Build Field And Engine Specific Maintenance Based On Trend Analysis

Brian Snyder
Michael A. Devine

Power sector

April 2014

Summary

Establishing site- and engine-specific maintenance plans based on trend analysis maximizes your equipment’s most valuable attribute: online production time.

introduction

For gas engine generator sets, the most valuable commodity is time, specifically uptime to generate power and revenue. Therefore, the goal of engine maintenance is to achieve the highest possible uptime. In essence, high uptime spreads capital and operating costs over more kilowatt hours, reducing the key variable of cost per kilowatt hour.

Replacement parts, fluids, supplies and labor costs are important, but uptime is even more important. Unplanned downtime, especially major breakdowns and long-term repairs, therefore of course need to be avoided. Additionally, this means that planned downtime must be kept to a minimum appropriate for the type of application. Maintenance according to manual requirements based on the engine owner’s manual, as opposed to maintenance according to perception, is not the key to achieving optimal uptime. Instead, the key is to apply best practices for optimal maintenance intervals and to integrate as many service tasks as possible into such scheduled maintenance work. The way to do this lies in trend analysis – understanding how the engine responds to fuel, load, environmental conditions and other factors, and adjusting maintenance efforts accordingly.

Understand the risks

A basic mistake users make is treating their industrial engine generator sets with the same maintenance mindset as a personal vehicle. Today, car and light truck engines require little attention other than an oil change. Insufficient maintenance can result in costly but rarely catastrophic consequences.

Industrial gas engines are far more complex and riskier: an inadvertent major failure can cost tens of millions of dollars in repair costs, more than downtime and production stoppages.

The pitfalls of too little maintenance are obvious, and too much is too expensive: if equipment is serviced more often than is really necessary, labor is wasted, production is lost through prolonged planned downtime, and consumables And spare parts are also discarded before the end of their useful life. Depending on the engine’s operating conditions, performing scheduled maintenance as required by the manual may mean too little or too much maintenance.

Monitor trends

The recommendations in the owner’s manual (prescribed intervals for oil/filter and spark plug changes, valve adjustments, overhauls, and other tasks) are often very conservative and intended to provide safety for engines in remote areas that are not frequently visited, or for owners who lack Engines that track basic condition indicators with time or precision provide safety.

In reality, there is no one-size-fits-all maintenance solution. Certain engines, such as those used in landfill or wastewater treatment digesters, may require shorter maintenance and overhaul intervals primarily due to fuel quality issues. On the other hand, for engines running on clean natural gas, less maintenance is typically required: operators can safely extend published intervals through trend analysis without increasing risk. Below are the basic items to monitor.

lubricating oil

Lubricant is easy to overlook, yet it is the lifeblood of your engine. Lubricants make up a large portion of maintenance costs, but are also critical to protecting and responding to engine conditions

Lubricating oil selection

Good maintenance starts with selecting the right lubricant for the engine and application – providing necessary lubricant life and component protection. Choosing a lubricant for an industrial engine generator set is more complicated than picking up a tub of 5W-30 from an auto parts store.

Engine manufacturers publish model- and application-specific oil specifications for properties such as ash content, lubricity, and operating temperature. Lubricating oils contain a variety of additives designed to increase oil stability, prevent acidification and extend service life under a range of operating conditions. New petrochemicals are frequently introduced to the market. Care should be taken to select a lubricant supplier that has a history with the engine manufacturer and understands the lubrication requirements of the engine in its specific application.

Oil sample analysis

Oil sample analysis is probably the most valuable engine trending tool. An engine oil analysis is similar to a medical blood test and is just as important in assessing the health of your engine. Engine oil analysis results provide important information to help set the most appropriate oil change intervals and can alert you to potential failures.

A key engine health indicator in natural gas engine oil is the metals, namely iron, chromium and copper. The presence and content of these substances, usually expressed in parts per million (ppm), can help indicate which parts are wearing as expected and which are showing abnormal wear. Harmful acids can also be detected by analysis as a decrease in total base number (TBN) (TBN is a measure of buffering) and an increase in total acid number (TAN) (TAN is a measure of acid content) (TAN is usually not included in the base Oil analysis, which must be specifically requested) The analysis can also detect ethylene glycol, which indicates a coolant leak, and silicon, which may be a sign of a leak in the intake system or a damaged air filter. Oil life is a function of the volume of oil in the engine, engine workload, environmental conditions and fuel quality. Oil can be safely stored in an engine until certain measurements reach the end-of-life limit set by the engine manufacturer. In engines fueled by pipeline gas, the most common end-of-life limitation is oxidation, the gradual breakdown of the oil caused by heat. The next most common is nitrification, caused by incomplete combustion or fuel contaminants. Oil sample analysis can be used for trend analysis in two situations.

