Justification for machine condition monitoring

Justification for machine condition monitoring

Continuous operational condition monitoring implies that machine condition data are collected and evaluated in real time. Of special interest here are data that could rapidly change and are tell-tale signs of unexpected degradation of the condition of the machine. Vibration amplitude excursions indicate rapid changes and will give early warning of disaster. A timely shutdown will minimize possible consequential damage and the resulting extended downtime.

Both frequency and duration of shutdowns can be reduced with carefully targeted planning of maintenance work. Such detailed planning allows streamlined manpower and spare parts allocation. But, intelligent allocation requires information on machine condition to project wear progression and probable time to failure.

In condition-based maintenance, machines are only shut down if their condition demands it. Parts are only changed if a damage criterion is reached and the total anticipated survivability of parts in terms of remaining life and wear reserves are being exploited. Thus, then, achieving a reduction in material costs may only be possible if one has reasonably accurate data on the machine's condition. One obviously gages the existing wear potential in "traditional" wear parts, e.g., piston rings or packing, and thereby optimizes machine operating life.1

Efficiency is another indicator of machine condition. The primary purpose of efficiency monitoring in fluid machinery is to record changes in process or machine parameters that influence energy transfer to the medium being moved or compressed. Low efficiency causes fluid temperature rise and increases power consumption.

Reciprocating gas compressor efficiency may be determined, for example, by finding how indicated compression power relates to the driver power consumption.

What to monitor and why. Recall that the objectives described here are general and not specific to reciprocating compressors; they apply to numerous industrial machines. However, designing an effective strategy for machine condition monitoring systems parallels those used for reciprocating compressors. In these positive-displacement machines, their mechanical features—and perhaps their most vulnerable parts—must be analyzed in terms of maintenance frequency.

This was done by Dresser-Rand who, in 1997, made a survey of200 operators and designers on the causes of unscheduled shutdowns of reciprocating compressors. The results (Fig. 1) clearly indicate the relative maintenance intensity of certain component groups.
These statistics show that eight component groups were responsible for 94% of unscheduled shutdowns. Valve defects are obviously responsible for most of the unscheduled maintenance events. More recent experiences indicate similar results, even though the absolute involvement ratio of valves has dropped slightly due to the use of new materials.2

Monitoring systems are being marketed for every component listed in the statistics for rotating machinery, but the specific machine requirements must be considered when choosing a system. For example, analyses of entire subassemblies or operating point-specific threshold checks are appropriate to detect valve damage on reciprocating compressors.

As of2006, the three most important methods are:

•    Valve pocket temperature measurement

•    Vibration analysis

•    p-V diagram analysis.

Using suitable temperature probes and measuring the temperatures upstream and downstream of the valves is the simplest and most cost-effective method of determining valve condition.

LITERATURE CITED
1    Klein, U., "Vibration diagnostic assessment of machines and plant," Verlag Stahleisen GmbH, Dusseldorf, 1998.

2    Leonard, S. M., "Increasing the Reliability of Reciprocating Compressors on Hydrogen Services," Presented at the NPRA Maintenance Conference, May 20-23, New Orleans, Louisiana, 1997.

3    Nickol, J., "Reciprocating compressors in process installations: Availability of 24.000 hours, Utopia or reality?" Presented at the 1st EFRC-Conference, Nov. 4-5, Dresden, 1999.

Batch standards find their way into nonbatch industries

Ask a control engineer from the petrochem industry if he/she has heard of the ISA-88 and 95 standards and he/she will likely say "yes," followed by a comment along the lines of "they are the batch standards and don't apply to us." It is true that both these standards came from the batch environment and were designed to address batch issues. More and more engineers from petrochem companies are showing up at common-interest groups for the standards. Even at this year's WBF (formerly World Batch Forum) North American conference, held in Baltimore, it became clear that these standards are opening eyes in industries and applications outside of batch. Not only are they able to provide structure and portability to many types of sequential operations in continuous processes, but they are also able to provide a design methodology for many other applications within the manufacturing environment. Control and application engineers in many companies not traditionally linked with batch per se have found that these batch control standards' models and structures have helped in developing detailed control functions' design for their nonbatch processes. Modularization of the control functions-based ISA-88 batch control standard can improve a control project whether it is a batch, continuous or discrete process.

