Dpt_Principle

Differential Pressure Transmitter (DPT) using orifice plate. Principle The Orifice Plate is a device that houses, and accurately positions for measuring flow within a pipe or tube. The orifice meter consists of a flat orifice plate with a circular hole drilled in it. There is a pressure tap upstream from the orifice plate and another just downstream. There are in general three methods of placing the taps. The coefficient of the meter depends upon the position of taps. When placed, an orifice plate will produce a differential pressure by abruptly constricting the fluid flowing through it. The differential pressure is measured across the plate through taps precisely located on the orifice plate. An orifice plate installed in a line creates a pressure differential as the fluid flows through it. This differential pressure is measured via impulse lines by a differential pressure transmitter which converts it into an analogue or digital signal which can be processed to provide a display of the instantaneous rate of flow. The relationship between the rate of flow and the differential pressure produced is very well understood and is fully covered by comprehensive national standards. The relevant standards are BS 1042 and the equivalent ISO 5167. One of the principle advantages of orifice plates manufactured and installed following these standards is that they do not require calibration. This means that orifice plates are very cost effective on larger line sizes. b) Vortex Flow meter Principle An everyday example of a vortex shedding phenomenon is a flag waving in the breeze: the flag waves due to the vortices shed by air moving across the flagpole. Within the flow meter, as flowing liquid moves across the tiny strut or “bluff bar”, vortices are also shed but on a smaller scale. The meter transmits an ultrasonic beam through the vortex pattern downstream of the strut. As vortices are shed, the carrier wave of the ultrasonic signal is modified. This change in the carrier wave is measurable and moves in proportion to the number of vortices shed. Digital processing enables the vortices to be counted, and this value is converted into a velocity. Software converts velocity into a volumetric flow rate, in units of measure selected by the operator. Vortex flow meters utilize the smallest strut in the industry, which allows for high sensitivity; superior performance at very low flow rates; large turndown ratios; and low pressure drop. Operating measure Liquid level in vessels is a very popular application for differential pressure transmitters, where they are often the most cost-effective solution. Fluid within a tank develops a head pressure that can be measured at the bottom of the tank. Head pressure is a function of the fluid’s height and density. If the tank is an open or vented vessel, the level measurement could be performed with a gage pressure device, but as is typical with most tanks, the amount of pressure produced is often very low. Head pressures of less than 400 in. H2 O is very common. Gage pressure transmitter spans are usually much larger. To improve measurement precision, it is often better to use a more sensitive DP transmitter and vent the low pressure port to atmosphere.     If the tank is a closed vessel, a DP transmitter is an even more cost-effective solution. When kept in a sealed tank, liquids release vapor until pressure equilibrium is achieved. The vapor pressure rests on top of the fluid and adds to the head pressure reading that is measured at the bottom of the tank. To calculate a true fluid level, vapor pressure needs to subtracted from the total head pressure. Here again, rather than using two independent gage pressure transmitters, a single DP transmitter can be applied to receive both the total head pressure and vapor pressure. The differential pressure is representative of the net head pressure, which indicates liquid level.   REFERENCES 1) http://www.engineeringtoolbox.com/flow-meters-d_493.html 2) http://www.sensorsmag.com/sensors/leak-level/the-principles-level-measurement-941 3) http://editionsbyfry.com/Olive/ODE/CTL/default.aspx'href=CTL/2010/08/01&pageno=56&entity=Ar05600&view=entity 4) http://www.omega.com/literature/transactions/volume4/T9904-07-DIFF.html 5) http://www.maritimesuppliers.com/imidocs/0699p031%20orifice%20plate%20flowmeters.pdf 6) http://www.daniel.com/Products/Gas/Orifice/Simplex/Manuals/08004B%20Daniel%20Simplex%20Orifice%20Plate%20Holder.pdf 7) http://www.freepatentsonline.com/3999435.pdf 8) http://www.SpiraxSarco.com