Flow and Differential pressure
Differential type flow measurement are employed in many industries like chemical and petrochemical plants, Refineries, water treatment plants etc.Flow measurement using primary elements like orifice plate ,venturi tube ,pitot tube etc are examples of differential pressure type flow measurement .In this method Differential pressure created with the help of these primary elements is measured using a differential pressure transmitters and it is converted to flow as differential pressure varies with flow.Some examples of primary elements for flow measurements are described in this article .
A differential pressure is created in a fluid stream using the primary elements like orifice plate or ventury tube etc.Actually they are flow restrictions which creates high pressure and low pressure points in a flowing fluid in a pipeline. These high pressure and low pressure streams are fed in to a differential pressure transmitter through proper tappings or impulse tubes.These transmitters measures the differencial pressure created .and this DP is related to flow as
Q = k√DP
Where Q is the flow rate
,DP is the differential pressure
and k ,a constant.
DP transmitter either converts this DP in to flow as per this relation and send a signal which is proportional to flow. Or it transmitts a signal which is proportional to DP and this is further converted in to flow in control system side.
Primary elements for flow measurements
The function of primary elements to create a Differencial pressure in a flow pipeline . various types of flow restrictions are used according to the conditions and factors A brief note of some of them are given below.
1.Orifice plate
Thin metallic plate with a hole to allow the passage of fluids through it. It is usually placced between the pipeline flanges .Its presence generates a differential pressure in a fluid stream .Different types of orifice plate are available and we can select them depending on the applications and nature of fluid motion . orifice types are named based on the shape of hole in it like concentric orifice, eccentric orifice,segmental orifice etc. Each one of them are suitable for specific applications .
Ventury tube
It is so constructed that it can be readily fitted or integrated with the pipeline flanges .see figure below .It consists cylindrical inlet part followed by a convergent entrance known convergent cone a cylindrical throat and a divergent outlet part known as divergent cone . The inlet and outlet part of the section has the same size as that of pipeline .so it can be so easily fit into the pipeline flanges .
Pitot tubes
Pitot tubes are a popular device for measuring fluid velocity It is mostly used in aircrafts, submarines etc to measure their speed .Also it is a a primary element for flow measurement applications .In Differential pressure type flow measurement it is used to create the static and total Pressure heads whose difference can be interpreted as flow. It consists of two tubes one being perpendicular to the flow steam and receives a head equivalent to static pressure . Another one is a tube facing to the flow stream .And the head developed in this tube is the total Pressure Or stagnant Pressure . Stagnant means the fluid become stagnated at this tube because it’s motion is restricted causing the whole kinetic energy of the fluid entering is converted in to potential energy .Thus a head equivalent to the velocity of the fluid is developed in addition to the static head .So the head developed inside this tube is the total head .
Thus Total head = Static head +dynamic head
Dynamic head = Total head – static head
So the difference in heads ,ie difference in value of the Total and static head gives the velocity head from which we can calculate the flow rate by the same flow and DP equation .
F = K √DP
Flow nozzle
Flow nozzle ,a cone shaped restriction which is used as a primary element for fluid flow measurement .see below figure .The inlet section which has large cross sectional area is gradually converging into throat where the area is minimum .Hence a high pressure and low pressure area are being created in inlet and section and throat respectively High pressure and low pressure tappings are fed to the DP measuring device and flow rate is deduced as
Flow = K √DP
