Study of Inherent Characteristic of Control Valve

Study of Inherent Characteristics of Control Valves: Recent Advancements and Applications in Process Control

Control valves are critical components in process control systems, playing a vital role in regulating fluid flow, pressure, and temperature in various industries, including chemical processing, crude oil, natural gas, and water pipe systems. Understanding the inherent characteristics of control valves is essential for optimal process control, efficiency, and safety.

Inherent Characteristics of Control Valves

Control valves exhibit unique characteristics that affect their performance, including:

1. Flow Characteristics: The relationship between valve opening and flow rate, which can be linear, equal percentage, or quick opening.
2. Pressure Drop: The decrease in pressure across the valve, which affects the valve's ability to control flow.
3. Rangeability: The valve's ability to control flow over a wide range of operating conditions.
4. Repeatability: The valve's ability to consistently return to the same position for a given signal.
5. Hysteresis: The difference in valve position for a given signal, depending on the direction of travel.

Recent Advancements in Studying Control Valve Characteristics

Recent research has focused on developing advanced methods for characterizing control valve performance, including:

• Computational Fluid Dynamics (CFD): Numerical simulations to predict valve flow behavior and pressure drop.
• Artificial Neural Networks (ANNs): Machine learning algorithms to model valve behavior and predict performance.
• Internet of Things (IoT): Integration of sensors and actuators to monitor and control valve performance in real-time.

Applications in Process Control

Understanding control valve characteristics is crucial for developing effective process control strategies in various industries, including:

• Chemical Processing: Control valves regulate fluid flow, temperature, and pressure to optimize chemical reactions and product quality.
• Crude Oil and Natural Gas: Control valves manage fluid flow, pressure, and temperature to ensure efficient transportation and processing.
• Water Pipe Systems: Control valves regulate water flow, pressure, and quality to ensure safe and efficient distribution.

Incorporation of Control Valve Characteristics in Process Control

To develop effective process control strategies, control valve characteristics must be incorporated into the control system design. This can be achieved through:

• Valve Selection: Choosing valves with the appropriate characteristics for the specific application.
• Control System Design: Designing control systems that account for valve characteristics, such as flow characteristics and pressure drop.
• Tuning and Optimization: Tuning and optimizing control systems to account for valve characteristics and ensure optimal performance.

Understanding the inherent characteristics of control valves is essential for developing effective process control strategies in various industries. Recent advancements in studying control valve characteristics, such as CFD, ANNs, and IoT, have improved our ability to predict and optimize valve performance. By incorporating control valve characteristics into process control system design, tuning, and optimization, industries can ensure optimal performance, efficiency, and safety.

Experimentation Procedure to Determine Inherent Characteristics of Control Valves

Determining the inherent characteristics of control valves is crucial for understanding their behavior and performance in various process control applications. The experimentation procedure outlined below provides a comprehensive approach to determining the inherent characteristics of control valves.

Pre-Experimentation Preparation

• Valve Selection: Select a control valve that is representative of the type and size used in the specific application.
• Test Rig Setup: Set up a test rig that includes a fluid source, piping, fittings, and instrumentation to measure flow rate, pressure, and temperature.
• Instrumentation Calibration: Calibrate all instrumentation to ensure accuracy and reliability.

Experimentation Procedure

1. Flow Characterization Test:

    - Set the valve to a fully open position.

    - Measure the flow rate through the valve at various pressure drops.

    - Plot the flow rate versus pressure drop to determine the valve's flow characteristic (e.g., linear, equal percentage, or quick opening).

2. Pressure Drop Test:

    - Set the valve to a fully open position.

    - Measure the pressure drop across the valve at various flow rates.

    - Plot the pressure drop versus flow rate to determine the valve's pressure drop characteristic.

3. Rangeability Test:

    - Set the valve to various positions (e.g., 10%, 20%, ..., 100% open).

    - Measure the flow rate through the valve at each position.

    - Plot the flow rate versus valve position to determine the valve's rangeability.

4. Repeatability Test:

    - Set the valve to a specific position (e.g., 50% open).

    - Measure the flow rate through the valve at this position.

    - Repeat the measurement multiple times to determine the valve's repeatability.

