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General Information on Rotameters

Rotameters come in a variety of sizes and materials, but they all have a tapered tube and a float. The "float" is the thing that rises and falls inside the tapered tube. The float is in a continuous state of dynamic equilibrium whose vertical position in the tube is the balance point between the upward force of the fluid, or gas, and the downward force of gravity on the float. The point of equilibrium is strongly affected by specific gravity and viscosity changes.

The float material is selected to provide a certain weight, which when used with a specific tube taper will provide the proper range. In large, 10" and 5" scale units, the most common float material is 316 st. st. or Hast C. However, aluminum, PVC and Kynar are also used. In small rotameters using ball floats common materials are st. st., black glass, tungsten carbide, red sapphire and tantalum.

The accuracy statement is a function of scale length and tube material. A glass tube is always a higher accuracy tube than a molded plastic tube.

Rotameters have accuracy's stated as a % of the full scale value, which provides increasing error as a % of flow rate as the flow rate drops. Try to size a rotameter so as to operate in the upper 50% of the range to minimize this effect. For a discussion on how accuracy's are defined, see our "Practical considerations in choosing a flowmeter".

Large rotameters:
       10" scale length ...... glass tube is 2% FS, plastic is 2-3%
        5" scale length ....... glass tube is 3% FS, plastic is 3-4%

Small purge rotameters:
        5" scale length ....... glass & plastic are about the same at 4% FS
        3" scale length ....... glass & plastic are about the same at 6% FS
     1.5" scale length ....... glass & plastic are about the same at 10% FS


Viscosity handling characteristics vary with the float design.

   Ball floats ......... no viscosity immunity at all. (Only found on small meters.)
   Cylindrical floats ........ large "drag" surface, little immunity.
   Sharp edged floats ..... maximum immunity to viscosity. (1" and larger)


Practical viscosity limits using sharp edged floats:

    1/2" size ......... 10 cP
    3/4" size ......... 15 cP
    1" size ............ 20 cP
    1.5" size ......... 40 cP
    2" size ............ 60 cP


Construction
- if rupturing of the tube can in any way create a hazard, either use a protective case design or switch to a metal tube meter. If severe water hammer occurs in the line, the glass or plastic tube is what normally ruptures.


Float instability occurs on gas applications with low pressure on the discharge port, typically, in situations where the discharge is to atmosphere or to the intake of a vacuum pump. The easiest way to stop float cycling is to reposition the rotameter to a higher pressure part of the loop, or to install a valve downstream of the indicator to provide some back pressure. Be sure to tell us if you anticipate having this type of installation.

The Hedland meter is a special form of rotameter with a spring replacing gravity. On gaseous application, it should have a 25 psig backpressure to avoid instability.


Sizing and Correcting Liquid Service Rotameters. Sizing is the process of selecting a rotameter. If your fluid is water, you can select the appropriate tube and float directly from the specification sheet. If your fluid is not water you need to calculate the equivalent water flow rate, and then select the tube and float with your calculated data.

Example - You want a rotameter for isocyanate, which has a specific gravity of 1.25 @ 25 C. Your max flow rate is about 20 GPM.

                                                                                                sp.gr. of isocyanate
Equivalent flow rate water   =   Max flow isocyanate  X  ---------------------------------
                                                                                                   sp.gr of water

                                                                         1.25
Equivalent flow rate water   =   20 GPM x ----------  =   20 GPM x 1.118 = 22.36 GPM                                                                          1.00


You would specify a rotameter tube and float combination that would place 22.36 GPM close to the top of the scale. Never put your maximum flow at the top of the scale, the operator will think that it is stuck there and giving a incorrect reading. Normal practise is to size for operation from 20 to 80% of meter span (range).


CORRECTING:

Correcting is what you do when you have a rotameter and changed the fluid going through it. For example, in the above we have sized a rotameter for isocyanate at 20 GPM. Let's say that you have received the rotameter and you need to use it on fresh water service.

All you need to do is take the above correction factor, i.e. 1.118, and divide the indicated flow rate by it.

