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Valves, Instrumentation, and Control
Types of valves
Components of valves
Instrumentation: on-line meters
Process control review
Valves, Instrumentation, and Control Learning Objectives
At the end of this section, you should be able to…
Describe the major types of valves, their operation and characteristics, and draw their standard symbols on a P&ID.
Select an appropriate valve for a specific application.
Identify the function of the major components of a valve.
Select trim and K
List the four most commonly measured variables and draw the symbols for those meters on a P& ID.
Describe the principals of measurement.
Define the variables involved in an PID loop and draw appropriately on a P&ID.
Define tuning.
List the common process problems that lead to unstable PID loops.
References
Chapter 15 in Turton et al., Analysis, Synthesis, and Design of Chemical Processes.
Process Engineers: Take Control! Cecil L. Smith. Chemical Engineering Progress. August 2000.
Valve Types
Ball Valve
Butterfly Valve
Gate Valve
Globe Valve
Check Valve
Parts of a Valve
3. End fitting: must be specified when buying the valve - butt weld end, compression flange, pipe thread, quick disconnect
4. Material: closure member, housing, seat – stainless steel
5. Packing/seals: seals stem, replaced
6. Seat: where the closure members seals against the valve housing
Ball Valve
Gate Valve
Butterfly Valve
Check Valves
Globe Valve
Rupture Disk (not a valve – ruptures at a set pressure)
Valve Symbols
Instrumentation- Form groups of two.- List as many process conditions that can be measured on-line as you can.- List the four most common.
Instrumentation
Most commonly measured process conditions: temperature, level, pressure, flow rate, pH.
Other variables include composition, moisture, specific gravity, viscosity, dissolved oxygen.
Temperature
Thermocouple
Two wires of different metals welded together at the end
When the junction is heated a small electric current proportional to the junction temperature is generated
Types of thermocouple refer to the types of metal – common J,H, K
Thermowell is a sleeve that the thermocouple sits in
Issues – low current, fouling of thermowell
Thermister/Resistive thermal detectors (RTD)
Consists of a resister that have a high coefficient of resistance
Resistance = f(temperature)
Flow Rate
Historically the most common method to measure flow rate is the orifice plate.
However, more sophisticated methods are becoming more common because they have less pressure drop.
Orifice Meter
Calculating pressure drop across and orifice meter
P (psi) = K*(/1726)(velocity2)
K=orifice coefficient usually 0.6-0.8
=density (lb/ft3)
velocity=velocity through the orifice ft/s
for estimation use orifice ½ of the pipe diameter
Example: assume 4 inch line, 200 gpm, orifice diameter = 2 in, orifice radius = 1 in = 0.083 ft
velocity=(200gal/min)(ft3/7.48gal)(min/60 sec)(1/pi(0.0832ft2)=20.6 f/s
P (psi)=0.7*(62.4)(20.62)/1726=10.7 psi
Process Control: A Review
Assignment: Read the Smith article and Turton’s chapter 15 about process control.
Process Upsets
T, F, reaction rate, P etc. may change and effect product quality. These variables may change due to equipment failure, intentional production rate changes, cooling water changes, etc. These changes are called process upsets.
Controlled Variables
Controlled variables are variables that we want to maintain at constant or specified values (T, P, flow rate, level, etc.).
Manipulated Variables
Manipulated variables are variables that we intentionally change to maintain our controlled variable at a constant value. We often manipulate the values by opening or closing a valve.
Measured Variables
Measured variables are variables that we measure with a meter (often often the controlled variable or a variable that we use to calculate the controller variable).
Controller
A unit that reads an INPUT signal and a SET POINT, compares the two, performs a calculation on the difference, and sends out an OUTPUT signal.
Common type of controller: PID = proportional, integral, derivative
Controller
Controller Operation
If the INPUT is different than the SET POINT, the OUTPUT signal is changed.
If the INPUT signal is the same as the SET POINT, the OUTPUT signal remains the same.
Types of Control:Feed Back
When the controlled variable changes or is different than the set point, the controller adjusts the manipulated variable to bring the controlled variable back to the set point value.
The controlled variable must change (be different than the set point) for control action to be taken.
Feed Forward
The measured variables are used to change manipulated variables before changes in the controlled variable takes place. Especially useful when there are long lag times in the process.
All factors likely to cause a change in the controlled variable must be taken into account.
Some type of model of process behavior/dynamics must be known, and the accuracy of control is directly linked to the accuracy of control.
Cascade
Uses controllers in series. The first controller’s output is the set point to the second controller.
Reduces lags and allows finer controls, but is more complicated than single loops.
Distillation Control
Purity of distillate will decrease and amount in overhead will decrease.
Valve CV-1 will open to increase reflux rate in response to concentration change.
Level in reflux drum will drop, so CV-3 will close to reduce overhead flow rate.
Pressure will decrease causing CV-2 to close and cooling water flow rate to decrease.
Level and purity in the bottoms will increase causing CV-5 to open increasing bottoms flow, and CV-4 will close reducing steam
Control Exercise
Form groups of two students
Develop a feedback control loops.
Draw your PFD with control loop on the board.