Pneumatic & Electric Ball Valve - How They Work

 

 

Ball valves can be combined with a pneumatic actuator (pneumatic ball valves) or an electric actuator (electric ball valves) for automation and/or for controlling remotely. Depending on the application, automating with a pneumatic actuator vs an electric one may be more advantageous, or vice-versa. In this article, we will compare the two options. 

• Table of Contents
• Ball valve overview
• Actuator overview
• Pneumatic actuators
• Electric actuators
• Combining an actuator and a ball valve

 

Ball valve overview

A ball valve is a quarter-turn valve that controls the flow of a media by having a hollow rotating ball, as seen in Figure 2. The figure shows the main components of a manual ball valve in a sectional view. When the hollow portion of the ball is in line with the flow (pipe or hose), the valve is open and the media can flow through. The valve closes when the solid portion of the ball is in line with the flow, which is done with a 90-degree rotation (hence the name quarter-turn valve) of the ball.

It is also possible to position the valve between fully open and fully closed, which allows you to regulate the flow more precisely. Typical ball valves have two ports, one for an inlet and one for an outlet. However, three ports (L or T) are also available, and depending on how the valve is assembled and installed will determine how the 90-degree rotation of the ball directs the media flow. Four-port ball valves are possible but rare.

Ball valves have a valve stem, which is attached to the ball and controls its rotation. In Figure 2, the valve stem is connected to a manual handle to actuate the valve. However, the valve stem can also be connected to a pneumatic or electric rotary actuator to spin the stem to open and/or close the ball valve automatically and/or remotely.

 

Actuator overview

A valve actuator is a device that is used to remotely control a valve. If it controls a quarter-turn valve, the actuator is known as a quarter-turn actuator. Instead of a manual lever, you can mount an actuator on the valve to automatically and/or remotely control it. Actuators use a power source to generate the torque that is required to operate (rotate) a ball valve. For most actuators, the power source is either pneumatic, electric, or hydraulic (not discussed in this article). The difference in this power source makes different designs, which each have different advantages and disadvantages for certain applications (discussed below). Aside from the torque generating component, an actuator may have other features such as position indicators and manual override.

 

Pneumatic actuators

Pneumatic actuators control ball valves by the conversion of compressed air energy to mechanical motion. A rotary mechanical motion is required in a ball valve for a 90 degrees turn. Pneumatic actuator ball valves can be single-acting or double-acting. A single-acting pneumatic actuator uses a single compressed air input to turn the valve and a spring to return the valve to the normal position. A double-acting pneumatic actuator has two compressed air inputs to turn the valve and return the valve to the original position.

 

Operating principle

The most common mechanism for a pneumatic actuator for ball valves is the rack and pinion mechanism. This comprises of the rack (a linear gear) and the pinion (a circular gear) (figure 4). The rack is attached to a piston which is pushed by compressed air to achieve linear motion. This linear motion is converted to circular motion by the pinion. The pinion drives the stem of a ball valve to open and close positions.

To control the pneumatic actuator for ball valves, the compressed air is regulated by solenoid valves. Electrical signals from the controller energize the solenoid valve to either open or close positions allowing compressed air to flow through to both piston sides of the pneumatic actuator. The piston pushes the rack which turns the pinion connected to the stem of the ball valve.

 

 

Electric actuators

Electric actuators convert electrical energy into rotary force by the use of an electric motor to turn the ball valve through 90 degrees. They are energy-efficient, clean, and a quiet method of valve control. The electric motor can be powered by an alternating current (AC) or a direct current (DC). It is housed in a robust, compact housing that also contains other components of the actuator such as gearings, limit switches, wiring, etc. The whole assembly is connected to a valve through a compatible connection interface.

 

Operating principle

The electric motor generates a torque, which is transmitted by a shaft connected to the valve stem. This rotates the ball valve. To achieve the required torque, a system of gears is connected to the electric motor shaft. The torque capacity is an important specification for selecting an actuator. It must be higher than the required torque (breakaway torque) to turn the ball valve by a certain percentage often specified by the ball valve manufacturer. The breakaway torque is the minimum torque required to turn the ball valve usually in the fully closed or fully open static positions.

The speed of operation (the response time) of an electric actuator is inversely proportional to the torque of the actuator. The gear system defines the relationship between speed and torque. A higher gear ratio would result in more torque but a lower response time.

Electric actuators can be powered from a 12, 24, and 48V direct current and 24, 48, 120, 130, and 240V alternating current. Limit switches are installed to stop the current to the motor when fully closed and open. Electric motors can be used to carry out modulating control. This is used to accurately position the valve at any point between fully opened and fully closed positions (i.e. between 0° and 90°). This is useful for regulating the flow rate through the valve. A positioning circuit board (PCB) is installed in the electric actuator to modulate the electric motor. 

