This paper will address
- Knowing when to do a pH sensor calibration versus a calibration check
- How to properly clean a pH sensor
- How to perform a pH sensor calibration
- A decision tree for step by step guidance
The phrase in the above title is actually incorrect in its sequence of wording. All pH readings are supposed to be taken and accepted only when the pH sensor is clean. After all, a contaminated pH sensor may yield an incorrect reading. So one must make sure the sensor is clean before doing a calibration. Once a pH sensor is installed in the process and operating, how do you determine when it is time to take the sensor out of the process and do a cleaning, or a calibration? Does one perform both a cleaning and a calibration or just a cleaning, or just a calibration, or does one just perform a calibration check in buffers or...?
This is something that can be quite confusing, especially when the operational practices and procedures documented by your company's Quality Control or Environmental Practices department may not be specific enough when they describe the procedure or the timing on when to conduct the pH calibration and maintenance. Inversely, the procedures may be too specific, detailing many more procedures and operations than are actually required.
In practical terms, users must develop their own maintenance and calibration schedule. This schedule is accomplished by taking the pH sensor out of the process after a set amount of time, perhaps after a day or two to perform a visual inspection of the sensor. If after inspection you find no debris or fouling on the electrode and reference surfaces with the naked eye, rinse the sensor off in distilled water and perform a buffer check.
This paper will address the application of Guided Wave Radar (GWR), also known as Time Domain Reflectometry (TDR), in your steam loop. Included will be discussions of how this technology functions and differs from more traditional forms of level indication.
The heart and soul of any boiler based power generation system is the steam loop or circuit. Without the proper availability of water in this system, efficiency suffers. In more extreme circumstances damage to other components from either too much water (carryover) or too little water (low water condition) will occur and shorten a boiler's lifespan. In the most extreme situation a dry fire accident could occur resulting in severe damage and personal injury.
Level indication in the steam loop is critical, yet the methods employed to measure it have been slow to evolve or change. Some of that has been due to code requirements (PG-60 of the ASME Boiler and Pressure Vessel Code) or a simple lack of confidence in "new" technology. It has only been in the past 15 to 20 years (recent in terms of boiler/steam loop history) that technologies such as magnetic level gages or differential pressure devices have been used in place of direct reading glass gauges on applications such as feedwater tanks, high pressure preheaters or hotwells. These same devices are now utilized for drum level indication as well. The most recent addition to the technology basket for steam loop applications has been Guided Wave Radar. Used in conjunction with other technologies it is seen as a reliable cost effective choice for redundant level measurement in all steam loop applications, including drum level.
The technology advancements in measurement instruments and final control elements provide greater process insight, reduce engineering costs and contribute to improving the overall operational performance of the plant. These instruments are often collectively referred to as smart devices.
Process measurements are instantaneous but analyzer responses never are. From the tap to the analyzer, there is always a time delay. Unfortunately, this delay is often underestimated or misunderstood.
Time delay is defined as the amount of time it takes for a new sample to reach the analyzer. One way to control time delay is with a regulator. Regulators control pressure, and pressure in an analytical system is closely related to time. In the case of gas systems with a controlled flow rate, the lower the pressure, the shorter the time delay.
Delay may occur in any of the major parts of an analytical instrumentation (AI) system, including the process line, tap and probe, field station, transport line, sample conditioning system, stream switching system, and analyzer.
Human operators are a key part of any process control system. As such, they constitute part of a complex, causal chain of overall system processing. Human machine interfaces (HMIs) form a key link in that chain by bridging the physical world where processes reside with the perceptual reconstruction and representation of those processes in the heads of human operators and supervisors.
If an HMI design gives rise to a flawed or inaccurate representation of a process, then error and suboptimal task performance may result. HMIs have become increasingly important links in this chain for two reasons. First, the arrival of distributed control systems (DCS) in the 1970s distanced operators from the physical entities they controlled, requiring all interaction be mediated by HMIs. Second, the ongoing introduction of complex automation into process control is increasingly changing human operators into supervisors. Supervision has complex decision-making requirements that must all be conveyed via HMIs.
Download this entire white paper to learn more.
Dirk Beer, Harvey Smallman, Cindy Scott, Mark Nixon
The editors of Control and Control Design compiled this special report from the 21st annual Automation Fair event, hosted by Rockwell Automation, in Philadelphia, November 5-8. This interactive PDF includes more than 20 articles documenting highlights ranging from executive keynotes and new product announcements to vertical industry forums and Rockwell Automation Process Solutions User Group and Safety Automation Forum meetings.
Halloween in New Orleans this year saw more than the usual array of costumed revelers on Bourbon Street. Indeed, hundreds of process automation professionals descended on the Crescent City to exchange ideas and best practices at Yokogawa's 2012 User Group Conference and Exhibition. The Control editorial team was on hand and developed this exclusive report of presentation highlights--on topics ranging from safety system risk management to the latest in data acquisition technology.
