The second feature from the series of two on garbage disposal
KUALA LUMPUR, Dec 29 (Bernama)-- The climate changes and high-moisture waste are the factors that made garbage disposal dumps the sole method for efficient garbage disposal in the country.
The other method, the incinerator like that used in Langkawi is unable to meet the disposal of more than 19,000 tonnes of garbage generated daily by the nation, apart from consuming a lot of power.
The cost for constructing an incinerator is also much higher when compared to that for constructing a garbage disposal dump. The former is also able to dispose garbage of much lesser amount.
The other factor is that the incinerator is not cost-effective when it comes to garbage disposal.
"A RM1.5 billion incinerator can only dispose up to 1,200 tonnes of garbage daily, but a sanitary garbage disposal dump built at the cost of RM120 million like that at Bukit Tagar is able to dispose more than 3,000 tonnes of garbage daily," according to civil engineer Mohd Fatimi Said.
According to a source, the government will upgrade 30 of the 175 existing waste disposal dumps into sanitary facilities by the end of 2010. The move will not include the nine sanitary garbage disposal sites to be constructed.
Among the locations marked for the construction of these sanitary garbage disposal dumps are Sungai Udang (Melaka, Lahat (Ipoh, Perak), Bukit Jembalang ( Kemaman, Terengganu) and Ladang Tanah Merah (Negeri Sembilan).
Plus the seven existing sanitary garbage disposal dumps, this will bring the number of such facilities nationwide by the end of the 9th Malaysia Plan (9MP) to 46, said the source.
MANAGEMENT STRATEGY
For the Solid Waste Management and Public Cleansing Corporation (PPSPPA), the agency has initiated several measures to overcome the shortage of garbage dumpsites and existence of illegal dumps in the country.
Among them is the implementation of solid waste management strategy based on 'waste hierachy' to reduce the volume of garbage sent to disposal sites, as practised in many developed nations.
Among the measures undertaken are the reduction of waste at the source at all premises, encouraging reuse and boosting the quality of items that can be recycled.
It is learnt that the approach is taken as it did not involve provisions of the Solid Waste Management and Public Cleansing Act 2007 (Act 672).
The government has encouraged the private sector to invest in green technology in the effort to boost efficiency of more environmental-friendly energy usage towards facing the changes in the global climate.
At the recent United nations Organisation Conference On Climate Changes in Copenhagen, Denmark (COP 15), Prime Minister Datuk Seri Najib Tun Razak stated Malaysia's commitment to cut the percentage of carbon dioxide emissions by 40 per cent by the year 2020 with assistance from the developed nations.
RECYCLE
According to news reports, the government has carried out a pilot project on waste separation at source in Putrajaya with the aim of improving public awareness on recycling to reduce the volume of waste needed to be disposed.
The joint-venture effort by the PPSPPA, Solid Waste Management Department, Putrajaya Corporation, Alam Flora and Konsortium SSI-Schaefer is aimed to reduce by 40 per cent the volume of garbage sent for disposal.
"The Solid Waste Management and Public Cleansing Act focuses on recycling and has a special allocation for separation of wastes at the source," Housing and Local Government Minister Datuk Seri Kong Cho Ha was quoted as saying by the media.
The pilot project that began last Aug 29 involved 170 apartments at Precint 8, 276 houses (Precint 9) and 105 houses (Precint 10) where each household was provided with two garbage bins of different types, one for organic waste and the other for non-organic waste that can be recycled.
This way, the respondent would be able to recycle and reduce the amount of rubbish sent to disposal sites and at the same time the organic waste can be turned into compost and used for other purposes.
In other words, the separation of garbage at source will assist to lengthen the lifespan of a garbage disposal dump.
SOLID WASTE MANAGEMENT CORPORATION
Chief Executive Officer of PPSPPA Datuk Zaini Md Nor said the corporation viewed the issue based in the overall context and not limited to collection of garbage and construction of dumps.
He said the PPSPPA, established under Act 673 - the Solid Waste Management and Public Cleansing Corporation Act 2007, that began operations on June 1 2008 is responsble to monitor, supervise and enforce solid waste management and public cleansing in the country.
Among its roles is to enforce Act 672 and make improvisation recommendations whenever necessary.
