the seal pot vent line will prevent removal of the process fluid, contaminate the seal pot safe barrier fluid and expose the plant and its personnel to a flammable or toxic fluid release in the event of an outer (atmospheric side) seal failure. 

It is recommended that a local, laminated, warning sign be positioned at the pump. In addition, a short ‘mini information’ presentation by site rotating machinery specialists has been effective in making operators aware of this potential safety hazard.

Dual un-pressurized seals (tandem) rely on a buffer fluid at or near atmospheric pressure to lubricate the atmospheric side seal. This buffer fluid is circulated via a pumping ring from the seal to the seal reservoir and back to the seal (in a closed loop).

Plan 52 – dual un-pressurized seal using synthetic buffer fluid to lubricate the atmospheric side seal. A pumping ring in the seal circulates the buffer fluid (pressure less than the seal chamber) to the reservoir.

The reservoir is at atmospheric pressure (less than seal chamber pressure), so the leakage across the process side seal faces migrates into the seal reservoir, and will either increase pressure, level, or both in the reservoir. Since every seal does leak a certain amount, it is essential to have the reservoir vented to a flare or vapor collection system. If the reservoir is allowed to reach the seal chamber pressure, the atmospheric side seal will most likely fall (if it hasn’t already) as it is not typically designed to handle seal chamber pressure.

If this is a concern in the plant, you may want to consider requesting the seal vendor to redesign the atmospheric seal to handle maximum seal chamber pressure. In addition, as the process side seal leaks in this flush plan, the atmospheric side seal will essentially be sealing the pumped fluid, exposing the plant to the release of flammable and/or toxic vapors.

Monitoring of seal leaks can be done by checking the level and pressure of the reservoir, as one or both may increase in the event of excessive leakage. The seal vendor (or support system vendor) may supply high level and/or pressure switches which would alert the operators to a seal leak. It is highly recommended to specify this instrumentation in new projects, as it will cut down on the already high workload of operators (if the alarm sounds then the level or pressure can be personally verified in the field).

In addition to checking for excessive leakage (pressure or level increase), temperature in and out of the seal can help verify proper circulation. In a reservoir that is not cooled, there should be a temperature drop of approx. 1 to 2

F) from the seal outlet to the seal inlet. If there is no temperature drop (or if the temperature increases), this indicates zero or possibly reverse circulation.

It has been our experience that Plan 52 flushes may not be vented properly (blocked in) and level may be low or at zero in the reservoir. For these reasons, it is very important for the operators to be trained to understand the necessity of monitoring this system.

Most plants have experienced a release of process fluid from dual un-pressurized (tandem) seal systems when the vent line had been improperly closed off. Lack of awareness of dual un-pressurized (tandem) seal system operational principles has resulted in flammable and toxic fluid releases in many plants.

This best practice has been recommended since the mid-1980s. Since that time, this best practice, when implemented by plant operations, has resulted in optimum dual un-pressurized (tandem) seal safety and reliability (MTBFs > 48 and as high as 80 months).

Venting of seal pot to prevent contamination

Always execute the following seal installation best practice guidelines for component and cartridge mechanical seals to achieve maximum pump mechanical seal MTBFs:

Component seal faces must be cleaned with clean cloth and rubbing alcohol (or alcohol wipe) just before installation. Proper care must be taken to not touch the faces after cleaning.

Limited grease should be used on seal gland ‘O’ ring as it can migrate to the seal faces and cause damage. Proper practice is to sparingly apply grease to ‘O’ ring then install it in the gland. This will ensure ‘O’ ring will stay in the groove when installing the seal.

Use liquid soap sparingly on sleeve ‘O’ ring to assist in installation.

It is recommended that this best practice be incorporated into the plant maintenance procedure manual for mechanical seal installation.

Refer to the vendor instruction book and obtain all required mechanical seal components.

Have a clean area available for the components and ensure that there are no finger prints, oil or grease on the primary seal faces. Wipe with alcohol using a lint free cloth if necessary.

Primary seal face flatness will have been confirmed by the seal manufacturer and should be within 1-2 helium light bands. For this reason, the primary seal faces must be free of fingerprints, oil and/or grease.

Inspect the rotating head assembly as follows:

Secondary ‘O’ ring in pusher seal – inspect for discontinuities, confirm proper material and durometer. (Note: Do not lubricate ‘O’ ring.)

Ensure that all springs (if a pusher seal) are sitting upright and have not been dislodged from their counter-bores.

Confirm that shaft or shaft sleeve surface finish, where secondary ‘O’ ring or Teflon wedge will ride, is maximum of 32 rms, using a comparator.

Clean and de-burr the shaft area and seal chamber area as required using a solvent that will be compatible with the process fluid. (Note: Assure all piping/tubing is isolated from flush/quench ports. Plastic plugs need to be installed in all flush/quench after isolation of piping/tubing, up to the point of the piping/tubing reinstallation.

Install the seal head carefully being sure not to damage the secondary ‘O’ ring, or the ID of the primary ring on the shaft/sleeve.

Proper spring compression check (‘working height’) – obtain dimension from seal drawing which shows the distance between the back of the seal retainer and the seal chamber face. Then fit the bearing bracket and shaft up against the casing and mark (with bluing pen) the point on shaft or sleeve directly below the seal chamber face. Remove shaft bearing bracket from casing and mark the dimension where the back of the retainer sits when assembled.

Attach seal head to shaft/sleeve and ensure that set screws are installed on clean and undamaged shaft areas. If this is not possible, carefully de-burr with a file and smooth out with emery cloth.

