Thursday, July 27, 2017

Pneumatic Control Valve Positioners

smart valve positioner for pneumatic process control valve
Smart valve positioner
Courtesy Rotork
Valve positioners can provide process operators with a precise degree of valve position control across the valve movement range, as well as information about valve position. A relationship exists between applied pneumatic signal pressure and the position of the valve trim. The relationship between the two elements is dependent upon the valve actuator and the force of the return spring reacting to the signal pressure. In a perfect world, the spring and pneumatic forces would reach equilibrium and the valve would return to the same position in response to an applied signal pressure. There are other forces, however, which can act upon the mechanism, meaning the expected relationship between the original two elements of pressure and position may be offset. For example, the packing of the valve stem may result in friction, or the reactive force from a valve plug resulting from differential pressure across the area of the plug may be another.
While these elements may seem minor, and in some cases they are, process control is about reducing error and delivering a desired or planned output. Inclusion of a positioner in the valve assembly can ensure that the valve will be set in accordance with the controller commands.

Each positioner functions as a self-contained small scale control system. The first variable in the positioning process is the current valve position, read by a pickup device incorporated in the positioner. A signal which is sent to the positioner from the control system, indicating the desired degree of opening, is used as the setpoint. The controller section of the positioner compares the current valve position to the setpoint and generates a signal to the valve actuator as the output of the positioning process. The process controller delivers a signal to the valve, and then the positioner takes that signal and supplies air pressure required to accomplish the needed adjustment of the stem position. The job of the valve positioner is to provide compensatory force and to act as a counterbalance against any other variables which may impact valve stem position.

Magnetic sensors can be employed to determine the position of the valve stem. The magnetic sensor works by reading the position of a magnet attached to the stem of the valve. Other technologies can be employed, and all have differing ways of overcoming degrees of inaccuracy which may arise with wear, interference, and backlash. In addition to functioning as a positioner, control valve positioning devices can also function as volume boosters, meaning they can source and subsequently ventilate high air flow rates from sources other than their pneumatic input signal (setpoint). These devices can positively affect and correct positioning and velocity of the valve stem, resulting in faster performance than a valve actuator solely reliant on a transducer.

The inclusion of a positioner in a control valve assembly can provide extended performance and functionality that deliver predictable accurate valve and process operation. Share your valve automation requirements with a knowledgeable valve automation specialist and combine your process knowledge and experience with their product application expertise to develop an effective solution.

Wednesday, July 19, 2017

Condensate Return in a Steam System - Basic and Essential

food and dairy production plant
Efficient production of steam and return of condensate
are essential to the operation of this and many other
industrial operations.
Many industrial processes and plants, as well as commercial buildings, utilize steam in their operations. The generation and use of steam is one of the oldest industrial processes and is so well understood that it may be considered more of a utility than part an industrial process. Whatever the case, if your process or installation uses steam, then steam is a necessary input for successful operation. Keeping your steam system performing at capacity frees up time and resources for the more complex aspects of your work.

If steam is not consumed directly by the process as a component input, it is steam's heat of vaporization that is utilized by the operation. Efficient use of steam as a heating medium results in the conversion of vapor to liquid (water). Returning the liquid condensate back to the boiler for conversion to vapor again is a necessary step in the efficient operation of a closed loop system.

Condensate return systems are certainly not high technology, but keep in mind that a steam system may be the lifeblood of not just one, but many operations throughout a plant. Avoiding downtime in the steam system, of which the condensate return system is an integral part, ranks highly on the list of "Important Things for Plant Operations". Condensate return is critical.

Three general methods are employed to move the condensate from a collection vessel, a trap, to the feedwater side of the boiler. Gravity can be used when conditions permit. A pressure motive return arrangement uses steam pressure and a coordinated valve sequence to drive the condensate through the piping system and back to the boiler. Condensate pumps can also be employed as a positive means of moving condensate through the return piping system.

What are some strong attributes of a good and reliable condensate return pump?
  • Minimize or eliminate cavitation at high temperatures. Cavitation will impede pump performance and cause premature deterioration of pump and drive components.
  • Ability to handle a high load during cold starts through motor and pump design.
  • Design and configuration to handle high temperatures without deterioration of pump and motor.
  • Develop higher pressure at lower motor speeds for extended service life.
  • Avoidance of mechanical seals below water line.
  • Consider a single unit with dual pumps for handling high loads and extending service life.
Specifying and installing a solidly designed and built condensate return pump may require an investment of your time and consideration. The return on that investment will be reduced maintenance, repair, and downtime. hare your steam system challenges, from end to end, with knowledgeable application specialists. Combining your intimate operational knowledge and experience with their deep product knowledge and experience with many installations will yield a good solution.

Monday, July 17, 2017

Filled Impulse Lines With Pressure Sensors

industrial process measurement and control pressure transmitter
Pressure sensors or transmitters are installed directly to
process lines or vessels, or remotely using impulse lines.
Image courtesy Azbil NA, Inc.
Pressure sensors intended for use in industrial process measurement and control applications are designed to be robust, dependable, and precise. Sometimes, though, it is necessary or beneficial to incorporate accessories in an installation which augment the performance of pressure sensors in difficult or hazardous environments. There are some scenarios where the sensor must be isolated from the process fluid, such as when the substance is highly corrosive.

