Why Gap voltage is setting at -10V in Bently Nevada vibration probe

 Gap voltage of Bently Nevada vibration probe is usually set at -10V. Reason for setting at -10 volt is as we know, proximitor allows probe to work from -2 volt to -18 volt.

So voltage difference of 2 to 10 and 10 to 18 is 8 volt. When setting at -10 volt, probe can equally measures vibration at both sides (I mean when shaft is going away from probe or towards probe).

Suppose we adjust probe at -9 volt. Probe can detect vibration?

Yes, it can can detect vibration but its one side will be margin of 7 volt and other side margin of 11 volt.

At 11 volt margin, probe will detect vibration excellent but at other side,7 volt side, it has limitation, when more vibration it may not detect.

RTD Temperature to Resistance calculation formula with Example

RTD

RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its temperature changes. The resistance increases as the temperature of the sensor increases. The resistance vs temperature relationship is well known and is repeatable over time. An RTD is a passive device. It does not produce an output on its own. External electronic devices are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage

Resistive Temperature Detectors (RTDs) relate resistance to temperature by the following formula:

   RT = Rref[1 + α(T − Tref)]

Where,

RT = Resistance of RTD at given temperature T (ohms)
Rref = Resistance of RTD at the reference temperature Tref (ohms)
α = Temperature coefficient of resistance (ohms per ohm/degree)

Example:

The following example shows how to use this formula to calculate the resistance of a PT100 RTD with a temperature coefficient value of 0.00392 at a temperature of 35 degrees Celsius:

Assuming Temperature Reference = 0 Degrees

For PT100 RTD the Rref = 100

RT = 100 Ω[1 + (0.00392)(35 − 0)]

RT = 100 Ω[1 + 0.1372]

RT = 100 Ω[1.1372]

RT = 113.72 Ω

For Temperature to Resistance conversion also the same above formula applies.

The above given is a basic equation only for RTD calculation.

Note:

1.The above RTD calculation tool designed for a standard PT100 sensor.

Electromagnetic flow Meter

 

PRINCIPLES

Electromagnetic flow meters detect flow by using Faraday's Law of induction.

According to this principle, when a conductive medium passes through a magnetic field B, a voltage E is generated which is proportional to the velocity v of the medium, the density of the magnetic field and the length of the conductor.

Inside an electromagnetic flow meter, there is an electromagnetic coil that generates a magnetic field, and electrodes that capture electromotive force(voltage). Due to this, although it may appear as if there is nothing inside the flow pipe of an electromagnetic flow meter, flow can be measured.

There is one important point when using electromagnetic flow meters. Because electromagnetic flow meters are based on the laws of electromagnetic induction, conductive liquids are the only liquids for which flow can be detected. This electromagnetic flow meter being non intrusive type, can be used in general for any fluid which is having a reasonable electrical conductivity above 10 microsiemens/cm.

Under Faraday's law of induction, moving conductive liquids inside of a magnetic field generates an electromotive force (voltage) in which the pipe inner diameter, magnetic field strength, and average flow velocity are all proportional. In other words, the flow velocity of liquid moving in a magnetic field is converted into electricity. (E is proportional to V × B × D)

As the flow changes, the electromotive force (voltage) captured by the electrodes changes as follows.

Instrumentation Codes

CODES AND STANDARDS FOR CONTROL AND

INSTRUMENTATION (Part-1)

Temperature Measurements

1. Instrument and apparatus for temperature measurement - ASME PTC 19.3

(1974).

2. Temperature measurement - Thermocouples ANSI MC 96.1 - 1982.

3. Temperature measuremnet by electrical Resistance thermometers - IS:2806.

4. Thermometer - element - Platinum resistance - IS:2848.

Pressure Measurements

1. a) Instruments and apparatus for pressure measurement - ASME PTC

19.2 (1964).

b) Electonic transmitters BS:6447.

2. Bourdon tube pressure and vacuum gauges - IS:3624 - 1966.

3. Process operated switch devices (Pr. Switch) BS-6134.

Flow Measurements

Instruments and apparatus for flow measurements - ASME PTC 19.5 (1972) Interim

supplement, Part-II.

Measurement of fluid flow in closed conduits - BS-1042.

