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LEVEL MEASURING INSTRUMENTS
INTRODUCTION LEVEL MEASUREMENT IS GAUGING THE POSITION OF THE TOP SURFACE OF LIQUID OR SOLID RELATIVE TO A FIXED REFERENCE OR ZERO POINT.
MEASUREMENT OF LIQUID LEVEL IS THE MEASUREMENT OF LIQUID HEIGHT.
UNITS OF LEVEL MEASUREMENTS: The level is actually a height, therefore unit of length should be used. Unit of length in SI is metre (m). So the level can be given in millimetres, centimetres or metres.
water Zero point
Oil Zero point
For very low pressure For pressure vessels
The transparent tube is made of glass or plastic.
Sight glass is giving direct level measurement.
High pressure vessels have special types.
Sight glass can give wrong reading if the tapings are plugged.
Float and Tape method
Take-up Reel with Indicator
•This is used to measure the levels in un pressurised vessels.
•It consists of a float connected to a tape reel. The reel has a mechanical indicator that gives the reading of the level in the tank.
•As the level of the tank changes , the float position also changes. This change is noted on the indicator of the take-up reel.
Float and Cable method
Float Scale Weight
Steel wire is attached to the float and passed over pulleys. Indicator moves along the scale. It is attached to the wire. The weight serves to keep the steel wire taut.
The displacer is lowered into the fluid in a tank and the weight of the displacer is directly acting on the linkage. As the level in the tank change the weight acting on the linkage also changes due to the change in buoyant force.
The change in the weight is proportional to the change in level.
This is a commonly found instrument in PDO operations.
When a body is dipped or floated in a liquid, it experiences an upward force from the liquid .This force is known as buoyant force.
A displacer mechanism consists of a solid cylinder of some lightweight material and connected to a meter through a linkage mechanism..
This can be used to measure interface level also. E.g. Water and Oil Interface Level measurement
HYDROSTATIC PRESSURE METHODS
THE HYDROSTATIC PRESSURE THAT A FLUID EXERTS DOWNWARD IS PROPORTIONAL TO THE HEIGHT OF THE FLUID COLUMN. THE DOWNWARD PRESSURE OF THE FLUID DOES NOT INCREASE AS THE HORIZONTAL SIZE OF THE COLUMN INCREASES. BY NOTING THE CHANGE IN PRESSURE AT THE BASE OF THE CONTAINER, WE CAN INDIRECTLY MEASURE THE LEVEL. BY USING A PRESSURE SENSITIVE GAUGE AT THE ZERO POINT OF A TANK, WE CAN MEASURE THE LEVEL.
Pressure exerted by a column of liquid depends upon the height of the liquid column and density of the liquid
Pressure = Height X Density of the liquid x Gravitational Constant
P = h x x g P = Pressure in Pascals h = height of the liquid in metres = density in kg/m3 g = gravitational constant ( 9.81m/s2)
h = p/ x g Find out the level of a tank contain oil , when base pressure = 0.4 bar ( 40,000 Pa) density = 800 kg/m3 g = 10 m/s2.
40,000 / 800 x 10 = 5 m.
Differential pressure type
This method can be used to measure tanks and containers levels. One leg of the DP cell will be connected at the bottom of the tank and other leg will be open to the atmosphere. The DP is proportional to the tank level.Atmospheric pressure effect is nullified.
D/P = h . . g
Open tank Atmospheric pressure
Differential pressure method can be used to measure the level of pressurised tanks and separators. One leg of the DP cell is connected to the top of the vessel and other leg will be connected at the bottom. The level is proportional to the DP. Gas pressure effect on the D/P cell is zero.
D/P = h . . g
Line filled with liquid
D/P cell leg filled with liquid (wet leg)
Some gas can condense in the measurement line. The L.P line is filled with the liquid. Meter zero is adjusted to compensate the L.P liquid pressure.
Ball Float Level Switch
It can only show if the level is above or below the place where it is attached.
Ball Float Level Switch
Round float is attached to the liver. Liver is attached to the flange by a bending tube. Bending tube is flat and lever can pivot around it. A rod which is immovably attached to the lever, goes through the tube. The rod moves along with the float. The movement of the rod is sufficient to operate the microswitch. This is a switch with a very small operating stroke. The bending tube transmits the movement of the float in a pressurised area to the outside. The lever is secured by a bracket. Ball floats are usually fitted outside the tank for easy maintenance.
