Specifying control valves and control system achitecture Flashcards
Control the flow
Control valves
Ensure there is no backflow
Non return valves
Perform a safety function
Safety critical valves
Signal from the controller
4-20mA analogue
The part that modulates (obstructs) the flow rate with either linear or rotary motion
Valve plug/disc
Pneumatic, hydraulic or electrically powered device that supplies force and motion to open or close a valve
Actuator
The pressure containing part of the valve. Contains the components that contact the fluid
Body
The connector from the actuator to the inside of the valve - transmits force to move the plug/disc
Stem
Closure for valve body, through which the stem passes
Bonnet
The surface that the valve plug contacts when the valve is closed, thus forming the seal
Seat
Surrounds the plug and has openings that determine flow
Cage
Specifying a control valve
- Type of fluid (Viscosity, SG of fluid)
- Maximum and minimum flow-rate
- Input P at maximum and minimum flowrates
- Pressure drop at specified max and min flowrates
- Valve body and seal material (contact materials)
- Valve type (globe, diaphragm, gate, or pinch)
- Valve flow characteristic
- Fail position. (fail closed, open or in position)
- Mechanical stops
- Temperature of fluid
- Connections (Inlet and outlet pipe size)
- Certification required
- Type of seals
- Tagging requirements
- Named manufacturer
- Manual, air actuated (3-15 psig), 4-20 mA, Hart
Used to convert a signal in electrical current to a pneumatic (pressure) signal
Normal range of pneumatic pressure is 3-15psig
I/P (current pressure) converters
The valve is closed as pressure is applied
Normally open/fail open if air fails
Direct acting
The valve is opened as pressure is applied
Reverse acting
Normally closed/fail closed if air fails
Used by most control valves which is spring loaded. Air displaces it in proportion to the air supply pressure
For large valves considerable forces
are sometimes required. In this
case, higher pressures can be used
with an I/P or P/P converter used
Pneumatic diaphragm
For rotating valves such as butterfly valves
Rotary control valves
- Pneumatically controlled piston for large vales that require greater force to move
Piston actuator
Eliminate the need for a compressed air system and avoid inefficiencies due to frictional losses in transmission. They can be more reliable, require less maintenance and be more tolerant to environmental temperature and pressure variations. They offer superior control and are very easy to integrate into new control systems.
Fewer limitations on size of valve
Electric Control Valves
For diaphragm actuators are often used as adjustable travel stops. They also provide a ready means of positioning the control valve in emergency
Handwheels
Closing of the valve moves the system curve … the Head vs Flow curve
up
Opening of the valve moves the system curve … the Head vs Flow curve
Down
Three different “speeds” of control valve opening
- Quick opening
- Linear
- Equal percentage
Causes the difference in valve opening
- shape of the plug
- seat arrangement
- Provides a large part of its flow capacity with relatively small lift as the valve opens
- Slower thereafter
Quick opening
Valve plug is shaped so that flowrate is directly proportional to the valve lift (% travel)
Linear
Valves have a plug shape so that each increment in valve lift (% travel) increases the flowrate by a certain percentage of the previous flow
Rate of change in flowrate increases with valve opening
Equal percentage
Normally the valve only controls the flow rate correctly at openings of between ….. of the fully open position.
20-70%
Ratio of normal maximum operating flow to minimum controllable flow
Turndown ratio
Important factor used for sizing valves
Cv of the valve
Cv = 1.16Q √ SG/ Delta P
SG = specific gravity of the fluid
By using this, the valve stem travel is no longer a linear function of the pressure which changes the valve opening characteristic
Control valve cams
Non return valve types
- Swing check valve
- Wafer type check valve
- Disc check valve
- Flap type check valve
- Spring lift check valve
- Lift check valve
Types of safety critical valves
- Block valves
- Emergency Block Valves
- Zone valves
- Fail safe valves
- ESD valves
- Excess flow valves
Separate one item of equipment and seals against liquid flow
Block valves
Separate one item of equipment from another
during fire or other emergencies. These are usually arranged as fail safe emergency block valves (FSEBV)
Emergency Block Valves (EBV)
Isolate units; generally located at the end of pipe racks and at ground level. Also known as battery limit valves.
Zone valves
Fail in a fully opened or fully closed position upon loss of power. The correct failsafe position must be determined by process hazard evaluation
Fail safe valves
Ball valves that close rapidly; and seal in
both the up and down stream directions. Same as block or zone valves. Important for SIL
ESD valves
Close automatically if fluid flow rates exceed a critical value. The critical flow rate of excess flow valves depend upon the physical characteristics of the fluids as well as flow rates
Excess Flow Valves