5. Hysteresis Test:

    - Set the valve to a specific position (e.g., 50% open).

    - Measure the flow rate through the valve at this position.

    - Gradually increase and then decrease the valve position while measuring the flow rate.

    - Plot the flow rate versus valve position to determine the valve's hysteresis.

Importance of Experimentation

Determining the inherent characteristics of control valves is essential for:

1. Valve Selection: Understanding a valve's characteristics ensures that the correct valve is selected for a specific application.
2. Control System Design: Knowing a valve's characteristics enables the design of control systems that account for the valve's behavior.
3. Process Optimization: Understanding a valve's characteristics allows for optimization of process conditions, such as flow rate and pressure.
4. Troubleshooting: Knowing a valve's characteristics facilitates troubleshooting and maintenance, reducing downtime and improving overall process efficiency.

To study the inherent characteristics of the control valve, equal %, and linear experiment modes are conducted by a unique experimental setup:

With two control valves, one operates based on air-to-close mode and the other with air-to-open mode. when air is supplied to the diaphragm of the control valve it results in closing then it has equal percentage characteristics and in vice-verse, it has linear characteristics. Pneumatic actuators are used to control the air supply to the control valves. In general tap water is circulated with a pump from the bottom of the receiving tank to the supply tank. By opening the manual gate valve water from the supply tank is passed to the receiving tank through the rotameter and control valve. Inlet pressure at the control valve can be measured in terms of the water column. By air regulator stem of the control valve is moved and adjusted for the required flow rate. Stem opening in terms of mm can be observed by the scale fitted near the stem.

Theory of inherent characteristic determination: 

The fluid flow rate in a pipeline is controlled with the help of automated control value in modern industries. Extremely powered actuators and pneumatic singles by means of pressurized air, hydraulic, etc allow the control room operator to open or partially open and close the valve. The amount of fluid passing through a valve at any time depends upon the opening between the plug and the seat. Hence there is a relationship between stem position, plug position and the rate of flow. The relation between the flow through the valve and the valve stem position (or lift) is called the valve characteristic.

In general, the flow through a control valve for a specific fluid at a given temperature can be expressed as: Q = f1(L,p0,p1)
Where Q is the volumetric flow rate, L is valve stem position or lift, and p0 and p1 are upstream and downstream pressures.

The Inherent flow characteristic of control valve is the relation developed between the flow of fluid and the valve movement at constant pressure drop across the valve ( fixed upstream and downstream pressures). Hence, the inherent characteristic is, Q = f2(L)
It can also be written as m = Q/Q_max = f(L/L_max )
m = f(x)
Where Q_max is the maximum flow when the valve stem is at its max lift L_max (valve is fully open),
m is fraction of maximum flow, Q/Q_max and x is the fraction of maximum lift, L/L_max

Operation procedure to determine inherent characteristics of the control valve:

• Open the manual plug valve of equal percentage (air-to-close) control valve.
• Open the valve up to 14 mm travel (fully open).
• Adjust the regulatory valve at the inlet of the control valve to maintain the flow at 400 LPH. Note down the pressure drop.
• Slowly increase the air pressure by the air regulator and close the control valve to travel the stem by 2 mm.
• The pressure drop across the valve will increase. Maintain the pressure drop by adjusting the regulatory valve. Observe the flow rates.
• Take the observations at each 2 mm stem travel till the valve is fully closed by repeating the above step.
• Plot the graph of flow % of maximum versus valve lift % of full lift.
• Repeat the experiment for the linear valve (air to open).

A model table for parameters observation:

Stem lift, mm                     Air to Close                                               Air to Open
                            Pressure in mm     H2O Flow in LPH        Pressure in mm       H2O Flow in LPH
14
12
10
8
6
4
2
0

The formula for Valve coefficient calculations:

G = Sp.g = 1 for water.
Q = m3/hr = LPH/1000.
Cv = 1.16 Q √(G/∆P)
∆P = ∆P in mm of H₂O / (10.33 x 103)

Model table to represent the results in table form:
 
Stem lift, mm                   Air to Close                                   Air to Open
                         Flow in LPH        ∆P, mm H2O          Flow in LPH      ∆P, mm H2O


14
12
10
8
6
4
2
0
RESULT: The inherent characteristics of the air-to-open and air-to-close valves are verified

Wetted Wall Column Experiment and Set UP

A wetted wall column is used to study the distillation, gas absorption and vaporization operation of various chemical systems, to obtain data and correlation between components present in different phases that exist in equilibrium conditions during the operation. The experiments of wetted wall columns provide data for the design of separation operations. 


