MOV (Motor Operated Valve), Theory of operation

Introduction:
Electric actuators allow valves to be remotely operated and, by use of a motor, allow rapid valve operation that otherwise would not be feasible.
This section deals with the control circuit and operation of the actuator in the motor mode. Although there are several power sources used in actuators, this section will be directed strictly to the use of electric motors.

Control Components:

In the circuit shown above, control power is transformed off incoming motor leads. The stop switch is normally closed allowing a current flow path to exist up to the open and closed switches. The circuit as shown is deenergized with the valve in the full open position.
When the close direction switch (close switch) is closed, the closing coil of the Reversing Starter is energized. This will close the main line (motor leads) "CL" contacts to start the motor in the close direction, close the "CL" contact around the close switch (seal-in contact) and open the "CL" contact (electrical interlock contact) in series with the open coil. The actuator will continue to position the valve in the close direction until the torque and/or limit switch detect binding or full stem travel, the respective contact will then open deenergizing the close coil which results in the main line contacts opening, the seal-in contact opening and the electrical interlock contact shutting.
The actuator can then be operated in the open direction in the same manner as described above. In the mid-position, the actuator can be operated in either direction.

Power Supply: 
The function of the power supply is to supply the energy required to operate the valve to which the actuator is attached. Although there are many sources of power, the assumption is made that 240 Volt Alternating Current (VAC), which is a typical selection, is the power source.

Motor:
The function of the motor is to convert electrical power to mechanical power. Reversing any two of the three leads to the motor will result in a change in the direction of rotation. The typical motor on Limitorque actuators is limited to a 15 minute duty cycle which must be considered when performing maintenance.

Overload Heater Coils:
Overload heater coils (thermal overload relays) are a form of protection in the event of excessive motor current. Care must be exercised when sizing the heaters due to their time delay.

Reversing Starter:
Reversing starters have two separate functions: 1) to interchange power leads which change the direction of rotation, and 2) to provide mechanical and electrical safety interlocks that prevent the contacts for both directions being closed at the same time which would cause a direct short between phases. The operation of the reversing starter is based on using a small control current to control the larger motor current through electromagnetic switching. The coils shown in Figure 5-1 operate the main contacts of the starter when an open or close pushbutton is pushed. In addition, the seal-in contacts and contacts labeled "CL" and "O" are operated by the same coils in the reversing starter.

Control Transformer:
The function of the control transformer is to reduce the control voltage to a lower and safer level. Normally, the primary windings of the transformer is connected to two phases of the motor power. The secondary windings provide the control voltage as single phase, normally 115 VAC. The primary side may have two fuses for protection, while the secondary side normally has one.

Stop Pushbutton(s):
The stop pushbutton(s) are always functional and are wired in series in the control circuit so that an operation by any one of them will open the circuit, which causes starter drop out and halts actuator operation. They are a normally closed momentary open contact which de-energizes part of the circuit when pushed. Stop pushbuttons are usually located at each of the operating stations and locally at the actuator.

Open and Close Pushbuttons:
The function of the open and close pushbuttons is to initiate operation of the control circuit, which will result in energizing the actuator motor. In typical applications, there are two sets installed - one at the valve (local) and one in the control room (remote). In some systems, only one set of open and close pushbuttons is energized at a time. See REMOTE/LOCAL switch.

Contactors Auxiliary Contacts:
The function of seal-in contacts is to allow the person operating the actuator to release the open and close pushbuttons without having the actuator stop. This allows electric controls to stop the operation automatically at a pre-set condition without operator intervention. As an example, once energized, the actuator can be stopped by either the torque switch or limit switch depending on set-up of the valve-actuator. The seal-in contacts are labeled "O" and "CL" in Figure 10-1 and are in parallel with either the open or closed switch.

Remote/Local Swith:
This is a selector switch which determines the location of control for the actuator. If the remote location is selected, the local control pushbuttons will not work.

Overload Contacts:
The function of the overload contacts is to protect the circuit from an overload condition by interrupting the control circuit. The contacts are an integral part of the sensing heaters, normally 1 per phase. Great care must be exercised when working on equipment which is protected with heaters which reset automatically when they cool. Some nuclear plants do not have protective heaters and some have them only when the actuator operates in a certain direction.

