Tuesday, May 30, 2017

Rack and Pinion Style Pneumatic Valve Actuator

pneumatic rack and pinion valve actuator
One example of a pneumatic rack and pinion valve actuator
Courtesy Rotork
Three primary kinds of valve actuators are commonly used: pneumatic, hydraulic, and electric.
Pneumatic actuators can be further categorized as scotch yoke design, vane design, and the subject of this post - rack and pinion actuators.

Rack and pinion actuators convert linear movement of a driving mechanism to provide a rotational movement designed to open and close quarter-turn valves such as ball, butterfly, or plug valves and also for operating industrial or commercial dampers. The rotational movement of a rack and pinion actuator is accomplished via linear motion and two gears. A circular gear, known as a “pinion” engages the teeth of one or two linear gears, referred to as the “rack”. Pneumatic actuators use pistons that are attached to the rack. As air or spring power is applied the to pistons, the rack changes position. This linear movement is transferred to the rotary pinion gear (in both directions) providing bi-directional rotation to open and close the connected valve.
rack and pinion gears animation
Rack and pinion gear
Courtesy Wikipedia

The actuator pistons can be pressurized with air, gas, or oil to provide the linear the movement that drives the pinion gear. To rotate the pinion gear in the opposite direction, the air, gas, or oil must be redirected to the other side of the pistons, or use coil springs as the energy source for rotation. Rack and pinion actuators using springs are referred to as "spring-return actuators". Actuators that rely on opposite side pressurization of the rack are referred to as "direct acting".

Most actuators are designed for 100-degree travel with clockwise and counterclockwise travel adjustment for open and closed positions. World standard ISO mounting pad are commonly available to provide ease and flexibility in direct valve installation. NAMUR mounting dimensions on actuator pneumatic port connections and on actuator accessory holes and drive shaft are also common design features to make adding pilot valves and accessories more convenient.

Pneumatic rack and pinion actuators are compact and effective. They are reliable, durable and provide good service life. There are many brands of rack and pinion actuators on the market, all with subtle differences in piston seals, shaft seals, spring design and body designs. Some variants are specially designed for very specific operational environments or circumstances.

Share your process valve control and automation challenges with application experts, and combine your process experience and knowledge with their product application expertise to develop effective solutions.

Wednesday, May 24, 2017

Desuperheating and Attemperation of Steam

electric power plant
Electric power generation plant
Steam heats or powers a respectable swath of industrial operations, plus there is electric power generation. Steam is an important sort of "back office" component of the lives of many dwellers in modern economies.
What is steam?
Sorry, but we need to get everybody on the same page here. Steam is water vapor, produced by the application of heat to water. In order for steam to do work and serve as a useful energy source, it must be under pressure. There can be applications that employ steam at atmospheric pressure, but most are pressurized.

The heat goes on, the water boils, steam is produced and flows through the piping system to where it is used. Sounds simple, sounds easy. It is not. There are intricacies of designing and operating a steam system that determine its raw performance, as well as how efficiently it uses the fuel or other heat source employed to boil water. Steam utilization equipment is also carefully designed to provide its rated performance when supplied with steam of a given condition.

Steam at any given pressure has a saturation temperature, the temperature at which the vaporized water content of the steam is at its maximum level. Heat steam above its saturation temperature and you have superheated steam. Cool it below the saturation temperature and vapor will start to condense. The way in which the steam is to be used determines whether, and how much, superheat is desirable or necessary.

  • Turbine operations benefit from properly superheated steam because it avoids exposure of the turbine to liquid water droplets, generally a source of surface erosion and other accelerated wear.
  • Heat exchanger performance is based upon certain inlet conditions, one of which is the degree of superheat.
  • Maintaining sufficient superheat throughout a continuously operating steam system minimizes the need for, and size of, a condensate return system
Processes are designed to deliver a predictable output when provided with known inputs. In the case of steam, the temperature of the steam may be an input requiring control. This brings us to attemperation, which in the case of steam most often refers to lowering the temperature of a steam supply. Attemperation and desuperheating (reducing the degree of superheat) are accomplished in a similar fashion, but with differing objectives. Attemperation involves simply controlling the temperature of the steam, without any direct regard for the level of superheat. Desuperheating, as a control operation, is not directly related to the temperature of the steam, just the degree by which it exceeds the saturation temperature at the current condition. For attemperation, steam temperature measurement is all that is needed. For desuperheating, pressure and temperature measurements are needed. Decreasing the temperature of superheated steam will naturally reduce the amount of superheat.

Some process requirements may focus on temperature of the delivered steam, without regard to superheat level. Others will rely on a specified level of superheat. The application scenarios are vast, with equipment available to accomplish whatever is needed. 

Either operation can be accomplished with some sort of heat exchanger that extracts heat from the steam. A more flexible option relies on the addition of atomized water to the flowing steam to manage temperature or superheat level. Share your steam system challenges with experts, combining your own facilities and process knowledge with their product application expertise to develop effective solutions.

Tuesday, May 16, 2017

Shell and Tube Heat Exchangers

diagram of shell and tube heat exchanger
Shell and tube heat exchanger diagram
Cars are something which exist as part of the backbone of modern society, for both personal and professional use. Automobiles, while being everyday objects, also contain systems which need to be constantly maintained and in-sequence to ensure the safety of both the machine and the driver. One of the most essential elements of car ownership is the understanding of how heat and temperature can impact a car’s operation. Likewise, regulating temperature in industrial operations, which is akin to controlling heat, is a key process control variable relating to both product excellence and operator safety. Since temperature is a fundamental aspect of both industrial and consumer life, heat management must be accurate, consistent, and predictable.

