This article will take an in-depth look at rotameters.
The article will bring more detail on topics such as:
This chapter will discuss what rotameters are, their construction, and how they function.
A rotameter is a device that measures the flow of fluid volume per unit time in a closed tube. There are diverse rotameters applications, including chemical injection/dosing and tank blanketing. A rotameter can be said to be a gauge for measuring fluid flow using its graduated glass tube, which contains a free float.
There are purge applications of a rotameter whereby it is used to keep process lines clear. In simple flow measurement, an alarm or an electrical output makes it possible to continuously check flow conditions and control them.
A rotameter contains a transparent tube which is tapered and is placed vertically in a way that it has a small diameter at the bottom. This tube changes its cross-sectional area in order to affect the float by giving it a constant drop. In order to create minimum hindrance to the flow a proper perfect shape of the float is placed into consideration.
On the outer margin of the glass tube a linear scale is marked. The conical tube can be made up of plastic, metal and glass with all of them having different types of uses. Opaque liquids are contained and used in metal tubes whereas gas and liquids are used in the glass tube. Metals of different densities can be used in the construction and these include lead and aluminum where stainless steel floats are mostly used.
Fluid enters the tube from the bottom and moves towards the top escaping through. This fluid is the one whose flow is to be measured. The float will rest at the bottom of the tube when there is no flow in the instrument. In such a situation the total diameter of the float is nearly equal to the inside diameter of the glass tube.
The flow area of the annular opening increases when the fluid enters the tube thus also making the float move upwards. It is moved upwards until the lifting strength produced from the difference in pressure across its upper and lower surface begins to equal to the float weight. The lifting force and the pressure difference will temporarily increase due to the increase of the flow rate in the rotameter. Afterward the float travels to the top and increases the area on the annular opening. Due to this, the lifting force which had been increased will now decrease and the force of the fluid will become the same as the float weight. The difference in pressure remains the same by changing the area of the annular opening in relation to the flow rate. The flow rate indication is provided by the scale marked on the glass tube.
When using rotameters, calibration must be undertaken for a given gas or fluid at a given set of conditions. Normally the conditions are written on the sides of the flow meter along with its range of flow and the units of measurement. In the use of rotameters, one is always advised to correct the flow tube readings according to any changes which come from flow conditions. Usually manufacturers detail the required corrections for the meters, but this is not always the case.
One of the formulas used in the rotameters is:
Q=kA√gh
Where:
Q = volumetric flow rate
k = a constant
A = annular area contained between the float and the wall of the tube
g = the force of gravity
h = the pressure drop of the float
Because of its advantages, the rotameter is the most widely used variable area flow meter. It consists of a float that moves through a tube as the fluid passes through. More flow due to volume exerts a greater pressure on the float, thus lifting it even higher. In liquids, buoyancy becomes one with the velocity of the flowing liquid, thus raising the float. In gasses buoyancy is left out as the speed of the gas, and the pressure assigns the float to a certain height.
Commonly, the tube is set up in a vertical manner with no flow and having the float at the bottom. But just as the fluid starts passing through the float begins to elevate up to the top of the tube. The height gained by the float as it moves is generally proportional to the rate at which the fluid flows. The process reaches equilibrium when the upward force now equals the weight of the float, also giving the float a fixed position with no movement. At this moment, readings can be easily taken and these include readings of the density and the fluid’s resistance to flow (viscosity).
Using flow regulation valves, one can manually adjust the flow in the rotameter. The name rotameter was gained from the early designs where the early equipment had free floats which rotated in relation to the change in gas and fluid pressures. Due to the rotation, the name Rota-meter then came up.
Generally used fluids such as air and water already had their calibration data and reading scales provided together with the rotameters. Part of this standard information such as the calibration tables, standard flow values, nomographs, and slide rules are normally provided by the manufacturers.
The characteristics of a rotameter include:
The area is proportional to the volume flowing in a unit of time through a variable meter thus making these meters have increments of equal scale. The linearity of a rotameter scale is amiss at about 5%.
The loss of pressure through the float is constant in the variable area meter. At a higher flow rate, the differential in the meter increases due to the friction losses in the fittings.