 

Combining an actuator and a ball valve

Although actuators and ball valves are separate components, they are most often used together. Therefore, it is more convenient to get them as a package to ensure conformity. Combining an actuator with a ball valve gives you an automatic ball valve that can be controlled remotely. The actuator and the ball valve have a connection interface to connect them. The connection interface comprises of a shaft, or stem, to connect the valve ball, and a flange to bolt the actuator to the valve. This interface may be brand-specific or standardized to standards. You can mount a brand-specific actuator on a compatible brand-specific valve. On the other hand, different ball valves and actuators can be interchanged as long as they follow the same standard.

 

Comparison between pneumatic and electric ball valves

The following are some of the comparable features of pneumatic and electric ball valves:

1) Rotation speed
The rotation speed is the speed at which the ball of an actuated ball valve makes a complete rotation (90-degrees). Typically for the same size units, the rotation speed of an electric ball valve is lower than that of a pneumatic ball valve.

2) Life span
The life span of equipment is the time that the unit is fully functional and operational. Pneumatic ball valves have fewer components and are easier to maintain; hence they have a longer life span than their electric counterparts. Electric actuators have several components that need maintenance, like the electric coil, electronic driver, mechanical actuator, etc.

3) Precision
Precision, or modulation, is for units that stop at a partially open point (i.e. 20-degrees open) to more accurately regulate the flow. Both pneumatic and electric actuators are precise in operation, but motorized ball valves have higher levels of precision. An electric ball valve is capable of opening and closing by very precise degrees. Pneumatic actuators carry out modulation by controlling the air pressure at the inlet port. Leaks or pressure fluctuations can easily affect the valve’s position. Electrical actuators, on the other hand, use exact electrical control signals to carry out control. Additional information can be found in our electrical modulating ball and butterfly valves article.

4) Energy consumption
Energy consumption is the energy required by the actuator to rotate the valve. In comparison, the energy consumption of an electric operated ball valve is less than pneumatic actuated ball valves. In pneumatic actuators, the entire air compression system (compressor, filters, lubricators, power, etc.) accounts for their high energy consumption.

5) Fail-safe
This is a safety feature designed to automatically open or close a valve in case of a power failure. It is typically easier and cheaper to feature a fail-safe mechanism on a pneumatic ball valve than on a motor actuated ball valve. Pneumatic acting actuators are very common and make use of a spring to return to the base position and are ideal as a fail-safe solution. Electric actuators with a fail-safe mechanism can operate with a battery or a spring and are usually more expensive than the pneumatic solution.

6) Cost
The cost of a pneumatic ball valve is usually lower than an electric one because the actuator design is less complex. However, this doesn’t take into account the costs of the components of the pneumatic system, such as the compressor, air preparation, pipes, etc. When no pneumatic system is available near the valve, usually electric actuation is preferred. The operation of a pneumatic valve is more expensive in the long run due to the higher energy consumption and energy losses that are a result of generating compressed air.

7) Position feedback
Position indicators indicate the position of the actuator at any given time. They are usually placed atop the actuator for high visibility. Most pneumatic actuators can be equipped with a limit switch on top for electrical feedback. Many electrical actuators have internal limit switches for position feedback. However, more basic actuators do not have this feature.

8) Size/torque range
Torque is the rotary force a ball valve requires to turn. Pneumatic actuators offer a much higher torque per unit size than electric actuators. Therefore, for applications requiring a large valve or high torque typically a pneumatic ball valve is a better option.

9) Hazardous conditions
An electric ball valve has to be NEMA/ATEX certified before it can operate in hazardous conditions. Pneumatic actuators, however, are more widely available with ATEX certification. Also, they neither generate nor are affected by electromagnetic disturbance. Unlike their electric counterparts, pneumatic actuators are not sensitive to wet environments, neither are they subject to overheating.

The Ultimate Guide to Select Swivel Joints

 

 

What is a Swivel Joint?

Swivel joints, also known as rotary unions are used in applications where a constant transmission of fluids from a stationary source to a rotating source is required without cross-contamination or leakage. Typical applications use swivel joints to allow for 360-degree rotation while preserving hoses from getting tangles as components turn. In return, mechanical stresses that would result from hose twisting, bending, and stretching can be relieved.

Swivel joints are engineered to operate at a wide range of pressure and temperature for a variety of conditions and environments. Based on industry requirements, the swivel joint can be designed to have multiple passages and can transfer different types of fluid simultaneously at various rotational speeds. Typically, as the number of passages increases, the size increases, and speed will be lower.

