There have been whole books written about differential pressure (DP) flow devices (orifices, pitot tubes, flow nozzles, venturi meters, elbow taps, wedge meters, averaging pitot, etc.) I only have space to hit the highlights and suggest that you pick up any of several books on the subject if you want to know more about this type of meter.
Concept: Just like every other flowmeter, a differential pressure flowmeter has strengths and weaknesses. The DP meter is one of the oldest means of flow measurement, and many derivatives of the basic meter exist. This section will try to briefly provide the strengths and weaknesses of the offerings and alert you to the possible pitfalls of using this type of meter.
Details: Nearly all differential pressure flow devices use some version of Bernoulli’s Law. Bernoulli’s Law states that the energy of a fluid is composed of three types: static head, pressure, and velocity. If a fluid is forced to pass through a restricted area, then the velocity will increase in that restriction. Because the overall energy must remain the same, the pressure of the fluid will drop as the velocity increases. The pressure drop can be measured to calculate flow, but the pressure drop increases as the square of the flow, so if the flow doubles through a DP meter, the pressure drop will increase by FOUR times.
The differential pressure can be generated in a number of ways. An orifice plate is the most common, but flow nozzles, averaging pitot tubes, wedge meters, venturis, and many others can be used. It is even possible to measure the differential pressure across the inside and outside of an elbow to determine flow rate.
At a high level, here is a list of the pros and cons of this type of meter:
- Works in gas, steam, and liquid applications.
- Is insensitive to fluid conductivity.
- A well designed orifice installation can be extremely accurate, especially if the meter uses a honed meter run and temperature and pressure compensation. Such meters are often used for custody transfer applications.
- Flow nozzles, pitot, and averaging pitot tube meters can generate very low permanent pressure drops. (Note that the turndown of these meters can also be quite limited.)
- Averaging pitot arrays can accurately measure gas flow in odd shaped ducts with minimal meter runs.
- If the line size is large (greater than 6”), the economics favor differential pressure meters over vortex meters. Pitot tubes, averaging pitot tubes, and several insertion type nozzles can be used in extremely large pipelines at relatively low cost.
- Orifice meters do NOT have a low flow cutoff like vortex meters. If a high turndown is required, a high and low range transmitter can measure the DP across the same orifice plate to provide continuous flow measurement through a wide range of flow rates.
- A segmented wedge meter can be combined with capillary seals to provide flow measurement of viscous, sticky fluids such as tar and sludge. This same arrangement can also handle some solids entrainment.
- Because the DP transmitter is usually well removed from the process, DP flowmeters can be used in a wide range of temperatures and pressures.
- Integral flow orifices can measure extremely low flow rates, well below those measurable by vortex meters.
- The square relationship between differential pressure and flow greatly limits the turndown of most DP meters. The pressure drop rises quickly as the flow increases.
- The permanent pressure drop of most orifice plates is about two-thirds of the measured differential pressure. The energy cost can be significant over the life of the meter.
- DP meters are sensitive to installation. The length of the meter run, the location of the transmitter, the method of running the impulse lines, and several other items can greatly impact their accuracy.
- The impulse lines on liquid and steam meters usually require freeze protection. Impulse lines are also prone to plugging in many applications.
- Like a vortex meter, the orifice meter requires an upstream and downstream meter run to establish a good flow profile. Such a run must usually be at least 25 diameters upstream and 10 diameters downstream. (Note that these distances depend upon the particular type of DP meter used.)
- For lines smaller than 6″, a vortex meter is usually a more economical choice if the low flow cutoff is not an issue.
Watch-Outs: The slightest bit of erosion on an orifice plate or pitot tube nozzle can have an enormous impact on accuracy. In addition, the sizing calculations are made for a very specific set of pressure, temperature, and density conditions. If these conditions can vary, the engineer should measure these parameters and adjust the flow rate calculation.
Exceptions: Each type of differential pressure flowmeter has a variety of pros and cons but the sheer number of types provides many options to the engineer. Few processes exist that cannot be handled by at least one version of the DP flowmeter.
Insight: Orifice type meters have been around for a long time and have been extremely well studied. Hundreds of constants and factors now exist, which can be included in the sizing calculations to make them highly accurate.
Rule of Thumb: Vortex meters have been gradually replacing orifice type meters, especially with the advent of the vortex meter made for steam applications. However, there are many applications where only a certain type of DP meter will suffice, and the DP meter (in its many variations) remains one of the most common flowmeters in the world today.