“ AQUOIL” OIL / WATER INTERFACE DETECTOR
BRASS WEIGHT IS HAVING A ZINC TIP. A SMALL MILLIAMMETER IS ATTACHED TO THE TAPE HOLDER.
PRINCIPLE OF OPERATION THE MILLIAMMETER INDICATES THE FLOW OF CURRENT WHEN THE ZINC TIP TOUCHES WATER. REMEMBER THE PROCEDURE FOR USE. MEASUREMENTS:- 1. SCREW THE CONNECTING CLAMP TO THE TANK SHELL. 2. TAKE THE NORMAL READING OF THE OIL LAYER (B). 3. TAKE THE INTERFACE READING-DEFLECTION (A). 4. A-B WILL GIVE OIL LEVEL. 5. TAKE ANOTHER READING OF THE BOTTOM OF THE TANK (C). 6. C - A WILL GIVE WATER LEVEL. 7. READING “B” CAN BE COUNTER CHECKED BY NOTING THE OIL MARK IN THE TAPE. THE QUANTITY CAN BE CALCULATED FROM THE CALIBRATION TABLE OF THE TANK.
In oil and gas industries the measurement of flow is very important.
Flow is the measurement of fluid movements.
We can measure the flow rate or total volume.
Flow rate is a measure of how much fluid is passing by a given point over a specific period of time.
Total volume is a measure of the accumulated volume of fluid that has passed a given point over an extended time period(for E.g.24 hrs).
The unit for volume flow rate in the SI is m3/sec.
The mass flow in the SI is kg/sec.
There are two major types of flow meters.
Direct flow meters and indirect flow meters.
• Flow Instruments:-
• In general, there are two major types of flow meters - direct flow
measurement type (Positive Displacement Meters) and those that
measure flow of fluids indirectly or indirect flow meters.
Direct volume measurement.
To transfer a fixed quantity to B Take a known capacity cup and transfer the desired amount, say 5 cups, one cup at a time. Fluid is transferred in discrete units and the total amount is determined by adding together the number of individual units.
• Positive displacement meters are direct flow measurement devices.
• There are many different types available, Sliding vane type, Oval gear type, etc.
• It consists of an external housing and an internal measuring unit, and a mechanism to drive a counter.
• The liquid enters into a measuring chamber and the counter counts each time the chamber fills and the total volume/flow can be calculated by adding each counts.
POSITIVE DISPLACEMENT METHOD
SLIDING VANE TYPE
Shaft (Counter Drive)
Indirect flow measurement:-
• In this, flow is not directly measured. Some related characteristic of fluid flow, such as, fluid velocity, pressure or fluid level is considered to determine the volume by using charts or formulas.
•Velocity depends on pressure differential. Because the cross sectional area is constant,average velocity is an indication of its flow rate. Liquid flow rate = Average velocity area
• One of the most common devices used in indirect flow measurements is the Orifice plate meter and Orifice plate.
Orifice plate meters
In normal practice the orifice tapings will be taken from the bottom of the liquid flow lines and the DP transmitters will be mounted below the tapings to avoid gas entering.
In gas pipelines the tapings will be from the top of the pipeline and DP transmitter will be mounted above the tapings to avoid liquid entering into the tapings.
Orifice plate meters are indirect flow meters. A pressure differential is created by inserting an Orifice plate into the pipeline. As per the principle the square root of Differential Pressure across the orifice is proportional to the flow (velocity).
A differential pressure transmitter receive the signal from the orifice tapings and transmits the DP.
Flow rate will be proportional to the DP.
Orifice plates are available in different sizes.
The pressure drop is very high across an orifice plate. This is the main disadvantage.
Orifice plates need straight pipes minimum 6D-27 D before and 10 D-50 D after the meter. If a bent comes very near to the meter the reading will not be accurate.( D=Dia of the pipe)
Orifice plates can give problems when you use for oil or gas which is having solid particles. This particles may accumulate at the upstream of the orifice plate.
Flow =Co * P1-P2 = Co * DP
Co = Orifice constant and depends upon the properties of the fluid, diameter of the pipe and orifice, etc.