PPSPPA is also responsible to inculcate public awareness and encourage public participation for sustainable management of public waste and cleansing such as reduction in waste and lodging reports on illegal dumping.
The corporation is also responsible for new technology-related research, such as recycling technology, 'waste to energy' technology, and waste treatment facilitating technology.
According to Zaini, in the short run the corporation would manage the implementation of closing non-operating garbage disposal dumps, upgrading and construction of new disposal sites as planned by the Solid Waste Management Department in 9MP.
"The corporation is the implementor of policies determined by the department", he said.
The PPSPPA also collaborated with the concessionaires to improve the garbage collection schedule that frequently received complaints from the public apart from providing feedback to the local authorities.
The garbage collection issue should be dealt first before the public's mentality on recycling is changed.
CLEAN MALAYSIA
The corporation, with the staff count of 900 at 52 district and state offices nationwide, is optimistic towards implementing its responsibilities in making Malaydia a clean country as required by its vision.
"To make the vision a success, we need the support of the society", said Zaini, adding that PPSPPA is conducting a survey on public awareness for recycling that involved more than 45,000 respondents nationwide since last August.
"The study is expected to be completed by end of this month. Through it, we will know the level of public awareness, problems and their suggestions in this issue", said Zaini.
Friday, January 1, 2010
Monday, November 30, 2009
Poor Controller Tuning Drives Up Valve Costs
How to get six-month paybacks by reducing stroking, air consumption, and wear
George Buckbee, PE
Published in Control Magazine April 02, 2002
The control valve is the workhorse of process control. In most modern PID control loops, the sensor is solid-state, as is the controller. The valve is the only component with moving parts. Therefore, it is the most subject to wear and tear, and requires frequent attention to stay in top condition. Unfortunately, it is also the most expensive component in the loop both to purchase and to repair.
Poor controller tuning contributes to wear and tear, often forcing the valve to move five or 10 times more than is actually required for good control. This article assesses the cost of poor tuning and provides some direction for the person looking to reduce valve maintenance costs.
For a PID control loop, the valve is usually the most costly component, typically representing 50% to 75% of the purchase cost of the control loop. However, it often represents closer to 90% of the ongoing maintenance costs.
These costs are well understood. Valve seals wear, packing leaks, components corrode, and linkages and springs suffer the stresses of thousands, even millions of cycles. In most industrial plants, a planned maintenance program combats the effects of this wear. It is typical for plants to spend on average between $400 and $2,000 per valve per year.
A good maintenance program will evaluate valve performance and routinely rebuild or replace valves that are out of specification. Work orders are written and the jobs are scheduled during plant shutdowns. Plant maintenance personnel are usually stretched quite thin during these shutdowns, so much of the work is completed with overtime pay or through third-party contractors, further increasing the cost.1
Wear and tear is caused by excessive operation of the valve. If the valve is constantly in motion, then the springs and linkages are being constantly stressed and fatigued. Positioner arms and other devices with moving components are also being worn in the process.
As Free as the Air We Breathe?
A well-hidden cost of control valves is compressed air. Large, expensive compressors operate with electricity 24 hours a day. If you know your plant's cost per kilowatt-hour, you can estimate the cost per year for 1 standard cubic foot per minute (SCFM) of air: 0.25 hp/SCFM x 0.745 kW/hp x 24 hr/day x 365 day/year x cost/kWh.
At a typical industrial rate of $0.06/kWh, this is roughly $98/year for each SCFM. And this doesn't count the capital, depreciation, and maintenance costs for the compressor, dryer, and distribution system. To determine the cost of air for control valve actuation, we need to know a little about the size of the valve actuator. Table I provides some typical air uses, based on actuator size and type.
Since it costs only $100 to $200 per year to tune and optimize a control loop, the savings on air alone justifies the effort with more than 200% payback.
The amount of valve movement plays a significant role in the amount of air required for the valve. For good control, of course, the valve must move. But just how much the valve must move depends on many factors, including the size of the valve, the process gain, and the size and frequency of disturbances and setpoint changes.
Top Control's experience is that most controllers, when properly tuned, require the valve to move between 1,500% and 20,000% per day; half of the time responding to noise, and half of the time to disturbances. A poorly tuned controller may move the valve two to 50 times as much.
