The Importance of Controls, Instrumentation, and Automation in Hygienic and Sanitary Processing Systems

Instrumentation is used in all manners of process operations to achieve many different objectives, primarily related to product flow and the condition of the process system. 

However, much of the instrumentation used in the industrial processing world is not suitable for use in the hygienic and sanitary processing industries because it cannot be cleaned sufficiently after operations to ensure the requisite purity in the final product. 

The focus of this article, then, will be on instrumentation that is commonly used in the hygienic and sanitary processing industries, and what types of instruments are suitable for which applications.

So, with these two objectives in mind, sanitary process instrumentation is used for one of two things:

• First, instruments of various types are used to monitor the overall performance of a processing system, testing for fluid factors such as viscosity, conductivity, and turbidity. These measurements allow plant operators to optimize fluid flows to achieve both of the objectives noted above.

• Secondly, and with a particular focus on achieving the product safety objective, instruments allow process plant operators to generate the maximum efficiency in clean-in-place (CIP) systems. CIP systems are generally more efficient if they are automated, reducing the guesswork and inconsistencies resulting from human error. A properly automated CIP system delivers consistent cleaning results, allowing operators to monitor and control the critical parameters needed for proper cleaning. The old saying is that TACT is needed for a CIP system to work at its optimal level: Time, Action, Chemicals, and Temperature. Proper instruments help plant operators hit the ideal mark for TACT.

Instrumentation maximizes a sanitary processing system’s efficiency, providing real-time data about a number of fluid parameters, including:

• Overall system temperature
• Supply and return line temperatures
• Tank levels
• Supply flows
• Chemical compatibility
• Line pressure

The subsequent sections of this article will discuss the type of instrumentation most used in sanitary processing systems, explaining how each instrument works and what it is used for.

An Overview of Process Instrumentation

1. Flow Meters

In CIP operations, flow meters are used to measure the flow rate of a fluid through a piping system. They are used to ensure a flow rate that is sufficiently turbulent to provide proper cleaning, as well as the optimal flow required for the efficient use of spray devices. They also precisely control the rinse steps for CIP.

For purposes of product processing, flow meters are also used to show the total volume of product that was processed through the line. This measurement is used for helping to optimize production outputs.

It should be noted that total fluid volume is different than fluid velocity or turbulence, so measuring each may be important to achieving both CIP efficiency and production output goals.

There are various types of flow meters, operating on different principles. The most common types of flow meters used in hygienic processing applications include the following:

Mass flow meters

These types of flow meters measure the flow rate of a fluid directly, and are the only type of meter that truly measure mass—all the others measure volume. There are two types of mass flow meters. 

1. Thermal flow meters use a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element, and the conducted heat is directly proportional to the mass flow rate. 
2. Coriolis flow meters measure the mass of a fluid flowing through a vibrating U-shaped tube, causing it to twist. This is the so-called Coriolis effect, which measures the deflection of the tube to provide an accurate measure of fluid flow. 

Mass flow meters are highly accurate, often to within +/- 0.1%, regardless of the type of fluid being measured. They may also be used with all sorts of fluids, including non-conductive fluids, but with viscous fluids, pressure drop does need to be taken into consideration (pressure head loss can occur). On the downside, mass flow meters are very expensive.

There are a number of factors to consider when choosing a particular type of fluid flow meter, including the following:

• Accuracy
• Cost
• Flow rate range
• Head loss
• Maintenance
• Lifetime requirements

Generally speaking, each of these factors are inter-related (i.e., the cost of a particular flow meter will increase with required accuracy and greater lifetime).

Most importantly, though, pick a flow meter that is compatible with the product being measured (e.g., is the product conductive? what is its viscosity?). And, choose a meter that is easily cleanable.

2. Level Sensors

Level sensors detect the level of liquids and other fluids (e.g., slurries). The level measurement can be either continuous or point value. Continuous level sensors measure fluid levels within a specified range and determine the exact amount of a substance in a certain place. In contrast, point-level sensors only determine whether a substance is above or below the sensing point. These types of sensors are generally used to detect whether fluid levels are excessively high or low.

In determining the type of level sensor that is appropriate for a given application, the first question to answer is whether one is interested in measuring for control, or measuring to determine inventory. 

Put another way, how much product do I want to bring in, versus how much product do I have now that is ready to package and ship? 

Answering these questions will help to determine the correct type of level sensor. Level sensors are widely used in the dairy, food & beverage, brewery, and life sciences industries.

In CIP operations, level sensors

• Monitor the tank levels in the wash and rinse tanks
• Signal valves to turn on and off automatically to add water or chemicals, ensuring a sufficient volume of CIP fluids to run a cleaning cycle

In product processing applications, level sensors

• Keep pumps from running dry
• Keep vessels from overflowing
• Automate filler applications with minimal product losses due to their ability to receive precise fill/empty signals

In brewery operations in particular, level sensors play a key role in both wort boiling and lautering.

There are a wide variety of level sensors, employing a range of technologies. All level sensors, though, are based on one of four different types of technologies: hydrostatic level, radar, potentiometric, or capacitance. 

Some of the most common level sensors used in sanitary applications are:

Point level sensors

Float type

A mechanical sensor using a magnet sealed inside a float, with measurement calibrated against an activation scale. These work well with a wide range of liquids, including corrosives, but should not be used with high viscosity (thick) liquids or sludges.

Pneumatic

These sensors use a column of air to depress a diaphragm against a switch. These are suitable for use with highly viscous liquids, and provide a low-cost technique for point-level monitoring. Diaphragms can puncture, however, and compromise the sanitary nature of the sensor.