Your oil analysis protocol should begin by testing a sample of clean (unused) engine oil that you plan to use. All oils have different additives and chemical compositions; these oils need to be evaluated and the chemicals they produce known before being used in an engine. The results of the clean oil test provide a baseline against which to compare the results of future analysis of used oil samples.

The used oil should then be analyzed every 250 operating hours to determine parameters for setting scrap limits and trends for specific oil formulations. Technicians should take samples when the oil is warm and well mixed to ensure that the sample truly reflects the condition of the oil in the crankcase.

The oil should then be analyzed at half its expected life to verify the trends initially seen. Once the scrap limit has been determined, it should be prudent to shorten the replacement interval slightly to ensure safety. Once a clear trend is identified, sample the oil at each change to verify that the trend is maintained.

Be sure to pay attention to the total number of hours the oil has been sampled – oil age can greatly affect analysis results. High-quality analytical laboratories list the latest test results alongside previous test results. They are also equipped with

A trained and certified oil technician who knows the specific engine model and will review the results, look for anomalies, and advise the user on where the situation is headed

There are a few other key points to remember about engine oil and analysis:

Just as no two people have exactly the same blood chemistry or blood pressure, no two engines operate in exactly the same way or have exactly the same wear characteristics. Even two engines of the same model in the same location do not operate in the same mode. . What is normal for one engine may not be normal for another, so engine trends need to be analyzed individually.
When checking oil sample analysis, the contaminants to consider may not come from within the engine, but from the surrounding environment. For example, working in dirty areas may increase silica levels; working near chemical plants that emit chlorine compounds may accelerate oil acidification.
Please check carefully before sending an oil sample. If large wear particles are found, notify your laboratory and have them investigate the cause.
There are many benefits to using an original equipment oil filter and subscribing to your engine manufacturer’s oil analysis service. In this case, having factory-trained, qualified technicians perform the analysis and correctly interpret the results, thereby enhancing warranty support: If the engine manufacturer’s oil filter fails due to a defect, the manufacturer not only reimburses the failed filter engine, and will also compensate for the damage caused to the engine.

spark plug

Proper spark plug operation is critical to gas engine performance, fuel economy and emissions. Today’s advanced lean-burn engines may use conventional J-gap spark plugs or prechamber spark plugs. In a prechamber design, the spark plug draws air and fuel through a small hole into a chamber surrounding the electrode. During ignition, the pre-combustion chamber protects the flame from being “blown out” by turbulence in the cylinder. The growing flame is then ejected through the small hole, igniting the air fuel throughout the cylinder.

Spark plug wear varies greatly with factors such as engine load coefficient and fuel quality. For example, the life of a J-gap spark plug might be 3000 to 5000 hours in a low compression engine and 2000 to 4000 hours in a high compression engine.

As with engine oil, spark plugs can be changed more often than the intervals listed in your owner’s manual. Good trend data can help operators set optimal replacement intervals for specific equipment. J-gap spark plugs must be readjusted to the correct specification at each service. Operators can monitor and track spark plug wear trends by measuring and recording the gap at each service and observing the general condition of the spark plug. Good maintenance procedures require regular cleaning of spark plug electrodes.

Spark plug performance can also be predicted by monitoring the secondary transformer output, which is usually read as a percentage by engine software or a SCADA system. A new spark plug usually registers at about 25%. As a spark plug ages, the electrodes degrade and the gap increases, thereby increasing the voltage required for the spark plug to jump across the terminals. When the secondary voltage exceeds 90%, the spark plug will fail, and it is easy to reach the best time to readjust the gap or replace the spark plug.

If your engine is unstable or running poorly, checking the exhaust temperature is a good way to troubleshoot the problem. A drop in temperature can indicate a faulty spark plug or a misfire. Deciding when to replace all spark plugs is a judgment call. Generally, if only one spark plug fails, and it has failed far short of the expected replacement interval, it is more cost-effective to replace only that spark plug. However, especially for a short period of time, when three or more plugs fail, it is a reliable sign that all plugs are nearing the end of their life.

