Key points
Although it is over 100 years since the first Differential Pressure Flow Measurement devices were used, they still form the largest installed base of all flow measurement loops, especially in the process and chemical, oil and gas sectors.
Steve Gorvett, product manager for DP flow and temperature for ABB Measurement & Analytics in the UK, outlines the main options on offer and explains how developments in technology are steadily broadening the appeal of the meters into other sectors.
The origins of differential pressure measurement can be traced back to 1738, when Italian mathematician Daniel Bernoulli determined that the total energy (kinetic + pressure + potential) within a flowing fluid is constant.
Some 60 years later, another Italian – Giovanni Battista Venturi – determined that a fluid flowing into a restriction gains kinetic energy or velocity at the expense of pressure and that some of the pressure is recovered when the fluid leaves the restriction.
More than 270 years after Bernoulli developed his theory, flowmeters based on the differential pressure measurement principle continue to account for the majority of the global flowmeter market. In 2007, a survey carried out by flowresearch.com estimated that the global market for differential pressure devices is actually growing by around 6% per year, with a total value of over $1bn.
Differential pressure flowmeters explained
To achieve a differential pressure measurement requires two key items of equipment.
Firstly, a primary element is needed which creates a pressure drop. Essentially, this element creates a restriction or reduction in the flowline which causes a difference between the upstream and downstream pressures.
This restriction can vary in shape, according to the type of element being used and the type of medium being measured. More about the different elements available is included later in this article.
The difference in pressure is measured by a differential pressure transmitter, which can then be used to calculate the flow. Recent developments in transmitter technology have transformed the possibilities for differential pressure measurement.
With the arrival of multivariable transmitters, for example, users can now measure not just differential pressure, but also static pressure and temperature, all within a single unit, giving them additional data that can be used to further enhance their process.
A crucial factor in selecting differential pressure flowmeters is the issue of pressure drop.
Although creating a local drop in pressure is an integral part of the measurement principle it is important to ensure that any permanent loss of pressure is minimised in order to maximise the energy of the flowstream.
Certain types of differential pressure flowmeters are better than others at maximising this recovery. For example, venturi flowmeters, with their much larger pressure recovery section, offer a much lower pressure drop than orifice plate flowmeters.
One of the biggest benefits of differential pressure flowmeters is their inherent versatility. They can be used to measure the flow of gases, steam and liquid, giving them a wider field of applications than any other type of flow meter.
This versatility, and the relative simplicity of the differential pressure principle, has led to DP flowmeters enjoying the largest market in terms of both sales and installed base.
In particular, they are in widespread use throughout the oil and gas industry and are continuing to be installed apace as the demand for energy drives the construction of new oil and gas exploration facilities.
Though the oil and gas market continues to represent a core area, new applications in new sectors are also being opened up by recent developments in differential pressure flowmeter technology.
Choosing the right type of differential pressure flowmeter
Choosing the best type of flowmeter for any application requires careful consideration of a range of factors. Where differential pressure flowmeters are concerned, there are a range of sensors each of which have their own advantages and drawbacks for certain types of applications.
The main types of differential pressure flowmeters are:
- Orifice plate flowmeters
- Pitot tubes
- Venturi tubes
- Flow nozzles
- Wedge elements
Orifice plate flowmeters
Orifice plate flowmeters are the most commonly used of all DP devices. Their durability and lower purchase cost compared with other meter types mean they are widely used throughout industry.
They are ideal for a wide range of high temperature, low velocity applications measuring clean, dirty or corrosive liquids, including viscous liquids, as well as vapours and gases.
Orifice plate flowmeters typically comprise of flat metal disks with a precision hole drilled in the centre, the size and profile of which depends on the application.
The disks themselves are often held in place by a carrier assembly fitted into the pipeline. Various types of orifice plate flowmeters are available, including concentric, eccentric and segmental, each with their own set of advantages for particular applications.
One key drawback with conventional orifice plate flowmeters stems from the number of separate components required for an installation. A typical orifice plate flowmeter installation consists of the orifice plate, orifice flanges, an isolation valve, valve manifold, the DP transmitter, interconnecting tubing and couplings.
When connected together, all of these present potential leak points which can affect the accuracy and reliability of the flowmeter, as well as leading to fugitive emissions of the liquid or gas being measured.
They can also suffer from a slow speed of response, due to issues such as long or blocked impulse lines, or impaired signals due to condensation in gas system impulse lines or gas bubbles in liquid lines.
These problems have been overcome with recent advances in orifice plate flowmeter technology.
ABB’s OriMaster flowmeter, for example, features a single piece design combining all the major components needed for an orifice plate installation.