A common design of heat exchangers used in the oil refining and chemical processing industries is the shell and tube heat exchanger. A pressure vessel, the shell, contains a bundle of tubes. One fluid flows within the tubes while another floods the shell and contacts the outer tube surface. Heat energy conducts through the tube wall from the warmer to the cooler substance, completing the transfer of heat between the two distinct substances. These fluids can either be liquids or gases. If a large heat transfer area is utilized, consisting of greater tube surface area, many tubes or circuits of tubes can be used concurrently in order to maximize the transfer of heat. There are many considerations to take into account in regards to the design of shell and tube heat exchangers, such as tube diameter, circuiting of the tubes, tube wall thickness, shell and tube operating pressure requirements, and more. In parallel fashion to a process control system, every decision made in reference to designing and practically applying the correct heat exchanger depends on the factors present in both the materials being regulated and the industrial purpose for which the equipment is going to be used.

The industrial and commercial applications of shell and tube heat exchangers are vast, ranging from small to very large capacities. They can serve as condensers, evaporators, heaters, or coolers. You will find them throughout almost every industry, and as a part of many large HVAC systems. Shell and tube heat exchangers, specifically, find applicability in many sub-industries related to food and beverage: brewery processes, juice, sauce, soup, syrup, oils, sugar, and others. Pure steam for WFI production is an application where special materials, like stainless steel, are employed for shell and tube units that transfer heat while maintaining isolation and purity of a highly controlled process fluid.

Shell and tube heat exchangers are rugged, efficient, and require little attention other than periodic inspection. Proper unit specification, selection, and installation contribute to longevity and solid performance.

Wednesday, May 10, 2017

High Pressure Valves for Industrial Processes and Operations

engineer working on pump and piping system oil refinery
Industrial operations present substantial
challenges to engineers and equipment
I am convinced that there is a valve out there for every conceivable application. Of course, that is not literally true, but there is an enormous array of manufacturers producing countless valve variants to meet specific requirements of the many industrial fluid processing applications.

A valve installed in a fluid process needs not only to perform its intended control function, but to stand up to the impact of several physical challenges.
cutaway view of high pressure angle valve for industrial process control
Cutaway view of high pressure angle valve
Courtesy Flowserve - Kammer
  • Temperature
  • Pressure
  • Corrosion
Any combination of these factors in the extreme can call for the use of a severe service valve. A good match between the valve ratings or capabilities and the demands imposed by the process conditions is essential for achieving safe operation and a reasonable useful valve lifespan.

Valves designed to handle very high pressure will exhibit specific attributes designed to accommodate the imposed physical stress. Body construction, assembly hardware, seats, and trim will all be noticeably heavier, stronger.

Rely on a valve specialist to contribute product expertise to the valve selection process. Combine your own process knowledge and experience with their product application expertise to develop an effective solution.



Tuesday, May 2, 2017

Operating Principles and Application of Vortex Flowmeters

vortex flow meter for steam gas or liquid
Vortex Flow Meter
Courtesy Azbil NA
To an untrained ear, the term “vortex flowmeter” may conjure futuristic, potentially Star Wars inspired images of a hugely advanced machine meant for opening channels in warp-space. In reality, vortex flowmeters are application specific, industrial grade instruments designed to measure an important element of a fluid process control operation: flow rate.

Vortex flowmeters operate based on a scientific principle called the von Kármán effect, which generally states that a fluid flow will alternately shed vortices when passing by a solid body. “Vortices” is the plural form of vortex, which is best described as a whirling mass, notably one in which suction forces operate, such as a whirlpool. Detecting the presence of the vortices and determining the frequency of their occurrence is used to provide an indication of fluid velocity. The velocity value can be combined with temperature, pressure, or density information to develop a mass flow calculation. Vortex flowmeters exhibit high reliability, with no moving parts, serving as a useful tool in the measurement of liquid, gas, and steam flow.

While different fluids present unique challenges when applying flowmeters, steam is considered one of the more difficult to measure due to its pressure, temperature, and potential mixture of liquid and vapor in the same line. Multiple types of steam, including wet steam, saturated steam, and superheated steam, are utilized in process plants and commercial installations, and are often related to power or heat transfer. Several of the currently available flow measurement technologies are not well suited for steam flow applications, leaving vortex flowmeters as something of a keystone in steam flow measurement.

Rangeability, defined as a ratio of maximum to minimum flow, is an important consideration for any measurement instrument, indicating its ability to measure over a range of conditions. Vortex flowmeter instruments generally exhibit wide rangeability, one of the positive aspects of the technology and vortex based instruments.

The advantages of the vortex flowmeter, in addition to the aforementioned rangeability and steam-specific implementation, include available accuracy of 1%, a linear output, and a lack of moving parts. It is necessary for the pipe containing the measured fluid to be completely filled in order to obtain useful measurements.
Applications where the technology may face hurdles include flows of slurry or high viscosity liquids. These can prove unsuitable for measurement by the vortex flowmeter because they may not exhibit a suitable degree of the von Kármán effect to facilitate accurate measurement. Measurements can be adversely impacted by pulsating flow, where differences in pressure from the relationship between two or more compressors or pumps in a system results in irregular fluid flow.

When properly applied, the vortex flowmeter is a reliable and low maintenance tool for measuring fluid flow. Frequently, vortex flow velocity measurement will be incorporated with the measurement of temperature and pressure in an instrument referred to as a multivariable flowmeter, used to develop a complete measurement set for calculating mass flow.

Whatever your flow measurement challenges, share them with a flow instrument specialist, combining your process knowledge with their product and technology expertise to develop effective solutions.