ISA-88 in continuous control. The ISA-88 batch control standard is providing significant benefits to users and suppliers of batch control systems worldwide. Although the standard is primarily designed for batch processes, it is also being applied successfully in various manufacturing industries. This is because the structure required for flexible manufacturing mirrors the structure required for many batch processes, even though the underlying process is often continuous or discrete. The standard allows collapsing hierarchy levels, where one or more levels of functions may be eliminated, as needed, making it very adaptable for many different applications.

Batch control engineers have long joked that a continuous process is a huge batch process with very long cycle times. That may be a somewhat facetious statement, but continuous processes do use sequences in many of their operations and continuous manufacturers are turning to ISA-88 as a way to provide a standardized means of designing and implementing these operations across their sites.

Continuous processes use sequenced operations for startups, shutdowns and transitions from one product to another. The latter often means manipulating many setpoints, changing lineups and the possibility of introducing error. Some continuous manufacturers have found that using an ISA-88 approach to design and implement the recipes for each new transition state helps to reduce the time taken for a transition and for the process to come back to a steady state. It also reduces potential errors from shift to shift and operator to operator.

The same is also true of startups and shutdowns. Continuous processes run for so long between planned shutdowns that manufacturers have turned to ISA-88 to help with the design of sequences to ensure that startups and shutdowns are done safely, with minimum disruptions.
A different view of process analysis. ISA-88 is also starting to find application in another "bastion" of the petrochem world—process analysis. Analyzers in the process industries are small "batch" processes. A sample is taken from a line, transported to an analyzer, the analysis is performed and the data sent to the control strategy (after error checking). The "batch" standards may be able to improve cycle times and get better coordination between process and analysis. Paul McKenzie introduced the concept of a batch analytical recipe (BAR) in his keynote speech at the WBF forum and said that the whole process of setting up an analyzer should lend itself to an ISA-88 approach. The instrument setup, sample preparation and system suitability criteria all lend themselves to an ISA-88 approach and definition using the BAR concept.

ISA-95 as a design and integration methodology.

As the petrochem companies and other traditionally continuous companies wrestle with integrating their business and control systems, many of them are turning to ISA-95 as a methodology to help them in this process. In fact, many of the major ERP systems' suppliers are using the B2MML schemas developed by WBF's XML working group to help facilitate these integrations—be they from batch, continuous or discrete industries.

It is clear that batch control problems and their solutions are not unique to batch processes and the so-called batch industries. They can also be applied to continuous and discrete processes, packaging lines and material storage facilities. The batch control standards committees always believed that the batch standard models could also be applied to areas that are not strictly batch. In fact, recently the Make2Pack joint working group between the WBF and the Open Modular Architecture Controls User Group has developed standards for packaging machine control based on ISA-88 standards. It is also clear that the desire for refining and petrochem companies to integrate their business and control systems will inevitably pull them closer to the ISA-95 standards.

Maybe they should change the name of the standards to "manufacturing standards".

Balance between interchangeability and heating value

Gas interchangeability is a common issue for natural gas (NG) distribution networks. Interchangeability must be addressed in major gas markets in which gas is supplied from many different sources with different qualities. In the US and the UK, gas interchangeability is yet to be fully resolved regarding national quality specification guidelines with respect to liquefied natural gas (LNG) imports and nonconventional gas sources.

Many international gas markets have adopted interchange-ability parameters to ensure end-user protection in dealing with multiple supply sources. Practically all global gas markets (and even some US regions) have adopted applying interchangeability parameters in contractual terms and conditions. The most commonly used reference is the Wobbe Index. This index adjusts the heating value with relative gas density and does consider burner performance. It is internationally the most widely accepted measure of interchangeability.

LNG parameters. Particularly in the UK, the Wobbe Index is frequently used as a parameter to determine the upper limit (major constraint) on imported rich gases. However, most US gas specifications are based upon heating value—not the Wobbe Index. Historically, this practice has been adequate because most NG supplies were sourced from interstate transmission pipelines and had very homogeneous gas compositions.