Electrical Interlock Contacts:
The electrical interlock contacts, (contactor auxiliary contacts), prevent both the open and close contactors from operating at the same time. If the relay is protected by mechanical means, the electrical interlocks operate as a backup. The contacts are normally CLOSED contacts that open when the associated contactor operates. The open auxiliary contact is wired into the close circuit, and the close auxiliary contact is wired into the open circuit. They are labeled "O" and "CL" in Figure 10-1 and are in series with either the open or closed coils of the reversing starter.

Lights:
The lights' functions are to give approximate valve position information and as a useful tool for troubleshooting the actuator. The lights indicate the point where a particular rotor operates when activated by the limit switch, and is the same point where associated actions, if any, should be activated by contacts on the same rotor. Normal operation has the open rotor turning off the red closed light and the close rotor turning off the green open light, with both lights on between the open and closed position. The contacts are normally aligned with the motor contacts on the rotor and are 90 degrees off from the spare and torque switch bypass contacts. In actuators where the functions are divided by the use of additional rotors, (4 train limit switches) the lights may not function at the same time as the rest of the contacts. The lights may be driven by relays or actuated by external switches on the valve. There are many different control circuit arrangements.

Limit Switches:
There exists two (2) types of limit switches in electric actuators, 1. Position Limit Switches, 2. Torque Limit Switches.

1. Position Limit Switches:
The function of the geared limit switch is to count turns of the drive sleeve in order to keep track of valve position, to shut off power to the actuator motor at the proper stroke position, to turn indicating lights on and off at the proper positions, and provide interlocks, etc, as required. The limit switch is a relative mechanism and proper operating points must be set to match any desired valve positions.

The rotors are a form of drum switch with 4 contacts each. Normally 2 make and 2 break, although other Vendors will provide other configurations. There is usually one rotor for the open position which trips at the full open position, and one rotor for the close position which trips at the full close position. The finger base is where the wiring is connected to the limit switch.
The indication contacts operate opposite from their expected manner. The indication contact on the close rotor turns off the open light when the valve is closed, and the indication contact on the open rotor turns off the close light when the valve is open. Both lights are on during in between open and closed travel.
The interlock, or spare, contact is located above the indication contact and is 90 degrees out of phase with the motor and light contact. When the motor and light contacts are CLOSED, the spare contact is OPEN.
The purpose of the torque switch bypass contact is to act as a bypass around the torque switch FOR THE OPPOSITE DIRECTION to allow for high starting torque situations. The open rotor has the close torque switch bypass contact and the close rotor has the open torque switch bypass contact. These contacts allow a valve to be opened or closed when there is a high pressure across the valve, thermal expansion of the stem, or some other requirement.
The torque switch bypass is normally set by percentage of valve stroke. For example, if the drive sleeve of a valve turns 100 turns from full open to full closed, then 10% bypass will be ten drive sleeve turns from the open or closed position. The bypass is normally percentage-based for the shut seat only, because of pressure differentials across the valve adding to the torque load for opening the valve. The problem with adding any percentage bypass when using a two train limit switch is you have to trip the associated rotor early. Example: the torque switch is bypassed for 10 percent of valve stroke when opening a valve, the close rotor has to trip at 90 percent of the closing stroke. This requires the motor contact on the close rotor to be jumpered and have the torque switch shut off the actuator, plus you get a closed light indication at 90 percent closed instead of 100 percent. For this reason, many plants have gone to 4-train limit switches where the light indication, or the bypass, is placed on a different rotor.

2. Torque Limit Switch:
The torque switch used has two possible functions. The first, on torque closed valves, is to ensure that the valve has sufficient and accurate thrust on the valve stem to guarantee seating. The second, on limit controlled valve operations, is to ensure that the actuator and valve are protected from possible excessive thrust. The double-contact torque switch, with one set of contacts being for the open direction and the other being for the close direction, is normally used.

