Understanding upstream and downstream pipe diameters for flow measurement
One of the most common issues with flow measurement is an incorrectly sized flowmeter, followed by poor installation. While there are some pretty creative ways to get flowmeters piped, this neglects the fact that there are certain installation best practices that should be followed for each flowmeter type and those should be adhered to whenever possible.
Incorrect flowmeter installation
After all, you went through the trouble of selecting the proper meter for an application, why not ensure it’s installed in a manner that will produce optimal measurement results? A lot has changed over the years in terms of flowmeter technology; however, you can’t argue with physics. Fluid flowing through a pipe generally assumes a desirable flow profile at or near the center of the pipe. Flow disturbances (distortion and swirl) can occur via improper flowmeter installation, thus reducing measurement accuracy. Aside from improper sizing, material compatibility, and meter misapplication, a number of issues with flow measurement arise from improper piping practices.
A lot of time, effort and money can go into the purchase of a flowmeter. However, it seems that not as much consideration is given to its installation. Perhaps this is due to limited changes that can be made to existing piping (time and expense), the end user having a technology they are not familiar with and making assumptions based on what they have done in the past, or a mechanical contractor having limited experience with various flowmeter technologies, etc.
In this case, when talking about improper installation, the focus is on the placement of the meter in the process piping and its relation to valves, elbows, strainers, reducers, and a host of other devices that can be inserted into the piping that can have an effect on the flow profile. There are some general guidelines and best practices that all users should follow. The old adage, "If you don’t know something, ask," applies here. There are no bad questions when trying to determine proper meter placement.
Ask the manufacturer
Each technology has specific recommendations for meter placement and recommended pipe diameters upstream and downstream. Perhaps one of the most overlooked resources is speaking with the account manager/sales resource for that particular vendor. Most, if not all, flow measurement professionals have been through extensive training on their products and are familiar with a host of applications. They should be the first call when you have a question or concerns about where the meter should be placed in the pipe. In many cases, these professionals will assess each installation and give you the recommendations needed to ensure proper performance.
One size does not fit all
In this case, one technology does not fit all applications. We all have our favorite technology that we swear by, but you can’t fit a round peg in a square hole and the same goes for flow meters. Just because there’s a spare meter lying in the storeroom doesn’t mean it will be the most suitable. There are many options to consider, especially if you have existing piping that can’t be modified easily. It’s important to start thinking about the importance of installation effects and how this could impact the type of meter that is chosen.
If you find that your flowmeter performance is suffering from installation effects, all is not lost. Ideally, the meter could be moved to a more suitable location which would ensure proper upstream and downstream straight pipe. Another option would be to change the type of meter or change to a different design of the same kind of meter. For example, changing from a standard vortex to a reducer vortex.
Upstream and downstream requirements
As there are multiple flow technologies available, so too are the recommendations for proper meter placement in process piping. While each manufacturer may have different guidelines, below is a general overview of typical upstream and downstream piping requirements to get started.
For an orifice plate, the straight run required depends on both the beta ratio of the entire installation and on the type of upstream components in the pipeline. If you are using a ‘standard integral’ orifice plate with an upstream inlet side beta of .4, straight run requirements can range up to 20 straight pipe diameters and 10 straight pipe diameters downstream. However, if you are using a conditioning orifice plate with a .4 beta, straight run requirements range from 2-6 straight pipe diameters.
Magnetic flowmeters are fairly forgiving in terms of varying flow profile. To ensure specified accuracy over a broader range of process conditions first, ensure that the meter is piped in a manner so it always remains full. Second, install the flowmeter a minimum of five straight pipe diameters upstream and a minimum of two straight pipe diameters downstream from the electrode plane.
Vortex flowmeters like a fully developed flow profile and typically require long straight upstream piping runs. Elbows, reducers, etc upstream of the sensor will distort the flow profile and so adequate straight runs need to be accounted for. Typical recommendations suggest 35 straight pipe diameters upstream and 5 straight pipe diameters downstream.
If you don’t have sufficient upstream straight run, some manufacturers allow for K factor corrections that could reduce this requirement. If using temperature and pressure compensation to derive mass flow, then the temperature and pressure transmitters should be installed downstream of the flowmeter at a minimum of 4 to 6 straight pipe diameters.
Coriolis flowmeters are designed to measure mass flow and density directly. Their measurement is insensitive to fluid profile, have very few installation limitations and therefore don’t require upstream and downstream straight pipe runs. The main guidelines for coriolis sensors are to make sure the sensor tubes remain full. If installed in a vertical pipe, make sure the fluid (liquid and slurries) flow up and gasses flow down and do not use the meter to align misaligned pipe.
Turbine flowmeters must be installed in a manner that minimizes measurement errors caused by turbulence or damage to the moving parts caused by high flow rates. It is also recommended that a strainer is used upstream of the turbine meter to capture any contaminants that could damage the moving components. Sufficient back pressure should also be maintained to avoid any potential flashing or cavitation.
The general guidelines suggested by manufacturers of turbine flowmeters are 15-20 straight pipe diameters (inclusive of the strainer) straight pipe runs upstream and 5 straight pipe diameters straight pipe runs downstream. The upstream requirement can increase if, for example, there are two elbows in different planes (up to 50 straight pipe diameters). Alternatively, the upstream straight run requirement can be reduced if some form of flow straightening is used, down to 10 straight pipe diameters upstream.
Ultrasonic flow meters
Ultrasonic flow meters requires fully developed flow conditions to ensure the meter will perform as specified. There are two basic types of measuring principles, doppler and transit time, with the latter being more commonly used for fiscal custody transfer applications of gas and petroleum liquids. As with other technologies, these sensors do require adhering to basic installation guidelines to minimize errors caused by flow profile disturbances. However, these requirements vary by manufacturer, which could lead to some confusion.
If this is a fiscal custody transfer application of either a liquid or gas, then make sure to be fully versed with the current standards that are available. In this case, the AGA Report No. 9 Measurement of Gas by Multipath Ultrasonic Meters or the API Chapter 5, Section 8 Measurement of Liquid Hydrocarbons by Ultrasonic Flowmeters using Transit Time Technology.
Consulting the standards
There are a number of professional organizations that offer guidelines and standards for a majority of the flow technologies being used. Some of these are ISA, ISO, API, AGA, ASME, and ANSI. When in doubt, consult these guidelines as they have been tested and provide a very good starting point to ensure proper flowmeter performance. Some of the most common guidelines/standards are listed below:
- Coriolis – ISO 10790
- Critical nozzles – ISO 9300
- Differential pressure – ISO 5167
- Electromagnetic – ISO 6817
- Positive displacement – ISO 2714
- Thermal mass – ISO 14511
- Turbine – ISO 2715
- Ultrasonic – ISO 17089
- Variable area – ISO 11605
- Vortex – ISO/TR 12764
- ASME MFC-3M-2004
- ASME MFC-6-2013
- ASME MFC-11-2006
- ASME MFC-5.1-2011
- ASME MFC-16-2014.
By asking the right questions, consulting with the manufacturer, and having a good understanding of the existing guidelines and standards, you should be well on our way to getting the most for your flowmeter investment.
Jeff Weiss is an account manager with Cross Company. He has over 20 years of experience in the process control industries in business development and management roles. This article originally appeared on Cross Company’s Innovative Controls blog. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, firstname.lastname@example.org.
Cross Company is a CSIA member as of 2/2/2017