Dynamics and Control of Chemical Processes Degree in Chemical Engineering Unit 8. Control System Instrumentation

G784 – Dynamics and Control of Chemical Processes 1. Transducers and Transmitters

Figure 1. A typical process transducer

Transducers and Transmitters

- Fig. 1 illustrates the general configuration of a measurement transducer; it typically consists of a sensing element combined with a driving element (transmitter)

- Transducers for process measurements convert the magnitude of a process

variable (e.g., flow rate, pressure, temperature, level, or concentration) into a signal that can be sent directly to the controller

- The sensing element is required to convert the measured quantity, that is, the process variable, into some quantity more appropriate for mechanical or electrical processing within the transducer

G784 – Dynamics and Control of Chemical Processes 1. Transducers and Transmitters Transducers and Transmitters

- Before 1960, instrumentation in the process industries utilized pneumatic (air pressure) signals to transmit measurement and control information almost exclusively

- These devices make use of mechanical force-balance elements to generate signals in the range of 3 to 15 psig, an industry standard

- Since about 1960, electronic instrumentation has come into widespread use Sensors The main categories of measurements used in process control are: temperature, pressure, flow rate, liquid level, and composition

G784 – Dynamics and Control of Chemical Processes 1. Transducers and Transmitters Sensors Selection Criteria:

1. Measurement range (span).

The required measurement range for the process variable must lie entirely within the instrument´s range of performance

2. Performance. Depending on the application, accuracy, repeatability, or some other measure of performance is appropriate. For closed-loop control, speed of response is also important

3. Reliability. Manufacturers provide baseline conditions. Previous experience with the measurement device is very important

4. Materials of construction.

The instrument may need to withstand high temperatures, high pressures, and corrosive and abrasive environments. For some applications, seals or purges may be necessary

5. Prior use.

For the first installation of a specific measurement device at a site, training of maintenance personnel and purchases of spare parts might be necessary

6. Potential for releasing process materials to the environment.

Preventing exposure to fugitive emissions for maintenance personnel is important when the process fluid is corrosive or toxic. Sterility in bioprocesses must be maintained

G784 – Dynamics and Control of Chemical Processes 1. Transducers and Transmitters Sensors Selection Criteria:

7. Electrical classification. If the sensor is not inherently compatible with possible exposure to hazards, suitable enclosures must be purchased and included in the installation costs

8. Invasive or non-invasive. The insertion of a probe (invasive) can cause fouling, which leads to inaccurate measurements. Probe location must be selected carefully to ensure measurement accuracy and minimize fouling

G784 – Dynamics and Control of Chemical Processes Table 1. On-line Measurement Options for Process Control

Temperature

Flow

Pressure

Level

Composition

Thermocouple Resistance temperature detector (RTD) Filled-system thermometer Bimetal thermometer Pyrometer -total radiation -photoelectric -ratio Laser Surface acoustic wave semiconductor

Orifice Venturi Rotameter Turbine Vortex-shedding Ultrasonic Magnetic Thermal mass Coriolis Target

Liquid column Elastic element -bourdon tube -bellows -diaphragm Strain gauges Piezoresisitve transducers Piezoelectric transducers Optical fiber

Float-activated -chain gauge, lever -magnetically coupled Head devices -bubble tube Electrical (conductivity) Radiation Radar

Gas-liquid chromatography (GLC) Mass spectrometry (MS) Magnetic resonance analysis (MRA Infrared (IR) spectroscopy Raman spectroscopy Thermal conductivity Refractive index (RI) Capacitance probe Surface acoustic wave Electrophoresis Paramagnetic Chemi/bioluminiscence Tunable diode laser absorption

G784 – Dynamics and Control of Chemical Processes 1. Transducers and Transmitters Transmitters

- A transmitter usually converts the sensor output to a signal level appropriate for input to a controller, such as 4 to 20 mA

- Transmitters are generally designed to be “direct acting” - In addition, most commercial transmitters have an adjustable input range (or span) - For example, a temperature transmitter might be adjusted so that the input range of a platinum resistance element (the sensor) is 50 to 150 ºC

- In this case, the following correspondence is obtained:

Input

Output

50 ºC

4 mA

150 ºC

20 mA

- This instrument (transducer) has a lower limit or zero of 50 ºC and a range or span of 150 ºC – 50 ºC = 100 ºC

- For the temperature transmitter discussed above, the relation between transducer output and input is:

Km =

20 - 4 = 0.15 mA /º C 150 - 50

- The gain of the measurement element Km is 0.16 mA/ºC. For any linear instrument (Eq. 1):

Km =

range of instrument output output range = range of instrument input span

G784 – Dynamics and Control of Chemical Processes 1. Transducers and Transmitters Transmitters

Figure 2. A linear instrument calibration showing its zero and span

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements - Every process control loop contains

a final control element (actuator), the device that enables a process variable to be manipulated

