It's surprising to learn how many people responsible for a boiler's performance feel that fuel flow is an accurate measurement of the effectiveness of the boiler. Or how many times boiler efficiency calculations don't include compensation for pressure and temperature. Knowing the real efficiency of your boilers is paramount to knowing how much money you're saving, or losing, in your everyday operations.
There are a lot of factors to consider when determining the real efficiency of boilers. These factors become more critical when changes occur in the economic landscape, such as the rising cost of gas. While gas is certainly more environmentally friendly than other fuels, the current price of gas may cause you to take a new look at burning oil or coal. Because gas fuels contain a lot more hydrogen, boilers burning gas typically average 3% less efficiency than those firing oil or pulverized coal. Plus, the fuel price per Btu of gas may be considerably more than for oil or coal. This difference is actually causing some larger industrial companies to allocate more production to plants that use fuels other than gas.
Whatever fuel you choose to burn will certainly affect the overall efficiency of your boilers, but not as much perhaps as other factors, such as not compensating for pressure and temperature changes in your plant's environment.
Efficiency Variations: Everyday Examples
To illustrate how boiler efficiency can fluctuate based on variations in plant conditions-even the weather-take the example of your personal automobile. Especially for older cars, the efficiency of the engine changes as the weather changes. You may notice that your own car seems to drive differently and get better or worse gas mileage at different times throughout the year. To compensate for this, you can change the octane of the fuel you use. You can even alter your driving methods somewhat to increase your gas mileage, and thus your engine's efficiency. A few older cars had indicators of gas mileage so one could monitor the car's performance and manually make changes as appropriate. Nowadays, most new cars include oxygen and temperature sensors that automatically improve engine efficiency. But, unless automakers begin to include on-board efficiency calculations (which I'm not sure any of us would use anyway), you're stuck with whatever efficiency your engine is able to muster.Thank goodness that isn't the case with your plant's boilers. Like your automobile's engine, boilers are affected by changes in fuel, pressure and temperature. But, unlike your automobile's engine, you can actually do something significant about it.
Many people don't know this, however. They treat their boilers with no more interaction than their car. They just accept whatever efficiency seems to be "built-in" with the design of the boiler. To be sure, each boiler does have its limitations. But plant engineers and maintenance personnel have the ability to work within these limitations and have a real impact on boiler efficiency and online control by using methods such as monitoring stack oxygen, as well as pressure and temperature compensation for fuel and air flows. Efficiency is well within your control-if you know how to measure it, and how to use the measurements to provide a more accurate indication of your actual boiler efficiency.
There's no logical reason to run a boiler like an automobile. "Fuel and air in, energy out" is not the end of the story when it comes to boilers. Good thing, too. Poor gas mileage in an automobile won't cost you nearly as much as poor boiler efficiency. Depending on the size and the number of boilers in your plant, a change in fuel price and a few percentage points of lost efficiency can mean hundreds of thousands of dollars lost over a year's time-dollars that could be contributing to your plant's bottom line.
How to Keep Profits from Going Up the Stack
It starts with proper plant design. If you're designing a new plant, consider yourself lucky. You have a great deal of control over desired efficiency by including energy saving accessories or by choosing more efficient engines. If you're not designing a new plant, that is if you're entrusted with running an existing plant, then you're pretty much stuck with the current design and its associated limitations. But not to worry. There are plenty of areas where you can wring additional efficiency out of your current equipment. You just have to know where to look and what to do.The second step in the efficiency improvement equation is plant (boiler) operation. This is where much of the boiler's energy goes up the stack. Not to say that operators are purposely running boilers inefficiently. On the contrary, operators have learned over the years to do simple tweaking here and there to compensate for rumblings and puffs and all sorts of strange happenings in the boilers. Keeping the boilers up and running, and of course operating in a safe manner, is the main priority.
But wouldn't it be nice if there was a way to spend just a little extra time gathering information about the boiler's operation, just a few telltale measurements that could make the difference of several percentage points of efficiency, and as stated before, those all important profits? Well, there is.
