Why it pays to consider self-averaging pitot tube flowmeters
Today’s industrial operators continue to need to find ways to reduce their energy consumption and emissions to atmosphere wherever possible without impacting on their competitiveness. This has seen a growing desire for proven, reliable measurement technologies that can help deliver accurate data, which in turn can then be used as the basis for future performance improvements. Steve Gorvett, Differential Pressure (DP) Flow and Temperature product manager for ABB Measurement & Analytics in the UK, explains how self-averaging pitot tube flowmeters offer a flexible, versatile and cost effective way of addressing these needs.
Although it is over 100 years since the first DP devices were used, their inherent benefits mean they still enjoy the largest installed base of any flowmeter technology, especially in the process, chemical and oil and gas sectors. Although other technologies such as electromagnetic, Coriolis and ultrasonic flowmeters are enjoying steady growth, they are still restricted by factors such as size, materials of construction and non-ideal process conditions, none of which apply to DP flowmeters.
One type of DP flowmeter that is enjoying growing popularity is the Torbar self-averaging pitot tube.
In common with all differential pressure measurement devices, the Torbar works by partially obstructing the flow of a gas or liquid in a pipe.
Spanning the entire pipe diameter, it comprises of four main components, namely a single piece outer impact tube, an internal averaging tube, a low pressure chamber and the head, which is in turn connected to a DP transmitter.
The single piece outer impact tube incorporates a number of sensing holes, or ports, facing the upstream flow.
These ports are positioned at equal annular points in order to capture as much of the flow profile as possible in order to produce the most accurate measurement of flow conditions.
Torbar meters work by measuring the difference between the combined static and velocity pressure on the upstream face of the device and the static pressure on the downstream side. As multiple pressures are collected on the upstream face from across the flow profile, they first have to be averaged to produce a single mean value. This is done at two points, first in the outer impact tube and then, more accurately, within the internal averaging tube. During this ‘secondary averaging’ stage, the various pressures from the multiple ports are brought together in a common manifold. The output is then taken from the neutral position of the manifold to produce the high pressure side of the DP measurement.
The low pressure component is produced from the static pressure, which is sensed by a single hole on the downstream side of the outer impact tube.
The DP output is then relayed to a differential pressure transmitter to generate an electrical signal proportional to the flow rate. With certain designs and applications, the DP transmitter can be directly mounted onto the Torbar via an integral three or five valve manifold to form a compact flow meter.
The use of profiled flat edges around the side of the device and adjacent to the downstream port helps to create a fixed separation point for the flow stream, producing a stable pressure area within which the static pressure can be measured.
By maintaining a more constant flow co-efficient at high velocities, the use of the flattened edge design produces a typical turndown of 10:1, enabling flow to be measured across a very wide range.
Versions are also available for bi-directional flow applications, where the meter is supplied with the same number of holes on the upstream and downstream faces.
Advantages over other flowmeter types
Torbar flowmeters offer a number of advantages over other flowmeter types, including:
Minimal pressure loss
Many flowmeter technologies introduce pressure loss into a system. Pressure losses equate to energy losses and increased costs. Valves, pipe friction, reducers, expanders, and measuring devices such as flowmeters all increase the Permanent Pressure Loss (PPL) in the system. Some flowmeters require upstream reducers and downstream expanders to operate properly.
The key is therefore to select flowmeters which introduce the lowest possible pressure losses.
For new processes, engineers often consider PPL when designing a system because it's important in sizing the pump (liquids), compressor (gases), or boiler (steam) to meet the process conditions and to deliver the desired pressure and/or flow. For operating processes, PPL leads directly to the need for additional energy usage, which can equate to significant increased annual operating costs. By minimizing pressure losses in a process, engineers can cut the need for top-up pumping or compression, as well as reducing their environmental impact. In the case of steam boilers, which are expensive, the ability to retrofit existing flowmeters with those having low pressure losses can boost the effective boiler capacity.
By selecting flowmeters with low pressure losses, engineers can:
- Reduce pumping/compressing cost
- Increase capacity
- Minimize compressor, pump or boiler size.
The amount of pressure lost in a flowmeter depends on three factors: the fluid density, the square of the fluid velocity (Vf)2, and the degree of obstruction to fluid flow, (Kmeter).
