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 of time in a closed tube. There are several types of rotameter applications, including chemical injection/dosing and tank blanketing. A rotameter is a gauge for measuring fluid flow using a graduated glass tube with an enclosed free float.
Also known as variable area flow meters, rotameters are used to measure liquid or gas volumetric flow rates as they pass through the tapered tube of the rotameter. The flow of the liquid or gas raises the meter’s float, increasing the area through which the media may pass. The larger the amount of flow, the higher the float is raised.
A rotameter can be used for purge applications to keep process lines clear. In simple flow measurement, an alarm or an electrical output makes it possible to check flow conditions and control them continuously.
A rotameter contains a transparent, tapered, vertical tube with 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. The float is shaped to create minimum hindrance to the flow.
A linear scale is marked on the outer margin of the glass tube. The conical tube can be made up of plastic, metal, or glass, with all of them having different uses; for example, opaque liquids are used in metal tubes, whereas gas tubes contain other gasses and liquids. In addition, metal tubes might use metals of different densities, including lead and aluminum, while the floats are mainly made of stainless steel.
Fluid enters the tube from the bottom and escapes through the top. This fluid is the one whose flow is 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 making the float move upwards. It moves upwards until the lifting strength produced from the difference in pressure across its upper and lower surfaces begins to equal the float weight. The lifting force and pressure difference will temporarily increase due to the flow rate increase in the rotameter. Afterward, the float travels to the top and increases the area in the annular opening. Due to this, the lifting force will 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 scale marked on the glass tube indicates the flow rate.
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 using rotameters, one is always advised to correct the flow tube readings according to any changes in 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, a 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 vertically with no flow and the float at the bottom. But just as the fluid passes 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, including 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.
Generally used fluids such as air and water already have their calibration data and reading scales provided together with the rotameters. The manufacturers normally provide part of this standard information, such as the calibration tables, standard flow values, nomographs, and slide rules.
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 capacity range of the flow meters.
Installations can be made without considering the connections or lengths of straight pipe procedures or following the meter.
Oil, tar, sulfuric acid, and black liquor are corrosive liquids that can be correctly handled in an area meter.
The flow rates can handle a very low-pressure loss by placing floats on larger gauges.
A rotameter’s characteristics include its simple construction, thus providing a low cost. It is easy to take readings because the rotameter contains a linear scale with most meters. Accuracy, which is within ±2% of the total reading, allows a little room for errors in the readings. Lastly, the rotameter is an easy-to-install instrument.
The components of a rotameter include:
To obtain strong and uniform tubes, the metering tubes are created in a mandrel and annealed to have no internal stresses. This process leads to the metering tubes having greater reproducibility and more interchangeability. In addition, conical metering tubes with curved elements are created to extend the graduations at the lower end of the range. 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 requirements are noticeably high. In some rotameter designs, plastic tubes are used due to their lower cost and high impact strength.
The floats are made using materials that resist corrosion and capacity modification, such as stainless steel. 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 is a constant downward force, whereas buoyancy is a constant upwards force. Drag force is a variable force having an upward direction. They are constructed using metals of different densities, like lead and aluminum; glass and plastic can also be used. Floats are designed to be spherical for small flows.
The instruments must contain audible or visual alarms to alert users of a dangerous situation. The instruments must also contain controller functions for the proper use and tuning of the instruments. These help the devices receive an input signal that can be processed into an output signal for easier communication. To change the commands being processed by the rotameter, the instrument must be programmable. This is achieved by inserting a built-in microprocessor in the programmable meters. These microprocessors can be adjusted electronically depending on the materials, ranges, and outputs.
A rotameter must contain some recorder or totalizer functions, as these totalize the amount of material and media controlled. A recorder function can be placed to process data logging and record this data in a computer system so it can be used in the future, even if the rotameter is not in use. This same function can later produce a summary of the data in the form of displayed charts or tables.
Finally, rotameters can accommodate use in sanitary environments such as medical or food processing places.
A laboratory rotameter can be calibrated to an accuracy of about 0.50% AR over a 4:1 range. 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 vary too much. This point depends on the equipment's design, as those that use ball measurements are the most sensitive, and larger ones are less sensitive. The viscosity limit is usually determined by the float shape and the material making up the rotameter. Once the instrument passes through its viscosity limit, the viscosity readings will need 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 will correct the fluid density. Low-viscosity fluids such as gasoline, jet fuel, and other light hydrocarbons 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.
