Comparison of Valve Actuator Features

Spring and diaphragm
Advantages:-

• Lowest cost
• Ability to throttle without positioner Simplicity
• Inherent failure-mode action
• Low supply-pressure requirement
• Adjustability to varying conditions
• Ease of maintenance

Disadvantages:-

• Limited output capability
• Large size and weight

High-pressure spring and diaphragm
Advantages:-

• Compact, light weight 
• No spring adjustment needed 
• Costly cast components not needed 
• Inherent fall-safe action
• No dynamic stem seals or traditional stem connector block needed
• Design can include integral accessories

Disadvantages:-

• Requires high supply pressure 40 psig (2.8 bars) or higher
• Positioner required for throtting

Pneumatic piston
Advantages:-

• High force or torque capability 
• Compact, light weight
• Adaptable to high ambient temperatures 
• Fast stroking speed 
• Relatively high actuator stiffness

Disadvantages:-

• Fall-safe requires accessories or addition of a spring
• Positioner required for throttling
• Higher cost
• High supply-pressure requirement

Electric motor
Advantages:-

• Compact 
• Very high stiffness 
• High output capability 
• Supply pressure piping not required 

Disadvantages:-

• High cost
• Lack of fail-safe action
• Limited duty cycle
• Slow stroking speed

Electrohydraulic
Advantages:-

• High output capability 
• High actuator stiffness 
• Excellent throtting ability 
• Fast stroking speed 

Disadvantages:-

• High cost
• Complexity and maintenance difficulty
• Fail-safe action only with accessories

Vortex flow meter

·         Vortex flow meter operates on the physical principle of the Karman vortex street. When a fluid flows pass a bluff body, vortices are alternately forced on the side of that body and then detached or shed by the flow.

·         The frequency of vortex shedding is proportional to the mean flow velocity and therefore the volumetric flow (with Reynolds > 4000) St = Fd/V

·         Vortex frequency = St.v/d

                 St = strouhal number (dimensionless)

                 v = velocity fluid (m/s)

                 d = width of bluff body (m)

·         Alternating pressure change caused by the vortex are transmitted via lateral port (sensor) into the bluff body.

·         The sensor detects the pressure pulses and converts these into electrical signals.

·         They are typically available in flange size from ½ inch to 12 inches.

·         In gas services frequencies are about 10 times higher than in liquid applications.

·         The proportionally between object width (d) and vortex street wavelength {(l) – (lambda)} is called the “strouhal number” (S), approximately equal to 0.17

 ls = d             l» d/0.17

Type of vortex flow meter sensor  

        i.Thermal sensing

      ii.Mechanical sensor

    iii.Capacitive sensor

    iv.Piezoelectric sensor

      v.Strain gauge sensor

    vi.Ultrasonic sensor

 Thermal sensor

·         Thermostats are using (negative temperature co-effective)

Mechanical sensor

·         Also called shuffle ball sensor

·         A magnetic ball or disc moves from side to side due to vortices.

·         This movement is detected by a magnetic pick up.

·         The main problem of stream the movement of the ball or disc can be slowed by condensation.

Capacitive sensor

·         A stain less steel diaphragms are welded onto the side of bluff body and the assembled filled with oil and sealed.

·         Purring vortex shedding diaphragm deflects and transfers through the internal port from one side to the other.

·         When diaphragm deflects there is a change in the capacitance between the diaphragm and electrodes.

·         Capacitance is inversely proportional to the distance between the electrodes and directly proportional to the plate area.

·         Modern capacitive sensors use with superheated steam for temperature upto 427° C.

Piezoelectric sensor

·         Piezo element produces voltage output that is proportional to applied pressure.

·         Whilst piezo ceramic material produces a high output it’s hawing limited operating temperature range (about 250° C).

·         Lithium niobate (linbo3) piezoelectric material offers only medium output but operating temperature range is above 300° C.

·         These type sensors are unsuitable for temp. below -40° C

·         This sensor same like capacitive sensor.

Strain gauge sensor.

·         The vortex created by bluff body causes the body itself to be displaced by small amount of the order of 10µm.

·         This elastic movement can be detected using strain gauges attached directly or indirectly to the bluff body.

·         Movement of the body produces a change in resistance of the strain gauge.

·         Main drawback is upper temperature limitation of strain gauge (about 120° C).

Ultrasonic sensing

·         Ultrasonic transmitter and receiver placed behind the bluff body.