The torque switch is operated by the axial motion of the worm in both directions. The contacts on the torque switch are double break contacts and are not self-wiping, which can lead to continuity problems in some actuators because of an oil film forming on the contacts.
Torque switches are provided with limiter plates which limit settings to a safe value, and prevents overtorquing a valve in case the setting screws come loose. There is no overtorque protection from incorrectly set or installed torque and limit switches.
The torque switch performs as though it senses torque, although it is simply sensing how far the worm shifts on the wormshaft, which depends on the resistance of the drive sleeve to turning. When the torque or thrust reaches a preset amount, the torque switch will open. The torque switch doesn't care how the torque and thrust forces are applied and used. If packing is too tight, if valve components are blocking the stem during the valve stroke, or if the stem threads are damaged, the torque switch will still operate at the set amount of torque.

Testing after Maintenance:
One of the most critical steps after performing maintenance is the initial operation and follow-up testing. Many newly rebuilt actuators, and/or associated valves have been damaged the first time they are run after having been repaired because of mistakes in the testing process.
An actuator should never be run until the limit switches and direction of rotation of the motor have been checked. The limit switch functions should be checked in manual by positioning the valve at the desired positions and observing that the switches operate. Something as simple as leaving the set rod screwed in at the wrong time can cause major damage, and the only way to verify this is by manually checking.
The most important operational check is the direction of rotation test. If the motor runs the actuator in the wrong direction for the full stroke, the only protection will be the motor overloads. None of the other designed-in protection will protect the actuator due to the operating protection being on the wrong side of the control circuit.
The most effective steps for testing after maintenance are:

1. Manually open the valve and verify the open limit switches.
2. Manually close the valve and check the close limit switches. Observe any binding or resistance to operation.
3. (The above two can be reversed depending on starting position.)
4. Manually mid-position the valve for the first electrical operation. If there are problems with the settings, the valve will not immediately be in a position which could cause damage.
5. Energize the actuator and verify the indication is correct for mid-position. This will indicate that the limit switches are in an expected position.
6. Prepare for the first electrical run by placing your hand physically on the STOP control, so that the actuator can be stopped immediately if a problem develops.
7. Check the direction of rotation. Press the CLOSE button and verify that the actuator rotates in the correct direction, then stop the actuator. The easiest way to verify the direction of rotation is to compare drive sleeve rotation by motor to drive sleeve rotation in manual after checking the arrow on the handwheel for proper rotation.
8. If the direction of rotation is incorrect, stop the actuator IMMEDIATELY and reverse two of the motor leads for opposite rotation. If the actuator is allowed to run to the point where the limit or torque switch should stop the operation, the actuator will continue to run because the protective and controlling features are on the wrong side of the control circuit.
9. If the direction of rotation is correct, start the actuator in the close direction. Be ready to stop the actuator if the motor begins to sound like the load is increasing beyond an acceptable point, otherwise, let the motor run until the limit or torque switch stops the motor.
10. Verify that the shut indication is correct. If not, the limit switch is out of adjustment.
11. Verify that the valve is fully closed by placing it in manual and turning the handwheel in the close direction. If it is not, the limit or torque switch will need to be adjusted.
12. Operate the actuator in the open direction. Be ready to stop the actuator if the motor begins to sound like the load is increasing beyond an acceptable point, otherwise, let the motor run until the limit or torque switch stops the motor.
13. Verify that the open indication is correct.
14. Verify that the valve is in the correct open position by placing it in manual and checking in the open direction.
15. Operate the actuator from open to closed as necessary to verify that everything is functioning.
16. Place the actuator in manual. Note any problems in going from electric operation to manual. Operate electrically. Verify the operation of the declutch components in going from manual to electric. This will insure that the actuator is ready to be released for unrestricted use.

MOV (Motor Operated Valve), specification

MOVs are widely used primarily for isolation purposes in the process industries.

MOVs consists of two main parts, valve and electric actuator (including auxiliary gearbox). The valves to be actuated include gate, globe, butterfly, triple offset, ball and plug type valves.

Electric actuators can be defined into two (2) basic types:

1. Multi Turn 
A multi-turn actuator is an actuator which transmits torque to the valve for more than one complete revolution.

 2. Quarter Turn
 A quarter turn actuator is an actuator which transmits torque to the valve for only one quarter (45 degree) of a complete revolution. withstanding thrust.