This eliminates the need for users to source and install a separate manifold, transmitter and impulse piping, typically cutting the cost of installation and commissioning by up to 50%.
The all-in- one design also solves the previous problems associated with impulse lines, leading to enhanced accuracy and a faster speed of response.
Pitot tubes
As a low cost, easily installable flowmeter with a low pressure drop, pitot tubes present the ideal solution for measurement of clean liquids, gases or steam. Although less accurate than orifice plate flowmeters, they can be readily installed in most locations, including square or rectangular pipe ducts.
Pitot tubes measure flow by sensing the difference between the impact pressure of the flow and the static pressure.
There are two main types of pitot tubes. Single port pitot tubes measure impact pressure, with the tube inserted into the flowstream with its sensing port facing the direction of flow.
A second tube is then inserted to measure static pressure. The flowrate measured is proportional to the difference between the impact pressure and the static pressure.
These devices are limited by their ability to only measure at the actual point of impact.
In contrast, averaging pitot tubes feature multiple impact-sensing ports along their length, positioned at equal, calculated points, and include both an outer and inner impact tube.
The inner tube provides secondary averaging of the various sensor holes, making averaging pitot tubes ideal for coping with the non-uniform flow profile conditions inherent in the short pipe runs found in most applications.
Venturi flowmeters
As one of the oldest types of flowmeter, venturi tube flowmeters represent the workhorse of the flow measurement world for measurement of clean liquids and gases. Despite their maturity, they continue to enjoy market domination in the oil and gas industries in particular, where their solid performance, low pressure drop, reliability and durability make them ideal for the arduous demands characteristic of oil and gas applications.
The general design of a venturi tube is simple. It consists of a parallel inlet section of the same diameter as the pipe, followed by a coned reduction (typically 21° included angle) and a parallel throat section.
The outlet is also conical, but with a shallower angle (typically 15°) than the inlet. The outlet cone either finishes at the same diameter as the pipe (referred to as a classical venture – the most common design) or at a diameter smaller than that of the pipe (a truncated or short venturi).
The high (i.e. upstream) pressure tapping is located just before the inlet cone and the low (downstream) pressure tapping is at the centre of the throat section.
In recent years, the use of venturi meters has been extended into the field of sub-sea measurement and processing, where their attributes make them particularly suitable for the hostile environments found in such applications.
They can also be designed for the high process and ambient pressures routinely found in sub-sea applications and are suitable for the multi- phase fluids found in production (oil + gas + water + sand). It is also relatively easy to integrate specialist sub-sea DP transmitters, cabling systems and manifolds with the Venturi assemblies.
Flow nozzles
Flow nozzles are essentially flow tubes with a smooth entry and sharp exit. Flow nozzle type flowmeters are most commonly used for high velocity flow measurement of non-viscous liquids or gases in applications where there is a risk of erosion or cavitation that would damage an orifice plate.
Typical examples include flow testing on steam raising plant. With no sharp edge that could degrade over time and affect performance, flow nozzles offer excellent long-term accuracy. They also offer a relatively low pressure loss.
Wedge flowmeters
Featuring a v-shaped flow restriction, these types of flowmeter are particularly well-suited to measuring more demanding types of fluids, such as those with a high solids content, high viscosity or high levels of erosive/abrasive particles. They are also ideal for measuring air flows.
Achieving optimum performance
Choosing the appropriate DP flowmeter sensor is just one aspect. To optimise the installation, it’s important to consider a range of other factors as well.
These include transmitter turndown. Traditionally, older transmitter technologies have measured to an accuracy of ±0.2% of the calibrated span. With advances in transmitter technology, this has now improved base accuracy to ±0.025% of the calibrated span.
Another consideration is the minimisation of leak paths, both for improved health and safety and enhanced process efficiency. The answer here is to select a DP primary, such as the wedge, that can be fitted with remote diaphragm seals.
Alternatively, users can opt for an all-welded design Featuring integrated welded manifolds, these types of devices reduce the leak paths associated with traditional manifolds.
With the introduction of second generation multivariable pressure transmitters, it is possible to measure mass flow from any DP primary device. This technology can be applied retrospectively, making it ideal for existing installations where enhanced performance through measurement of mass flow is required.
At a glance
The below table provides a quick guide to the various types of differential pressure flowmeters available and the types of applications where each can be applied:
Summary
The above information provides a basic guide to the various types of differential pressure flowmeters available and their general suitability for particular types of application.
With decades of experience in the design and manufacture of a full range of flowmeter technologies for both industrial and utilities applications, ABB is well placed to advise you on the best solution for your requirements.
