Key points
Differential pressure (DP) flow measurement technology has been and remains the most widely used approach as plants work toward a higher level of operational excellence.
It’s not hard to see why. Engineers and technicians understand how it works, it can handle any kind of fluid (liquids, gases, steam, etc.), installations can be built in any size, and it even lends itself to DIY setups.
At the same time, it has a few drawbacks. DP flow meters cause line pressure loss, they need long straight pipe sections to ensure accuracy, and many of those DIY setups are maintenance headaches with leaky impulse lines. Over the decades, instrumentation providers have worked hard to preserve the basic advantages of DP flow technology while mitigating its drawbacks.
Some of the first things users wanted to get rid of were the troublesome external impulse lines and long straight pipe sections. To accommodate those requests, Flow Meters that use four orifices rather than a single orifice to create the necessary DP signal have been introduced.
They mount the DP transmitter directly to the flange with integral impulse lines eliminating separate pipe taps and tubing. The result: a leak-proof installation that can be installed between two flanges anywhere in the piping where there are at least two diameters of straight pipe on both sides. This simplifies installation and operation by allowing a measurement to be added just about anywhere.
DP technology is scalable to any line size, but when users have to work with large pipe diameters, extra fittings and flanges get expensive. For these installations, using an Annubar™ averaging Pitot tube approach avoids expensive pipe modifications. All it needs for mounting is a small hole through the pipe wall to insert the Annubar sensor element.
For large diameter pipes the cost of an Annubar flow meter is exponentially less than other primary elements. The configuration also includes integral impulse lines for leak-proof operation. The Annubar flow meter option is an excellent choice for applications of any size where it is critical to minimize pressure loss since it represents the smallest pipe obstruction.
In situations where process needs call for more than a simple volumetric flow measurement, using a multivariable DP transmitter setup can provide a fully compensated mass flow reading. By adding a temperature and static pressure sensor for the fluid, and a few fluid characteristic values, it is possible to calculate many additional variables, including mass flow, energy flow, totalized flow, process temperature, static pressure, and more.
These capabilities working together with sophisticated transmitter technology have produced a broad product line featuring high turn-down ratios, device diagnostics and robust mechanical strength supported by exceptionally stable and maintenance-free performance, able to boost plant profitability year after year.
Users have leveraged these accurate measurements to deliver more precise process control and optimisation while reducing production outages and maintenance costs.
Yes, DP flow technology is alive and well and shows no signs of being surpassed for flow measurement versatility.
Selecting the right Differential Pressure Flowmeter
Achieving a differential pressure measurement requires two key elements – a primary element, which creates a restriction or reduction in the flowline to cause a pressure drop – and a differential pressure transmitter to calculate the flow.
Selecting the right differential pressure flowmeter requires an understanding of the key factors that can affect their suitability for specific types of applications
Calculating the Differential Pressure
The relationship between velocity and differential pressure provides the basis on which all differential pressure devices operate.
When the measured fluid flows at a velocity through the restriction, the area of the fluid path is reduced, causing the fluid to move at a higher velocity to maintain the same flowrate.
As the velocity increases, the kinetic energy also increases, causing a consequent reduction in the pressure energy. This creates a lower pressure in the meter throat compared to that upstream of the throat.
Each DP device deviates to some extent from the calculated relationship, which is based on the ratio of restriction diameter to pipe diameter. One reason is that when the fluid passes through the restriction, it continues to ‘converge’ for a short distance. This means that the minimum diameter of the fluid “jet” (called the vena contracta) can be smaller than the throat of the restriction and the velocity in it is therefore higher, as shown in the following diagram:
Consequently, the actual pressure reduction is greater than that calculated from the restriction diameter. To correct for this, a Coefficient of Discharge is applied.
The ideal value of this coefficient would be 1.0 but the actual value varies from one class of DP device to another. It also varies within a given class of device, depending on the -ratio (the ratio of the restriction diameter to the pipe bore diameter).
When using the meter to measure flowrate, it is necessary to know the differential pressure generated, which is usually achieved using a differential pressure transmitter. Know your Reynolds Number
Reynolds Numbers are a means of comparing the dynamics of two or more flow systems which are geometrically similar but dimensionally different. To select the appropriate flowmeter, it is necessary to calculate the Reynolds number of the application.
This is the ratio of momentum against viscosity and can be obtained by calculating the minimum and maximum fluid flow and viscosity figures of the application. A general guide for matching DP flowmeter selection to Reynolds Numbers is outlined below:
| Primary element | Recommended service | Minimum Reynolds value |
|---|---|---|
| Orifice plateSquare edge concentricConical/quadrant edge concentricEccentric/segmental | Clean liquids, gases & steamViscous liquidsLiquids & gases containing secondary fluid phases | ≥ 2000≥ 50010,000 |
| Pitot tubes | Clean liquids, gases & steam; contaminated gases | 12,000 |
| Venturi | Clean & dirty liquids, gases, steam & viscous liquids | Varies according to specification (size, flow capacity etc.) |
| Flow nozzles | Clean liquids, gases & steam | 50,000 |
| Wedge elements | Dirty liquids, gases, steam, slurries & viscous liquids | 500 |
Reynolds Numbers allow a common fluid such as water to be used as the calibration medium both for liquids and gases. Gases, however, have very low viscosities and tend to be transferred at high pipeline velocities.
As can be quickly deduced from the Reynolds Number formula, this leads to very high Reynolds Numbers being generated, much higher than those normally achievable using water as the calibrating medium.
This often creates the requirement for the calibration medium to be in the same “phase” as the application to better establish its stability under close-to-operating conditions. These factors together lead to some devices being calibrated using gas, a complex and costly operation often requiring the use of specialist third party gas calibration centres.
If a water calibration is acceptable, some have suitable water calibration rigs in-house, enabling them to offer economic flow calibrations.
Calculating Density with Differential Pressure
Temperature can introduce error to a hydrostatic level measurement, specifically, when temperature changes, the density of the fluid being measured can also change, which means the level calculation being made by the instrument is being made with the wrong density value, creating an inefficiency and opening the door to a potential overfill.
That’s a major problem, so what can operators do when they know the density is constantly changing (either due to temperature or process conditions) and the level output is incorrect?
Do the maths
A differential pressure sensor can be used to measure hydrostatic level and calculate density. As long as liquid covers both mounting ports of a differential pressure sensor, users can calculate the fluid's density.
To account for liquids of changing densities, users can rearrange the basic hydrostatic level formula and solve for density. In order to complete the equation, however, one needs to know the fluid level. But how is one supposed to know the fluid level if the density is changing? This is where differential pressure (DP) comes to the rescue.
A differential pressure sensor measures the pressure at two different points. As long as these two points are covered by the fluid, the height of the measurement becomes the distance between the mounting ports and the pressure used for the calculation is the difference between the two, or the DP.
So now that we have the level (distance between the ports) and the pressure (the differential pressure measurement), the only variable left in the basic hydrostatic pressure formula is density, which we can now solve for using the other two variables.
So, with a DP measurement one can calculate the changing density of the fluid.
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