As domestic NG supplies lag behind demand, the US gas supply mix will increasingly involve more imported LNG, and pipeline-gas compositions will become less homogenous. Many participants in the US gas market have called for pipeline standards clarification on issues surrounding gas interchangeability to ensure consistent safe and reliable supply sources.

International standards. Different gas specifications and standards for the global expanding supplies markets will have an effect on the NG markets. Quality issues on LNG deliveries will be more complex and critical as receiving terminals balance and cope with different supply chains.

The growing receipt of high-heating-value, rich LNG adds challenges to receiving terminals that are connected to low-calo-rific-value gas pipeline networks. This situation for both ends of the LNG supply chain must be addressed.

Shipping side. LNG producers must decide whether to produce more than one LNG specification to satisfy all potential customers in Asia, Europe or North America. Such actions would involve additional capital investment and operating costs to produce several LNG products with varying qualities. Shipping only one quality (LNG product) could potentially limit marketing opportunities in an expanding commodity market.

Receiving side. Many US gas pipelines can accept rich gas from LNG terminals with high heating value limits—1,050 Btu/ scf—1,070 Btu/scf. Imported LNG from many sources can have

heating values ranging from 1,080 Btu/scf to 1,160 Btu/scf. To import rich (high-Btu) LNG and still comply with gas send-out requirements, receiving terminals must reduce the gas Btu value.

Options. The most common reduction approach is injecting inert gas (e.g., nitrogen) up to the pipeline content limits—usually 2% to 3%. Air injection is also technically feasible, but if it is only used for minor heating value adjustments to avoid exceeding the minimum free-oxygen content specifications, which are 0.01—0.2%. The Btu reduction by adding inerts is limited to 20-30 Btu/scf. This may be sufficient to adjust quality for some LNG streams, but insufficient for others. Under the current US heat content-based pipeline tariff specifications, few imported LNG supplies meet restrictive gas quality provisions. In 2005, only LNG from Trinidad and Tobago could be directly delivered into most gas markets along the US East Coast.

Conversion for interchangeability. However, if US pipelines translated current heat content specification into a corresponding Wobbe Index (1,330—1,370 Btu/scf), most global LNG supplies, when blended within allowable concentrations of an inert gas, could meet the tariff specification for interchange-ability on the Wobbe Index basis.

Extraction. In cases where inerts cannot meet the US heating value constraints, C2 and heavier components must be removed at extra costs to the LNG receiving terminal. Experiences demonstrate that systems to reduce C2+ or C3+ gases at LNG receiving terminals are more cost-effective in terms of capital investment and operating costs than systems injecting inerts or boosting gas send-out pressure to achieve heating value constraints. Also, C2+ removal solutions yield a high-value natural gas liquids (NGLs) stream at the receiving terminal. The NGLs have an economical benefit and can further enhance the total profitability of the receiving terminal.

Most liquefaction plants are designed to serve clearly identified markets with long-term supply contracts. These contracts also specify the particular quality parameters that the plant must meet. Clarity of gas specifications throughout the US gas network and in supply contracts, incorporating Wobbe Index numbers and dew-point ranges, would assist LNG receiving terminal operators. Such changes could provide flexibility to terminal managers on how to optimize LNG inventories and provide reliable NG supplies to pipeline end users.

Detection, analysis, control

Process analytics is a central component in the monitoring and control of processes and it makes an extremely important contribution to higher quality, increased safety and greater efficiency.

MAXUM process gas chromatographs

Process analytics essentially performs four tasks within process control engineering: In conjunction with control tasks for optimizing the process and product, it is used to increase plant utilization, reduce energy requirements, guarantee smooth plant operation, and observe the required product specification.

Another task is quality monitoring and documentation for verifying compliance with the relevant regulations such as ISO. However, process analytics is important at the same time for environmental protection and process safety: On the one hand, in plant monitoring to protect personnel, equipment and the environment against possible damage from toxic or explosive substances, and, on the other hand, for emissions control to ensure that a plant's emissions are safely within the limits permitted by the authorities.