Selection of Electric Actuators per valves type and requirement: 
-Wedge Gate (Rising, non-rotating threaded stem) Torque Seated.
-Parallel Disc Gate (Rising, non-rotating threaded stem) Position Seated.
-Knife Gate (Rising, non-rotating threaded stem) Position Seated.
-Globe Valve (Rising, non-rotating threaded stem) Torque Seated. 
-Stop Check Globes (Rising, non-rotating threaded stem) Torque Seated.
-Butterfly Valves Position Seated

Information required to properly size the actuators:
-Voltage and Voltage Variance (IE, Plus 10%, Minus 20%)
-Modulating or Open – Close Application.
-Torque Required (and Max. Thrust if applicable)
-Shaft Diameter (and Thread design if Applicable).
-Operating Time Required. -Enclosure Classification (NEMA 4X, 7, IP 68)
-Maximum Line Temperature. -Safety Factor to be applied if applicable.
-Detailed Actuator Specification for the Particular Project. -An Important Sizing / Selection Consideration Involves High Temperature, Rising Stem, Torque Seated Valves. When the Line Temp Exceeds 800 Degrees, a Thermal Compensating ‘Floating’ Stem Nut Design is Warranted.

Electric Actuator controls:
-motor starters
-control power supply or transformer
-pilot devices / local controls
-A positioner is required to accept an analog signal (i.e. 4-20mADC).
-A digital communications board is required to communicate via ‘fieldbus’ or ‘two-wire.’

Motor controls can be located:
-at the motor (actuator)
-at a remote location

Intrusive actuators motor controls (basic contorls):

-Reversing starters
-Power supply (115V of 24VDC)
-Auto phase correction -Monitor relay
-Loss of phase
-Thermal trip -Torque trip
-Pilot devices / local controls
-Standard interface board (24 VDC or 115 V)
-NEMA 4X/6 Weatherproof enclosure
-Torque and limit Switches enclosed within the Actuator housing

Intrusive actuators motor controls (optional contorls):
-Control interface boards
    -Positioner (accept analog signal)
    -Fieldbus Protocols (PROFIBUS-DP -Modbus RTU -DeviceNet)
    -Accept emergency signal
-Overload relays
-Solid state starters / Thyristors
-High temperature rating
-Low temperature rating
-Enhanced corrosion protection
-Submersible enclosure
-Explosion proof
-Lockable tamper
-proof protection cover
-Ability to remote mount the motor controls

  Non-intrusive actuators motor controls (basic contorls):
-Reversing starters
-Power supply (24VDC or 115VAC)
-Auto phase correction -Monitor relay / fault relay
-Standard interface (24 VDC or 115 V contact closure)
-Pulse time
-Pilot devices / local controls
-Enclosure: NEMA 4X/6 Weatherproof enclosure
-Electronic Control Unit
-Output signals (programmable output relays) for Position, Torque, Fault, Running Open, Running      Close etc
-Analog torque feedback
-Analog position feedback
-LCD display
-Electronic nameplate
-Wireless Communications via Bluetooth
-Ability to generate and store torque curves
-Interface boards which can include:
   -Adaptive positioner
   -Process controller PID
   -Digital communications interfaces (PROFIBUS-DP, Modbus RTU, DeviceNet, Foundation
    Fieldbus)                
   -Accept emergency input signal
-Optional reversing starters based on Actuator size
-Overload relays
-Solid state starters / Thyristors
-Additional programmable relays
-Voltage Tolerance (i.e. +/- 30% of nominal voltage)
-High temperature rating
-Low temperature rating
-Enhanced corrosion protection
-Submersible enclosure
-Explosion proof
-Lockable tamper
-proof protection cover
-Ability to remote mount the motor controls

  Key Take aways:
Electric Actuators serve a key role in process industries
-Specifying the proper actuator features is important
-What is the application? Multiturn, Part-turn or damper
-What are the valve characteristics? Torque or Thrust
-What type of Motor Controls are Req‘d? Non-Intrusive or Intrusive
-What is the Control Interface? Contact Closure, Analog or Fieldbus
-What is the environment? Nuclear, Weatherproof, Explosion Proof, High Temperature, Low    Temperature or Submersible
-What is the Primary and Control Voltage? Are voltage fluctuations of concern?

Temperature Measurement (RTD)

Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance.
They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second. They are among the most precise temperature sensors available with resolution and measurement uncertanties or ±0.1 °C or better possible in special desions.

Usually they are provided encapsulated in probes for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function, such as a temperature controller or precision thermostat.

