Flow Control: Technology information & new products for fluid handling engineers

July 2006

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To Replace or Not to Replace
Addressing a DP Meter’s Low-Flow Performance

David W. Spitzer, P.E.

Instrument selection is an interesting topic because it encompasses many aspects of the installation, including the process, the environment in which the instrument must operate, maintenance requirements to keep the instrument operating properly, reliability, performance, and cost. Many of us routinely evaluate these and other parameters to select a particular instrument to specify and purchase.

To think of it in the abstract, the instrument selection process can be likened to starting a drawing on a blank page with only the edges of the paper to worry about. But what happens when you have to deal with an existing instrument? The analog would be one of starting with an almost fully drawn picture instead of a blank piece of paper. A salient question is whether to fix the picture or just throw away the paper.

I recently visited a water authority that was having problems achieving a reasonable water balance among its many flowmeters. This was a concern because the errors were large enough that the authority could over-bill or under-bill millions of dollars annually. The existing primary flow elements used differential pressure transmitters to generate a signal proportional to the flowrate. They were operating over a flow range of approximately 10 to 100 percent of full-scale flow.

Noting that the accuracy of differential pressure flow measurement systems tend to degrade at low flowrates, should these flowmeters have been replaced with flowmeters that utilize another technology that can achieve the desired 10-to-one turndown? The answer is maybe and maybe not.

A flowmeter station is comprised of one or more flowmeter runs in parallel so that the station as an entity can measure a wide range of flows. The desired flowrate through a given flowmeter at a given time is determined by the control strategy employed. Turndown requirements would be reduced if the control strategy could be modified such that the desired flowrate through a given flowmeter could be maintained in the upper part of its flow range. If this is the case, the existing flowmeter may be acceptable even though it may not have been the first choice for a new installation.

All may not be lost if the flow measurement system must operate over a wide range of flows. The primary element may well operate properly (hydraulically) at the lower flow rates. This should be verified to be sure that this is the case. If so, the problem can manifest itself in the relatively poor performance of the differential-pressure transmitter at low flowrates that produce low differential pressures within its range.

Replacement with another flowmeter technology is an option, but adding a second differential-pressure transmitter that can accurately measure differential pressures at low flows may be the pragmatic approach.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to... 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 for manufacturing and automation companies. He can be reached at 845 623-1830.

www.spitzerandboyes.com

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