In contrast to other DP meters, such as orifice plates, self-averaging pitot tube meters offer minimal irrecoverable pressure losses as well as being inexpensive and simple to install.
Savings can be achieved by opting for a self-averaging pitot tube meter over an orifice plate. In this example, using a Torbar meter reduced the permanent pressure loss by a factor of over 30. Steam was being supplied at a rate of 22,000 kg/hr at a density of 3.998 kg/m3 through a pipe with a diameter of 305mm.
With a Beta ratio of 0.633, the orifice plate introduced a differential pressure of 125 mbar and a resulting permanent pressure loss of 73 mbar.
In contrast, the Torbar introduced a differential pressure of 21.9 mbar and a resultant pressure loss of only 2.3 mbar.
Improved performance in non-ideal conditions
Providing the perfect conditions for flowmeters to function at maximum performance can often be difficult, especially where there is limited space available for piping and all of the other assorted fittings and accessories. In particular, many flowmeters require a specific number of pipe lengths upstream and
downstream of the point of installation in order to achieve the best possible flow profile for an accurate measurement to be derived.
Requiring fewer straight pipe lengths than many other flowmeter types, Torbar meters can overcome this challenge. They can achieve similar accuracy to other primary flow elements, particularly if they are in the same plane of a disturbance such as a pipe bend or elbow.
This accuracy is further boosted by the inclusion of secondary averaging. With standard Pitot designs, the upstream sensing hole nearest to the take-off point can have a greater influence on the pressure reading than the other holes, reducing accuracy in non-ideal conditions. By further averaging the pressures from the various sensor ports, the Torbar’s secondary averaging stage helps to overcome this problem, making it applicable for a much wider range of installation locations.
As an insertion device, Torbar flowmeters can be easily installed into both pipelines and circular, square or rectangular ducts from 15mm up to 8000mm (0.5 in to 315 in) in diameter.
Their (optional) retractable design also means they are easy to maintain, with none of the time, cost or technical difficulties associated with removing other forms of in-line flowmeter sensors. These designs also offer the ability for Torbar meters to be both installed and removed under pressure, eliminating the need for processes to be shut down whilst work is carried out.
The Torbar incorporates a number of features that enable it to offer a long service life with minimal maintenance. With no moving parts that can wear or fail, there is nothing that needs to be replaced. Various options, including a choice of stainless steel or exotic materials and versions for applications up to 1,200°C also means that the Torbar can withstand the rigours of harsh applications that can cause many other flowmeter types to fail.
Manufactured from a single length of tube with no joints, Torbar meters are also stronger than many other Pitot tube designs and have been successfully applied in many applications to replace other types of pitot tube that have broken in service. The robust design of Torbar meters means there are still many installations successfully using devices that were installed 10 or more years ago.
Where can the Torbar be used?
Torbar meters offer a versatile solution for measuring a wide array of gases, vapours and liquids. Fluids and gases that can be measured include natural gas; flue and exhaust gases; hydrocarbon, methane, nitrogen and carbon dioxide gases; petrol vapour; saturated and superheated steam; sea, potable and waste water; liquid oxygen, petrol and other substances such as nitric acid and red wine.
The low pressure drop characteristics of Torbars make them particularly suitable for measuring flows at low pressures. Their benefits also make them a cost effective solution for installations on large diameter pipelines.
The inherent benefits of Torbar meters make them suitable for use across a diverse range of applications where the gas or liquid completely fills the cross section of the pipe. Typical applications include:
- Liquid, gas and steam flow measurement
- Ventilation ducts
- Compressed air monitoring
- Water and other liquids at low and high pressure
- Biogas and flue gas
The wide range of flow measurement solutions on the market can often confuse customers when they are seeking the right solution for a particular application. In many cases, this can result in an over-engineered solution, pushing up costs both at the purchase and maintenance stages if the flowmeter is wrongly specified.
As a robust and simple flow measurement device offering low installation and maintenance costs, Torbar flowmeters can present the ideal solution for applications requiring a low cost, easily installable flowmeter with a low pressure drop.
JWF offers a wide choice of differential pressure (DP) flowmeters and primary elements to suit virtually any type of industrial application. Click here to visit the differential pressure flowmeter section.