±2% of the scale reading is the most used accuracy. Due to the user’s calibration and the scale length, this scale accuracy increases considerably.
0.5 cm3 /min of water and 30 cm3 standard/min of air are the large scale capacities range of the flow meters.
Installations can be made without taking into consideration the connections or lengths of straight pipe procedures or following the meter.
Oil, tar, sulfuric acid, and black liquor are some of the corrosive liquids and these can be correctly handled in an area meter.
The flow rates can be handled by a very low pressure loss by placing floats on larger gauges. Some of the rotameter characteristics include its simplicity in their construction thus providing a low cost. It is even way easier when taking readings as the rotameter contains a linear scale with most of the meters. Accuracy which is within ±2% of the total reading is contained thus allowing a little room for errors in the readings. Lastly the rotameter is an instrument which is easy to install.
The components of a rotameter include:
To obtain strong and uniform tubes, the metering tubes are created in a mandrel and annealed so as to have no internal stresses. This process leads to the metering tubes having greater reproducibility and more interchangeability. To extend the graduations at the lower end of the range, conical metering tubes with curved elements are created. Generally safety shielded glass tubes are used for measuring both liquids and gasses. Metal tubes are used in areas where opaque liquids are applied, where temperature and pressure requirement is noticeably high. In some rotameter designs, plastic tubes are used and this is due to their lower cost and their high impact strength.
The floats are made using materials that enable them to have resistance against corrosion such as stainless steel and also modification of capacity. These are grouped in terms of the capacities of the meters. The way in which a rotameter will be affected by changes in the viscosity of the fluid measured can be affected or determined by the shape of the float. Floats with sharp corners or edges are likely to be insensitive to changes in viscosity.
There are three forces that act on the float: weight, buoyancy, and drag force. Weight will be a constant force with a downward direction, whereas buoyancy will be an upward force in the direction and a constant type of force. Lastly drag force is a variable force having an upward direction. They are constructed using metals of different densities and these range from lead to aluminum also, glass and plastic can be put into use. Floats are designed in a manner whereby they are spherical in shape for small flows.
The instruments must contain audible or visual alarms used in alerting the users in situations of a dangerous condition occurring. For proper use and tuning of the instruments, the instruments must contain controller functions. These help make the devices receive some input signal that can be processed to produce an output signal for easier communication with the device. In order to change the commands being processed by the rotameter, the instrument must be programmable. And this is achieved by inserting a built-in microprocessor in some of the programmable meters. These microprocessors can be adjusted electronically depending on the different materials, ranges and outputs contained.
Recorder or totalizer functions must be contained in the instruments as these totalize the amount of material and media controlled. A recorder function is one which can be placed to achieve the process of data logging and recording this data in a computer system so that it can be opened and used in the later future even if not the rotameter is in use. This same function can later produce a summary of the data which had been recorded and this can be done in the forms of displayed charts or tables. The rotameters can accommodate the use in sanitary environments such as the use in medical or food processing places.
A laboratory rotameter can be calibrated to an accuracy point of about 0.50% AR over a 4:1 range. Whereas industrial rotameters are likely to be less accurate as these are 1-2% FS over a 10:1 range.
Flow rates can be manually set while tuning the valve opening and observing the scale. With small changes in viscosity, rotameters do not really vary too much. This point depends on the equipment's design as those that use ball measurements are most sensitive and those larger in size are less sensitive. The viscosity limit is usually determined by the float shape and the material which the rotameter is made up of. Once the instrument passes through its viscosity limit, the viscosity readings will have a need for them to be corrected.
If the fluid density is subject to change, one can use two floats. Of these two floats, one will depend on the fluid volume and the other one will be used to correct the fluid density. Low viscosity fluids such as gasoline, jet fuel, and other light hydrocarbons can work best with mass flow rotameters. The float position can be changed after the density changes due to buoyancy, which is caused by matching the float density with the fluid density.
Rota meters must be mounted vertically, with the taper's widest end at the top. Some of the options for mounting rotameters are insertion types, in-line flanged, in-line threaded, and in-line clamp.
