Measurement Technology for Process Automation

Anders Andersson

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351974486
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria elettronica e telecomunicazioni

Almost every industry that use liquids and gas in any form has a need to measure flow, temperature and pressure. This text is a practical guide on how to accurately use these measuring instruments to control processes in manufacturing industries for food, beverages, chemicals, pharmaceuticals, oil, water and waste water, power, etc. With higher prices of raw materials and more severe requirements for safety and environmental issues, there is a growing demand to measure with higher precision. The book includes a number of practical examples from various industries. It discusses how to comply with safety standards regarding measurements and explains how legal control systems apply to measurements. The aim is to help any process industry reduce the risk of high costs and damage to both people and equipment.

1

Application Examples

Always remember that safety comes first! But if it is safe to do a test, always try to do a simulation of the measurement or control system you are working on before starting up the real process. The trial-and-error method is probably the best and most efficient way to learn. So, if it is safe, try your ideas in the real world. For this reason, the first chapter in this book is all about practical set-ups, process circuits, control loops and typical installations of meters and sensors.

Hopefully, this will give you both inspiration and some new knowledge!

This book is about process measurement technology. This includes meters for pressure, temperature, level, flow and a few others. The focus is on processes running in pipes and tanks. We will discuss measuring methods, equipment, installation, maintenance and, perhaps most importantly, calibration and traceability.

Safety always comes first! Process pipes and tanks can contain hot, toxic substances at high pressure. Follow safety procedures, use protective equipment and listen to advice and instructions.

Designing a Process

Imagine a factory producing fruit juice. In this process, the people working in the factory use cleaned, rinsed and boiled fruits. The fruit concentrate is then mixed with water and sugar. Finally, the juice is packaged and distributed. This is a rather simple process, which could also be done at home in the kitchen. Everything we do at the factory is the same as what you would do in the kitchen, but now we’re going to make juice in greater quantities and in an industrial way. Of course, there are a lot of considerations when building this kind of process, especially when it comes to food and hygiene requirements. But for now, as well as for the rest of this book, we will focus on measurements and the problems around mixing, moving and distributing products, and how to control and measure these processes.

In the example illustrated in Figure 1.1, there are two tanks: one with fruit concentrate (2) and the other with sugar (1). The products are pumped to a new tank (4) where they are mixed with water (3) and heated. When cooking is completed, the bottles are filled with juice (5). To get all this to work, we need four different processes measuring circuits. First, we want to keep track of how much material is left in tanks (1) and (2). Then, we must mix the ingredients in the right proportions. During heating, we need to have the right temperature. Finally, we would like to know how much we put in each package so that we can put the right price on the products. These are the first four measurement examples on the following pages, where you can read more about how to do this in detail.

FIGURE 1.1

Mixing and batching.

In this specific example, it might be that the need for precision is not so important. But basically, the same measuring principles apply to almost any product manufactured. When it comes to costly ingredients, hazardous chemicals, medicines or products with extra high quality requirements, there are likely to be much greater demands on the measurements. Another aspect is that good measurement devices allow you not only to minimise errors but also to verify what has been done with the products, a type of quality traceability that is essential in some processes, like production of pharmaceuticals and food.

Mixing and Batching

Almost all manufacturing processes include mixing of different components. Often a product is mixed with water (diluted) to obtain a final product with the right concentration. However, the purpose of the process can also be to create a reaction, a particular product characteristic or the right flavour or texture. Process mixing is usually done in two different ways; in batches (in a tank, barrel or similar) or continuously in the ‘meeting point’ between two pipelines. In many cases, improper dosing of chemicals not only leads to a product of lower quality but can also result in a hazardous product. In such cases, there are of course higher demands on the process measuring equipment in use, to minimise the risk of incorrect mixing and avoid risks.

Two products, A and B, shall be mixed. The mixing is made in a tank (5) (Figure 1.2). As mentioned earlier, this method is similar to the way we would do at home in the kitchen when baking or cookin when ingredients are measured and mixed in a bowl. Here, in this process products A and B can be measured in several different ways, and in the case of large volume measurement, it is probably easiest to open and close valves (3) and (4) in two permanently installed pipelines. To know how long time the valves should be kept open, we can:

1. Read the level change in the mixing tank
2. Use flow (volume) meters (1) and (2) installed in each line

FIGURE 1.2

Mixing.

To make the process more automated, the valves can be controlled with preset counters connected to the flow meters. With a preset counter and valve actuators, the only thing the operator needs to do is to press START and the flow will then stop by itself when the right amount is delivered.

Reference, Read More

• To select the best flow meter, see Chapter 2.
• To find an appropriate level meter, see Chapter 5.
• For batching controllers and preset counters, see Chapter 7.
• To select a suitable control valve, see Chapter 8.

Advantage

• Mixing in a tank makes quality control easy because samples can be taken when mixing is ready (and the product is still in the tank).

Warning, Things to Consider

• Mixing in a tank causes interruptions in the production. If the mixing fails, it might happen that the complete batch and all tank content must be destroyed. If the raw materials tend to separate, a stirring device (such as a propeller) might be needed in the tank. Perhaps also cooling or heating is necessary to maintain product quality.
• Select dimensions, response times and filling times that correspond to the requested accuracy. Using level gauges for batch control is often difficult, but sometimes possible if the tanks are high with a small diameter. If using flow meters and high precision is needed, the flow meters must not be installed far away from the mixing point.
• When working with food products, extra care is needed for component and material selection.

Information Needed for Sizing and Selections

• You need to know tank height and diameter, as well as pipe diameter and minimum, normal and maximum flow rates in each line. Media and maximum operating pressure and temperature must be stated. Filling time for each batch might also be of value especially if very short (if less than 1 minute). Before selecting suitable flow meters, an estimate of the required accuracy, based on product quality requirements, is needed. Remember to check material properties when working with chemicals. Select signal input/output types so that sensors and controllers can operate together.

Continuous Mixing

Continuous mixing is a method used in newer production facilities, and often where production capacity is high. In this method, the various ingredients are flowing all the time. Flow rates are measured by flow meters (1) and (2) (Figure 1.3). The mixing is controlled by a control system (5) and two valves (3) and (4) according to a specific recipe, with mixing ratios specified as a %-ratio. The ratio controller can be a part of a large control system or a separate unit. In either case, the additive flow (in our example, the yellow colour) constantly adjusts to follow the main flow (the blue colour) to maintain a steady %-ratio in the final, mixed product (the green colour).

FIGURE 1.3

Continuous mixing.

Reference, Read More

• To select suitable flow meters, see Chapter 2. Basically, all kinds of flow meters can be used, but inductive and mass flow meters are common.
• To select control valves, see Chapter 8.

Advantage

• Continuous mixing allows for higher production, a flexible process with less need of space.

Warning, Things to Consider

• Make sure that products are well mixed. If needed, install a static mixer, a device that starts a rotation in the pipe which helps to mix the product. Product quality can be more difficult to check when the process is constantly running, and therefore a sampling valve is useful.
• Select pipe dimensions, meter sizes and flow rates that correspond to the required accuracy. Avoid long distances between the flow meters and the mixing point.

You need to know tank height and diameter, as well as pipe diameter and minimum, normal and maximum flow rates in each line. Media, maximum operating pressure and temperature must be stated. Before selecting suitable flow meters, an estimate of the required accuracy, based on product quality requirements, is needed. Remember to check material properties when working with chemicals. Select signal input/output types so that sensors and controller can operate together.

Filling

In a filling process, a certain amount (portion) is pumped or transferred into a package (or to another process). The first question to ask when designing such a system is: ‘How is each package sold; by weight or by volume?’ With constant ...