                                                                                         sp.gr. of water
Actual flow rate of water   =   Indicated flow   X   ---------------------------------
                                                                                      sp.gr of isocyanate

When you need to change the float material for chemical compatilbilty or to get the correct flow range. The sizing tables for larger rotameters are normally based on water service, using 316 st. st. floats. Let's say that you are going to run bleach through the meter and the 316 st. st. will be attacked by the bleach. The solution is to use a Hast C float.

If you are sizing a new rotameter, the factor is changed to:

                                                                         sp.gr. bleach x ( 316 SS sp.gr - sp gr water)
Water equiv flow  =  Max flow bleach  X  --------------------------------------------------------
                                                                         sp.gr. water x (Hast C sp.gr. - sp.gr. bleach)


From the table at the end of this section:  316 SS = sp.gr. 8.02
                                                                     Hast C = sp.gr. 8.94


Sizing calculations, for air at STP (14.7 psia @ 70 (F)

Select directly from the standard ranges provided by the manufacturer.

Sizing calculations, for perfect gases, i.e. argon, carbon monoxide, chlorine, helium, hydrogen, hydrogen chloride, nitrogen, oxygen.

The following discussion relates to sizing using volumetric units, i.e. CFM

The governing equation, written for conversion from a perfect gas to a air equivalent for sizing purposes is:

                                             (actual sp.gr.)          (air psia)          ( actual deg R )
Air equiv. = Actual flow x ------------------- x --------------- x ----------------------
                                               (air sp.gr.)          (actual psia )         ( air deg R )


Example - Assuming argon @ 70 (F, 30 psig, sp.gr. = 1.380, 76 SCFM

                                               1.380                  14.7                 (460degR + 70)
Air equiv. = Actual flow x ------------ x ------------------- x ----------------------
                                               1.000           ( 14.7 +  30 )           530deg R


Result:      Air equiv. = 76 SCFM x 1.175 x 0.5735 x 1 = 51.2 cfm



Sizing calculations, for imperfect gases.

The following discussion relates to sizing using volumetric units, i.e. CFM

Step 1 - From a table of the thermodynamic properties of your gas, determine the specific volume (cu.ft./pound) at the operating temperature and pressure.

Shortcut ... Most engineering handbooks will have a table giving the weight density of air over a wide range of temperatures and pressures. To determine the weight density of your gas, multiply the tabulated value by your specific gravity. Then, take the reciprocal to get the specific volume, i.e.

                                              1
Specific Volume = ----------------------
                                  Weight Density



Step 2 - Determine your specific gravity at the operating condition:

                                                        13.34
Operating sp.gr. = -----------------------------------------------
                                    Specific Volume in Cu. Ft./Pound

Step 3 - Determine the equivalent air sizing flow:

                                                                             Specific gravity of your gas
Equivalent air flow = Your flow in SCFM  X  -------------------------------------
                                                                                     Operating sp. gr.


Reference data for sizing rotameters

Float material            Sp. Gr.                          Gases Sp. Gr. @ STP
===========     =======                   ========================

316 SS                        8.02                          Air ....................................1.0
Hastelloy C                  8.94                         Ammonia (Anh) .............. 0.593
Teflon                          2.20                          Argon ...............................1.38
Glass                           2.53                          Carbon Dioxide ................1.52
Sapphire                     3.99                          Carbon Monoxide ............ 0.97
Carboloy                     15.00                        Chlorine............................ 2.45
Tantalum                    16.60                        Helium .............................. 0.138
Titanium                      4.50                         Hydrogen ......................... 0.069
                                                                     Hydrogen Chloride .......... 1.26
                                                                     Methane ........................... 0.554
                                                                     Natural Gas ..................... 0.60
                                                                     Nitrogen .......................... 0.967
                                                                     Oxygen ............................1.105
                                                                     Propane ...........................1.523

Bypass Rotameters

The bypass rotameter is a standard rotameter, fitted with a range orifice and designed to work in parallel with a mainline orifice plate producing 100 " w.c. at max flow. The purchase order must include the main line flow rate which is producing the 100" w.c. pressure differential.

If providing a 100" w.c. is a problem, perhaps the permanent pressure loss is to great, we can work down to 25" w.c. in most cases. However, this invariably means that the rotameter cannot provide a full 10:1 turndown.