When mounting insertion flow meters one must make sure they are perpendicular to the path of flow. These usually require a threaded hole in the process pipe. For In-line flanged flow meters we make sure they are parallel to the path of flow which must be between two already existing pieces of process pipes which are flanged. The path of flow must be parallel to the in-line thread flow meter as they are inserted into already existing two process pipes. Of the thread types, NPT is the most common one of them all.
The rotameter is made up of glass and therefore it requires great care so as to avoid breaking it. When operating the rotameter do not set it to 0 as it hinders the flow of the pressurized air causing great damage to the rotameter. Parallax error needs to be avoided by all means and therefore to see properly you may use an alcohol swab to clean the outer layer of the glass. The float has a tendency of getting stuck in the base of the flow meter leading to the blockage of air and no output from it. Inverting the flow meter upside down can be helpful to some extent in trying to move this float from the base.
Rotameters can be used in a variety of applications and each of the instruments has unique elements which meet these different applications. The types of rotameters include the glass tube flow meters, armored purge meter, and the flanged armored rotameter. These are explained in detail below.
It is used in turning off heavy equipment automatically when the flow of the bearing lubricant gets too low. It also shuts down electrical equipment when water that is being cooled decreases below a certain limit.
This is found in most municipal and industrial facilities and is used to measure liquids and gasses. It is installed as part of ovens and furnaces to check and keep track of the natural gasses flow thus making it possible to cool down fluids so as to protect equipment.
This is best to be used in applications where a low flow rate is required. Such applications include purging control lines and the enclosures of an instrument. This type of instrument is best in the fields of fluid sampling, measurement of the level, liquid specific gravity, and low flow uses of gasses and liquids. It consists of lengths of (112, 3, 10”) with connections of14” NPT.
This is best for municipalities and industries where it is applied in areas with low flow and high pressures. It is also used in gas analyzer systems and even where glass tubes can be unsuitable due to safety causes. It works well with measuring media which is cloudy and opaque. If the system’s condition is not well, the armored purge meter tends to purge the fluid thus making it an advantage of its use. One example of an armored purge meter is the 10A3200, which is available with NPT threads and a needle valve option.
This is usually used in industries and other automated systems which specialize in pharmaceuticals and petrochemicals. It measures opaque fluids under forceful conditions and fluids which are not conductors. The flanged armored rotameter is best suitable for high pressure uses. An example of the flanged armored rotameter is the FAM54 which includes flanged connections. Some of its uses include the use in optional alarms, HART communications, totalizer pulse output, digital display, and a transmitter.
These are used extensively and they are not only found in industrial areas but they can be used in laboratories and other pilot plants. Borosilicate glass is usually used to make the tube. Due to the factor of corrosion, there is now a need to manufacture the float using stainless steel, glass, and plastic. The floats reflect a specific reading on the scale due to the sharp or metering edges they have. A connection or an end-fitting is usually placed on the rotameter in relation to its field of use.
The most important part of standardizing is the tube float combination since this carries out the measurements. Lookup tables can be used for the conversion of the provided units into flows of the relevant fluids. For gasses that include nitrogen, oxygen, hydrogen, helium, carbon dioxide, and argon the correlation scales of the rotameter can be checked with the correlation tables. This way is more accurate as scales of liquids such as air or water would have been already determined under certain specific temperatures and pressures, but it can be less convenient.
Using plenty of floats we can measure different rates of the fluid’s flow. Placing a glass tube rotameter at eye level can make the scale readings to be taken and recorded more easily. Glass tube rotameters cannot be used for other types of fluids such as water over 90 °C in temperature, high pH, and wet steam which softens the glass of the tube. Glass is also dissolved by caustic soda and hydrofluoric acid and therefore for such fluids other different types of tubes must be used. The performance of the glass tube rotameter is limited by the limits in pressure and temperature of the glass tube where higher temperatures are the major limiting factor.
Glass tube rotameters can be effectively used in areas where several streams of gasses or liquids are being mixed together or transported through. Where one fluid is flowing out through more different channels, the glass tube rotameter can also be effective.
This chapter will discuss the applications and benefits of rotameters.
Rotameters are used in municipal places and industrial works for accurate level measurements. They are used in the purging of corrosive fluids. The rotameters do the measurement and control of machinery as they may shut down a cooling machine as it reaches a certain marked point. In some machinery, continuous lubrication is required, and rotameters are effective equipment in such situations.
