Flow measurement is a critical aspect of operations involving pumps, valves and other industrial systems. Users choosing flow meters to measure fluid flow must consider key factors to make the right decision. There are significant differences between meter designs, with each type of device having its own pros and cons.
Flow meters close the loop on pump systems, as information from these devices is used to regulate the speed at which pumps run and/or to modulate flow control valves. In addition, knowing the flow rate through a pump can be useful in determining if the equipment is operating properly or needs maintenance. Flow meters can be installed at various points in the pipeline system, starting at pump output and continuing downstream to ensure adequate flow is maintained.
Machinery and rotating equipment packages require sophisticated instrumentation, and special consideration should be given when selecting these component solutions. If any part of the flow instrumentation may be subject to vibration or pulsation in a machinery package, the affected equipment should receive extra attention.
Popularity of DP Technology
Many flow applications, including those involving pumps in skids and package systems, employ differential pressure (DP) flow meters, such as orifice plates and Venturi primary flow meter elements. DP flow transmitters are used to convert the DP signal at the primary to an electronic signal. DP meters employ a measuring technology that does not require moving parts in the flow stream.
The primary DP flow element is a physical restriction in the pipe that generates a pressure drop proportional to the square of the flow rate. The pressure drop is measured using a DP transmitter. Pressure and temperature measurements are often added in applications where operating conditions vary significantly to more accurately calculate the mass flow. The flow meter takes the square root of the DP and displays the results on a calibrated scale showing the flow rate.
The orifice plate is one of the most frequently used DP flow elements because of its low cost and adaptability. Limitations on this device include its susceptibility to damage by foreign materials entrained in the fluid, as well as the effects of erosion. Straight runs of upstream and downstream piping are required for an orifice plate.
A Venturi flow meter is used when high capacity and minimum head loss are critical factors. Its limitations include relatively high cost and the size or weight of installation. Straight runs of upstream and downstream piping are also required for a Venturi flow meter.
Advent of Cone Meters
One of the newest DP flow meter designs in widespread use is the cone meter. Resistant to the influence of asymmetric and swirling flow, this advanced instrument can improve the performance of older DP devices while enabling users to minimize installation and operating expenses and reduce total cost of ownership.
The cone meter is recognized for its performance characteristics, including accuracy, repeatability, wide turndowns and stable signals. The device can be used with an assortment of process media and handles the entire circumference of the media versus a specific target point within it.
This meter employs a cone-shaped element, which restructures the flow to a nearly flat profile—helping pinpoint accuracy. The cone element is placed at the center of the pipe, creating an annular space for the passage of fluid.
This approach allows for higher accuracy and lower permanent pressure.
The cone element conditions the flow at the same time it creates the pressure differential, providing smoother and less noisy DP readings.
When combined with today’s smart multivariable DP transmitters, high accuracy and high flow turndowns are possible.
The cone flow meter also acts as its own flow straightener, which reduces the need for straight pipe diameters both upstream and downstream of the installation site. The straight pipe requirement may be able to be eliminated in some cases.
The cone serves as a conditioning device as well as a DP producer.
A further benefit is that a cone meter can perform well over a wide range. The meter shows a linear (after square root) flow response in flow ranges that generate Reynolds numbers of approximately 5,000 to 6,000 and higher.
Although the cone meter tends to be more expensive than some other DP technologies, it provides valuable performance benefits and operational cost savings.
Cone Meter Vulnerability
Despite the advantages of cone flow meter technology, there is a potential vulnerability when the meter is exposed to certain stress events over its service life.
In high-stress circumstances involving vibration, pulsation or other rigorous conditions, the down-comer pipe may become detached from the meter body, causing the cone to flow downstream with the fluid.
One cone meter design that can be used for a range of applications, including high-stress situations, uses a new configuration that can overcome several possible obstacles to reliable performance. The configuration can typically handle vibration, unforeseen objects within the fluid stream, harmonics, system startup and other application-specific events.
This cone meter design employs three downstream support rods that are spaced 120 degrees apart radially from the center of the cone to the housing wall. This feature ensures the cone is centered, reduces bending stresses on the down-comer from handling by 10 times and reduces the chance of oscillation during operation.
The meter design has also been optimized so that the cone has the needed support to remain properly anchored. An upstream support added radially across from the down-comer pipe changes the constraint from a cantilever to a rigid support, which shares the axial load and cuts the operational preload by half.
Conclusion
Accurate and reliable measurement of various process parameters is required throughout pumping and pipeline systems. However, experience has shown that the measurement under certain flow conditions, such as those in a reciprocating compressor or pump, can be difficult.
Fortunately, several effective options are available to measure flow.
End users and skid builders can choose the right meter for the application and their budget.