Valve

Engine valves are designed to sink into their seats during daily wear and valve clearances need to be adjusted periodically to maintain an effective combustion seal within the cylinder. The rate of sinking varies with operating conditions, fuel quality, and other factors. The measurement of valve stem protrusion is essential for trend analysis.

The amount of valve stem protrusion shows the amount of wear that has occurred between the valve face and the valve insert. After the first 1000 hours on a new cylinder head, the initial valve stem protrusion should be measured, which is the normal “run-in” period for the valve to fit on the insert. This establishes a wear baseline. Afterwards, measurements should be taken when replacing spark plugs.

In addition to wear trends, valve stem protrusion measurements can also indicate impending engine failure. Over time, valve stem protrusion typically increases as the valve wears into the valve seat. A decrease in valve stem protrusion indicates a buildup on the valve surface that may be holding the valve open and causing valve burn. For example, prolonged light-load operation increases oil consumption and can lead to valve coking. Buildup on the valve surfaces also changes valve clearance, which alters valve timing, which in turn affects airflow and air-fuel ratio in the cylinder. Improper valve opening/closing sequencing can result in loss of compression, a rich fuel mixture causing knock, or too much inert exhaust gas in the cylinder at ignition, reducing power output.

Engine manufacturers set specifications for the maximum allowable valve stem protrusion. Measurements must be consistent and accurate. Ideally, the same person should always measure protrusion with the same tools and measurements. Protrusion is usually measured in millimeters, and different measurement equipment and techniques can significantly affect the accuracy of trend data.

cooling system

Coolant is an essential fluid with many uses. Its most obvious advantage is antifreeze, but it also increases the boiling point of cooling water and improves cooling efficiency. Just as important, it also contains additives that prevent mineral scaling in the coolant channels, lubricate the water pump seals, prevent rust, and prevent cylinder liner cavitation.

Select the type of coolant recommended by your engine manufacturer and mix it with deionized water in the appropriate proportions (a 50-50 ratio is typically used in cooler climates, but warmer climates year-round may require other mixtures). An easy way to do this is to use a pre-mixed coolant product. In any case, a coolant mixture that is too strong or too weak for your location can harm engine performance and life. Even in equatorial regions where antifreeze is not used, engines require the use of coolant conditioner to maintain optimal performance of the coolant.

Like engine oil, coolant should be analyzed regularly, at least once a year. An effective analysis program can help you verify correct coolant chemistry, monitor cooling system conditions, and correct coolant or cooling system problems before failure occurs. Analysis can detect symptoms of failure, such as inappropriate pH, unacceptable water hardness, the presence of sediment, low or high ethylene glycol levels, oil in the coolant, and elevated levels of lead, copper, and aluminum.

Cooling system pressure is also important. A leaking pressure control cap can lower the boiling point of the coolant, causing the water to boil and leak. Additionally, steam emerging from the cylinder head can limit cooling capacity and cause premature component wear.

All maintenance procedures that apply to ordinary cooling systems also apply to cogeneration units – the basic function of heat recovery is still to cool the engine.

While some manufacturers claim that their engines do not require top-end overhaul, all cylinder heads have moving parts that wear out. The valve spring lifts the valve up and down, and the valve rotator rotates about 3 degrees per stroke to prevent deposits from forming on the valve seat. Combustion heat causes molecular chemical effects on the exhaust valve and valve seat. The metal itself wears out due to repeated contact. Claims that a top-end overhaul is not required are often based on replacement of individual cylinder heads when worn or failed, often including extremely stringent or even unrealistic fuel quality specifications.

In-frame maintenance

An in-frame overhaul, as the name implies, involves repairing many of the engine’s internal systems without removing the engine block from the generator or base rails. An in-frame overhaul usually involves a lot of top-end overhaul work, including cylinder head replacement. An in-frame overhaul also typically includes replacement of the piston and connecting rod group, cylinder liners, turbocharger, and main and connecting rod bearings.

This process can typically be completed within 36 to 60 working hours; however, the accessibility of the engine and components and the skill level of the technicians used will have a large impact on the actual maintenance time. With good planning, this process can be completed within 24 to 36 business hours. Indicators for an in-box overhaul typically include oil consumption caused by cylinder liner vitrification (easily identified by bore range) and elevated levels of wear metals (aluminium, copper, chromium and iron) in the oil detected through oil sample analysis.