Complete range
Siemens offers its customers a comprehensive range of products and systems -from simple sensors to highly developed chromatographs: gas analyzers for monitoring explosion limits, analyzers for humidity and 02 traces, continuous gas analyzers (paramagnetic oxygen analyzers such as OXYMAT, NDIR analyzers such as ULTRAMAT, total hydrocarbon analyzers such as FIDAMAT) and process gas chromatographs (such as MAXUM and Micro-SAM) for different tasks.

Siemens is one of the leading suppliers of process analyzers and process analysis systems and supports its customers with application-oriented knowledge, tailor-made solutions, powerful devices and systems, and a global presence. Siemens is also the competent partner for efficient solutions with which

process analyzers can be integrated into automation systems in the process industry, and it also offers comprehensive services covering all aspects of process analytics. Partnership with Siemens in the area of process analytics provides customers with the certainty of being able to expand their competitiveness continuously and safely.

Improvements in floating-ring compressor seals

As is so often the case, compressor components undergo design changes as experience accumulates. Over the years, improved floating-ring shaft seals have become key elements in the uptime improvement inventory of modern plants.

Take, as an example, improvements in Elliott Isocarbon seals and associated seal systems. User feedback indicates that these seal configurations can, in fact, compete with dry gas seals in such areas as safety and reliability.

Safety attributes examined. While decades-old seal configurations (Fig. 1) may have resulted in mixed acceptance, the more recent versions of this product (Fig. 2) incorporate subtle (but important) changes in component geometry. Floating-ring seals are now favorably considered by many users for a number of reasons.

It should be noted that seal faces are made of stable materials. In Isocarbon seals, the rotating face is still made with cast iron, which is a very stable material that resists shocks and brittle fracture.

Dry gas seal rotating-face components are made of ceramics or tungsten carbide. Ceramics tend to be very brittle and sensitive to manufacturing variances. Unless expertly designed and manufactured, catastrophic failures and possible gas release can result with ceramics.

Reliability issues. A few additional advantages, disadvantages and concerns must be addressed when comparing floating-ring and dry gas seals:

•    Seal oil cools shaft ends; gas will not do so with the same effectiveness.

•    Vibration damping is enhanced with liquid (wet) seals. This may provide lower vibration through increased rotordynamic damping. However, as oil thickens with time, this damping varies. Such a change could influence equipment rotordynamics and create field problems. While dry gas seals have less damping, rotor characteristics do not change over time.

•    Process gas quality is essential to gas seal life. Unless absolutely clean, process gases can adversely affect gas seal operation. While this is generally not a major issue in the field if suitably designed filtration equipment is provided, filter system maintenance is a budget item that must be considered.

•    Owner-operators are often not allowed to service certain seal models. The gas seals are treated as a "black box" and must be returned to the manufacturer for refurbishment. With industry practices changing and owner-operator expertise often declining, some clients have opted to sign alliance agreements giving service and maintenance responsibilities to the vendor.

In essence, then, compressor sealing issues must be carefully weighed and modern technology updates are essential to making an informed choice.2 There can be no blanket endorsement of either sealing approach across the board. However, oil seals, including Isocarbon seals, can be very useful in certain
machines. They can be problem solvers in large compressors. One such application is in LNG where the advantages of damping and high cost of downtime justify a simpler oil seal arrangement. These seals also merit consideration where there is prolonged slow-roll, e.g., where steam turbine drivers are used and compressors are in hot stand-by service.

LITERATURE CITED

1    Bloch, H. P., A Practical Guide to Compressor Technology, 2nd Ed., John Wiley & Sons, Hoboken, New Jersey, 2006 (ISBN 0-471-727930-8).

2    Bloch, H. P., Compressors and Modern Process Applications, John Wiley & Sons, Hoboken, New Jersey, 2006 (ISBN 0-471-72792-X).

On seeking energy 'security'

I have to admit that I have problems with the concept of "security of demand." If you build a large, efficient production plant, you'll probably be justified in calling for the market to be able freely to choose your products. Yet, it seems sensible that, at times, governments will incentivize other choices if the net long-term result would threaten the general good. We learned that lesson from lead in gasoline, didn't we?

