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Differential Pressure (DP) Type Flowmeters

  • The transmitter measures DP by using 2 diaphragms. These 2 diaphragms are made into 2 capacitors from 2 reference plates. The configuration is shown below
    • clip_image00265
    • The separator liquid acts as a pushing medium for both pressures. The capacitance between the diaphragms are measured and presented into signal current.
    • The ventilation valve is not for line flushing. This is because the transmitter chambers are not flushed, only the impulse lines are flushed. Line flushing should be used done from the top of the transmitter.
  • It is a common practice that, for gas product, the orifice plate impulse line should be at the top, to provide water to go in the impulse line. The transmitter should be above the orifice plate so that unnecessary liquid from the product impulse line does not go inside the transmitter.
  • For liquid products, the impulse line on the orifice plate should be below or on the side and the transmitter too should be below the orifice plate.
  • For transmitters, the most common problem is electronic circuit failure. The diaphragm hardly fails. Drifts are a result due to dirt and very small corrosion.
  • Again I repeat, the diaphragm hardly fails. There is a very little possibility that when calibrating, the fluid inside is lost and pushing one diaphragm does not result in pushing the other side of the diaphragm. I don’t know what happens if this occurs because we have 2 capacitance. Hmm… this is the reason why they have 2 capacitors I guess. To detect if this liquid lost occurs, i.e. only 1 diaphragm is being pushed
  • Square rooting of signal is done on the DCS for old transmitter types. New smart transmitters have built in square root function.
  • To calibrate a DP type transmitter, it is important that the low side vent is on open air and the high side vent inserted with the calibrator. Open air is used as the reference DP because it does not change (i.e. is always 1 atm). So, when both vents are open ( or 1 vent is open and equalizing valve opened) , we set the zero reading DP. When equalizing valve is closed and high side vent is closed, we trap a pressure and start calibrating.
  • The orifice plate of a DP type transmitter has a tagging on them. The side of the tagging indicates the direction of flow (downstream).
  • While calibrating, if the low side vent is closed while high side is injected with pressure, nothing will happen because the air is compressed on both sides giving equal pressure. Not even a small flick on the signal or local indicator will occur.

Orifice Plates

  • Orifice plates are used to create a significant pressure drop on a flowing liquid/gas so that it can be measured by a Differential pressure instrument and converted into flow.
  • Orifice plates must be inserted between 2 orifice flanges.
  • The name plate of the orifice plate shows the upstream direction of the flow. It has details such as the orifice diameter, the word ‘square’, the word ‘upstream’, the tag number, the ANSI class rating, the nominal line size, the material of the orifice plate
  • Impulse lines can be tapped either from
    • Flange tap (1”/ +25.4 mm Distance) – Most of our plant is flange tap
    • Corner tap
    • D D/2 tap
  • The tap type contributes very little change to the measured DP
  • To calculate the whole size of the orifice plate, the following properties are required
    • Process Conditions, which are
      • The Operating Density
      • The Operating Pressure
      • Viscosity
      • Isentropic Exponent/K-factor (for gas)
      • Temperature
  • The maximum flow rate
    • For liquid, the operating flow rate or mass flow rate is required
    • For gas, the normalized flow rate is required
    • The Inner diameter of the Pipe
  • For high accuracy, Orifice plate shall be installed with a straight length requirement. ISO5167 specified that a straight length of 26D upstream and 7D upstream is required
  • If there is no straight length available, there is a new technology where an orifice with 4 holes can be used
  • Advantage
    • Cheap
    • Proven
    • Easy installation
  • Disadvantage
    • Low turn down ratio (3:1)
    • Effected by density change
    • Pressure loss in pipe
    • Need upstream and downstream straight length requirement
  • Types
    • Concentric
      • Orifice at the centre
      • Typical type. Used for ideal liquid
    • Eccentric
      • Orifice at the edge
      • For liquid containing solids, oil containing water
      • Used for slurry liquid and liquid with heavy solids
    • Segmental
      • Hole in the centre but in the form of a half circle (Since it’s a half circle, the hole will be at the bottom)
      • Used for VERY slurry liquid and liquid with heavy solids
    • General Rule, for normal liquids use concentric, but if got slurries or solids use eccentric. If verry slurry, used segmental
  • When to avoid orifice plate?
    • Highly Corrosive/erosive liquid
    • High turn down ratio
    • High viscosity or laminar flow. This is because the flow pattern of laminar flow is not evenly distributed. It is only distributed in the middle

Manifolds

  • Manifolds is a general term for any thing that has many parts and features. In instrumentation world, manifolds are normally referred to the an assembly of valves which connects between the instrument and the process line
  • The key features of manifolds are
    • They should have an isolation and ventilation point. This is normally referred to as block and blee
    • They should not leak. This is primarily done by the isolation valve packings which are made of either PTFE or Graphite. PTFE is cheaper but the problem is that they cannot withstand too much heat. However, since impulse lines is a dead leg, the heat will cool down with ambient temperature. The only problem is when flushing activity, where the process heat can damage PTFE
  • One rule of thumb to remember is that manifolds should withstand pressure up to 200 bar (at a 500 deg C. The pressure will be higher if temperature is lower but lets just put 500 degC as our rule of thumb
  • Connections to process are usually 3/8” tubes. This is because impulse lines are 3/8” in most standards (including PTS). However, old plants still use ½” for impulse lines
  • There are several types of manifolds
    • 2 way valve manifold is used for pressure gauge
    • 3 way valve manifold is used for DP, though not recommended as it does not have a venting facile.
    • 4 way manifold valve is used for DP. There is only 1 ventilation.
    • 5 valve manifold is used for DP also has calibration/ventilation facility
  • Manifold are used to provide an isolation, venting and also flushing service to a DP type transmitter

clip_image00285

  • The picture above shows the manifold configuration. Isolation valve is always the first valve to be seen from the product impulse line
  • In most applications, there are only 4 valves. This is because there is only one ventilation valve on the low side. To flush the high side, simply open the equalizing valve and close the lowside isolation valve.
  • In some applications, no equalizing valve exists. Instead, there are two vents. To equalize, simply open the 2 isolation valves. This is common in DP level instruments.
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