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Latest News from Cowan Dynamics

The Importance of Valve Positioners

The valve positioner is a feedback mechanism. It allows valves to be positioned precisely in accordance with the output signal of the controller, in the presence of major disturbances caused by unbalanced forces acting on the valve stem, changes in actuator temperature, etcetera. Even a relatively small actuator, that otherwise would have to be bench-set to an unacceptable degree, can be stroked precisely when a valve positioner is used to signal the valve position. Positioners are not now considered to be the universal solution to many problems. In fact, in some control circuits the effect of fitting a positioner can be detrimental. Whilst it is an advantage to use positioners in slow systems, it can be a disadvantage in the case of control loops with short reset times. A positioner...

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Maximize flotation cell mineral recovery

In this article you'll learn to maximize mineral recovery in a flotation cell. Dart valves have traditionally been used as level control valves in flotation cells. The dart valve positioning in flotation cells requires high consideration as the goal is to maximize mineral recovery. For this reason a reliable and accurate control valve actuator is crucial. Pneumatic actuators vs electro-hydraulic actuators for dart valve control  Traditionally dart valves use pneumatic actuators to control the flotation cell level.  However, due to air compression or elasticity, this method of automation can cause problems in accurate and efficient level control. The thrust due to the pressures at the bottom of the cell acts against...

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When to Use a Globe Valve Actuator

In this article, you will learn about the globe valve, when to use it, and globe valve actuator options based on the power source available. Globe valves are linear valves whose closing membrane is a disc or plug that opposes the flow inside the valve body, allowing more flow as it moves away from the seat. The seat is a ring installed in the center of the valve body, this design creates high energy loss due to the change in the flow direction and shortens the valve effective stoke. Globe Valve Parts diagram: They get their name from the shape of their spherical body. These valves can have resilient or metallic seats and are unidirectional as they must be installed with the pressure side under the seat. Normally in industry, they are flanged.  Also, they are offered in other connections...

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Why Use a Valve Actuator Limit Switch?

Why use a valve actuator limit switch? Valve automation is not simply the selection of the actuator. Control components are a big part of the complete actuation assembly.  When looking into the components that a valve automation system must have, we normally see solenoid valves which are used to control the pressurized fluid into the actuator. In addition, we also see what we often call position limit switches.  A valve actuator limit switch is a device that is designed to provide a remote signal to the control station, also known as PLC or DCS, regarding the valve position, often open or closed.  They can also be set to provide different position indications depending on the application. The switch will provide a signal on the position once the valve reaches it, therefore there will be...

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Knife Gate Valve Actuator Applications

In this article, you will learn about the various applications for knife gate valves & knife gate valve actuators. Knife gate valves are linear valves in which the closing membrane is a gate that is sharpened to better cut through the process media (such as slurry). The gate is connected to a stem and travels inside the valve body in which scrappers are often included to clean the gate as it moves from open to closed. They can be manual or automated (knife gate valve actuator), with metal or resilient seats, bi-directional, and come in wafer, lug and flange connections. Knife gate valves are very versatile and can be used in both isolation and control applications.  However, to provide good control characteristics the valve trims must be custom-made. Diagram of Knife Gate Valve Parts...

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Butterfly Valve Actuator Applications

What are the pros and cons of a butterfly valve? What options are available when selecting a butterfly valve actuator?  This short and informative article will explain it all. Butterfly valves are rotary or 90-degree valves where the closing membrane is a disc that is connected to the stem perpendicularly either symmetrically or asymmetrically. They can be manual or automated, with metal or resilient seats, bi-directional, and come in wafer, lug, and flanged connections. Butterfly valves are very versatile and can be used in both isolation and control applications. However, their control capabilities are limited. Butterfly Valve Parts Diagram Image Source from Velan Conventional resilient seated butterfly valves are designed primarily for fluid isolation as they have poor control...

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Valve Signature & Its Effect on Actuator Sizing

A valve is a pressure vessel with internal moving parts; and when they move, they encounter resistance from other parts and from the flow. The recording of the pressures and forces associated with overcoming these resistances can be referred to as “valve signature”.  For actuator sizing, it is critical to obtain accurate valve thrust/torque data. This is especially true for spring-return actuators as conservative or exaggerated valve thrust/torque data can lead to extremely oversized and expensive actuators. The thrust or torque required to move the valve, in either direction, will vary throughout its stroke. In industry, the following  terms are used to describe the various data points required for actuator sizing: (fig.1 - see table for details) The thrust or torque...

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What is a Scotch Yoke Valve Actuator ?

What is a scotch yoke valve actuator? Scotch yoke valve actuators are 90˚ driving mechanisms that can be powered by compressed air or hydraulic oil from an external source. Why consider a Scotch yoke valve actuator? Deciding which actuator is best for a certain application depends on several factors like environment, required accuracy, and close/open speed.  A scotch yoke valve actuator is engineered to automate all types of quarter-turn valves (butterfly, ball, plug, and damper) with various torque profiles to provide a “best fit” for every application. When choosing between a 90˚ actuator such as a Rack & Pinion or Scotch Yoke, the main difference between these actuators are the torques they can generate. The following table shows the difference between the two main types of 90˚...

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Valve Actuator Types for Industrial Process Control

Valves are used to control the flow of fluids in process control applications by opening, closing, or partially obstructing the fluid flow and controlling the line pressure. Depending on the flow requirements of the industrial process system, some method of changing the positions of valves is needed. Some valves are operated with hand wheels or levers. Other valves must be open, closed, or throttled frequently. Manually positioning valves in these types of situations is not practical and instead of using hand wheels or levers to control actuation, automated valve solutions such as using a valve actuator for positioning the valves. What is a valve actuator? A valve actuator is defined as a mechanical device that moves or controls a device such as a control valve. Actuators reduce the need...

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How Pneumatic Actuators Work

What powers them? Pneumatic Actuators are powered by compressed gas from an external source, most commonly dry clean air. Usually, a single air compressor can drive several pneumatic actuators at the same time which gives them an advantage over other types of actuators especially if the system has a large number of valves. For example, in an electrical valve actuator a source of power (motor) has to be installed on each valve.   Why consider a pneumatic actuator? Deciding which actuator is the best for a certain application depends on several factors like environment, required accuracy and closure/open speed. However the main difference between actuators is the thrust or torque they can produce. Here are general applications for the main types of actuators:   Hydraulic...

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