Electronic Measuring Instrument & Control Hardware/ Software

1. Automatic null balancing electrical measuring instruments - ANSI C 39.4

(Rev. 1973): IS:9319.

2. Safety requirements for electrical and electronic measuring and controling

instrument - ANSI C 39.5 - 1974.

3. Compatability of analog signals for electronic industrial process instruments -

ISA - S 50.1 (1982) ANSI MC 12.1 - 1975.

4. Dynamic response testing of process control instrumentation ISA - S 26 (1968)

pH Meter Working Principle, Calibration Procedure, Standard Solution Details

pH METER PRINCIPLE:

A pH meter is a scientific instrument that measures the hydrogen-ion activity in solutions, indicating its acidity or basicity (alkalinity) expressed as pH value. The principle of pH meter is the concentration of hydrogen ions in the solution e.g. it is the negative logarithm of an hydrogen ion. The pH range of solutions varies between 1 to 14, where 1 is the highest in acidic nature, and 14 is the highest in alkalinity.

Potentiometeric pH meter:

The pH meter measures the difference in electrical potential between a pH electrode and the reference electrode, that is why the pH meter is sometimes referred to as a “potentiometric pH meter”.

pH meter diagram:

Calibration of pH Meter:

Calibration of pH meter is a very important function that should be performed every day before performing any test on the pH meter.

Preparation of Standard Buffer

Buffer Solution pH 4.00 (20⁰C) Transfer the content of buffer capsule or tablet pH 4.00 into a 100 ml volumetric flask. Dissolve in about 80 ml of purified water, make up the volume to 100 ml with purified water & mix.

Buffer Solution pH 7.00 (20⁰C) Transfer the content of buffer capsule or tablet pH 7.00 into a 100 ml volumetric flask. Dissolve in about 80 ml of purified water, make up the volume to 100 ml with purified water & mix.

Buffer Solution pH 9.20 (20⁰C) Transfer the content of buffer capsule or tablet pH 9.2 into a 100 ml volumetric flask. Dissolve in about 80 ml of purified water, make up the volume to 100 ml with purified water & mix.

Calibration of pH meter:

Operate the pH meter and electrode system according to the manufacturer’s instructions or according to the applicable SOPs. All measurements should be made at the same temperature of 20° to 25°. The apparatus is calibrated with the buffer solution of potassium hydrogen phthalate (primary standard) (buffer pH 4.0) and one other buffer solution of different pH, preferably buffer pH 9.2. The measured pH of a third buffer pH 7.0 must not differ by more than 0.05.

Calibration Procedure:

·         The instrument is calibrated to pH 4, 7 or 9.2 but remembers to calibrate pH 7 first.

·         Dip the electrode in standard Buffer Solution of 7.00 pH value. 

·         Measure the temperature of the solution and place the temperature knob accordingly.

·         Bring the Function Switch of pH Mode.

·         Adjust the “Calibrate” control so that the display reads 7.00

·         Now again turn the Function Switch into Standby Mode.

·         Remove the electrode from 7 pH buffer solution and wash it with distilled water, soak & dry it.

·         Put the electrode in 4 pH buffer solutions.

·         Bring the Function Switch in pH Mode and Adjust the “Slope %” (Right side of the instrument) so that the display reads 4.00.

·         Remove the electrode from 4 pH buffer solution and wash it with distilled water.

·         Always keep the Function Switch at standby Mode after measuring the pH value.

Procedure and operation of pH Meter

·         Ensure the temperature of the Liquid being examined to 200-250C.

·         Immerse the glass electrode in the liquid to be examined.

·         Turn off the knobs to pH Checking & note.

·         When measuring the pH above 10, ensure that the electrode is suitable for use under alkaline conditions & apply any correction that is necessary.

·         Record the pH of the solution used to standardize the meter and electrodes at the end of a set of measurements. If the difference between this reading and the original value is greater than 0.05, the set of measurements must be repeated.

Types of pH meters

1.    Traditional pH Meter

2.    pen-like devices

3.    pH strips

4.    Holographic pH sensors

5.    Solid-state electrodes pH Meter

6.    Voltmeter display device.:

Frequently Asked Questions:

What is pH meter and what is it used for?

A pH meter is a device used for measuring the pH value of a solution. It consists of an electrode and voltmeter.

