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DIFFERENTIAL PRESSURE AS A MEANS TO MEASURE FLOW

Introduction | Maximum Flow Chart | Criteria needed to calculate bore size | Steam Properties |
Pressure Temperature Ratings for Carbon Steel Flanged Fittings | Readout Devices

For many troublesome gasses and fluids, the time-honored method for obtaining flow is to use a primary element, such as an orifice plate, a flow nozzle or flow tube, venturi or an averaging pitot tube. Inserted into a pipe, all of these devices will develop a differential pressure that can be equated to flow. The most common type of primary element for this purpose is the orifice plate, because it is the cheapest and easiest to obtain (it is perhaps also the most expensive to operate due to the pressure loss and resultant pumping costs). In any case, with all primary elements, the drop in pressure varies as the square of the flow (when the flow rate is doubled, the differential is increased four times). This is the reason for-the necessity of the non-linear square root dial on analog instruments, and a squareroot extractor (API, RiS) with a DP transmitter or a differential pressure transmitter with integral square root extraction, or a Smart Digital Panel Meter, Eke the Red Lion PAX with 16 segments of linearization possible. In providing a useful system, the following is of interest:

ORIFICE FLANGES:
Minimum size is 300# with DP taps located in the orifice flange one inch up and one inch downstream from the orifice plate. These are made for either piping or tubing, and you must specify the type, size and schedule.

ORIFICE PLATE:
Most common is the 1/8" thick concentric orifice plate in either 304 or 316SS (exception: for #2 Diesel and other hydrocarbon fuels, you should use a quadrant edge plate). Quality Products will provide the calculations and bore size required for a specific flow in a, specific pipe size. This is known as the beta ratio: that is the bore (or the diameter of the orifice plate hole) as compared to the internal pipe diameter, or O/ID For example, if the orifice hole is 2", and the pipe diameter is 4", 2 divided by 4 equals a beta ratio of 0.5. Recommended beta ratios are from .20 to .75. The following are representative for a variety of flows of water at 68 degrees F and 14.7 PSIA or 0 Gauge:

MAXIMUM FLOW - GPM
Differential In Inches of Water

In order to calculate the bore size necessary for the flow you wish at the differential you want, the following are the criteria to be obtained:

FOR LIQUID FLOW

• Maximum differential pressures in inches of water.
             OR
• Maximum liquid flow in GPM at the above differential.
  Specific gravity of the liquid at 60 degrees F.
  Pipe size and schedule, or I.D.
  Specific gravity of the sealing liquid at 60 degrees F. (if used)
  Temperature of the flowing liquid in degrees F.
  Specific gravity of the liquid at the flowing temperature.

FOR GAS FLOW

• Maximum differential pressure in inches of water
             OR
• Maximum gas flow in SCFH at the differential above
  Pipe size and schedule, or I.D.
  Specific gravity of the gas at 60 degrees F.
  Temperature of the flowing gas in degrees F. Line pressure in PSIG based on 14.73 psi absolute.

FOR STEAM FLOW

• Maximum differential pressure in inches of water.
• Maximum steam flow in PPH at the differential above.
  Temperature of the flowing steam in degrees F.
  Line pressure
  Pipe size and schedule, or I.D.
  Specific gravity of the sealing fluids at 60 degrees F. (if used)
  
• WE MUST HAVE ONE OF THESE TO SIZE

If, however, you are matching an orifice plate and flow to a dp cell or indicator you have on the shelf, you must watch the beta ratio and keep it between .20-.75.

Steam Properties:

• Lbs per Hour of Steam / 500 = Approximate GPM of Condensate
• Example: 500 lbs per hour steam / 500 = 1 GPM (Approximate)
  
• Steam is a gas and is perfectly transparent, colorless, dry and invisible. When partially condensed, as in contact with air, the mist or spray makes it visible.
  
• Dry steam contains no free moisture. It may be either saturated or superheated.
  
• Wet Steam contains free moisture in the form of mist or spray, and has the same temperature as dry saturated steam of the same pressure.
  
• Saturated Steam is steam in its normal state. Its temperature is that due to the pressure under which it is formed; that is, its temperature is the same as that of the water from which it is generated and upon which it rests.
  
• Superheated Steam is a steam of a temperature above that due to its pressure, having been further heated from another source, after leaving the water from which it is generated.

STEAM, WATER, AND OIL PRESSURE TEMPERATURE RATINGS
FOR CARBON STEEL FLANGED FITTINGS AND COMPANION FLANGES
(OTHER THAN RING JOINTS)

READOUT DEVICES:

Liquids: Barton, Dwyer Capsuhelic "B", Orange Research or Wika Differential Pressure Indicators with Square Root Dials. Barton Flow or standard DP Cells with Red Lion PAXP Digital Indicators.

Steam: Care must be taken that live steam does not hit the gauges or DP cells. Copper coils, filled systems or down-leg piping allows condensate temperature to drop to acceptable levels.

Switches: Barton ITS, Orange Research or Solon offer indicating switches. Mercoid and Solon offer blind switches.

Shut-Off Valves: Should be Ball Valves

Sealpots: Also known as condensate chambers. In steam flow, it is important to provide a constant water head on each side of the flow meter. You assure a constant water level and resultant head pressure with condensate chambers. Also, if you wish to seal out the measurand from the indicator or DP Cell, the sealpot can be filled with a compatible fluid. This is not needed with DP Cells, in general, due to their low displacement.

3 Valve Manifolds: The three-way valve allows on-site zeroing and calibration checks for DP cells. The 3 way valve will block the low pressure side, and open. In the high pressure port to both sides of the transmitter. The equalized pressure should result in a "zero output" signal.