The aim of the wetted wall column experiment : 
Evaluation of Mass Transfer Coefficient in a wetted wall column

Objective: The rates of diffusion into gases flowing through pipes are studied in wetted wall column.

Principle/Theory:

A thin film of liquid falling down inside of a vertical pipe through which the gas flows constitutes a wetted wall column. Wetted wall columns have been used as Hydrochloric acid, Ammonia, Acetone, Benzene and other volatile liquids absorbers. They have also been studied for theoretical studies for mass transfer because the interfacial surface between the phases is kept under control and is measurable.

The height of the wetted wall column required for mass transfer operations is excessive and consequently, this is not widely used, where large quantities of liquid or gas have to be handled, it would be necessary to arrange many vertical pipes in parallel and this leads to difficulties in the distribution of liquid into the inner surface of the tubes. The gas pressure drop for this is confined to skin friction effects, with few or no expansion or contraction losses
.

Mass transfer rates for fluids flowing through pipes have been studied more completely than in other cases.

The rates of diffusion into gases flowing through pipes have been studied in wetted wall columns.

A volatile liquid is submitted to flow down the inside surface of a circular tube, while a gas flows upward or downward through the center of the pipe. Measurement of the rate of evaporation of a liquid into the gas stream over a known surface permits calculation of the mass transfer coefficient for the diffusion of vapor into the gas stream. Since the liquid is pure, the concentration gradient for diffusion exists entirely within the gas phase, the mass transfer coefficient Kg may be calculated. Sherwood and Gillard conducted a series of experiments using a variety of volatile liquids with air in turbulent flow.

Here the mass transfer coefficient in the form of the dimensionless group is plotted against the Reynolds number of the gas for the system air-water (Sc = 60). For gases, values of Re from 2000 to 35,000 were covered and from 0.6 to 2.5 with gas pressures varying from 0.1 to 3 atm.

The equation which describes all the data for both liquid and gas flow is

(K_G d/DW (P_BM/P) (K_ld/D) = 0.023 Re^0.83 Sc^1/3

This empirical relation is quite remarkable in the manner in which it generally confirms the relationship between heat mass and momentum transfer developed theoretically. However, the evaporation of the volatile liquids in a wetted wall column results in the cooling of the liquids and consequent simultaneous heat transfer between liquid and gas. The heat transfer rates are given by the equation

hd / k = 0.023 Re^0.8 Pr^0.3

Owing to ripples and waves on the liquid surface.
Interphase Mass Transfer 

Equipment used: Wetted Wall column unit, Humidity meter

The material used: Water

EXPERIMENTAL PROCEDURE:

     1. Water is fed to the column at a rate at which complete wetting with a minimum of ripple formation is visible.

    2. The blower is started and the minimum flow of air is maintained.

   3. After about 5 minutes, when steady-state conditions reach, the humidity of air at the inlet and outlet is determined by the readings of the wet and dry bulb thermometers and by the use of a psychometric chart.

   4. Water flow rates and inlet and outlet temperatures are noted.

   5. The vapor pressure of water at different water temperatures is calculated.

  6. Calculations are made at different flow rates and values of Kg Vs N_Re are plotted on a log-log scale.

Here are the details of the observation table and the calculation format for your reference.