A way to aid pressure sensing instruments in situations where direct contact must be avoided is by using a filled impulse line. An impulse line extends from a process pipe of vessel to a pressure measurement instrument or sensor. The line can have a diaphragm barrier that isolates the process fluid from the line, or the line can be open to the process. There are best practices that should be followed in the design and installation of an impulse line to assure that the line provides a useful transmission of the process pressure to the sensor and whatever degree of isolation or protection is needed remains in effect.

The filled impulse line functions via the addition of a non-harmful, neutral fluid to the impulse line. The neutral fluid acts as a barrier and a bridge, allowing the pressure sensing instrument to measure the pressure of the potentially harmful process fluid without direct contact. An example of this technique being employed is adding glycerin as a neutral fluid to an impulse line below a water pipe.

Glycerin’s freeze point is lower than water’s, meaning glycerin can withstand lower temperatures before freezing. The impulse line connected to the water pipe may freeze in process environments where the weather is exceptionally cold, since the impulse line will not be flowing in the same way as the water pipe. Since glycerin has a greater density and a lower freezing point, the glycerin will remain static inside the impulse line and protect the line from hazardous conditions.

The use of an isolating diaphragm negates the need for certain considerations of fill fluid density, piping layout, and the need to create an arrangement that holds the fill fluid in place within the impulse line. System pressure will be transferred across the diaphragm from the process fluid to the fill fluid, then to the pressure sensor. It is important to utilize fluids and piping arrangements that do not affect the accurate transference of the process pressure. Any impact related to the impulse line assembly must be determined, and appropriate calibration offset applied to the pressure sensor reading.

An essential design element of a filled impulse line without an isolating diaphragm is that the fill fluid must be compatible with the process fluid, meaning there can be no chemical reactivity between the two. Additionally, the two fluids should be incapable of mixing no matter how much of each fluid is involved in the combination. Even with isolating diaphragms employed, fluid harmony should still be considered because a diaphragm could potentially loose its seal. If such a break were to occur, the fluids used in filled impulse lines may contact the process fluid, with an impact that should be clearly understood through a careful evaluation.

Share your pressure measurement requirements and challenges with experienced application specialists, combining your own process knowledge and experience with their technical expertise to develop an effective solution.

Friday, July 7, 2017

Industrial Uses of Steam - Part 2

industrial steam boiler gas fired
Steam is used throughout commercial, institutional, and industrial facilities in various ways. In addition to other direct pressure and propulsion/drive applications, steam can be utilized as ‘motive fluid’ to assist in the movement of liquid and gas streams in a piping system. Jet ejectors can pull vacuum in equipment like distillation towers, allowing for the separation and purification of vapor streams. Continuously removing air from surface condensers via steam results in the desired vacuum pressure on condensing turbines to stay uniform. The entrance and subsequent diffusion of the steam through an inlet nozzle results in a low pressure zone, where the air from the surface condenser gets transferred. Similarly, steam serves as the primary motive fluid for secondary drainers, which pump condensate out of vented receiver tanks, flash vessels, and other process control components susceptible to stall conditions.

Steam is applicable to a process called atomization, wherein steam mechanically separates a fluid. Burners use steam for atomization by having steam injected into the fuel, thus maximizing the efficiency of the unit’s combustion while concurrently minimizing soot production. These steam generators and boilers, powered by fuel oil, use steam atomization to partition viscous oil into smaller droplets. Flares, similarly, utilize steam atomization as an exhaust pollutant reducer. In said flares, typically, the waste gas mixes with the steam prior to combustion.

Along with motive fluid and atomization, steam is also a fantastic cleansing tool. The soot in soot blowers gets removed via a steam cleaning process. Oil or coal fuel sourced boilers need soot blowers to cyclically clean the furnace walls and eliminate combusted deposits. These regularly scheduled cleanings allow for the capacity, durability, and effectiveness of the boiler to remain consistent. The nozzle of the soot blower directs the steam, dislodging dry, sintered ash and slag. Hoppers then catch the dislodged substances and they are expelled with other combusted gases.

Steam can also add moisture to a process while simultaneously acting as a heat supply source. In paper production, paper moving over rollers at high speed is moisturized by the steam, ensuring that no miniscule breaks or tears are suffered during the production process. Pellet mills, which produce animal feed, directly inject steam to heat and, concurrently, add to the water content of the feed as the feed passes through the mill’s direct conditioner section. The water softens the feed and then partially gelatinizes the starch content, leading to firmer pellets overall.

Lastly, commercial and industrial facilities utilize low pressure steam as a primary source of seasonal heating and humidification. Finned or bare coils, coupled with steam humidifiers, condition the facility air, keeping the temperature regulated for both comfort and preservation of items like books and records. Steam coils heat the cool air, resulting in the relative humidity dropping. The controlled injection of dry, saturated steam allows for moisture addition to regulate the relative humidity in a smooth and precise manner via steam humidifiers installed in air ducts.

Share your steam generation and use challenges with steam system and combustion experts, combining your own knowledge and experience with their expertise to develop effective solutions.