Said another way, a properly tuned valve makes the equivalent of between 15 and 200 full strokes each day. This works out to between 1% and 14% per minute. Obviously, if we can reduce the valve movement, we can reduce the air usage. This in turn reduces valve maintenance needs, air costs, and process variability.
While we're on the topic of air losses: During a shutdown, assign someone to go hunting for air leaks. When the plant is quiet, they can quickly pinpoint air leaks (a spray bottle filled with soapy water helps). They can fix these leaks as they go, and the payback will be tremendous.
One other way to save on compressed air costs is to use an inlet guide vane to pre-spin the air entering the compressor. This reduces compressor-operating costs by approximately 10%.
How Tuning Affects Valve Costs
Of course, air costs are not the ultimate goal of the control system. A properly tuned controller will react when the process requires it, but will not over-react to process noise. In addition to tuning, a filter can be used to further reduce the noise that is seen by the controller.
Based on Top Control's experience, most control loops will see a two or three times reduction in valve movement when tuned properly. This happens through the proper use of P, I, and D parameters. Using these techniques, we have seen valve movement reduced by as much as 200 times.
Derivative (D) setting has the greatest impact on valve movement. Derivative action responds to the rate of change of the process variable (PV), or the error. Since noise represents quick changes to the PV, the derivative action responds quickly, moving the valve. In effect, derivative action will amplify noise.
George Buckbee, PE
Published in Control Magazine April 02, 2002
The control valve is the workhorse of process control. In most modern PID control loops, the sensor is solid-state, as is the controller. The valve is the only component with moving parts. Therefore, it is the most subject to wear and tear, and requires frequent attention to stay in top condition. Unfortunately, it is also the most expensive component in the loop both to purchase and to repair.
Poor controller tuning contributes to wear and tear, often forcing the valve to move five or 10 times more than is actually required for good control. This article assesses the cost of poor tuning and provides some direction for the person looking to reduce valve maintenance costs.
For a PID control loop, the valve is usually the most costly component, typically representing 50% to 75% of the purchase cost of the control loop. However, it often represents closer to 90% of the ongoing maintenance costs.
These costs are well understood. Valve seals wear, packing leaks, components corrode, and linkages and springs suffer the stresses of thousands, even millions of cycles. In most industrial plants, a planned maintenance program combats the effects of this wear. It is typical for plants to spend on average between $400 and $2,000 per valve per year.
A good maintenance program will evaluate valve performance and routinely rebuild or replace valves that are out of specification. Work orders are written and the jobs are scheduled during plant shutdowns. Plant maintenance personnel are usually stretched quite thin during these shutdowns, so much of the work is completed with overtime pay or through third-party contractors, further increasing the cost.1
Wear and tear is caused by excessive operation of the valve. If the valve is constantly in motion, then the springs and linkages are being constantly stressed and fatigued. Positioner arms and other devices with moving components are also being worn in the process.
As Free as the Air We Breathe?
A well-hidden cost of control valves is compressed air. Large, expensive compressors operate with electricity 24 hours a day. If you know your plant's cost per kilowatt-hour, you can estimate the cost per year for 1 standard cubic foot per minute (SCFM) of air: 0.25 hp/SCFM x 0.745 kW/hp x 24 hr/day x 365 day/year x cost/kWh.
At a typical industrial rate of $0.06/kWh, this is roughly $98/year for each SCFM. And this doesn't count the capital, depreciation, and maintenance costs for the compressor, dryer, and distribution system. To determine the cost of air for control valve actuation, we need to know a little about the size of the valve actuator. Table I provides some typical air uses, based on actuator size and type.
Since it costs only $100 to $200 per year to tune and optimize a control loop, the savings on air alone justifies the effort with more than 200% payback.
The amount of valve movement plays a significant role in the amount of air required for the valve. For good control, of course, the valve must move. But just how much the valve must move depends on many factors, including the size of the valve, the process gain, and the size and frequency of disturbances and setpoint changes.
Top Control's experience is that most controllers, when properly tuned, require the valve to move between 1,500% and 20,000% per day; half of the time responding to noise, and half of the time to disturbances. A poorly tuned controller may move the valve two to 50 times as much.
