• Search Flow Control:
The Leading Information Source for Fluid Handling

Current Issue

Subscribe

Subscribe to the
Flow Control E-News

seepex Progressive-Cavity Wobble Pumping Systems

Flow Control Sponsors

Bex Spray Nozzles Endress+Hauser Flow, Pressure, Level, & Temperature Instrumentation Sierra Instruments Viega Pipe Cutting Equipment Omega Engineering Flow, Level, Pressure & Temperature Instrumentation Sage Metering Thermal Mass Flowmeters Burkert Fluid Control Systems McCrometer Differential-Pressure & Magnetic Flowmeters
2009 | Flow Control: Volume XV, No. 7
More From This Issue
Subscribe Print Bookmark and Share

Considering Capillary Tubing

How the Application Affects Tubing Length Requirements
David W. Spitzer
In the last two months we examined the installation of impulse tubing in high-pressure steam flowmeter applications and removal of high-pressure steam flow transmitters from service. We found that measurement error can occur due to the different densities of the liquids in the impulse tubes in non-horizontal impulse tubing runs. A similar effect can occur when diaphragm seals are used with differential-pressure transmitters. Differential-pressure transmitters with diaphragm seals are not commonly applied to flow measurements, but they are quite commonly applied to level measurements.

A common “rule of thumb” is to purchase the capillary tubes of the same length for both the high-pressure and low-pressure sides of the transmitter. This would seem to make (mechanical) sense for flowmeters because the flowmeter element taps are generally close to one another, so the distance from the transmitter to each tap is similar.

In level applications, however, the nozzles can be in quite different locations. For example, a transmitter may be located one meter from the lower nozzle (near grade) and eight meters from the upper nozzle (near the top of the vessel). The “rule of thumb” would stipulate that the transmitter have (say) 10 meters of capillary tubing on both sides of the transmitter. This would seem reasonable for the upper nozzle and excessively long (and costly) for the nearby lower nozzle (where the capillary tubing would typically be coiled for convenience). However, this analysis only takes physical dimensions into consideration.

The capillary tubing contains liquid that transmits the pressure from the diaphragm seal to the transmitter. The density of this liquid changes with its temperature, so the installation should be designed to maintain the liquid in both capillaries at the same temperature. This is similar to the concept that was discussed for steam flow measurement transmitters.

However, the liquid in the capillary tubing is different from the steam flowmeter seal in the sense that the liquid in the capillary tubing is captive. Therefore, expansion of the liquid in one capillary will cause the liquid volume to change and affect the differential-pressure measurement. By nature, the differential-pressure transmitter will approximately cancel the expansion and density effects for capillary tubes of equal length (and liquid volume) when the capillaries are at the same temperature. However, unequal lengths of capillary tubing (and unequal liquid volume) on each side of the transmitter can affect the measurement because the different capillary lengths (and volumes) result in different amounts of expansion. Therefore, it is desirable for the capillary tube lengths to be not only equal, but also at the same temperature.

David W. Spitzer, 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 for manufacturing and automation companies. He can be reached at 845 623-1830.

More in Containment/Hose & Tube

Search for more Issue Archives

© 2012 EBSCO Industries, Inc. All rights reserved.
Grand View Media Group is a subsidiary of EBSCO Industries, Inc.

Site Design & Development by the Pinnex Group

Privacy Policy