Flow Measurements


Flow meters are generally divided in to two fundamental types 1. Quantity Meters. 2. Rate of flow meters

Quantity Meters : These are used to measure the quantity of flow that has passed certain point . in simplest form they can be visualized as gear pump (rotor) which is driven by fluid & hence quantity is measured by number of rotation ( cycles ) and fluid per cycle.

Rate of flow meters. Measure the velocity of fluid passing a certain point at given instant. From this rate of flow can be determined from velocity multiplied by area of passage. That is why they are classified as Inferential (volume inferred from the Velocity ) There are two fundamental components of the rate of flow meter. The Primary element : portion of instrument which enables flow into variable. e,g orifice & pressure tapping from Venturi. Secondary element : portion of Instrument measures the variable created by primary element. e.g differential pressure cell

Quantity meters are expensive devices, there fore Rate of flow meters are often used as quantity meters by fitting Integrator. Integrator , measures rate of flow for certain time interval and hence measures the quantity over certain period of time . It Should be noted that integration is general instrumentation operation whose use is not restricted to flow meters.

as we discussed above , rate of meters are inferential they can be further classified on basis of inferential instrumentation. Lets discuss one by one.

A. Inferential – Rotational This can be considered as Mechanical or electrical .

B. Electro – Magnetic Flow meter The Principle utilized is that of moving conductor (the liquid) in magnetic field generating a potential difference. In Simple arrangement shown the electro-magnets are supplied with current. There are two sensor electrodes . If B is the flux density of the field, v velocity of flow , d pipe diameter the n the suitable units the emf generated at any instant is given by e= Bvd

for constant B and d, e is directly proportional to v (v= Velocity)

C. Inferential – Differential Pressure Primary element : portion of instrument which enables flow into variable. e,g orifice & pressure tapping from Venturi.

Secondary element : portion of Instrument measures the variable created by primary element. e.g differential pressure cell

Why we have to use square root extractor ?

as we saw above in Primary element the quantity measured on scale will be non- linear for direct recorders due to square root relation and telemetering therefore correcting unit is to be fitted to for correct measurements .

when inferential devices utilized, with velocity sensors utilizing differential pressure techniques , the velocity is not directly proportional to pressure difference , or head. Velocity is related to the square root of pressure or head ie a curve of flow rate plotted against pressure or head is of parabolic form . this means that if pressure difference is used in sensor device connected to manometer or pointer through linear mechanism , the rate of flow scale on the manometer would have to be square root function , the scale division would increase in square root increments for equal increments of flow rate . from the aspect of display and continuous recording of flow rate the square root characteristic is not an embarrassment . if however the differential pressure signal has to be used in control system the square root is usually extracted to give signal which is directly proportional to the fluid flow rate . It is difficult to integrate reading of the system when the square root extraction is not applied .

Above diagrammatic sketch of one type Square root extractor utilizing pneumatic flapper nozzle position balance principle . The differential pressure from flow sensor acts on the horizontal lever B this effects the amount of air escaping from the nozzle and hence the pressure in the bellows. Pressure alternation in the bellows causes movement of the vertical lever A. The very small relative motion between lever A and B provide square root extraction. the out put air signal is directed to the measure element ( recorder ) or controller

Fig shows square root extraction technique using electrical force balance , the differential pressure, in electrical signal form represented as variable X in put, is applied to the left hand side. With the force balance B in equilibrium the output signal is variable square root of X. Variation in Input signal, causing unbalance , can be arranged to be re-balanced by suitable adjustment to output signal. This can be done in various ways , one method could be to connect the right hand of the beam to a differential capacitor in the output circuit . This s closed loop but without electrical connection between input detected signal and out put measured signal The force balanced system is simple analogue computer output can also be easily arranged for squaring, summing , etc by suitable wiring combination

Fig shows two arrangements of flow sensor / transducers units , each with square root extraction. for the pneumatic system valves marked A, B & C would be used in sequence open B, Close A and C, when taking the differential pressure transmitter out of pressure transmitter this valve sequence would also applied to the electrical system with steam flow measurements the pressure tapping from the flow sensor are lead to cooling reservoirs where in the steam condenses, Water only then acts on square root eliminator or recorder thus preventing damage.