Fluid Motion Control
Rupam Dutta
Senior Manager (Chemical)

Tribeni Deka
Manager (Chemical)

Bednidhi Sharma
Deputy Manager (Chemical)

A fluid is a substance that does not permanently resist distortion. An attempt to change the shape of a mass of fluid results in layers of fluid sliding over one another until a new shape is attained. The branch of engineering that deals with the behavior of fluids (liquids, gases, vapors) is called fluid mechanics. It has several sub disciplines, including aerodynamics and hydrodynamics.

Different types of fluids
Incompressible fluid: Its density changes only slightly with moderate changes in temperature and pressure. Liquids are generally considered as incompressible.

Compressible fluid: If the changes in density are significant, the fluid is said to be compressible.

Ideal fluid: An incompressible fluid having no viscosity is known as an ideal fluid. It is only an imaginary fluid.

Real fluid: Fluid which possesses viscosity is known as real fluid. All the fluid in practical are real fluids.

Newtonian fluid: Real fluid, in which shear stress is directly proportional to the rate of shear strain or velocity gradient, is known as Newtonian fluid.

Non Newtonian fluid: A real fluid, in which shear stress is not directly proportional to the rate of shear strain or velocity gradient, is known as Non Newtonian fluid.

Ideal plastic fluid: A fluid in which shear stress is directly proportional to the rate of shear strain or velocity gradient, is known as ideal plastic fluid.

Accurate measurement of flow rate of liquids and gases is an essential requirement for maintaining the quality of industrial processes. Diversified requirements of flow measurement exist, e.g. volumetric or mass flow rate; the fluid medium (gas or liquid); intrusive or nonintrusive etc. As such, there are different types of flow measuring techniques used in industries. While choosing flow-meters, one should consider some intangible factors, such as familiarity of plant personnel, their experience with calibration and maintenance, spare parts availability and mean time between failure history etc. at the particular plant site. While measuring the flow of compressible materials, volumetric flow is not very meaningful unless density/viscosity is constant. When the velocity (volumetric flow) of incompressible liquids is measured, the presence of suspended bubbles will cause error; therefore, air and gas must be removed before the fluid reaches the meter. In other velocity sensors, pipelines can cause problems (ultrasonic) or the meter may stop functioning if the Reynolds number is too low (in vortex shedding meters, RD>20,000 is required).

Common types of flow-meters that find industrial applications are :
(a) Obstruction type:Obstruction type flowmeters are of two types:differential pressure type and variable area type. Orifice meter, Venturimeter, Pitot tube fall under the first category, while rotameter is of second category. In all cases, an obstruction is created in the flow passage and the pressure drop across the obstruction is related with the flow. Orificemeter is used in Chemical, Petrochemical, water treatment plants, power generation, gas generation and distribution, etc. Venturimeter is used in aviation, automotive, chemical, petrochemical industries etc. Pitot tube is used to measure fluid flow, especially air flow in ventilation and HVAC systems. Variable-Area Flowmeters(Rotameters) provide practical solutions for many flowmetering applications.

(b) Inferential (turbine type):- This flowmeter is a simple way for measuring flow velocity. A rotating shaft with turbine type angular blades is placed inside the flow pipe. The fluid flowing through the pipeline will cause rotation of the turbine whose speed of rotation can be a measure of the flowrate. Turbine meters are suitable for extreme temperatures and pressures and applicable to liquids and gases. They can be installed easily and are well accepted in the oil and gas industries. (Fig. 1.1)

Figure 1.1:

(c) Electromagnetic: Electromagnetic flow meters detect flow by using Faraday’s Law of induction. Inside electromagnetic flow meter, there is an electromagnetic coil that generates a magnetic field and electrodes that capture electromotive force (voltage). Due to this, although it may appear as if there is nothing inside the flow pipe of an electromagnetic flow meter, flow can be measured. Electromagnetic flow meters also offer noninvasive sensing. They can be used with acidic, alkali, and ionized fluids with electrical conductivities ranging from 10 S/m to 10–6 S/m and with clean, dirty, corrosive, erosive or viscous liquids and slurries, but are not suited for use in hydrocarbon or gas flow measurement.