These days, some wise suppliers make it their practice to take into account the full range of stakeholders, include them where possible and even run an open book with customers. Not so the OPEC oil-producing nations. They simultaneously keep the world in the dark over the actual extent of their oil and gas reserves and raise a hue and cry over attempts by consumer nations to diversify their supplies.

Market sites. I know it's controversial for some, but I think sometimes you just have to give the free market a little tweak here or there. Supermarkets are, in fact, a super example of this. In the UK for a couple of decades, they've been building supermarkets on the outskirts of towns so that people can conveniently fill the trunk of their car without all the time-consuming social interactions necessary on the traditional high street.

This trend has market economics on its side. Out of town, groceries are cheap. However, for towns, there are hidden costs—and the result is well charted. The New Economic Forum calls it Ghost Town Britain: small shops close, the high street is successively hollowed out. It strips the character, diversity and social function from communities that have survived sometimes for a thousand years.

People are waking up to the downside of out-of-town stores and the private car use in small-town England. Planners are resisting pressure from developers and demanding that out-of-town stores move into town instead. Local communities are increasingly championing their high street fishmonger and small hardware stores.

Calls for security. So if Tesco, Sainsbury or Wal-Mart-owned Asda now started publicly kicking up a fuss about this backlash—demanding "security of demand" for their hypermarket developments—I think they'd be told to go to hell. In the global energy market, few people have the balls,

or the oil independence, to tell OPEC or Russia to go to hell. Nevertheless, I suspect it's a grudging, unspoken fantasy for European policymakers. I can only imagine that the rhetoric from Moscow or Vienna over recent months causes teeth to grind.

Policy bias. For example, the General Secretary of the OPEC cartel, Abdalla El-Badri, has warned that a combination of misplaced and unsustainable policies favoring biofuels and a corollary reduction in fossil fuel production could send oil prices "through the roof." He told the Financial Times in June that OPEC will invest $130 billion by 2012, but that these and further plans through 2020 for $500 billion of new projects could be revisited in light of current energy diversification strategies in consumer nations.

So, what's to be done? Well for one thing, OPEC could do us all a favor and publish data on its oil and gas reserves. That would make me feel more secure. Vienna could be as transparent about how much oil and gas it has, as it is in calling for consumer nations to be about their fuels policy.

It's said that it would take about a fortnight to replace speculation (and fear) with more or less hard fact were OPEC nations to publicly declare their oil reserves.

Perhaps then consuming countries would stop "discriminating against oil." Perhaps that would afford OPEC the security of demand it craves. Perhaps then, consumer countries could pursue policies like carbon sequestration, as OPEC suggests they should. Policies that "do not cause a huge dislocation of the current energy system."

But all of that assumes that OPEC's giant oil fields are not past their peak. And to get to the bottom of that, I guess one day soon I'm going to have to pitch Daniel Yergin's jovial assurances against Matt Simmons' alarm bells and do my best to see clearly into the opaque world of fossil fuel reserves.

Are NFPA standards enough?

The National Fire Protection Association (NFPA) 85 standard addresses the design, installation and operation of single- and multiple-burner boilers and other fired equipment. The NFPA 86 standard addresses similar issues for ovens, furnaces and fume incinerators. These codes are intended to prevent fires, explosions and implosions of equipment. Part of NFPA 85 outlines requirements for safety interlocks (Table 1). Interlocks protect the fired equipment from operating in dangerous modes. These standards are prescriptive; they stipulate specific equipment requirements such as a double block and bleed valve for a gas fuel train.

Some HPI operating companies view implementing specified interlocks as satisfying requirements for functional safety. While the NFPA listed interlocks are a good start to evaluating such hazards associate with fired equipment, they do not ensure that the risk has been reduced to a tolerable level. Several key concepts are not fully addressed by NFPA 85 including consequence severity and equipment reliability.