Note: If sleeve ‘O’ ring is installed, confirm proper material, size and durometer and apply a small amount of ‘Krytox’ or equal lubrication and install. Assure shaft surface is clean and free of any burrs and carefully install sleeve being sure ‘O’ ring is not damaged.

Install the stationary seal face into gland evenly to ensure face is not cocked. Assure that any finger prints are removed with alcohol and lint free towel.

Stationary ‘O’ ring checks – confirm proper material, durometer, and inspect for discontinuities.

Install gland to stuffing box using opposite and even tightening technique. Prior to doing so, a sweep of the seal chamber face is required to ensure proper sealing between gasket/’O’ ring on seal gland and seal chamber. A dial indicator should be attached to the shaft and the shaft will be rotated to cover a complete sweep of the seal chamber face. TIR should be no more than 0.005’.

After seal is completely installed, conduct a static pressure check. Consult the seal vendor for the proper pressure if static limit is not indicated on seal drawing. If any leaks are observed, consult seal vendor immediately.

Contaminated faces – can affect face flatness, resulting in excessive leakage. In addition, certain oils can set-up like adhesives and pull material out of the carbon primary ring faces after start-up, resulting in pre-mature seal failures.

Applying lubricant to secondary seals which can enter primary seal face areas – the same results apply as above.

Not setting proper spring compression: excessive compression – more seal wear. Low compression – will not provide sufficient face contact during start-up and standby conditions allowing excessive leakage.

Improper installation of mating ring into the gland resulting in excessive leakage due to improper face contact. Note that if back of mating ring is visibly ‘not flat’, the same result can apply. Stuffing box face TIR should be checked with dial indicator on shaft. Seal chamber face flatness should be no more than 0.005’ TIR.

Installation of gland to stuffing box not using equal and opposite tightening technique. Failure to tighten the nuts/capscrews properly can cock the mating ring, which will not allow for proper contact between the faces and result in excessive leakage between the seal faces.

Sleeve ‘O’ ring cut during installation, resulting in excessive leakage between shaft and sleeve.

Refer to pump vendor manual and maintenance records to ensure proper cartridge seal will be used.

Assure that the seal chamber is cleaned and shaft is de-burred to remove any sharp material that could damage seal sleeve ‘O’ ring.

Assure seal chamber is isolated from all flush and quench connections systems. Plastic plugs need to be installed in all flush/quench after isolation of piping/tubing up to the point of the piping/tubing installation.

Assure that the proper durometer, material and size ‘O’ ring with ‘Krytox’ or equivalent lubricant.

Install gland to seal chamber using equal and opposite tightening technique. Prior to doing so, a sweep of the seal chamber face is required to ensure proper sealing between gasket/’O’ ring on seal gland and seal chamber. A dial indicator should be attached to the shaft and the shaft will be rotated to cover a complete sweep of the seal chamber face. TIR should be no more than 0.005’.

Tighten set screws to engage shaft/sleeve by equal and opposite technique. Assure that a clean shaft surface is available for all set screw locations. If not, use a file and emery cloth to remove any damaged spots. Be sure to completely clean shaft to remove all debris.

Completely remove setting clips. If setting clips are slid up on the gland, they can potentially fall back down to the sleeve or drive collar resulting in an ignition source (the writer has experienced fires that have started due to this issue).

Failure to have and implement a required mechanical seal installation procedure has been a principal cause of low mechanical seal MTBFs (lower than 12 months).

FAI has used this best practice since the 1990s to ensure that plant mechanical seal installation procedures are in complete accordance with mechanical seal vendors’ recommendations and to ensure optimum installed seal safety and reliability.

Installation procedures for component and cartridge mechanical seals

For optimum mechanical seal MTBFs, execute the following mechanical seal monitoring best practices.

Confirm that the pump is operating in its Equipment Reliability Operating Envelope (EROE). Also called the ‘heart of the curve,’ it assures maximum centrifugal pump MTBF by avoiding all operating areas of hydraulic disturbances.

Check for a plugged flush line orifice by taking temperature reading across it.

Confirm seal chamber pressure is at least 345 kPa (50 psi) above the fluid’s vapor pressure.

If the flush system contains a cyclone separator or strainer, check for plugged components by taking temperature readings.

If the flush fluid is cooled, confirm the proper function of the cooler by checking the temperature drop across the cooler (should be approx. 10 to 38

F] normally) and the temperature rise of the cooling medium

Check the temperature difference on seal reservoirs (pots) between buffer/barrier in and out lines to ensure circulation to and from the outboard seal. If there is no temperature difference, the circulation through the buffer/barrier circuit has halted.

Always vent seal pot systems at the highest point to ensure pumping ring circulation.

For dual pressurized seal applications install a permanent differential pressure gauge to ensure that the seal pot filling pressure is not excessive, which will force open the inner seal.

We recommend that the above predictive maintenance (PDM) guidelines be followed for all centrifugal pumps in your facility at the following times:

After the pump operating conditions are normalized

Whenever the plant condition monitoring results indicate a change (in vibration, temperature, seal leakage, noise, etc.)

Failure to monitor centrifugal pump performance and the effect of the process conditions on pump flow rate are the major contributors to centrifugal pump mechanical component failure (seals, bearings, wear rings and impeller).

Most plant condition monitoring programs do not integrate centrifugal pump performance (operating point and produced head) with mechanical condition (vibration and temperature). Neglecting pump performance, in FAI experience, neglects consideration of approximately 80 percent of the potential root causes for mechanical seal failure.

This best practice has been used since the mid-1990s to recommend plant PDM practices for pumps with mechanical seals to optimize plant centrifugal pump safety and mechanical seal reliability (MTBFs above 48 and as high as 80 months).