Basics of Permanent Pressure loss in Differential Pressure Flow Meters

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The standard primary flow sensors commonly used in differential pressure flow meters are the orifice plates, flow nozzles and venturi tubes. These flow meters are often called "head loss" meters because there is a permanent pressure loss downstream these meters. In otherwords, upstream pressure never recovers to its original value downstream these meters. Various designs of these flow sensors are available which can provide the optimal meter for the desired operating conditions and requirements of the user. A critical factor in choosing a differential pressure flow meter is the pressure loss of the flow sensor. As a rule, when applying differential pressure devices, pressure loss must be small. This is because pressure loss means energy loss and higher pumping/compression costs.

The different installation versions of the primary flow sensors (orifice plates, flow nozzles, venturis) of differential pressure flow meters commonly used in flow measurement are tabulated below:
Primary Element Type of Primary Element
Orifice Plates Corner Pressure Taps
Flanged Pressure Taps
D and D/2 Pressure Taps
Flow Nozzles ISA 1932
Long Radius
Venturis Venturi Tube
Venturi Nozzle

The graph below shows the percentage pressure loss against beta ratio (beta = d/D) for orifice plates, flow nozzles and venturis:
From the permanent pressure loss graph above, we can deduce the following:
(a) Orifice plates are universal in their use in differential pressure flow meters but have the basic disadvantage of high pressure loss.
(b) Flow nozzles are characterized by lower pressure drops compared to orifice plates
(c) Venturis (Venturi tubes and Venturi nozzles) are characterized by much smaller pressure drops compared to orifice plates and flow nozzles.

Application Limits for Orifice Plates, Flow Nozzles and Venturi Flowmeters
The table below summarizes the various application limits with respect to bore or throat diameters(d), internal diameter of pipes (D), beta ratio (β =d/D) and pipe Reynold’s number (Re) for orifice plates, flow nozzles and Venturi flow meters:
Flow meter parameters Orifice Plates Flow Nozzles Venturis
Corner Pressure Taps Flanged Pressure Taps D and D/2 Pressure Taps ISA 1932 Long Radius Venturi Tube Venturi Nozzle
dmin (mm) 12.5 12.5 12.5 15 10 20 50
Dmin (mm) 50 50 50 50 50 50 65
Dmax (mm) 1000 760 760 500 630 1200 500
βmin 0.23 0.20 0.20 0.30 0.20 0.30 0.32
βmax 0.80 0.75 0.75 0.8 0.8 0.75 0.78
Re, Dmin 5,000 - 20,000 2,500 - 540,000 2,500 - 540,000 20,000 10,000 200,000 150,000
Re,Dmax 100,000,000 100,000,000 100,000,000 10,000,000 20,000,000 1,000,000 2,000,000



SelectIion Chart for Point Level Measurement Technologies

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Point level measurement is commonly done using the following technologies
(a) Capacitance sensors
(b) Nuclear sensors
(c) Vibrating fork sensors and
(d) Float switches
The above technologies are often best suited to certain process conditions or a combination of process conditions. To apply these technologies, some questions commonly asked include:
  1. Which level measurement technology best suits density changes in the process?

  2. Which technology will suffice for a changing dielectric strength of process fluid?

  3. Which technology can be best used where you have solids, dust, foam, slurries, emulsion, internal obstructions, vapors, viscous/sticky product?

  4. Which technology is best suited for high process temperature limits, high vessel pressure limits, low process temperature limits, low vessel pressure limits?

  5. Which level measurement technology can best resist noise (EMI, motors), product coating etc?

  6. Which technology is best suited a process where there is aeration, agitation, ambient temperature changes, or corrosion?

The above questions are

NEMA Insulation Classes for AC Motor Windings

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NEMA motor insulation classes describes the ability of motor insulation in the windings to handle heat. There are four insulation classes in use namely: A, B, F, and H. All four classes identify the allowable temperature rise from an ambient temperature of 40° C (104° F). As already noted in How to Read Electric Motor Nameplate Data, Classes B and F are the most common in many applications.

Temperature rises in the motor windings as soon as the AC motor is started. As shown in the table below, the combination of ambient temperature and allowed temperature rise equals the maximum rated winding temperature. Allowable temperature rise is made up of the maximum temperature rise for each insulation class plus a hot-spot over-temperature allowance. If the motor is operated at a higher winding temperature, service life will be reduced. As a rule,
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