Category Archives: Software

Instrument Calibration

A colleague recently asked a very important question:
How often should instruments be calibrated?

Many sites have requirements for calibration on certain instruments – such as fuel flow meters used for emissions reporting. But what about the rest?

Any sensor – temperature, pressure, flow, etc. – which provides input into your control system should be monitored regularly for accuracy. Things such as compressor inlet temperature, compressor discharge pressure and exhaust temperature – to begin with – all play a part in the load control of every type of gas turbine.

Annual calibrations are normally a good starting point – they provide you with a baseline for how each instrument changes over time. Reviewing these calibrations will give you a better understanding of how the working environment is impacting each sensor. In certain high stress applications – such as compressor discharge pressures on peaking units – calibrations and instrument adjustments may be needed more frequently than once a year. In low stress environments – such as cooling water temperatures – annual may be more than needed. But, if you lower the frequency of calibrations to less than annually, you may need to adjust your low-level alarm points to more readily catch instrument drift failures.

Please share how often you calibrate your site instrumentation here – I’d love to hear what your policies are. Do you calibrate all sensors or just a select few? Annually? Semi-annually (i.e. during each spring and fall outage)? Or “as needed”?

Do you utilize software tools, such as pattern recognition, to adjust your instrument calibration schedules?

I look forward to reading your replies.

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Heat Rate Improvement

Another good post on Heat Rate Improvement programs from EnergyPulse:

How to Build a Successful Heat Rate Improvement Program

Some of the comments after the article note that just watching heat rate is an ‘old paradigm’… but, sometimes the oldies are also goodies. In today’s world of automation, we are all susceptible to relying too much on technology and forgetting to think for ourselves.

In a world where many people will blindly follow whatever road their GPS takes them down, knowing how to read a map can still come in handy. Just recently, when asked to take us to the nearest Sonic Drive-in, our “Neverlost” system took us to the middle of an exclusive residential area where there were no restaurants for miles. We were lucky to get turned around and out of the neighborhood before the local sheriff showed up to escort us back to our hotel.

Had we known the general direction of the nearest strip mall, we might have questioned the GPS’s directions, and opted for the second Sonic on the list instead. But, we trusted the automation, and ended up on the wrong side of town. Luckily, not the wrong side of the tracks!

When controlling your equipment, knowledge of the process and expected trends in performance are essential to following the correct optimization recommendations. Automation and optimization systems for process control may work flawlessly 99% of the time, but that last 1%, when they take you in the opposite direction from intended they can potentially undo all the value of the other 99% – depending on the timing and source of the mis-calculation.

Automation systems are at the mercy of their incoming data. When a meter drifts or fails, the automation system may not recognize the error immediately. Recommendations for changes in control settings may go against the control room operator’s best judgment. If the operator blindly allows the plant to follow the automation signals, heat rate may be only one of the resulting casualties.

Continuous heat rate improvement programs need human involvement. This includes operators tracking real-time performance, maintenance and I&C personnel making sure data signals are accurate, and engineers supporting periodic detailed evaluations and capital improvement projects. Heat rate improvement is best accomplished as a team activity.

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Central performance monitoring makes the grade

I highly recommend reading this month’s Power Magazine – especially the article starting on page 66; Entergy’s ‘big catch”.  It’s a great example of how performance monitoring – with all the best tools and personnel – can make a huge impact on a plant’s performance and, more importantly for many, it’s reliability.

Most notably, the company in the article (Entergy) had setup a central performance monitoring and diagnostic center, where they had 24×7 support for equipment monitoring. The central staff was looking for performance losses, but also at reliability issues, such as increases in vibrations, temperatures, and other parameters which might indicate an imminent failure. The plant staff soon learned that this central group of professionals was there “not looking over their shoulder, but rater, watching their back.”

For a lot of plants, a central diagnostic center may be financially out of reach – although, when you consider the potential savings in forced outages and maintenance costs, it’s harder to make that argument. But, any additional observations you can make will support increased intelligence of plant operation which can lead to finding that abnormal condition before a catastrophic event occurs.

Just constantly trending your overall net heat rate in real time, where the control room operator can view it as time allows, will start the ball rolling. It’s not much – but it’s something. Small Steps. Kaizen. Once real time heat rate is being consistently monitored, you’ll start to see why a corrected heat rate can be helpful – changes in heat rate at full load become more apparent when corrected to a common baseline conditions.

Setting up a corrected heat rate trend does not need to be a large undertaking. Some information from the OEMs may be necessary, but again, taken in small steps, it can be fit into nearly any operating budget – and the time to payback is often very short. Errors in fuel metering are commonly found once heat rate is scrutinized relative to a set of reference conditions. Bad heat rate assumptions can lead to poor dispatch assumptions – which lead to operating in poor markets, or not operating in profitable ones.

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Plant Dispatch

Is it really March already?

I must be having way to much fun, because time really seems to be flying these days.

One item which keeps coming up, is plant dispatch.  What is the best way to dispatch a plant?  I would vote for efficiency based dispatch, of course.  Which means you need to know what your current efficiency (heat rate) is, and how that will change when you go to a new load – or need to operate under different conditions.

To that end, I’m working with a number of clients on putting together dispatch tools – all spreadsheet based.  In some cases, these are day-ahead spreadsheets, where the user enters the weather forecast for tomorrow, and the spreadsheet will spit back the expected capacities and heat rates for various “units” of the plant (base load, steam augmentation, duct burners, chillers, etc.).  In other cases, the user provides a long-range forecast, with expectations for different months or seasons, to see how the plant will do over the next 1, 5, 10 or 20 years.  When you add in expected gas and electricity prices, a picture of the overall plant economics comes into play. 

While some might call these “just spreadsheets” (which is what they are), they also lean on the power of VBA (visual basic for applications, or macros).  By putting the expected plant performance curves into VBA functions, you gain a lot of flexibility in how you can use the information.  You can determine when the plant will beat the spark spread and be in the money – when you can expect to be dispatched – when the best time to plan a major outage would be (when you expect not to be dispatched) – the list could go on…

I mentioned using these spread-sheets for day-ahead expectations, but they can also be used for hour-ahead expectations, or even current hour expectations.  Where are you versus where you expected to be?  How much can you offer the dispatcher if they call on you to “max out” your available capacity? 

Maybe you know of another application for these spreadsheet models – is there anything you need that you haven’t been able to find out  there in industry?

Or, do you use something besides a spreadsheet tool for these kinds of questions?

I’d be interested to hear from you.  I’m always looking for a better, more efficient, way to do things!

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