Said another way, a properly tuned valve makes the equivalent of between 15 and 200 full strokes each day. This works out to between 1% and 14% per minute. Obviously, if we can reduce the valve movement, we can reduce the air usage. This in turn reduces valve maintenance needs, air costs, and process variability.
While we're on the topic of air losses: During a shutdown, assign someone to go hunting for air leaks. When the plant is quiet, they can quickly pinpoint air leaks (a spray bottle filled with soapy water helps). They can fix these leaks as they go, and the payback will be tremendous.
One other way to save on compressed air costs is to use an inlet guide vane to pre-spin the air entering the compressor. This reduces compressor-operating costs by approximately 10%.
How Tuning Affects Valve Costs
Of course, air costs are not the ultimate goal of the control system. A properly tuned controller will react when the process requires it, but will not over-react to process noise. In addition to tuning, a filter can be used to further reduce the noise that is seen by the controller.
Based on Top Control's experience, most control loops will see a two or three times reduction in valve movement when tuned properly. This happens through the proper use of P, I, and D parameters. Using these techniques, we have seen valve movement reduced by as much as 200 times.
Derivative (D) setting has the greatest impact on valve movement. Derivative action responds to the rate of change of the process variable (PV), or the error. Since noise represents quick changes to the PV, the derivative action responds quickly, moving the valve. In effect, derivative action will amplify noise.
Saturday, November 21, 2009
SANITARY LANDFILL (SWM)
SOLID WASTE MANAGEMENT
Topic: SANITARY LANDFILL
Location: Jeram Sanitary Lanfill, Selangor
Sanitary landfills are sites where waste is isolated from the environment until it is safe. It is considered when it has completely degraded biologically, chemically and physically.The immediate goal is to meet, to the best extent possible, the four stated basic sanitary landfill conditions, with a longer term goal to meet them eventually in full. There are financial and other benefits to sites with long operating lifetimes (ten years or more). Large regional sites serving two or more cities could be economically beneficial, providing waste transport costs are not too high. Malaysian citizen now is about 23 million people. Solid waste that produce are rapidly increasing in such amount that it will reach the optimum.So, the issue now is that WHY MALAYSIAN PEOPLE DOESN'T SEEM TO UNDERSTAND ABOUT THE WASTE THAT THEY PRODUCE?
See the picture below how Malaysian people produced waste. Plastics, food waste, bottles,aluminium, tins, cans, paper, cardboard and etc is all dump in the landfill.picture show how the WORLDWIDE holding handle the sanitary landfill from the municipal waste until the leachate treatment plant to be discharge as clear water.
SO, PLEASE THINK BEFORE YOU THROW
Topic: SANITARY LANDFILL
Location: Jeram Sanitary Lanfill, Selangor
Sanitary landfills are sites where waste is isolated from the environment until it is safe. It is considered when it has completely degraded biologically, chemically and physically.The immediate goal is to meet, to the best extent possible, the four stated basic sanitary landfill conditions, with a longer term goal to meet them eventually in full. There are financial and other benefits to sites with long operating lifetimes (ten years or more). Large regional sites serving two or more cities could be economically beneficial, providing waste transport costs are not too high. Malaysian citizen now is about 23 million people. Solid waste that produce are rapidly increasing in such amount that it will reach the optimum.So, the issue now is that WHY MALAYSIAN PEOPLE DOESN'T SEEM TO UNDERSTAND ABOUT THE WASTE THAT THEY PRODUCE?
See the picture below how Malaysian people produced waste. Plastics, food waste, bottles,aluminium, tins, cans, paper, cardboard and etc is all dump in the landfill.picture show how the WORLDWIDE holding handle the sanitary landfill from the municipal waste until the leachate treatment plant to be discharge as clear water.