What is principle of pH meter ?

A pH meter is a scientific instrument that measures the hydrogen-ion activity in solutions, indicating its acidity or basicity (alkalinity) expressed as pH. The principle of pH meter is the concentration of hydrogen ions in the solution.

What is the best pH meter?

There are lots of pH meters available in the market in India, the best pH meter may depend on the usage of every individual, Somewhere a pen pH meter is best in use, whereas sometimes conventional pH meters is more suitable. some good examples of pH meters are as:-
Bluelab pH Pen Pocket Tester.
Hanna Instruments HI 9813-6N pH/EC/TDS Meter.
Bluelab Combo pH Meter.
Apera Instruments AI311 Premium pH Test Kit.
Oakton EcoTestr Pocket pH Meter.

What is full form of pH?

The full form of pH is the Potential of Hydrogen. It is a scale used to observe the acidity and alkalinity of a solution. The acidic solution has a higher number of hydrogen ion H+ and low pH.

What are the types of pH meter?

Traditional pH Meter
pen-like devices
pH strips
Holographic pH sensors
Solid-state electrodes pH Meter
Voltmeter display device.

What is the ph Meter definition?

Ph meter is scientific instrument used to measure the pH value of a solution. It measure the pH value by passing electric current through the measuring solution, and the electrode registers the activity of electrons and protons presents in the solution that posseses the pH value in number from 1 to 14.

What is digital pH meter?

Digital pH meter is broad term to define a class of pH meter. Those pH meters which shows the pH value instantly after dipping its knob in solution are called digital Ph Meter, they may digital ph meter with electrode or portable dry pH meters

Why P is small in pH value?

p is in the small letters because it refers to a word that is power, and H is in capital letters that refers to molecule Hydrogen. The pH term arrived from a French term puissance d’Hydrogen that means “power of Hydrogen” ion. In another theory, “pH” stands for the Latin terms pondus hydrogenii (quantity of hydrogen) or potentia hydrogenii (power of hydrogen).

SUMMARY:

pH meter is an essential part of the chemical industry, Food Industry, Pharmaceutical Industry, and Agriculture sector. It allows us to know the nature of the solvent and therefore provides insights to take further actions accordingly. If a product has low pH then the manufacturer can raise its pH with some alkali solutions. However, the pH meter looks like a very simple scientific instrument but its functionality puts it at the top position of Chemio-physics instruments. pH meter is a physics instrument but its working is chemistry oriented.

 

Instrumentation P&ID symbols

Instrumentation P&ID symbols

What is P&ID?

Piping and instrumentation diagrams, or P&IDs, are used to create important documentation for process industry facilities. The shapes in this legend are representative of the functional relationship between piping, instrumentation, and system equipment units. We've broken them down into seven main groups: equipment, piping, vessels, heat exchangers, pumps, instruments, and valves.

Equipment symbols

Equipment is comprised of miscellaneous P&ID units that don't fit into the other categories. This group includes hardware like compressors, conveyors, motors, turbines, vacuums, and other mechanical devices.

Piping symbols

A pipe is a tube that transports fluid substances. Piping can be made of various materials, including metal and plastic. The piping group is made up of one-to-many pipes, multi-line pipes, separators, and other types of piping devices.

Vessel symbols

A vessel is a container that is used to store fluid. It may also alter the characteristics of the fluid during storage. The vessels category includes tanks, cylinders, columns, bags, and other vessels.

Heat exchanger symbols

A heat exchanger is a device that's designed to efficiently transfer heat from different areas or mediums. This category includes boilers, condensers, and other heat exchangers.

Pump symbols

A pump is a device that uses suction or pressure to raise, compress, or move fluids in and out of other objects. This section is comprised of both pumps and fans.

Instrument symbols

An instrument is a device that measures—and sometimes controls—quantities such as flow, temperature, angle, or pressure. The instruments group houses indicators, transmitters, recordings, controllers, and elements.

Valve symbols

A valve regulates, directs, or controls the flow of a fluid by opening, closing, or partially obstructing passageways in a piping system.This category includes rotameters, orifices, and other types of valves.

Control valves Interview questions and answers

1. Control valves Interview questions and answers

   Question1. Why Do Different Control Valves Have Different Characteristics?