Temperature Measurement

Temperature is measured in industries extensively. The most common primary sensing elements for measuring temperature are Thermocouples and RTDs. There are several other temperature sensors also but right now we will be discussing only these two types.

A Thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control[1] and can also be used to convert heat into electric power. They are inexpensive[2] and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one kelvin (K) can be difficult to achieve. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Type K (chromel–alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.[5] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a step change in output when the magnetic material reaches its Curie point (around 354 °C for type K thermocouples).

Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C) causes an abrupt change in the characteristic, which determines the upper temperature limit.

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

Venturi Tube for Flow Measurement

Venturi Tube is also a very popular primary sensing element for flowmeasurement. It is mostly used for low pressure flow measurement like gas, air etc flow. If we compare it with the Orifice Plate, it works on the same principle but is a very precise form of it. In the same way Orifice Plate can be called the raw form of a venturi tube.
The basic working principle of both of them is exactly the same i.e, differential pressure.

Magnetic Flow Measurement

The most common flow meter apart from mechanical flow meters is the magnetic flow meter, commonly referred to as a "mag meter" or an "electromag". A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter requires a conducting fluid, e.g. water, and an electrical insulating pipe surface, e.g. a rubber lined nonmagnetic steel tube.

Vortex for Flow Measurement

Another method of flow measurement involves placing a bluff body (called a shedder bar) in the path of the fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The vortices trail behind the cylinder, alternatively from each side of the bluff body. This vortex trail is called the Von Kármán vortex street after von Kármán's 1912 mathematical description of the phenomenon. The frequency at which these vortices alternate sides is essentially proportional to the flow rate of the fluid. Inside, atop, or downstream of the shedder bar is a sensor for measuring the frequency of the vortex shedding. This sensor is often a piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a vortex is created. Since the frequency of such a voltage pulse is also proportional to the fluid velocity, a volumetric flow rate is calculated using the cross sectional area of the flow meter. The frequency is measured and the flow rate is calculated by the flowmeter electronics using the equation f = SV / L where f is the frequency of the vortices, L the characteristic length of the bluff body, V is the velocity of the flow over the bluff body, and S is the Strouhal number, which is essentially a constant for a given body shape within its operating limits.

Orifice Plate for Flow Measurement

Orifice Plate is the most widely used primary sensing element used for flow measurement. It works on the principle of differential pressure. An orifice plate is a plate with a hole through it, placed in the flow; it constricts the flow, and measuring the pressure differential across the constriction gives the flow rate. It is basically a crude form of Venturi meter, but with higher energy losses. There are three type of orifice: concentric, eccentric, and segmental.


An orifice meter is a device used for measuring the rate of fluid flow. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which says that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.

An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. As fluid flows through the pipe, it has a certain velocity and a certain pressure. When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta point (see drawing to the right). As it does so, the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation.

Flow Measurement

There exist about (four) 04 flow measurement techniques. All these techniques are based on different working principles. They are

Rotary Piston
Electromagnetic
Ultrasonic
Differential Pressure
Vortex

Among these above mentioned techniques, differential pressure is the most widely used flow measurement technique in the industry. This technique is based on the differential pressure principle. In this technique two different pressures are created by placing a hindrence in the path of flow. As a result of this restriction pressure tends to change as it passes the ristriction. The differential of the pressures before and after the restriction gives us the amount of flow.

The task of an electromagnetic flow meter is to measure flow volume of electrically conducting liquids, including water, food, beverages, chemicals, slurry, pulp & paper and mining slurries with magnetic particles. Whether it's a standard flow meter with modular pulsed DC technology, a high-performance AC pulsed flow meter, or a battery-operated electromagnetic water meter, an electromagnetic flow meter from world`s leading manufacturers facilitates flow rate management.

Interface Level Measurement

Key Applications of Interface Level Measurement Transmitter are liquids, slurries, powders, granules, food and pharmaceuticals, chemicals, hazardous areas.
The main purpose of this type of transmitters are to measure the interface level between two different liquids like oil and water.
Its benifits are:
Easy installation with verification by built-in LED
Low maintenance with no moving parts
Sensitivity adjustment
Integrated cable or PBT enclosure versions available
Intrinsically Safe, Dust Ignition Proof and General Purpose options available

Continuous Level Measurement Instrument

Continuous Level Measurement

Continuous level measurement is used for monitoring dynamic processes. These measurements are transmitted as an analog signal or digital value. Nowadays modern instrument manufactueres offers a comprehensive range of continuous level measurement transmitters based on a variety of technologies; ultrasonic, radar, guided wave radar, capacitance, gravimetric and hydrostatic.