Comprehensive overhaul

A complete overhaul is essentially a complete rebuild that brings the engine back to up-to-date condition. The comprehensive overhaul not only replaces the same components in the frame, but also includes the reorganization of the front gear transmission group (replacement of bearings, inspection of gears and replacement of worn gears), replacement of shock absorber pulleys, replacement of camshaft bearings, and inspection of the engine block. Liner bore inspection (to make sure the crank and cam bores are still parallel to each other) and many more include camshaft and crankshaft repair.

Unlike an in-frame overhaul, a full overhaul requires the engine to be disconnected from the generator and removed from the airframe or housing and transported to the controlled environment of an overhaul shop. For a typical 170mm bore, 16-cylinder engine, this process may take 200 to 250 operating hours, longer if there are more cylinders or abnormal wear. Calculated from the baseline setting of 1,000 hours, time indicators for a complete overhaul include a 300% increase in oil consumption or a 200% increase in the amount of oil sent to the blow-by gas recirculation system. Significant changes in exhaust emissions may also signal that it’s time for an overhaul, although this phenomenon alone may simply indicate the need for adjustments to the engine emissions system or control strategy.

It is wise to schedule inspections around the time of downtime as much as possible. For example, if a natural gas company will shut down a pipeline for maintenance approximately 2,000 hours before a scheduled turnaround, it may make more economic sense to complete the turnaround at that time than to restart and shut down several months later to complete the turnaround.

Computing costs

The success of a power generation project using natural gas engine generator sets comes down to the cost of electricity per kilowatt hour. Margins may be tight, but a tenth of the cost per kWh can mean huge sums of money over the life of a project. Typical capital costs for facilities and equipment vary widely, but are generally in the range of 3 to 3.5 cents per kWh, and remain essentially constant. On the other hand, maintenance costs are largely controlled by the owner.

How much does a good repair cost? Every application type and maintenance scenario is different, but a conservative rule of thumb is that for projects using natural gas generator sets, “full” maintenance costs are 0.7 to 1.3 cents per kilowatt hour.

Factors such as poor facility design that makes equipment difficult to maintain, poor-quality pipeline fuel, local parts taxes and import fees can all drive up costs.

How much does a bad repair cost? Basically no upper limit. Engine failure, premature overhaul due to rapid component wear, prolonged unscheduled downtime – all of this can add up to thousands of dollars in damages and, on large projects, even millions of dollars in hard costs and Lost revenue, greatly increasing the cost per kWh.

Good maintenance doesn’t have to be difficult. Good maintenance requires trend-savvy methods, effective scheduling, well-trained personnel, high-quality replacement parts and consumables, and access to technical support from the engine manufacturer. Trend-based maintenance will help improve production time and reduce costs throughout the life of a power generation project.

Computing costs

The “B useful life” concept: comparing apples to apples

Like any industrial product, manufacturers of engines place demands on reliability and durability: continuous trouble-free uptime. To evaluate these claims, it helps to develop some objective measures by comparing “apples” to “apples.”

B Service life is one such measure, a scale from 0 to 100 that refers to the percentage of an item that can be used normally during a given useful life. For example, engine components are often designed for the age of the B10. This is the time (in hours of operation) at which 10% of the components will need repair or replacement and 90% of the components will continue to function.

The design of a machine, like an engine, always presents a compromise between durability and cost. For example, in space exploration or air travel, critical components are manufactured with a service life close to zero—nearly 100% of the components will be used until their design life. This is very expensive but necessary because failure is unacceptable and often life-threatening.

In industrial engines, on the other hand, component failure is undesirable and costly but usually not life-threatening. If an engine is manufactured with a service life close to zero, its price will be very expensive and few users can afford it.

Now, suppose an engine manufacturer introduces a 20,000-hour top-end overhaul interval, and a competitor with a similar engine introduces a 30,000-hour top-end overhaul interval. Both statements may be correct, but based on different B age criteria. That said, the 30,000 hour claim is probably based on the B50: only half of the components last the advertised life.

Therefore, among these more reliable claims, it is wise and legal to ask each manufacturer the following question: What B age is this claim based on? In this way, B age lets buyers know if apples are actually being compared to apples to apples. In fact, in the manufacturer’s operation and maintenance manual, some have a service life of B10, while others have a B50.

service reference

SEBU6400-05

SEBU8554-03

SEBU7681-17