   Answer :
   • Some valves have an inherent characteristic that cannot be changed, such as full port ball valves and butterfly valves. For other valve types, such as globe, the characteristic can be changed to suit the application.
   • Ideally the inherent valve characteristic should be chosen to give an installed characteristic as close as possible to linear (see inherent vs installed characteristic). This enables the loop to remain tuned at all conditions with the same calibration settings.

2. 
   

3. Question2. Definition Of Linear And Equal Percentage Characteristic?

   Answer :
   Linear – For equal stem movements the change of flow resulting from the movement is constant throughout the stroke.
   Equal Percentage – For equal stem movements the change of flow resulting from the movement is directly proportional to the flow rate immediately before the change took place.
   Besides the loop gain and installed characteristic considerations, equal percentage valve trim will generally give better rangeability and better control at low flow rates. Linear trim will give better control at flow rates over 50% of the valve capacity. 

4. Question3. What Is The Trim In A Control Valve?

   Answer :
   The trim consists of the parts of the valve that affect the flow through the valve. In a standard globe valve the trim would just be the plug and seat. In a special valve the trim would consist of the plug, seat and retainer (or disk stack).

5. Question4. Why Is Reduced Trim Required In Control Valves?

   Answer :
   • Control valves are sized according to the application requirements and must satisfy both Cv and velocity criteria.
   • Reduced trim is used where it is necessary for the valve to have a Cv capacity smaller than the maximum possible in that size of valve.
   • The most common reason for reduced trim is that the flow rate is low for the size of valve required – particularly where 25mm valves have been specified as the smallest size to be used. Some plants stipulate that no control valve should be less than two sizes smaller than the line size, other that the valve should not be less than half the line size.
   • The second reason is that on high pressure drop gas or vapour applications the valve invariably is sized on the outlet port velocity limits and the Cv required is much less than the full bore Cv.

6. Question5. What Is Meant By Critical Pressure And Critical Temperature?

   Answer :
   Critical temperature is that above which a fluid cannot be liquefied by pressure alone. Critical pressure is the equilibrium or vapour pressure of a fluid at its critical temperature.

7. 1. Question6. Are Safety Valves, Regulators And Isolating Valves All Examples Of Control Valves?

      Answer :
      Normally the term control valve is used to describe a valve that controls flow with an externally adjustable variable restriction. Safety valves and isolating valves should not be referred to as control valves without a qualifier such as safety control valve or on/off control valve. Regulators should be referred to as self-regulating control valves to avoid confusion.

   2. Question7. Is Flow Through A Control Valve – Turbulent Or Laminar?

      Answer :
      • Flow through control valves is almost always turbulent.
      • Laminar flow takes place with liquids operating at low Reynolds numbers. This occurs with liquids that are viscous, working at low velocities. Laminar flow in gases and vapours very seldom will be experienced in process plants.

   3. Question8. What Is Cavitation?

      Answer :
      Cavitation is a condition that occurs in liquid flow where the internal pressure of the liquid, at some point falls below the vapour pressure and vapour bubbles form and at some other point downstream rises above the vapour pressure again. As this pressure recovers so the bubbles collapse, and Cavitation takes place
      It is possible to predict where cavitation will occur by looking at the pressure conditions and the valve recovery factor. However, it is important to recognise that the damage that occurs is dependent on the energy being dissipated and is thus flow dependent.Cavitation sounds like stones passing through the valve.

   4. Question9. What Effect Does The Positioner Cam Have On A Valve Characteristic?

      Answer :
      The feedback cam in the positioner controls the relationship between the control signal and valve position. With a linear cam at 50% signal the valve will be 50% open.
      It is possible to alter the apparent characteristic of a valve by changing the shape of the cam e.g. for a ball valve that has an inherent equal percent character it is possible to make it appear linear so that the flow rate through the valve at 50% signal is half of the maximum flow – the valve will however only be 25% open to achieve this result.
      From the control point of view there are advantages in doing this, but changing the valve characteristic and keeping the linear cam in the positioner is a better technical solution if it is possible.

   5. Question10. What Is Flashing?

      Answer :
      Flashing is a condition that occurs with liquid flow where the pressure falls below the vapour pressure and remains below it. There are then two phases flowing (i.e. liquid and vapour) downstream.
      Severe damage can occur inside a valve due to erosion caused by the impact of liquid droplets travelling at high speeds.