OBSERVATIONS:

S.No	Air flow rate from M2 Qa lpm	Water flow rate Qw lpm	Air temperature		Water temperature 0C	Air PD mm H20 across the orifice
			Inlet	Outlet	T1 T2	M1 M2
			Td1tw1	Td2 tw2
1
2

Length of the tube =

Diameter of tube = 

From the readings, T_d1, t_w1 and T_d2 , t_w2  and from a psychometric chart,

Partial pressure of water vapor at inlet = P1 KN/m2
Partial pressure of water at outlet         =   P_B KN/m2

DATA ANALYSIS:

Mean air pressure in column P_t = P_B *(DP1/2)X1000/13600*1.013*100/760 KN/m2

Air flow rate =
9.22 X SQRTDP2/10⁶ kgmoles/sec

Driving force at inlet of air= Dp_W1 = PW1 –pw1

 Where,

pw1 =pure component vapour pressure of water at outlet water temp T1

PW1 = Partial pressure of water at the bottom

Driving force at the outlet of the air

DpW1 = PW2 –pw2

where,

pw1 =pure component vapor pressure of water at outlet water temp T2

PW2= Partial pressure of water at the top of the column

DPwm =  (DpW1 - DpW2)/ ln (DpW1/DpW2)

Amount of water evaporated

NW = QA* (pw2/Pt – pw1/Pt)

In terms of mass transfer coefficient Kg, the rate of mass transfer is given by

 Nw = KG * A* DPwm  

where A = p DL m2 where d = i.d of column and L = Effective length of column

For each flow rate of air ,Kg can be calculated.

Pam = (Pt – pw1) – (Pt –Pw2)/ ln (Pt – pw1/ Pt –Pw2) is also calculated

To plot Gillard correlation, Kg * (d/DW)* (Pam/Pt) * (rADw/m_air) ^0.44

Where Dw  = diffusivity of water vapor in air = 0.13X10^-4 m²/sec

m_air =viscosity of air = 1.85X 10-3 kg/m/sec

Nre = dG/ mair   = d.Q_A.A_C/m_air  

Ac =cross section area of column

d = diameter of column

Kg * (d/DW)* (Pam/Pt) * (rADw/m_air) ^0.44   Vs N_re is plotted on a log –log sclae.

This value is compared with reported values.

RESULTS:

Reported value   =

Calculated value = 

Wetted Wall Column Set Up

The above setup is used to calculate the experimental mass transfer coefficient of the liquid and gas system at different temperatures and different flow rates; of course, the main application of the wetted wall is to determine the data of the gas and liquid mass transfer coefficient. 

A blower is provided with two valves at suction so that when the studies are focused on vapors of a chemical substance that are stored in a storage vessel and the vapors from the vessel are sucked by the blower and passed into the column or if the vapors are required to be mixed with air than an option at the discharge line of the blower which is facilitated with nozzle helps in mixing the vapor with air.

A heater is provided at the discharge line to supply heat to the vapor or air to maintain the temperature of the system and the whole pipeline is insulated to prevent loss, by using the rotameter, the flow rate of the vapor can be controlled.   

PSYCHROMETRIC
CHARTS

Application of wetted wall column:

Wetted wall column equipment, industrial application

The Heart of Thermal Phosphoric Acid Production: Wetted Wall Column Technology

Thermal phosphoric acid production relies on a pivotal component: the combustion chamber, intricately designed and integrated with a wetted wall column. This innovative setup facilitates the transformation of phosphorus into phosphoric acid, a crucial process in various industries.

The Wetted Wall Column: A Masterpiece of Engineering

The column's clever design enables efficient phosphoric acid production. Here's how it works:

Step-by-Step Process

1. Combustion Chamber: Phosphorus and air ignite, producing a hot gas mixture.

2. Column Inlet: The combusted mixture enters the top of the column.

3. Water Flow: A thin film of water cascades down the column's walls.

4. Reaction Zone: Phosphorus pentoxide reacts with water, forming phosphoric acid (H3PO4).

5. Product Collection: The resulting phosphoric acid collects at the column's base.

Key Benefits

1. Efficient heat transfer and reaction rate optimization.

2. Minimal energy consumption.

3. High-purity phosphoric acid production.

4. Compact design, reducing spatial requirements.

The Science Behind the Process

Phosphorus pentoxide (P2O5) reacts with water (H2O) to form phosphoric acid:

P2O5 + 3H2O → 2H3PO4 (Hydration reaction of phosphorus pentoxide)

Industrial Applications

1. Fertilizer production.

2. Pharmaceutical manufacturing.

3. Food processing.

4. Detergent industry.

The wetted wall column is the linchpin of thermal phosphoric acid production. Its ingenious design ensures efficient, high-quality acid production, making it an essential component in various industries