The various types of level measurement used are:
· Float & tape type
· Displacer type
· Differential head type
· Purge type
· Radar type
· Servo type
· Ultrasonic type
· Level switches
Float & tape type
This is the simple and oldest method available for indication, generally used in storage tanks. Even though they are very simple, only a very few are used now. In certain cases, the liquid level of the tank raises or lowers the float, which moves the indicating cable. Cable movement drives the indicator up and down the gauge board on the side of the tank.
The liquid level in inches, feet, meters or decimeters is shown by the indicator position on the gauge board. These types are used in quarries for water level measurement.
Float & tape type ATG (Automatic tank gauging) are used in Benzene & PIBU tanks, Where the level measurement is displayed using a dial and counter built into the gauge head. The gauge head is installed at the tank side. When coupled with suitable electronic transmitter, a signal 4-20 MA can be transmitted to a remote point. In these types of gauges, the stainless steel float is attached to a stainless steel perforated tape to detect the liquid level. The float follows the liquid level as it rises and falls due to the constant pullback tension provided by a powerful negator spring. The precisely perforated tape engages a pin on a sprocket wheel that it turns drives the counter assembly.
Displacer type
Displacer type level measurement instruments are widely used in the refinery. This works on the Archimedes principle. When a body is submerged in a liquid, it suffers a loss of weight proportional to the weight of the liquid displaced by it. The body is termed as displacer. The torque tube is the best-known method of using displacers for level measurements. The torque tube twists a specific amount for each increment of buoyancy change. The displacer rod is designed to absorb the side ways forces and minimize friction by a knife-edge bearing. They are calibrating for a particular specific density and any variation from the calibrated density induces error in the reading.
Calibration of level transmitter
A. Externally mounted displacer and chamber
Let us assume displacer length is 800mm and the specific gravity of the liquid is 0.8. For calibration, using water, proceed follows:
- The total height to which water is to be filled in the chamber is determined by multiplying displacer length and specific gravity of the liquid, i.e. 800 X 0.8 = 640 mm
- From the center marked on the float chamber, measure length equal to 400mm downwards, and mark that point as 0%.
- From zero mark measure 640mm and mark that point as 100%.
- Similarly from bottom calculate 25%, 50% and 75%, by dividing the total length equally.
- Connect a white transparent tube to the bottom of the chamber with draining facility. Set the density dial at 1.0
- Add water from the top of the chamber and check the output of the transmitter at zero mark, it should be4 mA. Adjust zero.
- Take level to the 100% mark and check whether the output is 20mA. If not adjust the PB dial to have 20mA.
- Repeat step (f) and (g) till you get satisfactory result.
- Now check the reading up scale from 0 to 100% in steps of 25% and down scale and note down the readings. Change the specific gravity to 0.8. The instrument is now ready for installation.
As the float and the chamber are mounted inside, the procedure described in (A) cannot be followed. From the data provided by the manufacture, the weight of the float can be calculated. Let it be W1. Calculate he loss of weight when it is fully submerged in a liquid of specific gravity. Let it be W2.
- To the torque tube, which the displacer is connected, attach a pan to carry weights.
- Add weight W1 in the pan and check the output. It should be 4mA. Otherwise adjust zero of the instrument.
- Remove weights W1 and add weight equal to (W1-W2) check the output. It should be 20mA. Otherwise adjust p.g. dial to have 20mA.
- Repeat steps (c) and (d) till the reading match.
- Add weights equal to W1 – W2 , W1 – W2 , ¾
4 2
(W1 – W2) and check the outputs. It should be 8, 12 and 16 mA.
- Instrument is now ready for installation.
C. Calibration procedure for inter-face level measurement
Let us take an example to illustrate this. Consider two liquids of specific gravity 1.0 and 0.9 and the interface level is to be measured. Let the float length be 800mm. Since we are using water as the calibration liquid, the level up to which the chamber is to be filled with water in case of specific gravity of liquid 0.8 is the rate of the specific gravities multiplied by the float length.
i.e. 0.8 X 800 = 640
1
The second liquid is water itself. The level of water to be taken on to the chamber should be equal to the length of the float. When the chamber is filled with the second liquid of specific gravity 1.0, there will be no trace of liquid of specific gravity 0.8, as it will float over water. This corresponds to 100% level for calibration. Similarly, when the chamber is full of liquid (specific gravity 0.8), the second liquid will be below the first liquid. This corresponds to zero level.
When there is a mixture of two liquids inside, the loss of weight is the addition of loss of weight in liquid I up to which the float is immersed and loss of weight in liquid II up to which the float is immersed.
Note the Center of the float and from center mark 400mm downwards to identify the bottom of the float.
- From this mark measure 640mm, upwards and this will be 0% point.
- From the bottom mark measure 800mm and mark this position as 100%.
- Take water in to the chamber and when the level raises up to 0 mark, note down the output of the transmitter. It should be4 mA.
- Increase water level up to 100% mark and note down the transmitter output. It should be 20mA.