Assuming the boiler or boilers are being operated as well as can be expected, it might be time to ask, "How well are they really performing?" As mentioned before, simply measuring fuel flow or fuel/air mixture won't get you a truly accurate indication of boiler performance. Remember the example of the automobile engine? With gas prices soaring and winter upon us, now would be a good time to look at how you're compensating for the changes in weather. You might find that your boilers will run more efficiently if you can measure more accurately the REAL fuel and steam flows-that is, those that include pressure and temperature compensation.
People who are serious about efficiency are getting a better handle on what it is they're making. They've learned that actual flow rates vary depending on ambient conditions. And they've learned how to measure them accurately. As an example, one client's numbers showed that its gas-fired boilers were 91% efficient. Since 86% was about the maximum efficiency to be expected (per the manufacturer), we questioned the data. It turned out that after adding pressure and temperature compensation, the actual efficiency was closer to 81%. That 10% swing was unknowingly costing the plant a lot of money.
Example: Instrument type: VORTEX
Fluid type: Natural Gas
Base pressure: 0 psig
Base temperature: 60 deg F
Actual temperature: 40 deg F
Actual pressure: 30 psig
Density ratio: 2.92
Measured flow = 1000 actual cubic feet per hour at flowing conditions
Compensated flow = measured flow x new density ratio divided by old density ratio
New density ratio = (actual pressure) x (base temperature) divided by (base pressure) x (actual temperature)
New density ratio = (30 psig + 14.7) x (60 deg F + 459.67) divided by (0 psig + 14.7) x (40 deg F + 459.67) = 3.16
Compensated flow = 1000 x 3.16/2.92 = 1082 scfh
Difference = 100 x (1082 - 1000) divided by 1000 = 8.2%
So, after working to make sure boiler operation is on target, improving measurement accuracy is the next step in improving boiler efficiency. And to improve the accuracy of your measurements, you first have to make them. That means if you don't have pressure and temperature transmitters in your flow loops, you need to get them. Like everything else, you need the right tool for the job. And don't assume the factory settings will work year-round. You'll have to adjust the settings for actual plant conditions. Also, don't assume every meter will compensate for ambient conditions. They won't. Vortex meters work well to measure velocity, but most don't have built-in compensation capability. If you want to limit the readjusting you have to do, look for one of the newer, intelligent meters-preferably one with steam properties on board. With this type, once you set it up, it has automatic adjusting parameters. Just remember this-if you don't have a precise measuring device, you're more than likely to get inaccurate measurements. And inaccurate flow measurements will fool you into thinking boiler efficiency is better or worse than it really is, which really makes your decisions tougher.
Figure 1 shows an example of one of the newer, intelligent measurement devices on the market. The Rosemount 3095 MVT Multivariable Mass Flow transmitter is a great instrument for measuring steam and fuel flow. Advantages include:
- A 1% accuracy or better over an 8:1 flow range (64:1 DP Range)-with this device, the boiler can be operated over wider ranges with greater turndown, while still maintaining accurate measurements.
- Four measurements (Qm, DP, P, T) are included in one device, providing real-time data to perform mass and heat balances across process units.
- Dynamically compensated mass flow is adjusted automatically for a number of variables, including velocity, compressibility and viscosity.
- Useful for gas or liquids.
- Has online diagnostics with self-testing. (For more information see www.rosemount.com.)
For oil fuels, consider a true mass flow meter such as one made by Micromotion (see www.micromotion.com).
Now that your boilers are operating well, and you have the ability to accurately measure flows, compensated for actual plant conditions of course, you're all set right?
Not quite. Having great measurement instrumentation will no doubt improve the operational ability of your plant personnel. But don't forget to look at the condition of other equipment in the plant. Take the valves for example. Old, sticking valves will affect flow control. Worse, they hide problems in the plant. (See www.frco.com/fisher.)
