Considering Downward Flow

Selecting flowmeters for pipelines where liquid flow is downward requires careful consideration, as such applications can pose measurement problems. For example, there is a pressure drop across flowmeter elements, such as an orifice plate, vortex shedder, or Coriolis mass flowmeter. In horizontal pipes, the pressure at no flow is the same throughout the pipe at the same elevation. At no-flow conditions in a vertical pipe, the pressure is lower at higher elevations. With upward flow, the total pressure drop is the sum of the pressure loss due to elevation and the pressure loss due to the flowmeter element.

Conversely, at no-flow conditions in a vertical pipe, the pressure is higher at lower elevations. With downward flow, the total pressure drop is the sum of the pressure gain due to elevation plus the pressure loss due to the flowmeter element. When the pressure loss across the flowmeter element is less than the loss due to elevation, the differential pressure across the flowmeter will be positive and result in a pressure gain. This can affect different flowmeters in different ways. In one downward flow installation, a vortex shedder was reputed to be in error by as much as 10 percent. In another installation, a Coriolis mass flowmeter measured accurately. When in doubt, the results of downward flow testing in a flow laboratory would likely provide useful information as to the suitability of the flowmeter for downward flow installations.

Accurate liquid flow measurement is generally predicated on the flowmeter being full of liquid. This poses a problem because many pipes that flow downward are not necessarily full of liquid. For example, a vessel overflow installation may keep the vessel full, but the pipe leading downward is likely partially full for at least part of its downward run. Flowmeters requiring a full pipe that are located where the pipe is not full will be in error. This is not necessarily due to the downward flow, but rather the partially full pipe.

Configurations where the pipe is partially full of liquid can be insidious. In one such application, the pipe rose, ran horizontally, and then fell vertically into a vessel. The flowmeter was located upstream of the control valve in the vertical pipe with flow downward. The flow measurements were in error by 30 percent to 50 percent. The problem was not in the technology because the same type of flowmeter performed accurately in a downward flow application where the pipe was full of liquid.

The lesson here is that you should be very careful when you apply flowmeters in downward flow applications.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting. He can be reached 845 623-1830.

www.spitzerandboyes.com