Acids Can Put Your Sensors in a Pickle

In many types of metals production, pickling is a process that is essential to removing impurities and contaminants from the surface of the material prior to further processing, such as the application of anti-corrosion coatings.

In steel production, two common pickling solutions or pickle liquors are hydrochloric acid (HCl) and sulfuric acid (H2SO4). Both of these acids are very effective at removing rust and iron oxide scale from the steel prior to additional processing, for example galvanizing or rolling. The choice of acid depends on the processing temperature, the type of steel being processed, and environmental containment and recovery considerations. Hydrochloric acid creates corrosive fumes when heated, so it typically must be used at lower temperatures where processing times are longer. It is also more expensive to recover when spent. Sulfuric acid can be used at higher temperatures for faster processing, but it can attack the base metal more aggressively and create embrittlement due to hydrogen diffusion into the metal.

Acids can be just as tough on all of the equipment involved in the pickling lines, including sensors. When selecting sensors for use in areas involving liquid acid solutions and gaseous fumes and vapors, care must be given to the types of acids involved and to the materials used in the construction of the sensor, particularly the materials that may be in direct contact with the media.

PressureSensor
A pressure sensor specifically designed for use with acidic media, at temperatures up to 125° C.

A manufacturer of silicon steel was having issues with frequent failure of mechanical pressure sensors on the pickling line, due to the effects of severe corrosion from hydrochloric acid at 25% concentration. After determination of the root cause of these failures and evaluation of alternatives, the maintenance team selected an electronic pressure sensor with a process connection custom-made from PVDF (polyvinylidene fluoride), a VitonTM O-ring, and a ceramic (rather than standard stainless steel) pressure diaphragm. This changeover eliminated the corroded mechanical pressure sensors as an ongoing maintenance problem, increasing equipment availability and freeing up maintenance personnel to address other issues on the line.

Blaise Pascal – The Ultimate Powerlifter

Our modern technological society owes a lot to the scientific work and inspiration of a 17th-century French mathematician and physicist, Blaise Pascal. Pascal was a pioneer in the fields of hydrostatics and hydrodynamics, which deal with the subject of fluid mechanics under pressure.

One of the most important physical principles he defined is known today as Pascal’s Law:

“Blaise Pascal Versailles” by unknown1

“A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.”

It is this characteristic of fluids held in containment that allows force applied to a fluid in one location to be delivered to another remote location. A well-known example would be the hydraulic brake system in a car. Mechanical pressure from the driver’s foot is transferred to the brake fluid through a master cylinder. This pressure is then instantly communicated to braking cylinders located at each wheel, causing them to apply mechanical force to press friction pads against a brake drum or rotor, thus slowing or stopping the vehicle.

In the industrial world, the compact yet incredible power of hydraulic cylinders is a constant source of awe and amazement. Through the magic of fluid power leverage via Pascal’s Law, hydraulic cylinders are capable of generating tremendous lifting forces to move massively heavy structures.

In order for such great force to be harnessed to do useful work, it must be kept fully under control. Force that is out of control is either useless or destructive. When it comes to controlling the movement of a powerful hydraulic cylinder, the piston/ram position must be continually monitored in near-real-time.

The most popular device for measuring cylinder position is called a Magnetostrictive Linear Position Sensor. Sometimes these position sensors are called LDTs (Linear Displacement Transducer) or MDTs (Magnetostrictive Displacement Transducer). All of these terms refer to the same type of devices.

BTL7

To get an idea of the power and control that is feasible with modern hydraulic cylinders and integrated cylinder position sensors, have a look at this amazing video from ALE Heavylift. The topside of a giant offshore oil platform was jacked up 131 ft (40 m) and then skidded horizontally a distance of 295 ft (90 m) to place it on top of its supports. Imagine the incredible synchronization of speed, position, and operational sequencing needed to safely lift and place such a massive structure.

For more information about magnetostrictive linear position sensors for hydraulic cylinders, visit the Balluff website at www.balluff.us.

1. “Blaise Pascal Versailles” by unknown; a copy of the painture of François II Quesnel, which was made for Gérard Edelinck en 1691. – Own work. Licensed under CC BY 3.0 via Commons – https://commons.wikimedia.org/wiki/File:Blaise_Pascal_Versailles.JPG#/media/File:Blaise_Pascal_Versailles.JPG

The Pressure to Step Up Performance

PressureSensorAppWe all know the roles of mechanical pressure gauges and switches. They either give us a visual indication of hydraulic pressure, or open or close a switch when reaching a certain threshold pressure. Electronic pressure transducers do the same, but more effectively and with a single component instead of two or three. Furthermore, an electronic pressure transducer provides more output variations, longer life, and greater accuracy.

Mechanical pressure gauges and switches still have their place in fluid power, but with more features and greater accuracy and life, transducers are being specified in a wide variety of applications. In an article recently published in Hydraulics and Pneumatics these applications and their conditions are discussed in greater detail. You can read the entire article on the Hydraulics and Pneumatics website.

 

Anatomy of a High Pressure Inductive Proximity Sensor

Some industrial applications will require a sensor with special properties. This type of sensor offering is needed especially when pressure comes to play. In a wide range of hydraulic cylinder and valve applications high pressure sensors are exposed to hostile environments and are subject to pressure that a standard sensor simply cannot hold up in. For example 350 bar of pressure can be detrimental to a standard sensor as it is not designed for a pressure application.

High pressure inductive sensors are designed to withstand the severe duty of a high pressure application with product features like corrosion – resistant housing materials, high strength ceramic sensing faces and special sealing techniques such as undercut housings with sealing and support rings. This is very important because not only do we need to have a sensor that can withstand pressure on the face of the sensor without damage we also need to make sure we can keep the hydraulic fluid inside the cylinder or valve where it belongs.

In the photo below you will notice the undercut area at the sensing face of the sensor along with an O-ring and supporting backing ring to make sure the application is sealed tight.

installation instruction Installation Photo

There are several common sizes for different types of cylinder and valves however the same principle applies. Below is an example of a flange mount style offering. This type of sensor takes a different design approach that is bolted to the top side of a cylinder with a sealing O-ring under the mounting point.

Strokemaster Diagram

strokemaster photo

It’s also important to know what form factor is needed when specifying a high pressure inductive sensor. Typically you will see pressure options from 50 up to 500 bar. The dimensions of the cylinder or valve will determine what type of high pressure sensor is needed.

HighPressureGroup

To learn more visit www.balluff.us.