Consequence severity. A key element missing is an evaluation of the consequence for a hazard. It is important to consider the magnitude of the consequence to ensure that the risk is sufficiently reduced. The location of fired equipment has a direct impact on the severity of the consequence. For example, consider two identical

boilers—one is located in a remote

area of the plant, and the other is centrally located near offices and manned work areas. If interlocks, operation and maintenance are identical, then the frequency of an accident would theoretically be the same. However, the consequence of the accident would be much higher for the boiler located in the more heavily manned area. This is why it is important to do a risk assessment on each piece of equipment.
Equipment reliability. The NFPA standards do not set specific requirements for the equipment used in the safety interlocks. There is guidance on some equipment, such as programmable logic controller (PLC) and flame detectors; yet, it is not complete. To ensure that the interlocks provide the require risk reduction two things are needed a target risk reduction and reliability calculations for the interlocks. Quantifying the consequences associated with the hazard and comparing it to tolerable risk guidelines will address the first issue. Reliability calculations that consider the equipment selected, testing intervals, testing effectiveness and mission time of the system will confirm if the interlocks are providing the required risk reduction.

The achieved risk reduction will vary greatly, depending on how the interlocks are implemented. To illustrate the impact of interlock design, consider a typical NFPA compliant interlock implemented in two different ways (Table 2). The first interlock is implemented with a pressure switch and control relays, while the second interlock is done with a safety-rated transmitter and PLC. Both interlocks have identical valve configurations. The first interlock achieves a risk reduction of only 12 while the interlock that contains the safety-rated transmitter and PLC achieves a risk reduction of 270. Clearly, both approaches can be considered an interlock for low pressure, but the one that takes advantage of safety-rated equipment provides much greater risk reduction.

[Best practice. A best practice seen in the field is a blended approach and yields a solution that includes the strength of NFPA standards and IEC 61511/ISA S84.00.01. This approach includes:

•    Verify that all pertinent NFPA interlocks are implemented

•    Include fired equipment in process hazard analysis

•    For hazards with significant consequences, do a formal safety integrity level (SIL) selection

•    As indicated by the SIL selection results, treat the affected NFPA interlocks as safety instrumented functions (SIFs).

•    Create safety requirement specifications (SRSs) and perform SIL verifications per IEC 61511.

This approach satisfies the requirements of the NFPA standards and provides alignment with the emerging functional safety standards.

Case 42: Torsional vibration problems can be costly

The results are hammering of the gears and fretting of the splines

It was then I learned that ifcii torsional vibration failures don't occur often, but when they do, they can be very costly to solve.

Torsional vibrations occur when a mass is twisted and released on a shaft and continues to vibrate, much like twisting a mass on the end of a coiled spring and releasing it. The vibration "dies out" unless a torsional frequency that coincides with one of the systems natural frequencies is present. When that occurs large vibratory amplitudes are possible and the system is said to be in resonance.

Fig. 1 consists of a pump, gearbox and diesel engine simplified into a two-mass system. The model considers the lowest of many natural frequencies; in this case, the lowest frequency is the most important.

The engine operates at 2,900 rpm and has a major harmonic of 4. This system is too close to resonance since 4 times 2,878 rpm = 11,513 cpm. Designing to keep resonance away from the operating range is usually the best approach.

The magnifier method1 can be used to approximate the resonance amplitude with engine damping only present (M= 25).

The 4th order excitation of this engine is ± 5% of the 6,300 in.-lb mean torque:
The mean torque on the gears at 2,900 rpm is 6,300 in.-lb and the vibratory torque is ±8,502 in.-lb.

"When the vibratory torque exceeds the mean torque such as is the case here, the gears and splines separate from their respective contact surfaces; they then cyclically load and unload at the resonant frequency. This results in "hammering" of the gears or fretting of the splines.

A complete analysis and torsional testing of this system are needed. Determining the engine excitation into a system is complicated2 and harmonics higher than the 4th order will, in fact, have to be considered

It follows that the services of an experienced torsional specialist are needed because a poorly thought-out modification may simply move the problems to another point in the system.

LITERATURE CITED

1    Sofronas, A., Analytical Troubleshooting of Process Machinery and Pressure Vessels: Including Real-World Case Studies, ISBN: 0-471-73211-7, John Wiley & Sons, p. 125.

2    Den Hartog, J. P., Mechanical Vibrations, 4th Edition, McGraw-Hill, p. 197.