Continuous level detection techniques based on radar, ultrasonic and hydrostatic principle are more popular in industries.

Radar level measurement is non-contacting and low-maintenance. As well, radar level measurement devices do not require any form of carrier medium and, the process environment (steam, pressure, dust or extreme temperatures) has practically no influence on measurement. There are a broad range of radar level measurement devices availablein the market that are highly reliable and effecient.

Ultrasonic level measurement is a highly cost-effective solution for short- and long-range measurement, even under difficult environmental conditions such as vibrations and dust. Ultrasonic level measurement is a non-contacting technology used in numerous industrial areas to monitor and control the level of liquids, slurries and solids.

The hydrostatic level measurement is a low-cost process for direct mounting or use with remote seals on tanks or containers. Gauge pressure transmitters are suitable for measurements on open containers; for closed containers, it is preferable to use the differential pressure transmitters.

Point Level Measurement

Point Level Measurement is a technique that is used to detect a particular level of liquid in a chamber. For example it is used to detect the high and low level in a tank. The modern techniques that are used in the industry nowadays are of Ultrasonic, Rotary Paddle, Capacitance, Vibrating and Ball Float type. Ball float is the oldest and most widely used technique for point level detection, and it is also considered very reliable too.

Level Measurement

Level Measurement in a process industry can be categorized into three (03) main types. They are Point Level, Continuous Level and Interface Level Measurements. Each type of measurement is further classified into different techniques.

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Pressure Measurement

Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges.

A manometer could also be referring to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. The term manometer is often used to refer specifically to liquid column hydrostatic instruments.

A vacuum gauge is used to measure the pressure in a vacuum which is further divided into two subcategories: high and low vacuum (and sometimes ultra-high vacuum). The applicable pressure range of many of the techniques used to measure vacuums have an overlap. Hence, by combining several different types of gauge, it is possible to measure system pressure continuously from 10 mbar down to 10−11 mbar.

Although pressure is an absolute quantity, everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero references, the following terms are used:

Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
Differential pressure is the difference in pressure between two points.
The zero reference in use is usually implied by context, and these words are only added when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings. Moderate vacuum pressures are often ambiguous, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equivalent to an absolute pressure of 30 inHg (typical atmospheric pressure) − 26 inHg = 4 inHg.

Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the tire pressure goes up. Some standard values of atmospheric pressure such as 101.325 kPa or 100 kPa have been defined, and some instruments use one of these standard values as a constant zero reference instead of the actual variable ambient air pressure. This impairs the accuracy of these instruments, especially when used at high altitudes.

Use of the atmosphere as reference is usually signified by a (g) after the pressure unit e.g. 30 psi g, which means that the pressure measured is the total pressure minus atmospheric pressure. There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).

A vented gauge pressure transmitter for example allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm, via a vented cable or a hole on the side of the device, so that it always measures the pressure referred to ambient barometric pressure. Thus a vented gauge reference pressure sensor should always read zero pressure when the process pressure connection is held open to the air.

A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure ranges such as hydraulics where atmospheric pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded.

There is another way of creating a sealed gauge reference and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then the output signal is offset so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference pressure transducer will never read exactly zero because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar.

An absolute pressure measurement is one that is referred to absolute vacuum. The best example of an absolute referenced pressure is atmospheric or barometric pressure.

To produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. If the process pressure connection of an absolute pressure transmitter is open to the air, it will read the actual barometric pressure.

process instrumentation

Measuring, positioning, recording and controlling are key parameters for all industrial processes. Thus, top priorities for process instruments are achieving high levels of precision and absolute reliability. Process instrumentation from worlds leading manufacturers satisfies these demands and provides an efficient means to increase plant efficiency and improve product.

Mainly there are four parametric measurements carried out in a process industry. They are

1. Pressure Measurement
2. Level Measurement
3. Temperature Measurement
4. Flow Measurement