A bypass rotameter installation requires a differential head producing device, generally a 300# orifice flange (weld neck or slip-on), orifice plate, the piping from the orifice flange to the rotameters and 2 block valves for this piping to allow isolation of the rotameter. The calibration of the range orifice on the rotameter is predicated on minimal pressure drop through the piping system to the mainline flange, keep the piping as short as possible.

If your customer requests that we provide the complete package, we can do so. We need to have the data to allow sizing the plate, i.e. what is the fluid or gas, flow rate, pipe size and schedule, gauge pressure, flow temperature, viscosity and specific gravity. If steam, we need the flow in pph, plus steam condition and, if superheated the degrees of superheat.


Rotameters ... Differences between King, SK and Blue White

A variable area meter is any flow meter which has a cross sectional area between a fixed and a moving surface, which varies as the flow rate increases or decreases. As an example, in the classic glass tube rotameter with a tapered internal diameter, the annular orifice between the float OD and the tube ID increases in area as the float rises with increasing flow rate.

The float rate is read from a scale, at the widest part of the float. At this point, the float is in equilibrium ... that is the forces acting downward are exactly counterbalanced by the forces acting upward. If we ignore the finer technical points which involve coefficient of discharge, fluid drag over different float shapes, etc., we can simplistically say that the downward forces are gravity and the weight density of the float. The upward force is the impact pressure exerted by the fluid velocity and the weight density of the fluid.

If the fluid specific gravity changes, the reading of the rotameter has to be corrected. To do this, take the indicated reading and divide it by the square root of the specific gravity.

Changes in viscosity affect the fluid drag on the float. As the viscosity increases, it will drag the float upward. The standard correction is use a float which has a sharp quadrant edge at the widest point, to lessen this effect.

To use the same tapered tube over a variety of flow ranges, you change the density of the float material. As the float specific gravity goes up, it's weight density goes up. This means that greater upward forces must be applied to reach the same point on the rotameter scale. Greater upward force means higher flow rates. If you examine the King literature, page 12 gives the standard flow ranges associated with a single tapered tube, available by using float materials of teflon (sp. gr. 2.2), 316 st. st.(sp. gr. 8.02) and Hastelloy C (sp. gr. 8.94). The heavier the float, the higher the flow range.


Hedland versus other rotameters:

Accuracy - Rotameters made by companies like Blue White and King use the force of gravity to provide the downward force, which means that they must be installed vertically. Hedland units compress a spring as the float moves up on the scale and can be mounted in any position. Because the spring compression must be limited to the linear part of the spring range, the scales are on the order of 1.5 to 2" in length. By contrast the King meter has scale lengths up to 10". (Some flow bench test rotameters have 24" scales). This is the major factor affecting accuracy ... if you have a 20-200 GPM range, what kind of reading accuracy can you get with a 1 1/2 or 2" scale length? The second factor is the repeatability of the force constant of the spring. The spring is operated within the linear portion (below the elastic limit), but not all springs have the exact same force constant and the scales are not made up on a per spring basis, therefore a error can be introduced. This same error is not present in standard rotameters using gravity.

Cleanliness of the fluid - The Hedland unit was designed specifically for use with hydraulic fluids to meet the requirements of a sister company producing hydraulic jack hammers. As such, fluid cleanliness was not a serious design consideration. Hedland recommends the use of a 200 mesh sieve or 74 micron filter if dirty fluids will be metered (exception - their meters intended for water service). By contrast a standard rotameter
has a tendency to be self cleaning and can pass good size particles. When a large particles enters it hits the float, causing it to rise and allowing the particle to pass by. The self cleaning aspect comes about because the fluid velocity increases around the float OD and this tends to scour the tube ID as well as the float OD.

High pressure - The Hedland unit shines in this area... 3000 psi.


Some general comments -

The limitations of the Hedland meter, i.e. short scale and spring based design are also seen in spherical rotameters of the type made by Erdco and Universal.

A variable area meter can be used for things other than measuring flows rate:

If you hold the flow rate constant and increase the density of the fluid, a greater force is exerted and the float will rise to a higher point of equilibrium. This means that you can use the rotameter as a specific gravity measuring device. Not so long ago, one could find rotameters calibrated in terms of specific gravity to determine acid strength.

Another way to use the rotameter is as a mass flow rate indicator. This never became popular because it requires that the float density be exactly twice the fluid density.




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