·         The vortex modulates the ultrasonic beam and the resultant output is the vortex signal.

·         This sensor system has a good turn down ratio.

·         The main problem associated with this technique is that extraneous sound sources can affect measurement.

            The majority of vortex meter use piezoelectric or capacitance type sensor. to detect the pressure oscillation around  the bluff body. 

The strouhal number and bluff body width and the cross sectional area of the flow meter are all constants (which is defined as “K”) the equation becomes

                  Q = F/K

“K” factor can be defined as pulses per unit volume such as pulses per gallons, pulses per liter, pulses per cubic feet, therefore one can determine flow rate by counting the pulses per unit time.

Vortex frequencies range from one to thousands of pluses per second depending upon the flow viscosity the character of the process fluid and the size of meter.

Hints of vortex flow meter:

·         The pipe Reynolds number should be above 30,000 minimum. This means vortex meters can only be used on low viscosity liquids. High viscous fluids (>3 pa.s (30cp)) and slurries are not recommended applications. (higher viscous having head loss)

·         The vortex shedding meter provides a linear digital output signal without the use of separate transmitter or converters.

·         There is no drift because this is a frequency system

·         The calibration of the meter is virtually independent of the operating conditions (viscosity, density, pressure, temperature and so on) whether the meter is being used on gas or liquid.

·         Low pressure (low density) gases do not produce a strong enough pressure pulse, especially if fluid velocity are low (if use the meter will be poor and low flows will not be measurable.

·         Vortex meter accuracy is based on the known value of the meter factor (K- factor).

Thermocouple Calibration

            A thermocouple can be any junction between two different metals and may be used to measure temperature. Each metal produces a different electrical potential that varies according to changes in temperature. This rate of change is different for each of the metals in the thermocouple, so a thermocouple produces a voltage that increases with temperature. You can calibrate a thermocouple by plotting the thermocouple's voltage-temperature curve.

Apparatus: 
Thermocouple; 
Temperature controllable bath (thermo bath)
Digital Thermometer;
Multimeter.

Procedure: 

Step 1
         Fill the thermo bath container with water and turn the thermo bath on. Heat the water to 30 degrees Celsius and turn the thermocouple device on. Connect each lead of the multimeter to one end of the thermocouple. This multimeter should be able to measure a voltage of 1 microvolt.

Step 2
         Place one junction of the thermocouple into the water and allow the voltage to stabilize. This occurs when the voltage stops fluctuating except for the last digit. Record the stable portion of the voltage from the multimeter.

Step 3
         Increase the water temperature to 35 degrees Celsius and record the stable voltage on the multimeter again. Repeat this procedure for each 5-degree increase in temperature from 35 to 60 degrees Celsius.

Step 4
         Measure the room temperature and look up the voltage for your thermocouple type at the room's temperature. For example, the voltage for a type K thermocouple at a temperature of 25 degrees Celsius is 1 millivolt. Add this value to each of the voltages you recorded in Steps 2 and 3.

Step 5
         Use the curve-fitting method of your choice to find the line that best fits your recorded data. The slope of this line provides the voltage increase for each degree of temperature increase. The voltage on a standard type K thermocouple should increase about 40 microvolts for every degree Celsius increase in temperature.

(As mentioned in the theory, the multimeter reading corresponds to the difference in temperature between the surroundings (room) and the bath. To calibrate the thermocouple, we have to take the room temperature into consideration to get the absolute value of temperature measured. Find the equivalent millivolt value for the room temperature from the corresponding Thermocouple table (E_RM). Then, add that millivolt value, corresponding to the room temperature, to every multimeter reading (E_MM). Tabulate the values.)

Observations (Type-K thermocouple used):
Room temperature : 20.5° C = 66.2° F; corresponding mV reading from tables E_RM = 0.818 mV

Bath Temperature,TB [° C]	Bath Temperature, TB [° F]	MM reading, EMM [mV]	Total emf, E = EMM+ ERM [mV]	Temperature corresponding to E, TTC [° C]	Temperature corresp. to E, TTC [° F]
30		0.361	1.179	29.4
35		0.579	1.397	34.8
40		0.788	1.606	39.9
45		0.995	1.813	44.9
50		1.202	2.020	49.9
55		1.407	2.225	54.9
60		1.613	2.431	59.8

Plot the measured bath temperatures values (T_B) on y-axis against the corresponding thermocouple emf (millivolt) values (E) on x-axis. Find the slope, intercept and the correlation coefficient of the curve-fitted line by any method. If the correlation coefficient is not very close to one, curve fit with higher order polynomial. Calculate the 95% uncertainty in temperature measurement of the calibrated thermocouple sensor if the obtained curve-fit correlation is used.