With the advancement of computer and data transmission technologies, systems formerly reserved for the office environment are now critical components of the manufacturing floor. The demands of factory automation, in addition to computer hardware and software, have brought the wire and cable networking products that interconnect these technologies into the industrial setting as well.
With the vast differences between an office and an industrial environment, networking cables such as gigabit Ethernet have had to adapt to these harsh new surroundings, not only from a physical perspective but from a performance perspective as well, in order to function reliably.
This white paper discusses the constructional differences between standard Gigabit Ethernet and the specifications required for similar cables utilized in an industrial manufacturing environment. Additionally applications for these ruggedized designs are also reviewed.
Every manufacturing industry is experiencing an increasing speed of business in several areas including changing schedules, customer needs, costs of materials, business models, and technologies. At the same time, many manufacturing sites - particularly in the discrete industries - have growing complexity in their operations which makes it more difficult to adapt. There are more SKUs and data to keep track of due to product proliferation, smaller lot sizes and compliance to government regulations.
The demands for improved speed and agility conflict with the plants' ability to respond. Visibility into current operations, including the control system, is the primary reason manufacturers buy Manufacturing Execution Systems (MES). This visibility provides the information necessary for informed decision making in real-time by all levels of personnel - plant floor to the executives.
MES applications contain the critical business processes for executing a production schedule. These systems perform the production-centric functions of planning, controlling, operating and informing. Control systems execute these functions to produce the goods needed to fulfill customer orders. By integrating MES with control systems, manufacturing becomes more agile for responding to change in this increasingly dynamic business environment. Integrating the control system with the MES allows for more effective and broader set of production management functions to improve operational performance.
To improve their response to operational issues, managers look to technology for connecting plant floor and business systems for automated business processes. Some manufacturers have implemented point solutions on a case-by-case basis. Because of the higher development costs and support issues, this approach is not acceptable. An integration platform is needed.
Water is a key element to life. It plays an important role in the world economy, as it functions as a solvent for a variety of chemical substances. 71% of the Earth's surface is covered with water and 97% of that water is in oceans and saline. Only 3% of the earths water is fresh and can be found in the polar caps, glaciers, ground aquifers, lakes, swamps and rivers. In parts of the world where there is limited or no access to fresh water, desalination is being used to convert saline water to drinking water. To manage resources and the flow of water, modern electrical pumps and control systems are employed. Water chemical compatibility and electrical interference are two major challenges for the control systems. Let's ook at two major sources of fresh water and the issues that can limit the performance of the control system.
Differential pressure (dP) sensors with electronic signal processing are increasing being used to monitor flow, filter condition and level. Since these devices offer linear and accurate output, they are also replacing the differential pressure switch that only support on-off condition and useless for closed loop control system. These dPs are often configured with expensive valves and fluid filled remote seals for added protection against corrosive media, radiation and/or extreme media temperature ranges when operating in demanding environments. In cold ambient environment specially operating in temperatures below -4 deg F (-20 deg C), the sensor need to be heated either by trace heater or within a heated enclosure to maintain the operation of the dP sensor. In addition to being expensive, these valves and seals tend to be bulky and require time to install and maintain. In many critical applications such as food and pharmaceuticals, filled fluids are a serious concern due to process contamination. In gaseous systems such as hydrogen and oxygen and semiconductor applications, fluid filled sensors are being banned since the leakage of fluid into the process could lead to an explosion and serious safety issues.
A new series of LVDT (linear variable differential transformer) based oil-less dP sensor with dual channel ASIC (applications specific integrated circuit) have been developed that can operate in a wide range of corrosive materials, radiation and temperature without any oil filling and bulky sealing systems. By encapsulating LVDT proven technology with digital compensation, the pressure sensors combine the benefits of friction-free operation, environmental robustness and unlimited mechanical life. By selecting the diaphragm thickness and material properties, Table 1 show the dP ranges that can be produced using the LVDT technology.
There are over two hundred pressure sensor suppliers around the world, offering products from a few dollars to thousands of dollars. A purchaser or engineer unfamiliar with pressure sensors can become overwhelmed with the price range, quality and options. The first step is to understand his/her application from the media being measured, to the desired electrical output for indication or control. The following is a guide through a variety of options to make a prudent decision.
Media is the most important item when selecting a pressure sensor for an application. Most sensor suppliers only sell sensors that are rated for benign environments such as clean, dry air. The next tier of suppliers will sell products that will handle mild environments through to difficult/corrosive environments. Clean water, steam, some forms of hydraulic oils and Freon can be considered mild environments. Difficult media tends to be corrosive liquids and gases such as hydrogen sulfide, hydrochloric acid, bleach, bromides, waste water and hydrogen. Wrongful selection of a pressure sensor can lead to catastrophic failure and serious injury. When unsure, ask the pressure sensor manufacturer to provide a chemical compatibility chart with their products. In fluidic systems, such as water and hydraulics, one must understand how the water hammer and pressure transients effect the pressure sensor.