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The pros and cons of rotameters

Also known as variable area flow meters. These instruments are perhaps the most simple technology for flow measurement. Despite its simplicity several factor may affect it so that it is not always the best option.

Fig. 01 A rotameter or variable area flow meter


Contents

How does it work?

Its working principle comes from a balance of forces acting on a float inside a vertical duct of conical geometry. This balance of forces, acting on the float, is as follows:

$F_G = F_D + F_B$        Eq. (01)

where $F_G$ stands for the gravitational force correspondig to the effect of the float weight acting downwards. $F_D$ stands for the drag force acting upwards due to the fluid traveling through the duct and passing the float. Finally, $F_B$ stands for the buoyant force acting upwards too due to density differences.

Fig. 02 Forces acting on the float inside the conical shape pipe.


For short, the fluid velocity, say $v_f$, appears in different terms of Eq. (01) so that it can isolated very easily.  The fluid velocity $v_f$ is then a function of the viscosity, the density of the fluid and the float, and of the cross section area of the float and the duct, which can be easily determined.

However, a further question remains, if you have the fluid velocity in a conical shape pipe, how can you read the flow rate in a scale in pipe wall?

Easy. You need to use repeatedly the equation,

$Q=v_FA$        Eq. (02)

at every height in the pipe where you want to know the flow rate. As you can see, the cross section area changes with height and so will the flow rate.

Some advantages of rotameters

  • Training is not needed (little, in fact) for measurement reading. You will need to know the part of the floatat which the reading has to be taken. There are several shapes of floats according to manufacturers and models.
Fig. 03 Some examples of floats in rotameters. Notice that the point at which flow rate must be read in the scale is diferent for every float (indicated by the horizontal line).


  • It is cheap. Although its cost shall depend on the operation conditions (maximum flow rate, pressure, temperature and fluid-instrument compatibility). Reliability is also an important parameter: prices may also vary from one manufacturer to another.
  • Different models/designs are available in the market.
Fig. 04 Some examples of rotameters. Some are for small flow rates, some for gasses and some others include switches too.


  • Some models/designs may offer one or two flow switches (which would increase its cost). These can be NO/NC contacts for triggering alarms, for example.
Fig. 05 Rotameter with two switches intended for triggering two alarms (low and high).


  • Rapid response (in the order of ms)
  • Cleaning is easy
  • Its installation does not require calibration. Also, wearing of the scaling in the instruments wall is not a common problem.
  • These can be used for liquids and gasses

Some disadvantages of 

rotameters


  • It is not portable instrument. There are no handheld versions or models.

  • It is in principle a visual in instrument because of its float so that it is commonly manufactured from transparent materials (some plastic) which makes it fragile with low resistance to stresses or fatigue.
Fig. 05 An example of a rotameter made of metal instead of plastic. It is a rare model. Notice that it works for slow flow rates.


        However,  there are with metal duct instead of plastic forcing the instrument to be digital. Convinience of these models may be restricted.
  • These type of instruments are restricted to open control loops. In other words, automation is not possible. At most, flow switches.
  • Flow rate data can not be automatically transferred to another device. It depends on a human to read the data from the scale
  • Because of the way in which the balance of forces is made, its installation has to be made vertically upwards only. Horizontal and inclined installation positions are forbidden.
  • Since it is, usually, plastic made, it has to be installed indoors. Otherwise sunlight will the polymeriq material causing reduction in wall transparency, reduction in pressure resistance or other damages.
  • It is not a precise instrument since human error can be introduced when reading at the scaling.
  • Scaling in the tube wall may not have the desired precision so that estimated reading of the flow rate is generally required.
  • It has to be installed in places with low vibration. Long term fatigue may crack the instrument so that leaking may appear.
Fig. 06 Cracks in a rotameter due to plastic fragility.


  • Its installation requires that no load should be exerted on the instrument (it will crack if so)
  • It has to be installed in places with good illumination and accesible to the operator so that meaurements ca be read
  • Dirt can be attache to the walls of the instrument making readings a bit difficult
  • For specific applications a supplier must be contacted. Material compatibility and operation conditions may be critical to know befor buying
  • It is restricted to clean liquids and gasses such as water or air (it depends on the model and manufacturer). In other words, if fluids different from water and air are to be used a specific adjustment/correction to every flow rate measurement must be performed (by a formula provided by the manufacturer)
  • If actual pressure and temperature conditions are different to those especified by the manufacturer a correction to every flow rate measurement has to be done too. 