In order to identify the concentration of known gasses in an atmosphere that contains multiple gasses, rotameters are used as gas analyzers. To measure effectively accurate density rotameters are also placed into use. Furnaces and gas burners in industrial sites need to be controlled so as to not damage equipment being used therefore to achieve such rotameters are used for monitoring. These equipment are also used in industries for refrigeration flow control.
A rotameter is easily available in the market and it is not costly. Fluids with a small or medium velocity e.g. velocity of 1 LPM to 10 LPM can be measured using a rotameter, thus making it a suitable instrument. The metal and glass cover of the rotameter is highly inert, making it easy for them to resist chemical reactions. Due to this inertness, the equipment is good to use where corrosive fluids need to be measured. Taking readings with the rotameter is way easier as it provides a linear measurement scale, achieving higher precision and accuracy.
They do not require any further external force outside of the measuring substance, thus reducing wrong readings. Their flow meters are versatile due to their small design. Therefore they can be used in a wide range of systems.
Despite the rotameter being useful in some areas it still has some of its own drawbacks and these are noted from the fact that its outer glass layer can easily break as the pressure in the fluid increases over time, as the instrument is being transferred from point to another and as it is being maintained. It can also be hard to take measurements if you use a fluid which is not transparent as you won’t be able to see the metering float properly. Parallax error can be easily gotten if you do not position or align your eyes to the same level as the metering float.
When taking measurements hold the instrument upright and avoid tilting so as to avoid accuracy during measurements. The resolution is nearly poor as compared to other measurement principles. The rotameter is not constructed like a rigid instrument as compared to other instruments like the venturi meter or orifice meter due to it using the glass tube. In situations where there are fluctuating rates in the flow of the fluid the rotameter can be a disadvantage to use. It is also not suitable for liquids which have suspended solid particles. It is also less accurate compared with a venturimeter and orifice meter.
There are parameters to be considered in the selection of the rotameters and some of these include flow rates. Two types of flow rates can be looked on and these are liquid volumetric flow rate and gas volumetric flow rate. Liquid volumetric flow rate is used only in rotameters which are liquid volumetric flow meters. This rate is expressed as the change in volume flow per unit time. Gas volumetric flow rate applies to rotameters which are gas volumetric flow sensors. It is also expressed in terms of the change in gas volume per unit time. Operating pressure must be considered and this is the total pressure of the process media the meter can withstand. This operating pressure is important as it can affect the gas by changing its density.
Fluid temperature is another specification that has the total temperature of the media being able to be monitored usually depending on liner and construction materials. Pipe diameter is important to consider during installation processes to know the right dimensions which are in relation to the right rotameter. Scale type must be indicated so as to help in record taking and these scale types include percentile. Valve requirements must also be noted, but the needle valve's most commonly used valve. For In-line flanged flow meters, we make sure they are parallel to the path of flow, which must be between two already existing pieces of process pipes which are flanged.
There are meters that do not require to be mounted directly to the process flow, these are non-invasive meters and can be used in closed piping systems. End fittings must be notable and these may include compression fittings which are used to prevent leakage by tightening down a sleeve over a joint. External clamp-on flow meters are non-invasive as these do not require to be mounted directly in the process flow, as these can also be used in closed piping systems. There is a specification of the socket weld or union which has the end fitting being designed or made in a manner for welding or soldering and can be a weld neck.
Operating conditions of the rotameter must be noted whereby it must have minimum and maximum rates of flow and be able to process temperature together with pressure. One must understand their needed accuracy in the use of the equipment and therefore get a rotameter which aligns with such. A rotameter must be able to provide direct and straightforward readings and this must be made aware of. The pipe size where it must be installed must also be noted down as each different rotameter will fit in its own different pipe. Flow regulation valves are optional, but these can be useful depending on the use of the rotameter.
In general, rotameters also known as variable area flow meters are instruments used in the measurement of the liquid or gas volumetric flow rate as either a liquid or gas passes through a tapered tube. The rotameter is best considered when the cost is to be kept at a minimum expense and also when high accuracy is not always required.
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