SO, PLEASE THINK BEFORE YOU THROW
Friday, November 13, 2009
Properly tuned and optimized PID loops increase product quality, plant uptime and throughput
Article by: John Gerry, President, ExperTune Inc., Hartland, Wisconsin
One of the most common approaches to controlling a field device is the proportional-integral-derivative (PID) controller. PID loops are everywhere in plants, controlling the critical flows, pressures, temperatures and compositions that make or break plant uptime and product quality. Use of the latest software tools helps get these loops tuned and optimized. To optimize the loop, the individual components of the loop must be working properly. This includes a valve (or drive) sizing with minimal stiction and hysteresis. Then, linearize the entire loop, not just the valve. Next, identify and eliminate interactions or cyclic upsets. Finally, tune the PID controller and set the optimum process variable filter. Improving any of these steps can have a significant impact on improving functionality in your plant.
PID loops are everywhere
in plants, controlling the critical flows,
pressures, temperatures and compositions
that make or break plant
uptime and product quality.
Get connected
There are a variety of ways to connect analysis software to your PID loops. Most analysis software supports DDE (Windows Dynamic Data Exchange) or imported plant data from plain ASCII files. Other software supports OPC (OLE for Process Control). OPC will eventually replace DDE as a standard way to connect your controller to a variety of Windows software. Some software includes wizards, which simplify the connecting process. There are also connections via direct drivers through a PC's serial port. The brute force approach is to use a small industrially hardened data acquisition system that reads voltage signals and includes jacks to clip in parallel across your control system's I/O.
Valve working?
Often overlooked, making sure your control valve works properly is key to getting the loop optimized. Hysteresis is unwanted slop or dead zone in the valve. If hysteresis suffering, and possibly cycling.
To check the hysteresis, follow these steps:
* With the controller in manual mode, move the controller output 10 percent and let the process variable (PV) settle.
* Move the output another 10 percent in the same direction. Wait for the PV to settle. Record how much it changed. Call this PV2
* Move the controller output back 10 percent. Let the PV settle. Record how much it changed from the last step as PV3.
* The hysteresis is 10 (1 - PV3/PV2) percent.
**Post-publication authors note: the 10% changes above result in a simple calculation, but are probably too large of a change to make in your plant. (ExperTune software includes a hysteresis checker that lets you make any size bumps)
If your hysteresis is more than one percent for valves with positioners or three percent for valves without positioners, consider repairing or changing equipment to reduce hysteresis and improve control. Often, the addition of a valve positioner will correct the problem. Caution: Use of valve positioners in flow loops is usually not recommended.
Analysis software can automatically find the hysteresis for you from less rigid test data. It will also check the sizing and transmitter spans. By adding the correct process variable filter, you can increase the life of your valve or drive without affecting loop performance.
Increase uptime and throughput: linearize that loop
Most loops are not linear; process gain changes as the measurement or controller output changes. Non-linear loops respond quickly, even to the point of oscillation, at one end of the range (or at one production rate) and are sluggish at others. A classic example is pH loops, but most loops are non-linear to some degree and can benefit by linearization.
The simplest approach to these loops is to tune the controller for the condition when the process gain is the highest. The result is sluggish tuning everywhere else. A more optimal strategy is adding a characterizer or linearizer to the loop. The combination of the characterizer with the non-linear process yields a linearized loop. With characterization, you tune the loop for great response across the entire range. This will provide better product quality, increase your uptime, and in many cases, allow you to increase your production rate without changing your equipment.
There are two type of characterizers: input and output.
Most loops need the output characterizer. If the linearity changes with load or production rate, use an output characterizer. Examples are: flow, pressure and temperature loops; jacket temperature in split-range chemical reactors; slave loops in cascades or any loop in which the setpoint will change. Use an input characterizer with pH, pIon, oxidation reduction and some temperature control loops on distillation columns.
To design the characterizer, you need data from your process. Output characterizers use plant data from the range of output you want to linearize. Input characterizers use lab titration data. The latest cutting edge software includes wizards that take you through the entire characterizer design process. The software does everything from collecting data to writing the characterizer XY points or equations and finally optimally tuning the controller.
pH Titration Curve
Figure 1. Titration points for a pH loop and the corresponding characterizer to linearize it
Figure 1 shows titration points for a pH loop (crosses on the graph) and the characterizer (redboxes) to linearize it. In this case, the characterizer output is a set of equations, but the user can also select XY points or other languages.
Implement an output characterizer by appending it to the controller output. An input characterizer is slightly more complex. With the input characterizer, characterize both the setpoint and the process variable. However, for display purposes, use the actual pH and setpoint before they are characterized. Also be sure that the setpoint entry is finished before the characterizer.
