Digital Valve Positioner Delivers Extended Functionality

digital valve positioner HART
Digital Valve Positioner
A valve positioner works in concert with an actuator to regulate the movement of an industrial process control valve in a predictable manner. Evolving from mechanical based designs, current state of the art positioners no longer employ mechanical means, such as cams, to establish positioner output. The latest models are software based and provide additional information and functionality to the user.
  • Loop powered device
  • Accommodates 4-20 mA input, as well as HART protocol for two way remote communications
  • Suitable for use with single or double acting actuators, linear or rotary motion
  • Certificates of approval for use in wide range of hazardous industrial environments internationally
Either of the input signal paths, analog or digital, can be utilized as the command source. When analog is selected as the input source, the 4-20 mA signal is considered the command signal. Selecting the digital source causes the level of the 4-20 mA input to be ignored and the signal sent via HART serves as the input command signal.

The software based units deliver enhanced functionality for in-field setup and calibration, as well remote monitoring and setup via HART. Multiple operating modes are accessible to the user to properly characterize valve position to desired curves, one of which can be customized. An analog feedback signal of the stem position is provided as an output.

There is more to learn. The document below goes into more depth about the features, specifications and options, with a good illustration of the user interface and screen shots taken from the SoftTools software suite that works with the digital positioner. Reach out to a valve control specialist with your requirements and challenges. The combination of your process knowledge and their product application expertise will produce effective solutions.

Type K Vane Actuator Provides Long Term Solution for Tough Power Plant Application

Rotork K-TORK vane type valve actuators on bottom ash spray valve control.
bottom ash spray valve control.
Rotork K-TORK vane type valve actuator has solved a difficult flow control application found in many coal-fired power plants – high-pressure bottom ash spray valve control.

High-pressure spray water is used to sluice bottom ash and pyrites from the boiler hopper bottoms and to carry the ash out of the plant. The valves used are typically ANSI Class 300 double-offset high-performance butterfly designs ranging in size from 3” to 12”, automated with double-acting actuators. They cycle from four to ten times per day and discharge to atmospheric pressure, creating a very high pressure drop. The flow media is re-circulated ash water that is abrasive and flows at pressures between 400 and 500 psi.

In many power plants around the world, K-TORK actuators have provided over 10 years of maintenance-free service whilst preserving the life of the valves and valve seats in this arduous duty.

Among the challenges, it is imperative that the valves close fully and with zero leakage in a high pressure drop state. If the valve disc moves even slightly from the seat, the abrasive, high-pressure water will “wire-draw” or cut the butterfly valve seat. Traditionally, rack-and-pinion or scotch-yoke actuators have been used in this application, but “slop” or hysteresis in the rotary-to-linear conversion allows for the pressure in the pipe to move the disc from the seat, often causing premature failure of the valve after a period of only three to twelve months.

The problem becomes more acute when multiple valves are leaking, lowering the available back-pressure at the header, which makes it difficult or impossible to move the ash from the boiler.

When assembled to the valve with a ‘No-Play’ coupling, the K-TORK actuator has zero lost motion, “slop” or hysteresis. The one-piece vane and drive shaft cannot be back-driven and will hold the disc of the valve firmly in place.

Additional challenges include the location of the valves on a manifold at the bottom of the boiler where space is critical and plant air can be poor quality. K-TORK provides a high torque-to-size ratio and the double-opposed lip seal design is forgiving to dirty or contaminated air.

Also, the low-friction performance of K-TORK provides a speed-controlled, smooth valve operation, eliminating the risk of water hammer created by the high pressure drop.

Finally, longer run time between shutdowns demands increased reliability from the equipment in these critical applications. In particular, as the number of plant maintenance personnel has decreased, actuators that reduce maintenance (seal replacement) time and work orders have a direct payback to the owner, especially when valve life can be significantly increased through improved actuator performance.

Share your fluid control and valve actuation challenges with a specialist, combining your process knowledge and experience with their product application expertise to develop effective solutions.

Concentric vs. Double Offset Butterfly Valve

high performance butterfly valve double offset double eccentric with actuator
High Performance Butterfly Valve
(double eccentric)
EBRO Armaturen
Butterfly valves serve a broad range of applications throughout fluid processing industries. Among their positive attributes, quarter turn operation and compact design make this valve type a primary selection for many shutoff applications.

Within the butterfly valve family, their are several basic design subdivisions that are substantial to the point of limiting or expanding the potential application of the valve. One of those subtypes is the double offset butterfly valve.

A general purpose (concentric) butterfly valve, often referred to as a resilient seated butterfly valve, has a disc connected to a rotating shaft. The rotational movement of the shaft positions the disc to fully obstruct the flow path (closed) or provide continuing degrees of open space in the fluid path cross section (open). The rotational movement of the shaft from fully open to fully closed is 90 degrees. The shaft is centered in the pipe bore and is also centered on the disc. This shaft orientation will cause the disc to contact the sealing surface throughout a considerable portion of the rotation nearing closure. This contact tends to accentuate wear on the resilient seat and limits the pressure rating for this type of valve.

A double offset butterfly valve is also known by two other common names, "double eccentric" and "high performance". The "offsets" refer to the position of the shaft relative to the pipe bore and the centerline of the disc. In this valve design, the shaft is behind the centerline of the disc and slightly to one side of the pipe bore centerline. This results in a different travel path for the disc as it moves from the open to close positions. The cam action movement of the disc limits its contact with the seat until the final few degrees of travel. The limited wiping contact of the seat tends to extend its useful life. Additionally, this type of closure movement affords the double offset butterfly valve a higher pressure rating than the concentric design.

Valve selection can be a challenging task, with numerous types and variants from which to choose. Share your industrial valve requirements and challenges with a valve specialist, combining your process knowledge and experience with their product application expertise to develop an effective solution.

Wetback vs. Dryback Firetube Boiler

firetube boiler
Victory Energy firetube boiler, Frontier series, will
be available in a dryback version
Differing boiler designs will provide their own array of positive attributes with regard to boiler efficiency, intended use, maintenance burden, and other factors. One distinguishing feature of firetube boilers is whether they are "wetback" or "dryback" design.

Not surprisingly, the difference between these two designs can be found at the "back" of the boiler, the end opposite where the fuel and air are introduced and ignited. The "wet" and "dry" designation indicates whether the firetubes, as they extend to the rear end of the boiler, discharge the hot gases into a reversing chamber surrounded by boiler water (wet) or partially isolated from it (dry).

A dryback boiler has a rear wall lined with refractory. The rear wall forms one side of the reversing chamber on a multi-pass boiler. While the refractory is certainly a point of long term maintenance, its presence as a heat sink has been noted to assist in maintaining setpoint pressure during a boiler off cycle. This tends to lengthen the off cycle of the boiler and provide some benefit. The dryback design will also have appropriately sized access doors or panels on the rear of the boiler to provide for maintenance, thus a space requirement particular to this design.

A wetback (thankfully, also called a water-back) boiler will generally have a reversing chamber constructed of steel that is contained within the confines of the boiler pressure vessel. There will be water between the reversing chamber and the boiler rear wall. Generally, the service access required at the rear of the boiler is less than for a dryback.

Victory Energy manufactures both wetback and dryback boilers in a number of configurations to meet a wide range of needs. Their extreme duty series, designated Frontier, has traditionally been a wetback boiler, but manufacturing of dryback versions is planned. The dryback product range will extend from 50HP to 2500HP and cover applications in the industrial and commercial spaces.