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

May 2007

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Measuring Pressures Below Absolute Zero
Instrument & Installation Requirements for Full Vacuum Applications

David W. Spitzer, P.E.

Selecting instruments for processes that operate below atmospheric pressure brings to mind a number of applications that make me think twice. Stay with me, and I’ll show you how to attempt to measure pressures below absolute zero — unsuccessfully, of course.

Distillation columns are used to separate the components of a feed stream that boil at different temperatures. “Heavy” liquid that boils at a relatively high temperature is boiled in the bottom of the column and “light” liquid that boils at a relatively low temperature is refluxed into the top of the column. Counter-flow contact of these streams results in the separation of the components.

Operationally, too much heat will cause heavy components to rise in the distillation column and can result in more impurities in the light product. High reflux will cause light components to fall in the column and can result in more impurities in the heavy product. Excessive reflux can cause the column to flood, whereby column operation is compromised. Flooding can be detected and potentially prevented by measuring the differential pressure across the distillation column.

Many distillation columns operate at elevated pressures, others at near-atmospheric pressure, while still others operate below atmospheric pressure. Differential-pressure transmitters with diaphragm seals are often used to measure the differential pressure across the column. This may be applicable for columns operating above atmospheric pressure. However, applying this measurement technique to columns operating below atmospheric pressure can present some interesting issues.

For example, assume the elevation difference between the column pressure taps is 15 meters, both diaphragms have eight meters of capillary tubing, the specific gravity of the diaphragm seal fill fluid is 1.00, and the differential-pressure transmitter is located 7.5 meters from each tap. Under atmospheric conditions, the pressures at the upper tap and low side of the transmitter are calculated to be approximately one and 1.75 atmospheres (absolute) respectively. Similarly, the pressures at the lower tap and high side of the transmitter are calculated to be approximately one and 0.25 atmospheres (absolute), respectively.

Now consider what occurs when the column is operated in a full vacuum at absolute-zero pressure. Under a full vacuum, the pressures at the upper tap and low side of the transmitter are calculated to be approximately zero and 0.75 atmospheres (absolute), respectively. Similarly, the pressures at the lower tap and high side of the transmitter are calculated to be approximately zero and -0.75 atmospheres (absolute), respectively. Wait a minute! The pressure cannot be less than zero atmospheres (absolute), so this instrument and its installation will not work under these conditions.

A potential solution to this problem includes mounting the transmitter at or below the lower tap and increasing the length of the capillary tubing. However, this approach will increase measurement error. Another option is to mount the transmitter on the lower tap with only one diaphragm seal. However, in this scenario ambient temperature changes will affect the measurement due to the expansion and contraction of the capillary fill fluid on only one side of the transmitter. A differential-pressure transmitter with purged impulse lines to the taps could also be considered, as can the installation of separate absolute-pressure transmitters on each tap where their measurements can be subtracted to obtain the column differential.

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 for manufacturing and automation companies. He can be reached 845 623-1830.

www.spitzerandboyes.com

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