 

How Does a Swivel Joint Work?

Swivel Joints come in different shapes and sizes based on the application and the environment where it is subjected to. While design consideration should be given to external factors, all the swivel joints have two main components: a shaft and housing.

The concept behind a swivel allows the shaft to rotate while the housing remains stationary in position. The shaft has drilled holes of varying size and depth starting from its top surface. Variable hole depths and markings define the flow path of fluid within the swivel. Through internal design, the fluid is carried through the shaft into and out of the swivel joint.

The housing unit includes machined passage and grooves to facilitate fluid transfer within the swivel and preventing cross leaks. Numbered markings are found on the housing outer diameter surface and the same can be found on the top surface of the shaft.

These numbers define where a user would expect the fluid to flow in/out between shaft and housing through the machined internal passages. A series of carefully selected internal components are fitted between the shaft and housing at specified locations. These include seals, snaps rings, O rings, wearings, small bearings in certain applications.

The selection of internal components is of vital importance while designing a swivel joint as extreme attention has to be given to the tolerances and internal design of the housing grooves. Failure to adhere to the recommended design and machining requirements results in leaking components.

Passage of power and signals are sometimes required for particular applications and industries. Slip rings can be integrated with the swivel joint through passing electric cables within the hollow inner diameter of the shaft.

 

How to Select a Swivel Joint?

As opposed to other rotary components in the same industries, a swivel joint mostly endures internal loads while operational. This is a direct result of pressurized fluid flowing within its internal components. To select an appropriate swivel joint, multiple influencing factors must be considered.

The most influencing factor on a swivel joint is the internal sealing solution between the shaft and housing. Very precise tolerances must be maintained during the machining phase to create the required grooves for seals and internal components to be fit.

Seals, o-rings, wear rings, and bearings are inserted within these internal grooves and must be able to withstand the pressures induced by the fluid Swivel joints are rated by the manufacturer as to their recommended operating pressures, temperatures, and speeds.

These values are directly related to the internal components specifications, geometric machining tolerances, and type of fluid used significant design and space considerations must be given for a swivel joint and its ports.

Based on application requirements, a swivel joint may have up to 9 different ports to supply fluid through different passages. A higher number of passageways result in significant size and material increase.  

Swivel joints can be modified in size and port sizes with respect to application requirements. Space restrictions, load requirements, duty cycle, and environmental surroundings all factors that are considered while selecting an appropriate component

Based on the requirement provided Slewmaster will be able to provide the best solution that suits the application.

In most cases, our standard in-stock swivels can be modified to fit into your application, and if required Slewmaster can provide a custom-built swivel that suits your needs. Further, Slewmaster can also cross-reference other manufacturers' Swivel Joint and provide an equivalent solution.

 

What are the Different Applications That Use Swivel Joints?

Due to a wide range of custom modifications that can be applied on a swivel joint, it can be found across many applications. Vacuum trucks and cranes that require their booms to rotate 360 degrees using a slew drive run into the problem of hose bending and tangling.

Introducing swivel joints to the system creates a smooth fluid flow for continuous rotation. Bottling lines use swivel joints to deliver fluid across to the different end locations. Design considerations are given to the different fluid viscosities to determine operating pressure requirements.

Farming and agricultural equipment utilize swivel joints in several ways including herd feeding and waste recycling. Welding and robotic arms often require electric passage and slip rings are used to allow for mobility and electric transmission.

 

How is a Swivel Joint Mounted?

Inspect the swivel joints and make sure that all the connections and passages are clean and free from any physical damage during shipment.

A flexible connection/ hose should always be used while installing a Swivel joint. While mounting the Swivel Joint, ensure that either shaft or housing are mounted in a manner that allows for some movement in order to accommodate any misalignment or run-out during rotation. It is recommended to fasten an anti-rotation arm to the stationary part of the rotary union.

When mounting the shaft and the housing make sure that pipe thread sealant is used on fittings and the fitting is properly tightened. For proper functioning of a swivel, it is required to ensure that the mounting flange or surface should be concentric to the axis of the swivel assembly.

After all, fittings are installed bolt the assembly down using the mounting flange or tapped holes provided on the swivel joint. It is recommended to perform a dry run after the swivel is installed to ensure proper mounting of the swivel joint assembly and to verify that there is an unintended movement of the swivel joint due to misalignment.

High pressure selected seals and internal components contain the fluid from leaking out of the closed system. If any leakage is found around any surfaces of the swivel joint, the manufacturer must be alerted immediately.