Product quality cycling
If a loop is cycling, the cause may be from interactions with other loops or from an upstream cyclic loop. State-of-the-art software tools help uncover these mysterious and hard-to-trace process cycling problems.
There are several tools available. One is cross-correlation analysis, which examines if a loop is affecting others. If it is, you can use a tool called "relative response time," a relative indicator of the speed of the control loop, to separate and decouple the loops. The smaller the relative response, the faster the loop. The higher the relative response value, the slower the loop.
To prevent interaction, adjust the tunings in the interacting loops so that the relative response time is different by a factor of five. The loops will then be responding at different times, reducing their interactions.
Another useful tool for pinpointing cycling is "power spectral density." It uncovers hidden cycles in the signal. Analogous to a prism that separates out the colors of white light, power spectral density mathematically separates normal operating data into its spectral or frequency components. A peak in the power spectral density indicates a cycling problem. Once you find a suspicious frequency, keep searching upstream until the peak is lower. The prior loop exhibiting the peak will be the problem.
PID tuning—the icing on the cake
Once your loop has only small hysteresis and is fairly linear, it's time to tune. The latest software tunes the PID controller and provides additional analysis—all from one loop test.
From the same test data used for tuning, some analysis software includes modeling, simulation and robustness plots. These let you see how the loop will respond offline before you use the tuning parameters in the plant. Robustness is an important consideration since there is always a tradeoff between fast response and sensitivity to the process changing.
Robustness plots graphically show how robust or sensitive the control loop is to process changes.
They also show how changing dead time or gain affect the stability of the loop.
The robustness plot has a region of stability and a region of instability. In Figure 2, the green area indicates instability (higher gain and dead time). The yellow area is stable. The line between indicates the verge of instability or continuous oscillations. For example, looking at the robustness plot, if the dead time were approximately 20 with a process gain of about 2.2, the loop would be on the verge of instability. In general, tune the loop to stay out of the red area in the plot.
The red and blue lines compare different tunings or uses of filtering. In our case, the red line is for a controller with a filter and the blue line for one without. Without the filter, the loop is slightly more stable.
How to make valves last (almost) forever
By simply filtering the PV, you might extend your valve's life by as much as a factor of three to five. Adding a correctly sized filter to the PV reduces the controller output jitters without degrading the loop's performance.
There is a fine line in choosing the correct size filter. If too small, it does nothing. Too large and the filter becomes slow enough to degrade the performance of the loop. State-of-the-art analysis software automatically finds the largest filter time possible without hurting your loop.
PlantA
Figure 2. State-of-the-art analysis software automatically finds the largest filter time possible without hurting your loop
In Figure 2, the red lines show responses and robustness with the filter. Blue lines are without the filter. In the lower graph (measured noise response), using the filter (red line) greatly decreases how much the valve will move. The "valve travel index" just above the lower graph assigns a numerical value to how much less the valve will move-in this case about 600 percent less. It also reverses 60 percent less. Adding the filter greatly reduces valve movement and reversals in this system. There is always a trade-off in process control and the robustness plot shows what we give up to get the increased valve life. In this case, not much. Looking at the robustness plot, the difference between the two systems is very small. Addition of the filter made the loop slightly less stable. Had we added a larger filter, the stability would decrease further.
Conclusions
By using state-of-the-art analysis software, you can derive the most efficient use of your equipment, and, therefore, reap the greatest efficiency benefits in your plant. Once optimized, your plant operations will improve more than you thought possible with higher quality than ever before. Find more information on the OPC Foundation at www.opcfoundation.org and on ExperTune at www.expertune.com. Figures courtesy of ExperTune, Inc.
One of the most common approaches to controlling a field device is the proportional-integral-derivative (PID) controller. PID loops are everywhere in plants, controlling the critical flows, pressures, temperatures and compositions that make or break plant uptime and product quality. Use of the latest software tools helps get these loops tuned and optimized. To optimize the loop, the individual components of the loop must be working properly. This includes a valve (or drive) sizing with minimal stiction and hysteresis. Then, linearize the entire loop, not just the valve. Next, identify and eliminate interactions or cyclic upsets. Finally, tune the PID controller and set the optimum process variable filter. Improving any of these steps can have a significant impact on improving functionality in your plant.
