Cowan Connection

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, 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. Conventional resilient and metal seated knife gate valves are designed primarily for fluid isolation as they have poor control characteristics. Their ‘blade-like’ gate design makes...

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. Conventional resilient seated butterfly valves are designed primarily for fluid isolation as they have poor control characteristics. The seat is an elastic single piece that holds in place the valve body. This design can have its integrity compromised if the valve is kept partially open. For this reason, butterfly valves that are designed for control...

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...

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. 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˚ actuators: How do they work? A scotch yoke actuator typically consists of a crank and piston system in a hollow cylinder. Pressure is applied to a side of the piston inside the cylinder. As a result, force is generated which...

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...

What are valves? Valves are equipment that are designed to control fluids, they can open, close, regulate, or isolate. They can be as small as fractions of an inch and as large as 360 inches. They can be as simple as the copper valve found in a hardware store and as complex as custom-made valves in exotic alloys for nuclear reactors. Valves are one of the oldest products known to man, they are found in many of our everyday activities such as turning on the water in the sink.  Main parts of a valve Each type of valve has its own design features which govern its main parts, however there are 4 main components that every valve regardless of its type. These parts are the stem, the closing membrane, the body, and the seat. The stem: Part that transfer the energy from the operator to the valve...

What is a gate valve? Gate valves are linear valves where the closing membrane is a wedged disc or gate which moves perpendicular to the flow, allowing more flow as it moves away from the seat. These valves usually have metal seats and are bi-directional, the gate is wedged into the seat for which reason they are thrust seated valves. Gate valves are designed to be used as isolation or on/off valves therefore, are usually in a fully open or fully closed position. Normally in industry, they are flanged but also are offered in other connections such as threaded or welded. Gate Valve Applications They are often found at the end of pipelines where they serve as a final barrier against leaks before connecting into other pipelines or tanks, also very commonly used as isolation valves to do...

What is a ball valve? Ball valves are rotary or quarter-turn valves. Their closing membrane is a bored pivoting ball that is supported on one or both top and bottom ends. When the valve is open, the bore in the ball is aligned with the flow of the pipeline, allowing the substance to pass through. When closed (by turning the stem 90 degrees), the bore becomes perpendicular to the flow and stops it. These ball valves can have resilient or metal seats, are usually bi-directional and come in different connection configurations. The most common ball valve configuration being flanged and threaded.   Ball Valve Design Conventional ball valves are designed primarily for fluid isolation as they have poor control characteristics. The seat and body of a ball valve can be easily eroded if the valve...