- Note down the output of the transmitter at 25%, 50% and 75% of the level. It should be 8, 12 and 16 mA respectively.
- The transmitter is ready for installation.
For pneumatic transmitters (still some are used in CDU1 & FCCU) instead of 4, 8,12,16 & 20 mA, 0.2, 0.4, 0.6, 0.8 & 1.0 Kg/cm2 respectively has to be set.
Differential head type level measurement
The application of displacer type level measurement is limited by length of the chamber in which the displacer if fixed.
One of the primary principles underlying industrial level measurement is that different materials and different phases of the same material have different densities. This basic law of nature can be utilized to measure level via differential pressure, that at the bottom of the tank relative to that in the vapor space or to atmospheric pressure
Level measurement based on pressure measurement is also referred to as hydrostatic tank gauging (HTG). It works on the principle that the difference between the two pressures (d/p) equal to the height of the liquid (h, in inches) multiplied by the specific gravity (SG) of the fluid
By definition, specific gravity is the liquid's density divided by the density of pure water at 68° F at atmospheric pressure. A pressure gage or d/p cell can provide an indication of level (accurate to better than 1%) over wide ranges, as long as the density of the liquid is constant. When a d/p cell is used, it will cancel out the effects of barometric pressure variations because both the liquid in the tank and the low-pressure side of the d/p cell are exposed to the pressure of the atmosphere. Therefore, the d/p cell reading will represent the tank level.
When measuring the level in pressurized tanks, the same d/p cell designs are used as on open tanks. It is assumed that the weight of the vapor column above the liquid is negligible. On the other hand, the pressure in the vapor space cannot be neglected, but must be relayed to the low-pressure side of the d/p cell. Such a connection to the vapor space is called a dry leg, used when process vapors are non-corrosive, non-plugging, and when their condensation rates, at normal operating temperatures, are very low (Figure 7-1C). A dry leg enables the d/p cell to compensate for the pressure pushing down on the liquid's surface, in the same way as the effect of barometric pressure is canceled out in open tanks.
It is important to keep this reference leg dry because accumulation of condensate or other liquids would cause error in the level measurement. When the process vapors condense at normal ambient temperatures or are corrosive, this reference leg can be filled to form a wet leg. If the process condensate is corrosive, unstable, or undesirable to use to fill the wet leg, this reference leg can be filled with an inert liquid.
These fill fluid generally used is Glycerin. In certain cases water, and sometimes the process fluid itself may be used as fill fluid. Generally a seal pot is also used when seal fluids are used so as to have not much variation in fluid head due to small leakage of fill fluid, reduction in temp and hence condensation in case of steam and easiness of filling.
The exact value of suppression or elevation is calculated as per the HP (high pressure) & LP (low pressure) impulse tubing length, range required and density of the fill liquid. Modern transmitters do not require any setting of zero suppression or zero elevation as such, but only assigning of LRV (low range value) and URV (upper range value).
The working and other details of differential pressure transmitter is explained in the section “pressure transmitters”.
When the process fluid is sludge, a viscous polymer or is otherwise hard to handle, the goal is to isolate the dirty process from the d/p cell. A flat diaphragm can be bolted to a block valve on the tank nozzle so that the d/p cell can be removed for cleaning or replacement without taking the tank out of service. If it is acceptable to take the tank out of service when d/p cell removal is needed, an extended diaphragm design can be considered. In this case, the diaphragm extension fills the tank nozzle so that the diaphragm is flush with the inside surface of the tank. This eliminates dead ends or pockets where solids can accumulate and affect the performance of the cell.
Diaphragm pressure seals are available with various fill liquids such as glycol and various oils. These seals are used when plugging or corrosion can occur on both sides of the cell. A broad range of corrosion-resistant diaphragm and lining materials is available. Teflon® lining is often used to minimize material build-up and coating. Level measurement accuracy does suffer when these seals are used. Capillary tube lengths should be as short as possible and the tubes should be shielded from the sun. In addition, either low thermal expansion filling fluids should be used or ambient temperature compensation should be provided, as discussed in connection with wet legs. If the seals leak, maintenance of these systems is usually done at the supplier's factory due to the complex evacuation and backfilling procedures involved.
When the process fluid is liquid nitrogen or some other cryogenic material (as in HPN plant), the tank is usually surrounded by a thermally insulated and evacuated cold box. Here, the low pressure (LP) side of a direct acting d/p cell is connected to the vapor space above the cryogenic liquid. As the liquid nitrogen approaches the HP side of the d/p cell (which is at ambient temperature outside the cold box), its temperature rises. When the temperature reaches the boiling point of nitrogen, it will boil and, from that point on, the connecting line will be filled with nitrogen vapor. This can cause noise in the level measurement. To protect against this, the liquid filled portion of the connecting line should be sloped back towards the tank. The cross-section of the line should be large (about 1 inch in diameter) to minimize the turbulence caused by the simultaneous boiling and re-condensing occurring at the liquid-vapor interface.