(d) Positive displacement (integrating): It is a type of flow meter that requires fluid to mechanically displace components in the meter in order for flow measurement. Positive displacement flow meters measure the volumetric flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes. These flowmeters are highly accurate and applicable to clean gases, liquids even viscous ones. Examples include heating oils, lubrication oils, polymer additives, ink etc. They are relatively expensive.

(e) Fluid dynamic (vortex shedding):Classified as a Vortex flow meter, this device utilizes a law, theoretically proven by Theodore von Karman in 1912. If a columnshaped obstruction (vortex shedder) exists in a flowing fluid, it will generate alternating vortices downstream. Therefore, detecting the number or pulse of vortices makes it possible to measure flow.

These flow-meters are applicable to many types of fluids, including high temperature gas and steam. Flow rate of highly viscous, dirty, corrosive, or abrasive fluids cannot be measured by this flow-meter.

(f) Ultrasonic Meters: These meters are non-intrusive i.e. no obstruction is placed in the fluid stream or any reduction of the flow channel. They create no pressure drop in the fluid. The rate of flow is measured from outside the tube. It uses sound waves to determine the velocity of a fluid flowing in a pipe. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe and its reflections from the fluid are the same. When the fluid moves faster, the frequency shift increases linearly. Though not highly accurate, ultrasonic flowmeters are commonly used to measure flow of water, molten sulfur, cryogenic liquids, and corrosive fluids. In oil and gas, wastewater treatment, power, chemical, food and beverage, pharmaceutical, metals and mining, pulp and paper industries this flowmeter finds application.

(g) Mass flowmeter (Coriolis). It measures mass flow rate of a fluid traveling through a tube. Coriolis flow meters work on the principle that the inertia created by fluid flowing through an oscillating tube causes the tube to twist in proportion to mass flowrate. Coriolis mass flowmeters measure directly the mass flow rate used mostly with small pipes. They are costly to install and operate. The industries in which they are used are chemical, oil and gas, food and beverage, pharmaceutical, pulp and paper, power, metals and mining, water and wastewater etc. (Fig. 2.1)

Figure 2.1:

Flow - control valves
Flow-control valves control the rate of flow of a fluid through a circuit. It accurately limit the fluid volume rate. Components which comprise almost all the flow-control valves have been depicted in the Fig 3.1

Figure 3.1:

Classification of Flow-Control Valves

1. Non-Pressure-Compensated Valves
These type of valves are used when the system pressure is relatively constant and motoring speeds are not too critical. The operating principle behind these valves is that the flow through an orifice remains constant if the pressure drop across it remains the same. In other words, the rate of flow through an orifice depends on the pressure drop across it. But here, the inlet pressure of the pump that remains constant. Therefore, the variation in pressure occurs at the outlet that is defined by the work load. This implies that the flow rate depends on the work load. Hence, the speed of the piston cannot be defined accurately using non-pressure-compensated flow-control valves when the working load varies.

2. Pressure-Compensated Valves
These types of valves overcome the difficulty caused by non -pressurecompensated valves by changing the size of the orifice in relation to the changes in the system pressure. This is accomplished through a spring -loaded compensator spool that reduces the size of the orifice when pressure drop increases. Once the valve is set, the pressure compensator acts to keep the pressure drop nearly constant. It works on the feedback mechanism from the outlet pressure. This keeps the flow through the orifice nearly constant

Functions of Flow-Control Valves:
1. Regulate the speed of linear and rotary actuators: They control the speed of piston that is dependent on the flow rate and area of the piston.

2. Regulate the power available to the sub-circuits by controlling the flow to them.

3. Regulate the pump flow proportionately to various branches of the circuit : It transfers the power developed by the main pump to different sectors of the circuit to manage multiple tasks, if necessary.

An understanding of fluid motion control is essential for accurately treating problems on the movement of fluid through pipes, pumps and all kinds of process equipment. In industrial control loops, the control in flow rates of incoming liquids or gases is very important to achieve the control objective Applying motion-control technology allows us to simultaneously increase machine productivity, reliability, product uniformity and customer satisfaction and loyalty while reducing shock and manufacturing cost.