PID loops are everywhere
in plants, controlling the critical flows,
pressures, temperatures and compositions
that make or break plant
uptime and product quality.
Get connected
There are a variety of ways to connect analysis software to your PID loops. Most analysis software supports DDE (Windows Dynamic Data Exchange) or imported plant data from plain ASCII files. Other software supports OPC (OLE for Process Control). OPC will eventually replace DDE as a standard way to connect your controller to a variety of Windows software. Some software includes wizards, which simplify the connecting process. There are also connections via direct drivers through a PC's serial port. The brute force approach is to use a small industrially hardened data acquisition system that reads voltage signals and includes jacks to clip in parallel across your control system's I/O.
Valve working?
Often overlooked, making sure your control valve works properly is key to getting the loop optimized. Hysteresis is unwanted slop or dead zone in the valve. If hysteresis suffering, and possibly cycling.
To check the hysteresis, follow these steps:
* With the controller in manual mode, move the controller output 10 percent and let the process variable (PV) settle.
* Move the output another 10 percent in the same direction. Wait for the PV to settle. Record how much it changed. Call this PV2
* Move the controller output back 10 percent. Let the PV settle. Record how much it changed from the last step as PV3.
* The hysteresis is 10 (1 - PV3/PV2) percent.
**Post-publication authors note: the 10% changes above result in a simple calculation, but are probably too large of a change to make in your plant. (ExperTune software includes a hysteresis checker that lets you make any size bumps)
If your hysteresis is more than one percent for valves with positioners or three percent for valves without positioners, consider repairing or changing equipment to reduce hysteresis and improve control. Often, the addition of a valve positioner will correct the problem. Caution: Use of valve positioners in flow loops is usually not recommended.
Analysis software can automatically find the hysteresis for you from less rigid test data. It will also check the sizing and transmitter spans. By adding the correct process variable filter, you can increase the life of your valve or drive without affecting loop performance.
Increase uptime and throughput: linearize that loop
Most loops are not linear; process gain changes as the measurement or controller output changes. Non-linear loops respond quickly, even to the point of oscillation, at one end of the range (or at one production rate) and are sluggish at others. A classic example is pH loops, but most loops are non-linear to some degree and can benefit by linearization.
The simplest approach to these loops is to tune the controller for the condition when the process gain is the highest. The result is sluggish tuning everywhere else. A more optimal strategy is adding a characterizer or linearizer to the loop. The combination of the characterizer with the non-linear process yields a linearized loop. With characterization, you tune the loop for great response across the entire range. This will provide better product quality, increase your uptime, and in many cases, allow you to increase your production rate without changing your equipment.
There are two type of characterizers: input and output.
Most loops need the output characterizer. If the linearity changes with load or production rate, use an output characterizer. Examples are: flow, pressure and temperature loops; jacket temperature in split-range chemical reactors; slave loops in cascades or any loop in which the setpoint will change. Use an input characterizer with pH, pIon, oxidation reduction and some temperature control loops on distillation columns.
To design the characterizer, you need data from your process. Output characterizers use plant data from the range of output you want to linearize. Input characterizers use lab titration data. The latest cutting edge software includes wizards that take you through the entire characterizer design process. The software does everything from collecting data to writing the characterizer XY points or equations and finally optimally tuning the controller.
pH Titration Curve
Figure 1. Titration points for a pH loop and the corresponding characterizer to linearize it
Figure 1 shows titration points for a pH loop (crosses on the graph) and the characterizer (redboxes) to linearize it. In this case, the characterizer output is a set of equations, but the user can also select XY points or other languages.
Implement an output characterizer by appending it to the controller output. An input characterizer is slightly more complex. With the input characterizer, characterize both the setpoint and the process variable. However, for display purposes, use the actual pH and setpoint before they are characterized. Also be sure that the setpoint entry is finished before the characterizer.
Product quality cycling
If a loop is cycling, the cause may be from interactions with other loops or from an upstream cyclic loop. State-of-the-art software tools help uncover these mysterious and hard-to-trace process cycling problems.