   6. 1. Question11. What Is Choked Flow?

         Answer :
         • Choked flow (otherwise known as critical flow) takes place in a valve when an increase in pressure drop across the valve no longer has any effect on the flow rate through the valve. It occurs when the velocity of the gas or vapour reaches sonic (Mach 1) at the vena contracta.
         • Choked flow is not necessarily a problem in valves but does need to be taken into account in the Cv calculations. For liquids, choked flow indicates the onset of full cavitation, which usually requires special steps to be taken to reduce damage.
         • With clean gases there is no problem with choked flow. Use the choked pressure drop in any equation to calculate Cv or flow rates. High noise levels may be generated.
         • Solid particles in gas flow will cause erosion due to the high velocities involved. With liquids full cavitation will occur when the flow is choked.
         • High recovery valves, such as ball and butterfly, will become choked at lower pressure drops than low recovery valves such as globe which offer a more restricted flow path when fully open.

      2. Question12. How Can Cavitation Damage Be Contained?

         Answer :
         Three methods exist for treating cavitation in control valves – the first is to ensure that the plug and seat are made of a material that can resist the damage (e.g. stellite hard facing). The second is to control where the bubbles collapse and keep this away from vulnerable components (see Cav Control trim). The third is to control the pressure drop and velocities to ensure that the liquid pressure does not fall below the vapour pressure – thus eliminating cavitation altogether.

      3. Question13. How Can Flashing Damage Be Contained?

         Answer :
         Flashing cannot be eliminated in the valve – if the downstream pressure is less than the vapour pressure then flashing will occur.
         To minimise the damage:-
         • Hard face trim (using hard facing materials such as Stellite, or Tungsten Carbide)
         • Use more erosion resistant body material
         • Increase size of valve, thus reducing the velocity
         • Use angle valve – flow over plug

      4. Question14. Definition Of Linear And Equal Percent Characteristics?

         Answer :
         Equal Percent characteristics.
         The change of flow resulting from a fixed increment of valve travel is directly proportional to the flow immediately before the change took place.
         Linear characteristics.
         The change in flow resulting from a fixed increment of valve travel is constant throughout the whole stroke.
         General rules.
         • Use Equal Percent if in doubt.
         • Use Linear for level control.
         • Use Equal Percent for pressure control.
         • Use Linear when the pressure drop across the valve is a large proportion of the total pressure drop.

      5. Question15. How Is The Characteristic Determined In A Globe Valve?

         Answer :
         There are several ways of altering the characteristic in a globe valve depending on the particular design.
         • The most common is to use the profile on the front of the plug head. In this case the seat ring and retainer are not changed. If the plug is cage guided the characteristic of the valve is usually determined by the retainer or disk stack with the plug having a flat face. As the plug moves up, it uncovers more flow paths.
         • A series of small holes at the bottom of the retainer with larger holes at the top will give a bi-linear characteristic, which can be designed to give results similar to equal percent.

      6. Question16. Is The Velocity Of A Fluid In A Control Valve Critical?

         Answer :
         The velocity is one of the more important considerations in sizing a control valve. For long life on liquid applications the velocity at the exit of the valve body should be less than 10 m/s. This compares with generally accepted line velocities of about 3 m/s, which explains why control valves often are smaller than the line size.
         On gases and vapours the velocity at the exit of the valve body should be less than 0.33 Mach (1/3rd of sonic) for noise control valves and less than 0,5 Mach where noise is not a consideration.

      7. Question17. What Is The Difference Between A Liquid, A Vapour And A Gas?

         Answer :
         These are all different states or phases in which a fluid can exist. H20 exists as a solid (ice), liquid (water), vapour (saturated steam), and a gas (superheated steam) – it depends on the temperature and pressure which phase is current. Practically the most significant difference between liquids and vapours/gases is the compressibility. Liquids are for most practical purposes incompressible where as the density of gas and vapours varies with pressure.