Does It Sound Like a Problem?
If you don't know you have a problem, it's pretty hard to fix it, isn't it? And if you don't fix the problems you don't know about, what do you think will happen over time? That's right. Slow, undetected decay in the plant's operation-decay that is extremely difficult to pin point. That's where the all-too-familiar tweaking or "operational adjusting" comes in. It should be called decay compensation.A lack of information about key components like valves and dampers can eventually become a severe safety concern. Not knowing the actual performance of equipment such as this can cause an operator to be unaware of a dangerous mixing of fuel and air. The little boiler rumblings and puffs may seem like aging pains, akin to creaks in our bones as we get older. But they're certainly far more serious and potentially threatening.
Rumbling due to combustion problems is a matter of degree. Minor vibrations aren't necessarily a big deal. But loss of flame is more serious. Rumbling may be a precursor to a puff or an explosion. Many people underestimate the foretelling nature of noises. They should not be ignored. Rumblings may mean the air/fuel mixture is not optimized. And it may mean your flow indications are inaccurate. This is why it's important to improve the accuracy of all your information. And pay attention to any unusual variations in the boilers when you run them up and down. Many operators ignore these telltale "voices." You can be sure they are warning voices, and eventually they may produce bad consequences.
Most boilers have burner management systems that shut down the unit when the aforementioned conditions arise. This is certainly good from a safety perspective. But how good is it when the systems are disengaged or bypassed because of annoying, frequent shutdowns? When the burner management system keeps shutting down the boilers, rather than bypass it as a nuisance, listen to what it's telling you. It's most likely doing the job it was designed to do. The problem isn't that it's too good at doing its job. The problem is that it's having to do its job too often. And that means there's another, more serious problem not being taken care of. Listen to your equipment and understand what it's telling you to ensure that you have the right information to know exactly what's going on.
Take stack O2 for example. How many people actually measure stack O2? If you don't know the amount of oxygen in the stack while burning fuel, then you don't know valuable information about the air/fuel mixture. And, as mentioned previously, if you don't know the accuracy of the mixture, then you don't know either how safe or how efficient your boilers are operating. This is not a good situation.
Some have attempted to solve this puzzle using extractive O2 analyzers. But unless plumbing leaks are routinely checked, what is actually being measured is the air leak-not the stack O2.
In-situ type O2 analyzers are the best, in terms of accurately measuring unburned O2 in the stack. The analyzer probe is actually mounted in the stack. This allows far less opportunity for plumbing leaks, which of course translates into greater accuracy-not to mention faster measurements and fewer maintenance headaches.
The self-calibrating type O2 analyzer is good. The manual type is less expensive, but all too often people forget to calibrate it. (For more information on analyzers, go to www.frco.com/rai.)
All In a Day's Work
It seems like there's a lot of complexity in ensuring accurate boiler efficiency. But it's really a lot simpler than it seems. And the payoff, not only in terms of cost savings, but also in terms of greater safety, fewer maintenance problems and a more efficient use of resources, is well worth the effort. Just remember:
- Optimize boiler operations according to design parameters.
- Choose robust, intelligent instruments and put them in the flow loops.
- Obtain accurate online measurements of fuel, air, and steam flow.
- Include pressure and temperature compensation.
- Measure and control O2 in the stack.
- Choose a good control system.
The last point may seem like an afterthought, especially since this article has focused so much on measurement accuracy. But remember the weak link theory. Your operations will only be as good as their weakest element. The best instrumentation in the world won't help you maximize operational efficiency if you have a control system that can't check calibration and do preventive maintenance from the control room. And if all the pieces aren't in place in a cohesive, uniform framework (see www.testdriveplantweb.com), you're accepting-and getting-far less out of your operations than you should.
As with all things, it's a matter of choice. What equipment you use in your job is your choice. But would you really choose to climb up stacks during inclement weather to recalibrate an instrument? The only thing that should be going up in this weather is your boiler's efficiency and your plant's profits.