Conductive 

Conductive level sensors are ideal for the point level detection of a wide range of conductive liquids such as water, and are especially well-suited for highly corrosive liquids such as caustic soda, hydrochloric acid, nitric acid, ferric chloride, and similar liquids. They are extremely safe since they use low voltages and currents. They have the additional benefit of being solid state devices, and are very simple to install and use.

Capacitance level sensors

These sensors excel in sensing the presence of a wide variety of aqueous and organic liquids, as well as slurries. The technique is frequently referred to as RF for the radio frequency signals that are applied to the capacitance circuit. This type of sensor is quite common in the sanitary industries, and works well when coated with viscous products such as ketchup.

Optical interface sensors

These are used for point-level sensing of sediments, liquids with suspended solids, and liquid-liquid interfaces. These sensors sense the decrease or change in the transmission of infrared light emitted from an infrared diode (LED). With the proper choice of construction materials and mounting location, these sensors may be used with aqueous, organic, and corrosive materials.

Finally, there are a number of level sensors which may be used for both point level detection and continuous monitoring, including the following:

Microwave sensors 

Ideal for use in moist, vaporous, and dusty environments, as well as in applications in which temperatures and pressures vary. They are largely unaffected by temperature, pressure, vacuum, or vibration. These sensors do not require physical contact with the process material, so the transmitter/receiver can be mounted a safe distance away from the process. These sensors, however, are relatively high priced, and set-up is complicated.

3. Temperature Sensors

Temperature sensors are used in both product processing and CIP operations to measure the temperature of process fluids. In considering the type of temperature sensor appropriate for a given application, it’s best to think in terms of temperature control versus temperature limit (in other words, do I want to modulate temperature, or do I want to use the temperature sensor as an on/off switch?).

For hygienic processing applications, temperature sensors may be manufactured from all-stainless-steel, and many models may also comply with the BPE standards used in the biopharmaceutical manufacturing industry.

Temperature sensors operate on a range of different principles, including the following:

Resistance Temperature Sensors (RTDs)

One of the most accurate types of temperature sensors and are quite common in the sanitary processing industries. In a resistive temperature sensor, the resistance is proportional to the temperature. This sensor is made from platinum, nickel, and copper metals, and can measure temperature in a wide range from -270°C to +850°C.

Infrared (IR) Sensors 

Electronic sensors used to sense certain characteristics in the surrounding environment (such as temperature) by either emitting or detecting IR radiation. IR sensors are non-conductive sensors, meaning they do not come into direct contact with process fluids in order to do their measurements. While a less-common type of temperature sensor than RTDs, IR sensors are often appropriate for use in instances where the temperature is difficult to measure.

4. Pressure Transmitters

Pressure transmitters (also called pressure sensors) are used for the precise measurement of pressures in pipes and vessels. They usually act as a transducer, generating a signal as a function of the pressure imposed. In process operations, pressure transmitters may be used for flow sensing, level/depth sensing, and leak testing, in addition to pressure sensing.

Common types of pressure transmitters include the following:

5. Conductivity Sensors

In process operations, conductivity sensors are used to measure the amounts of dissolved chemicals in aqueous solutions in order to determine the solution’s ability to carry an electrical current. This type of measurement is used to assess the purity of the liquid, and its safety or suitability for use. This type of sensor is also very commonly used in CIP operations.

There are two basic sensor types used for measuring conductivity, contacting and inductive. Each of these is described briefly below:

Contacting Conductivity Sensors

Ideal for use in pure and ultrapure water applications. They are highly sensitive to any ions present in the fluid, and provide the highest accuracy for low conductivity measurements. In CIP operations, conductivity sensors are used to measure the conductivity of the cleaning water.

Inductive Conductivity Sensors

Also called toroidal or electroless sensors, these have a wide range of capabilities, and are better suited for measurements in dirty, corrosive, or highly conductive solutions. They require less maintenance than do contacting sensors in the same environment. In CIP operations, inductive sensors measure the conductivity of the cleaning solution.

6. Turbidity Sensors

Turbidity is a measurement of suspended solids in a liquid, and provides an indication of fluid quality. To the naked eye, turbidity is the cloudiness or haziness of a fluid, similar to smoke in the air. Turbidity sensors, then, are used to measure the amount/number of particulate solids suspended in a fluid in order to measure the purity of the fluid and/or determine the level of contaminants in it.

All turbidity sensors work on the same basic principle. These meters shine a light source through a sample, and quantify the suspended particle concentrates. The more particles there are in a solution, the higher its turbidity.

In CIP operations, turbidity sensors are frequently used as a pre-rinse step to ensure that the process line has been flushed of all dirt before the washing cycle commences.

7. Refractometers

Refractometers are commonly used in food & beverage processing to ensure that the concentration of a dissolved solid, such as sugar, is at the required level to meet the specifications of the final product. In brewery applications, for example, refractometers are used to measure the specific gravity of the liquid before fermentation in order to determine the amount of fermentable sugars which will potentially be converted to alcohol. 

In health and life sciences applications, refractometers measure the total plasma protein in blood samples, and are used in pharmaceuticals manufacturing for the quality control of raw, intermediate, and final products.

Refractometers work by measuring the extent of light refraction (compared to a standard index) of transparent substances in either a liquid or solid state. This measurement is then used to analyze the fluid’s purity, and determine the amount or concentration of dissolved solids in it.