      8. Question18. What Is A Desuperheater And How Does It Differ From An Attemporator?

         Answer :
         • A desuperheater is a device that is used to control the addition of water to superheated steam to reduce the temperature to within 10°C of saturation.
         • An attemporator also adds water to steam to control its temperature but the set point temperature is higher and the downstream steam is still superheated.
         • Generally desuperheaters are used in process plants where the steam is used for heating. Attemporators are used more in power stations for interstage temperature control.

      9. Question19. What Is The Difference Between Installed And Inherent Characteristics?

         Answer :
         The inherent characteristic is a plot of the flow rate through a valve (or Cv) against percentage opening with a constant pressure drop across the valve.
         This is the result of a workshop test where the upstream and downstream pressure are held constant and the only variables are the flow rate and opening of the valve.
         The installed characteristic is the plot of flow against opening using actual pressure drops experienced in practice. Due to the fact that in most applications the pressure drop increases as the flow rate drops, the installed characteristic will normally change from =% towards linear, and from linear towards quick opening.

      10. Question20. Why Are Control Valves Sometimes Very Noisy?

          Answer :
          Noise is created by an object vibrating. Valve components will tend to vibrate whenever they are subjected to high velocity turbulent flow. Standard control valves will therefore tend to be noisy on high pressure drop applications particularly where flow rates are high, since the low pressure experienced downstream of the seat ring (at the vena contracta) is accompanied by very high velocities reaching as high as the speed of sound. Special low noise valves are designed to drop pressure gradually so that velocities are controlled at low levels.

      11. Question21. Can Two Control Valves Be Used In Series In High Pressure Drop Applications?

          Answer :

          Dropping the pressure across two valves rather than one is theoretically better. However, in practice, the two valves will not usually control well together unless the process can operate with a very low proportional band with slow response times.

          A better, and usually less expensive approach is to use a valve that is designed with multiple pressure drop restrictions inside the trim.

      12. Question22. Can Two Control Valves Be Used In Parallel To Handle High Turndown Applications?

          Answer :

          Two valves in parallel working on split range signals can give very high turndown capability. The situation that should be avoided if possible is that the larger valve operates in the “cracked open” position – one way to avoid this is to program the PLC or DCS to shut the small valve and use only the larger unit once the capacity of the small valve is exceeded.

          An alternative to two valves in parallel is to select a valve with a high rangeability such as a vee-ported ball valve.

      13. Question23. What Is The Difference Between Rangeability And Turndown?

          Answer :

          Generally the term rangeability is used to describe the capability of a control valve (i.e. the ratio of the maximum Cv of the valve to the minimum Cv at which it can control) whereas the term turndown is generally used to describe the requirement of an application (i.e. ratio of Cv at maximum conditions to Cv at minimum condition).

          Note that the rangeability of a valve must be greater than the ratio of the Cv of the valve when fully open to the calculated Cv for the minimum conditions of the application.

          • Turndown applies to the application and is the ratio of the calculated Cv at maximum conditions to the calculated Cv at minimum
          • Rangeability applies to the valve and is the ratio of the Cv of the valve fully open to the minimum Cv at which it can control
          • The rangeability of the selected valve must exceed the turndown requirements of the application.

      14. Question24. What Process Date Is Required To Size A Control Valve?

          Answer :

          • Medium – What is passing through the valve? – if it is a special liquid give specific gravity (at flowing temperate), critical pressure, vapour pressure and viscosity.
          • Pressures – What is the maximum pressure that the valve needs to be rated for? What are the upstream and downstream pressures for each of the maximum, normal and minimum flow rates.
          • Flow rates – Maximum, normal and minimum. The maximum is used to select the valve size, the minimum to check the turndown requirement and the normal to see where the valve will control.
          • Temperature – Maximum temperature for design plus temperatures at maximum, normal and minimum flow conditions.
          • Please see the relevant enquiry sheets for additional information that may assist in the sizing and selection of the control valve required.

      15. Question25. What Is Incipient Cavitation?

          Answer :

          Incipient means “starting” – “incipient cavitation” begins when the pressure first dips below the vapour pressure and continues until the flow becomes choked at which point “full cavitation” is said to take place.

      16. Question26. What Is The Difference Between A Diffuser Plate And A Choke?

          Answer :

          A diffuser is a plate with a large number of small holes in it that is installed in the downstream pipework. On gas and vapour applications it creates a back pressure between the valve and plate, and this enables a smaller value to be selected than would otherwise be possible, due to the lower velocity at maximum flow. The overall noise level produced will be lower as the overall number of pressure drop stages are increased.