Purge type:
Purge type (Bubbler tubes) provide a simple and inexpensive but less accurate (±1-2%) level measurement system for corrosive or slurry-type applications. Bubblers use air (sometimes water or flushing oil) introduced through a dip pipe. Gas flow is regulated at a constant rate (usually at about 500 cc/min). Generally a restriction orifice maintains constant flow, while the tank level determines the backpressure. As the level drops, the backpressure is proportionally reduced and is read on a calibrated transmitter. The dip pipe should have a relatively large diameter (about 2 in.) so that the pressure drop is negligible. The bottom end of the dip pipe should be located far enough above the tank bottom so that sediment or sludge will not plug it. Also, its tip should be notched with a slot or "V" to ensure the formation of a uniform and continuous flow of small bubbles.
In pressurized tanks, two sets of dip pipes are needed to measure the level). The two backpressures on the two dip pipes can be connected to the two sides of DP transmitter. The purge gas supply should be clean, dry, and available at a pressure at least 10 psi greater than the expected maximum total pressure required (when the tank is full and the vapor pressure is at its maximum).
These type of level measuring instruments are generally used in services where there are chances of fluid blocking the impulse tubing like in regenerator level, molten sulphur level, slurry services and also open vessels like water sumps.
Radar type:
Radar type gages are used in automatic tank gages (ATG). The different types of gages and their details are mentioned in section 3.6.
Servo type:
Servo type gages are used in automatic tank gages (ATG). The different types of gages and their details are mentioned in section 3.6.
Ultrasonic type:
API sump level at Wagon Loading is measured by VEGA Ultrasonic Level Indicator - VEGASON 51K
The continuous level measurement with Ultrasonic sensors is based on the running time measurement of ultrasonic pulses. Piezoceramic high-performance transducers emit focused ultrasonic pulses, which are reflected from the surface of solids and liquids. The meas. signal prepares a precise picture of the meas. Environment out of the running time and signal shape of
the reflected ultrasonic pulses.
Due to the physical sound velocity and the detected actual running time of the emitted sound impulses, the meas. Electronics calculates precisely the distance between transducer and product. The distance is converted into a level proportional meas. Signal according to the sensor parameter adjustment as exact scaled level.
The instruments operate with emitting frequencies from 16KHz to 70 KHz. to be prepared for the different distances and requirements.
As the sound velocity is subjected to a temperature influence, the transducer detects continuously the ambient temperature so that the level is provided precisely even with varying ambient temperature.
The indicating instrument at wagon Loading Control Room is VEGADIS 371EX, which connects to the VEGASON in a two-wired loop. VEGADIS is a digital indicating instrument with integral level switches and current output.
The instrument configuration parameters can be set /modified either by
- By MINICOM module, which is a detachable 6 key module with, display, which has to be plugged into the ultrasonic sensor or VEGADIS? OR
- By HART Handheld Configuration.
Level switches:
Level switches are widely used in a variety of process applications to give a contact output (open or close i.e., NO or NC) when the level reaches a particular level. They can be high or low switch. A high switch, switches the contact on the rising level and low switch generates the contact on the lowering level. These contacts will be suitably wired to other control systems for necessary actions (indication or control).
The buoyant force available to operate a float level switch (that is, its net buoyancy) is the difference between the weight of the displaced fluid (gross buoyancy) and the weight of the float. Floats are available in spherical, cylindrical, and a variety of other shapes. They can be made out of stainless steel, Teflon®, Hastelloy, Monel, and various plastic materials. Typical temperature and pressure ratings are -40 to 80°C (-40 to 180° F) and up to 150 psig for rubber or plastic floats, and -40 to 260°C (-40 to 500°F) and up to 750 psig for stainless steel floats. Standard float sizes are available from 1 to 5 inches in diameter. The float of a side-mounted switch is horizontal; a permanent magnet actuates the reed switch in it.
Floats should always be lighter than the minimum expected specific gravity (SG) of the process fluid. For clean liquids a 0.1 SG difference might suffice, while for viscous or dirty applications, a difference of at least 0.3 SG is recommended. This provides additional force to overcome the resistance due to friction and material build-up. In dirty applications, floats should also be accessible for cleaning.
The switch itself can be mercury (Figures 7-6A and 7-6C), dry contact (snap-action or reed type, shown in Figure 7-6B), hermetically sealed, or pneumatic. Displacer Switches
Whereas a float usually follows the liquid level, a displacer remains partially or completely submerged. The apparent weight of the displacer is reduced as it becomes covered by more liquid. When the weight drops below the spring tension, the switch is actuated. Displacer switches are more reliable than regular floats on turbulent, surging, frothy, or foamy applications. Changing their settings is easy because displacers can be moved anywhere along the suspension cable (up to 50 ft). These switches are interchangeable between tanks because differences in process density can be accommodated by changing the tension of the support spring.
No comments:
Post a Comment