Rotameters must be mounted vertically, with the widest end at the top. Some options for mounting rotameters are insertion, in-line flanged, in-line threaded, and in-line clamp.
When mounting insertion flow meters, one must ensure they are perpendicular to the flow path. These usually require a threaded hole in the process pipe. In-line flanged flow meters should be parallel to the path of flow, which must be between two existing pieces of flanged process pipes. The flow path must be parallel to the in-line thread flow meter as they are inserted into the two already-existing process pipes. Of the thread types, NPT is the most common.
A rotameter is made of glass, so care is required to avoid breaking it. When operating the rotameter, do not set it to 0, as it hinders the flow of the pressurized air and causes damage. Parallax error must be avoided by all means; therefore, to see properly, an alcohol swab may be used to clean the outer layer of the glass. Finally, the float tends to get stuck in the base of the flow meter, leading to blocked air and no output. Turning the flow meter upside down can be helpful in trying to move this float from the base.
Rotameters can be used in various applications, and each one has unique elements. The types of rotameters include glass tube flow meters, armored purge meters, and flanged armored rotameters. These are explained in detail below.
Glass tube rotameters are extensively used in industrial areas, laboratories, and other pilot plants. Borosilicate glass is usually used to make the tube. To resist corrosion, the float is manufactured with stainless steel, glass, or plastic. The floats reflect a specific reading on the scale due to their sharp or metering edges. 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 convert the provided units into flows of the relevant fluids. For gasses like 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 air or water would have already been determined under specific temperatures and pressures, but it can be less convenient.
Different fluid flow rates can be measured using many different floats. Placing a glass tube rotameter at eye level can make the scale readings easier to record. Glass tube rotameters cannot be used for other types of fluids, such as water over 194 °F (90 °C), high pH fluids, and wet steam, which softens the glass of the tube. Glass is also dissolved by caustic soda and hydrofluoric acid, so other tubes must be used. The performance of a 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 are effective in areas where several streams of gasses or liquids are mixed or transported, though they can also be effective where one fluid flows through several channels.
This is best for applications requiring a low flow rate, including purging control lines and instrument enclosures. This instrument is best for fluid sampling, level measurement, liquid specific gravity, and low-flow uses of gasses and liquids. It consists of lengths of (112, 3, 10”) with connections of 1/4” NPT.
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 keep track of natural gas flow, thus making it possible to cool down fluids to protect equipment.
It is used to automatically turn off heavy equipment 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 best for areas with low flow and high pressure within municipalities and industries. It is also used in gas analyzer systems and where glass tubes are unsuitable for safety concerns. These rotameters work well with measuring cloudy and opaque media. An advantage of an armored purge meter is that it purges the fluid if a system’s condition is not well. 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 non-conductive fluids. A flanged armored rotameter is most suitable for high-pressure uses. An example of a flanged armored rotameter is the FAM54, which includes flanged connections. This rotameter’s uses include optional alarms, HART communications, totalizer pulse outputs, digital displays, and transmitters.
A metal tube rotameter has a tapered tube made of steel and a float made of stainless steel or
polytetrafluoroethylene (PTFE). They are highly durable and long-lasting, with exceptional strength, and are engineered to be suitable for measuring corrosive or turbid liquids from high pressure or temperatures. Metal tube rotameters are explosion-proof with high accuracy and dependability, making them ideal for use in harsh, stressful conditions.
In a metal tube rotameter, a magnetic sensor and chip technology produces an LCD readout of the flow, accumulated flow, and percentage flow. A magnetic couple of the sensor provides a more stable signal transmission.
In non-ferromagnetic metal tube rotameters, the float uses a magnet with a follower magnet mounted outside the cone to track the float's position. The follower magnet is mechanically connected to a visual indicator or readout device.
This chapter will discuss the applications and benefits of rotameters.
Rotameters are used in municipalities and industries for accurate level measurements. They are used in the purging of corrosive fluids. Rotameters measure and control machinery; for example, they may shut down a cooling machine as it reaches a certain marked point. They are also effective equipment in machinery that requires continuous lubrication.
Rotameters are used as gas analyzers to identify the concentration of known gasses in an atmosphere that contains many. Accurate density rotameters are also used for effective measurement. Furnaces and gas burners in industrial sites need to be controlled not to damage equipment; therefore, rotameters are used for monitoring. This equipment is also used in industries for refrigeration flow control.
Despite rotameters being useful in some areas, they still have drawbacks.
Rotameters, also known as variable area flow meters, are instruments used to measure 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 and when high accuracy is not required.
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