David W. Spitzer

Process pressures and process temperatures are important measurements that are routinely used to monitor and control the process. Pressure and temperature measurement instruments are usually installed in convenient locations that process engineer or piping designer often selects.

Notwithstanding the above, a number of pressure and temperature measurements are used to compensate the raw flow measurement for the actual operating pressure and temperature. These measurements are especially prevalent when measuring the flow of gases in which the pressure and temperature vary during operation. In such installations, the pressure and temperature measurement taps should be located to obtain measurements from which an accurate, compensated flow measurement can be obtained. In a differential pressure flowmeter installation, where should the pressure and temperature taps be located?

A. Pressure upstream — temperature upstream
B. Pressure upstream — temperature downstream
C. Pressure downstream — temperature upstream
D. Pressure downstream — temperature downstream

Pressure measurements for flow compensation are usually located upstream of the flowmeter, although some flowmeters have internal taps for this purpose. A few flowmeters have equations for using a downstream pressure tap. In general, pressure measurements are located upstream of the flowmeter.

Temperature measurement devices are typically located in thermowells that project into the pipe and can distort the velocity profile in the pipe. Because flowmeter performance is often degraded by velocity profile distortion that enters the flowmeter, temperature measurement devices are usually located downstream of the flowmeter. In addition, the temperature measurement devices are generally located sufficiently far downstream of the flowmeter so that they do not affect the measurement.

Additional Complicating Factors
In a new installation, the instrumentation engineer can typically take control of the situation and appropriately locate the taps. However, all is not necessarily lost if the taps and/or instruments are already installed. The reason is that some flowmeters have equations for using downstream pressure compensation and the difference between the upstream and downstream gas temperatures may be small enough to be ignored. Keep in mind, however, that if an upstream thermowell creates a disturbance affecting the velocity profile that enters the flowmeter, the thermowell may have to be relocated.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.