          A choke is a restriction orifice and is a plate with one central hole. It is used with liquid flows and is also installed in the downstream pipe work to create backpressure. The purpose is to reduce the pressure drop across the valve at the maximum flow rate either to eliminate cavitation or to reduce the intensity of the damage to the valve.

      17. Question27. What Is A Field Reversible Actuator?

          Answer :

          The actuators for many control valves are either spring-to-open or spring-to-close. The Mitech control valve actuator has all the parts necessary to reverse the action – this will normally take place in a workshop on site.

      18. Question28. Will Separable Flanged Valves Seal In A Pipeline?

          Answer :

          The sealing face is part of the valve body and so the separable flanges are only there to hold the body in the line – they are not required to seal.

      19. Question29. What Is Vapour Pressure?

          Answer :

          • The terms vapour pressure applies to a liquid, and is the natural equilibrium pressure that exists inside a closed vessel containing the liquid.
          • Vapour pressure varies with temperature.
          • The vapour pressure of water at ambient temperature of about 25°C is in the order of 4 kPa(a). This means that water will boil at 25°C if the external pressure is reduced to an absolute pressure of 4 kPa. At 100°C the vapour pressure of water is 101 kPa(a), which means that water will boil at 100° C at sea level where the atmospheric pressure is about 101 kPa(a).

      20. Question30. Specific Gravity Is The Ratio Of The Density Of A Liquid To The Density Of Water – What Is The Specific Gravity Of Gas?

          Answer :

          The specific gravity of gas is the ratio of the density of the gas to the density of air both measured at standard conditions of 101,3kPa and 15°C .

      21. Question31. What Is Meant By Cryogenic?

          Answer :

          Cryogenic valves operate at temperatures below minus 100°C.

          These valves have extended bonnets to remove the stuffing box and actuator away from the source of cold and are made of materials such as stainless steel Monel or bronze that do not become too brittle at these temperatures.

      22. Question32. What Materials Can Be Used For Oxygen Service?

          Answer :

          • Monel, bronze and austenitic stainless steel (e.g. 316) are the best materials for oxygen service in order of preference. The higher the velocity the better the material to be used.
          • Velocities should not exceed 40 m/s in the valve body with Monel and bronze and should be less than 20 m/s with stainless steel.

      23. Question33. Why Do Oxygen Valves Require De-greasing?

          Answer :

          In the presence of most oils and greases oxygen will burn or explode. Even the oil deposited on a component by an uncovered hand is sufficient to cause a problem, which is why plastic gloves should be used when building degreased valves.

      24. Question34. Why Do Some Control Valve Actuators Have A Small Internal Fail Action Spring And Some Are External And Much Larger?

          Answer :

          A piston actuator piped up double acting and operating with full supply pressure of about 500 kPa is very stiff and can normally operate satisfactorily with the flow direction either under the plug or over. This enables the flow direction to be chosen to assist with the fail action, which means that only a small bias spring is necessary inside the actuator to start initial movement in the right direction in the event of air failure. In the case of diaphragm actuated valves, the stiffness is much lower and so the flow direction must always be under the plug, resulting in the need of a heavy spring to give fail closed action. This cannot be fitted inside the actuator.

      25. Question35. Why Is Live Loading Sometimes Offered On Valves?

          Answer :

          Live loading reduces the need for routine maintenance in the plant.

          Live loading is recommended on applications where a leak along the valve shaft would be likely to cause damage to the shaft and packing. High-pressure water and steam applications are examples of where live loading is advantageous.

      26. Question36. Why Is Energy Dissipation An Important Factor In Control Valve Selection?

          Answer :

          All Control valves cause pressure drop in the fluid as it passes through the valve. Since pressure is a form of Potential Energy, this means that a certain amount of energy is converted from potential energy into some other form. The higher the Pressure Drop and the greater the flow rate then more energy will be dissipated. Depending on the type of valve and the trim design this energy can cause significant damage to valve components due to cavitations and high velocities, or can be environmentally unfriendly because of high noise levels produced. Through the careful choice of valve type and correct trim design it is possible to minimize the adverse effects of high levels of energy dissipation.