NOTE: 
The multimeter voltage reading should be zero when you measure room temperate without the reference junction (the thermocouple connected directly to the meter) Then the reference temperature is the same as measured temperature, both equal to the room temperature, therefore the temperature difference and generated voltage should be zeros. However, your reading will never be exactly zero due to many reasons:

1.     if the tip of the thermocouple is wet it may be sub-cooled due to evaporation;

2.     the room temperature is non-uniform;

3.     the meter is not perfect (make a short-circuit to check its zero),

4.     the measured circuitry may be picking up some "noise," since it acts as an antenna; etc.

Cantrol Valve Benchset and Stroking

Benchset are the actual pressure ranges for travel of the actuator with no friction. These are the actual maximum and minimum pressures that move the actuator from end to end of its range of operation. This is typically performed on the actuator before it is mounted onto the valve.

Stroking is the pressure range for the operation with friction. Stroking takes into account the added pressure required to overcome friction forces when the actuator is connected to the valve assembly.

When stroking a valve, two sets of pressure ranges are produced. There is one pressure range that actuates the valve from fully open to fully closed. The other range is generated from actuating the valve from fully closed to fully open.

The operational stroking range adds pressure and flow effects.

With any type of friction, the problems arise where the pressure to drive the device in one direction will be different to that required to drive it in the other direction. To move the valve in the positive direction, then the force has to exceed the benchset pressure by the amount of the friction. To drive the actuator in the opposite direction, the force has to go below the benchset pressure by the amount of the friction. This is the hysteresis and applies when friction is present.

The amount of friction determines the amount of variation from the benchset pressure range.

RTD Wiring Arrangements(2 Wire,3 Wire,4 Wire Systems)

RTD connection to a Wheatstone Bridge:

• Two-wire
• Three-wire
• Four-wire

Two-wire measurement:

         This is the most basic type of connection for an RTD device. It is used in very simple, cheap applications. They minimize cost at the expense of accuracy. 

         The main problem with two wire measurement is that there is no accounting for the resistance, or even change of resistance in the sensing leads. The measuring device cannot differentiate between the RTD resistance and lead resistance.

Three-wire measurement:

         Three-wire measurement with an RTD device balances the resistances in the lead wires within the bridge. Even though this is a simple modification to the two-wire device, it has the added cost of requiring three wires to obtain the measurement.

         The concept of operation is quite simple in that one lead is measured in the top half of the bridge, with the other lead in the bottom half. Since the sensing distance and other effects are the same, the lead resistance from both sensing leads cancel.

Four-wire measurement - Switched:

         One of the limitations with the three-wire measurement, is that if the lead resistance is not the same or suffer different effects, then the measurement will be erroneous. The Four-wire measurement takes both sensing leads into account and alternates the leads into the upper part of the bridge.        

           By alternating, the lead resistance is effectively measured in both sensing leads, but is then cancelled out by taking the average of the two readings. This level of complexity does make four-wire sensing more expensive.

control Valve Leakage Classifications

          In many process applications, it is important that the control valve be able to completely stop fluid flow when placed in the “closed” position. Although this may seem to be a fundamental requirement of any valve, it is not necessarily so. Many control valves spend most of their operating lives in a partially-open state, rarely opening or closing fully. Additionally, some control valve designs are notorious for the inability to completely shut off (e.g. double-ported globe valves). Given the common installation of manual “block” valves upstream and downstream of a control valve, there is usually a way to secure zero flow through a pipe even if a control valve is incapable of tight shut-off. For some control valve applications, however, tight shut-off is a mandatory requirement. For this reason we have several classifications for control valves, rating them in their ability to fully shut off. Seat leakage tolerances are given roman numeral designations, as shown in this table

Seat Leakage Classifications

There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2 2006 (European equivalent standard IEC 60534-4).

The most common used are

• CLASS IV
• CLASS Vl

CLASS IV is also known as metal to metal. It is the kind of leakage rate you can expect from a valve with a metal plug and metal seat.