There are several tools available. One is cross-correlation analysis, which examines if a loop is affecting others. If it is, you can use a tool called "relative response time," a relative indicator of the speed of the control loop, to separate and decouple the loops. The smaller the relative response, the faster the loop. The higher the relative response value, the slower the loop.
To prevent interaction, adjust the tunings in the interacting loops so that the relative response time is different by a factor of five. The loops will then be responding at different times, reducing their interactions.
Another useful tool for pinpointing cycling is "power spectral density." It uncovers hidden cycles in the signal. Analogous to a prism that separates out the colors of white light, power spectral density mathematically separates normal operating data into its spectral or frequency components. A peak in the power spectral density indicates a cycling problem. Once you find a suspicious frequency, keep searching upstream until the peak is lower. The prior loop exhibiting the peak will be the problem.
PID tuning—the icing on the cake
Once your loop has only small hysteresis and is fairly linear, it's time to tune. The latest software tunes the PID controller and provides additional analysis—all from one loop test.
From the same test data used for tuning, some analysis software includes modeling, simulation and robustness plots. These let you see how the loop will respond offline before you use the tuning parameters in the plant. Robustness is an important consideration since there is always a tradeoff between fast response and sensitivity to the process changing.
Robustness plots graphically show how robust or sensitive the control loop is to process changes.
They also show how changing dead time or gain affect the stability of the loop.
The robustness plot has a region of stability and a region of instability. In Figure 2, the green area indicates instability (higher gain and dead time). The yellow area is stable. The line between indicates the verge of instability or continuous oscillations. For example, looking at the robustness plot, if the dead time were approximately 20 with a process gain of about 2.2, the loop would be on the verge of instability. In general, tune the loop to stay out of the red area in the plot.
The red and blue lines compare different tunings or uses of filtering. In our case, the red line is for a controller with a filter and the blue line for one without. Without the filter, the loop is slightly more stable.
How to make valves last (almost) forever
By simply filtering the PV, you might extend your valve's life by as much as a factor of three to five. Adding a correctly sized filter to the PV reduces the controller output jitters without degrading the loop's performance.
There is a fine line in choosing the correct size filter. If too small, it does nothing. Too large and the filter becomes slow enough to degrade the performance of the loop. State-of-the-art analysis software automatically finds the largest filter time possible without hurting your loop.
PlantA
Figure 2. State-of-the-art analysis software automatically finds the largest filter time possible without hurting your loop
In Figure 2, the red lines show responses and robustness with the filter. Blue lines are without the filter. In the lower graph (measured noise response), using the filter (red line) greatly decreases how much the valve will move. The "valve travel index" just above the lower graph assigns a numerical value to how much less the valve will move-in this case about 600 percent less. It also reverses 60 percent less. Adding the filter greatly reduces valve movement and reversals in this system. There is always a trade-off in process control and the robustness plot shows what we give up to get the increased valve life. In this case, not much. Looking at the robustness plot, the difference between the two systems is very small. Addition of the filter made the loop slightly less stable. Had we added a larger filter, the stability would decrease further.
Conclusions
By using state-of-the-art analysis software, you can derive the most efficient use of your equipment, and, therefore, reap the greatest efficiency benefits in your plant. Once optimized, your plant operations will improve more than you thought possible with higher quality than ever before. Find more information on the OPC Foundation at www.opcfoundation.org and on ExperTune at www.expertune.com. Figures courtesy of ExperTune, Inc.
Thursday, November 12, 2009
The truth
President Obama in Copenhagen November 10, 2009 : 4:29 PM
I was asked last week, "would it be helpful for President Obama to go to the United Nations conference on climate change in Copenhagen in December?"
The answer is, of course, yes, and I think he probably will. I certainly hope he will. Given how high the stakes of this international meeting are, I think it would make a big difference. Because we're the strongest economy, one of the two largest emitters of global warming pollution, and recognized as the natural leader of the world community, it's important for us as Americans to fully appreciate the role we play.
Since this is the most dangerous challenge the world has ever faced, and since the scientists have given us fresh warnings that we don't have a lot of time to reduce global warming pollution, it's really important for the United States to play the leadership role the rest of the world expects us to play.
President Obama's attendance in Copenhagen would be a clear signal of our leadership.
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