Pressure and temperature correction in rotameters


If during process operation demands using a rotameter in conditions different to the standard conditions some correction to every flow rate measurement must be introduced.

Fig. 06 Standard conditions for a rotameter in a label in the instrument.

These conditions are given by the manufacturer in the instrument manual or as a label in the instrument itself. These conditions are for pressure and temperature. If pressure and temperature are different to the standard conditions the corrected flow rated can be estimated from the formula,


$Q_{S}=Q_{NS}\sqrt{\dfrac{P_{NS} T_{S}}{P_{S}T_{NS}}}$        Eq. (01)


where the subscripts $NS$ and $S$ stand for non standard and standard, respectively. Also,

  • $Q_{NS}$ would be the actual flow rated at non standard conditions of pressure and temperature
  • $Q_S$ would be the corrected flow rate
  • $P_{NS}$ would be the actual absolute pressure at which the rotameter is being used. This is 14.7 psi + gage pressure (commonly used)
  • $P_S$ would be the standard pressure (as printed in the instrument manual or label). This is 14.7 psi (corresponding to 0 psig) (commonly used)
  • $T_{NS}$ is the actual temperature
  • $T_S$ is the standard temperature (as printed in the instrument manual or label)

An example for pressure and temperature correction on flow rate



Consider a rotameter for air flow rate measurement. between 10-100 SCFH air. The float indicates 60 SCFH air in the flowmeter scale. The standard conditions for this model are 14.7 psia and 530 R. The actual, non standard, conditions are 19.7 psia and 545 R. What is the corrected flow rate?


After substitution of the data into Eq. (01) gives the corrected flow rate to be: 68.5 SCFH air.


As you can see, an important difference in the measurement may be influenced due to non standard conditions of operation for the instrument.


Recommended installation of a rotameter



Since a rotameter is an invasive instrument catastrophic failures may lead to process line shutdown. A rotameter intended to monitor the flowrate of an important stream may require a reliable installation technique.

Fig. 07 Recommended installation for a rotameter.


First, at least two valves are required in the pipe where the instrument is to be installed. The first one is a gate valve whose purpose is to be always fully open and closed only in case of instrument failure. The second one can be either a throttling valve if there is an open loop so that the flow rate can be modulated easily but if you have a closed control loop then another gate valve is preferred.


As shown in Fig. 07 an extra throttling valve installed in parallel to the rotameter is advised as well. If the rotameter fails it can be directly isolated by means of the two afore mentioned valves and repaired. In the mean time, the parallel throttling valve can be used to modulated the flow rate so that the process shut down is avoided.


Valves before and after the rotameter are to be installed at convenient distance from the rotameter. You can safely consider 5D and 10D respectively. Some manufacturers also provide this information for their specific models.


Corrections due to fluids different to water or air



As mentioned early, rotameters are usually manufactured for water or air. So, can a different fluid be used in a rotameter? For short, the answer is yes. The flow rate readings should be corrected accordingly.


At this point, it should be mentioned that there are several formulas for the flow rate measurement correction so that the instruments manual must first be consulted.


As a caution you must consider not using a rotameter designed for gasses for liquid applications. Readings can be quite different.


Since different fluids have different specific gravity and this affects directly the float behavior. It is straight to think of a factor based on this fluids properties.


$Q_A=Q_I\dfrac{sg_C}{sg_A}$        Eq. (02)


where $Q_A$ is the actual or corrected flow rate corresponding to the new fluid, $Q_I$ is the flow rate read at the rotameter scale, $sg_C$ is the specific gravity of the fluid used by the manufacturer to set the scale and $sg_A$ is the specific gravity of the new process fluid.


An example of flow rate correction due to usage of a different fluid



Consider that you are intending to use a rotameter, whose manufacturer used water as the calibration fluid for setting the instrument flow rate scale, to measure the flow of hexane. What would be the hexane flow rate if the rotameter indicates 12.5 GPM (of water)?


First, for hexane sg = 0.67 while for water sg is about 0.99. Therefore, using Eq. (02) we obtain,


$Q_H=12.5\times \dfrac{0.99}{0.67}$

$Q_H=18.47 $ GPM


Check your knowledge on rotameters



Please, follow the link below to answer a quizz on rotameters and see how good you are on this subject


What you know about rotameters


This is the end. I hope you find this post useful.


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