CLASS Vl is known as a soft seat classification. Soft Seat Valves are those where either the plug or seat or both are made from some kind of composition material such as Teflon or similar.

Valve Leakage Classifications

Class I - Valve Leakage Classifications

Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made. Cass I is also known as dust tight and can refer to metal or resilient seated valves.

Class II - Valve Leakage Classifications

Intended for double port or balanced singe port valves with a metal piston ring seal and metal to metal seats.

• 0.5% leakage of full open valve capacity.
• Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 ^oF.
• Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

• Balanced, single port, single graphite piston ring, metal seat, low seat load
• Balanced, double port, metal seats, high seat load

Class III - Valve Leakage Classifications

Intended for the same types of valves as in Class II.

• 0.1% leakage of full open valve capacity.
• Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 ^oF.
• Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

• Balanced, double port, soft seats, low seat load
• Balanced, single port, single graphite piston ring, lapped metal seats, medium seat load

Class IV - Valve Leakage Classifications

Intended for single port and balanced single port valves with extra tight piston seals and metal to-metal seats.

• 0.01% leakage of full open valve capacity.
• Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 ^oF.
• Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

• Balanced, single port, Teflon piston ring, lapped metal seats, medium seat load
• Balanced, single port, multiple graphite piston rings, lapped metal seats
• Unbalanced, single port, lapped metal seats, medium seat load
• Class IV is also known as metal to metal

Class V - Valve Leakage Classifications

Intended for the same types of valves as Class IV.

• The test fluid is water at 100 psig or operating pressure.
• Leakage allowed is limited to 5 x 10^-4 ml per minute per inch of orifice diameter per psi differential.
• Service dP at 50 to 125 ^oF.

Typical constructions:

• Unbalanced, single port, lapped metal seats, high seat load
• Balanced, single port, Teflon piston rings, soft seats, low seat load
• Unbalanced, single port, soft metal seats, high seat load

Class Vl - Valve Leakage Classifications

Class Vl is known as a soft seat classification. Soft Seat Valves are those where the seat or shut-off disc or both
are made from some kind of resilient material such as Teflon. Intended for resilient seating valves.

• The test fluid is air or nitrogen.
• Pressure is the lesser of 50 psig or operating pressure.
• The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.

Typical constructions:

• Unbalanced, single port, soft seats, low load

                   It is from this leakage test procedure that the term bubble-tight shut-off originates. Class VI shut-off is often achievable only through the use of “soft” seat materials such as Teflon rather than hard metal-to-metal contact between the valve plug and seat. Of course, this method of achieving bubble tight shut-off comes at the price of limited operating temperature range and the inability to withstand nuclear radiation exposure.

Turbine flow meter:

• The flow of the liquid caused the rotor to spin at an angular velocity which is proportional to the velocity of the liquid. The speed of the rotor is detected by a pickup on the outside of the tube.
• The liquid may be swirling when its enter the meter which may speed up or slow down the meter. This can be corrected by using flow straightner (bundle of tubes element or radial vane elements). Straightening elements consist of a cluster of vanes or tubes inside a piece of straight pipe. This is placed at upstream of the meter. If a plain straight piece of pipe is before the meter, it should be about 20*diameters equivalent in length minimum.
• The vane is made of ferromagnetic material.
• A high frequency Ac voltage (10KHz) is applied to the coil.
• Generally turbine meters perform well, if the Reynolds number is greater than 4000 and less than or equal to 20000. Because viscosity, temperature variation affects accuracy.
• The turbine meter operating temperature ranges from -200 to 450 0C.
• Density changes do not greatly affect turbine meters.
• The number of pulses per unit volume is called the meters K-factor (pulse/gallon)
• Available in sizes from 5 to 600 mm.

Four main types of pickups:-

• Magnetic reed switch: magnets in the rotor cause the reed switch to on and off giving a digital output this device needs a power supply.
• Optical pickup (photo electric): A beam of light (usually infra red) is interrupted. ( the problem with this method is that the windows tend to become fouled, and so light transmission through the liquid is not to be recommended in many application).
• Inductive pickup: work on the principle of a moving magnet producing a flux which produces a voltage in an inductive pick up.
• Reluctive pickup: it works similar to inductive but the magnet is in the pickup ant rotor in magnetic.
• Radio frequency (RF): an oscillator applies a high frequency carrier signal to the coil in the pickup assembly, an passing of the rotor blades modulates the carrier.