- For most chemical and petroleum processes, the final control elements (usually

control valves) adjust the flow rates of materials, and indirectly, the rates of energy transfer to and from the process

Control Valves

- There are many different ways to manipulate the flows of material and energy into and out of the process; for example, the speed of a pump drive, screw conveyer, or blower can de adjusted

- However, a simple and widely used method of accomplishing this result with fluids is to use a control valve, also called an automatic control valve

- The control valve components include the valve body, trim, seat, and actuator Air-to-Open vs. Air-to-Close Control Valves

- Normally, the choice of A-O or A-C valve is based on safety considerations - We choose the way the valve should operate (full flow or no flow) in case of a transmitter failure or in an emergency situation

- Hence, A-C and A-O valves often are referred to as fail-open (FO) and fail-closed (FC), respectively

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements

Figure 3. A pneumatic control valve (air-to-open)

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements Control Valves

- The valve body contains an orifice that allows for the flow of liquids and/or gases - The trim modulates the flow rate and can be a plug, ball, disc or gate - The seat consists of protective material (typically metal or soft polymer) inserted around the orifice to provide a tight shutoff and to increase the life of the valve when corrosive or solid materials pass through it

- The actuator provides the force for opening and closing the valve Control Valves Dynamics

- It tends to be relatively fast compared to the dynamics of the process itself - The dynamic behavior of the control valve (and valve positioner) can be approximated by a first-order TF Gv(s) between the manipulated variable u(t) and the signal to the control valve p(t)

Kv U(s) = G v (s) = P(s) τ vs + 1 where τv << τp and τp is the largest process time constant

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements Example: Pneumatic control valves are to be specified for the applications listed below. State whether an A-O or A-C valve should be used for the following manipulated variables and give reason(s):

- Steam pressure in a reactor heating coil A-O to make sure that a transmitter failure will not cause the reactor to overheat, which is usually more serious than having it operate at too low a temperature

- Flow rate of reactants into a polymerization reactor A-O to prevent the reactor from being flooded with excessive reactants

- Flow of effluent from a wastewater treatment holding tank into a river A-O to prevent excessive and perhaps untreated waste from entering the stream

- Flow of cooling water to a distillation condenser A-C to ensure that overhead vapor is completely condensed before it reaches the receiver

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements Valve Positioners Pneumatic control valves can be equipped with a “valve positioner”, a type of mechanical or digital feedback controller that senses the actual stem position, compares it to the desired position, and adjusts the air pressure to the valve accordingly Specifying and Sizing Control Valves A design equation used for sizing control valves relates valve lift ℓ to the actual flow rate q by means of the valve coefficient Cv, the proportionality factor that depends predominantly on valve size or capacity (Eq. 2):

ΔPv q = C v × f ( ) × gS

- Here q is the flow rate, f(ℓ) is the valve characteristic, ΔPv is the pressure drop across the valve, and gS is the specific gravity of the fluid

- This relation is valid for nonflashing fluids - Specification of the valve size is dependent on the so-called “valve characteristic f” - Three control valve characteristics are mainly used

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements Specifying and Sizing Control Valves

- For a fixed pressure drop across the valve, the flow characteristic f(0 ≤ f ≤ 1) is

related to the lift ℓ(0 ≤ ℓ ≤ 1), that is, the extent of valve opening, by one of the following relations:

Linear : Quick opening : Equal percentage :

f = f =  f = R -1

where R is a valve design parameter that is usually in the range of 20 to 50

Figure 4. Control Valve characteristics

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements Rangeability The rangeability of a control valve is defined as the ratio of maximum to minimum input signal level. For control valves, rangeability translates to the need to operate the valve within the range 0.05 ≤ f ≤ 0.95 or a rangeability of 0.95/0.05 = 19 To Select an Equal Percentage Valve:

1. Plot the pump characteristic curve and ΔPS, the system pressure drop curve without the valve, as shown in Fig. 5. The difference between these two curves is ΔPv. The pump should be sized to obtain the desired value of ΔPv/ΔPS, for example, 25 to 33 %, at the design flow rate qd

2. Calculate the valve´s rated Cv, the value that yields at least 100 % of qd with the available pressure drop at that higher flow rate

3. Compute q as a function of ℓ using Eq. 2, the rated Cv, and ΔPv from (1.). A plot of the valve characteristic (q vs. ℓ) should be reasonably linear in the operating region of interest (at least around the design flow rate). If it is not suitably linear, adjust the rated Cv and repeat

G784 – Dynamics and Control of Chemical Processes 2. Final Control Elements To Select an Equal Percentage Valve:

Figure 5. Calculation of the valve pressure drop (ΔPv) from the pump characteristic curve and the system pressure drop without the valve (ΔPS)

G784 – Dynamics and Control of Chemical Processes 3. Dynamic Measurement Errors

Figure 6. Analysis of instrument error showing the increased error at low readings