Pressure & Temperature Tap Placement

David W. Spitzer, P.E.

In most applications, the pressure and temperature of a gas flow can vary — sometimes significantly. Often, these variations are not considered during design and only become apparent during operation. In one such application, the pressure of a gas of a reactant varied from 13 to 33 PSIG during actual operation. The yield loss due to flow measurement errors was significant because the flowmeter was designed to operate at a constant pressure of 28 PSIG. As you might suspect, such problems indicated that pressure and temperature compensation of the flow measurement was in order. Where should the pressure and temperature taps be located?
A. Pressure tap upstream; Temperature tap downstream
B. Pressure tap upstream; Temperature tap upstream
C. Pressure tap downstream; Temperature tap downstream
D. Pressure tap downstream; Temperature tap upstream
E. Taps can be located either upstream or downstream

Commentary
In general, the pressure taps should be located upstream of the flowmeter. As such, the tap would be located where there is a good velocity so the pressure should be stable and adequately represent the pressure in the pipe. This would seemingly eliminate answers C, D, and E. However, a few flowmeters actually have a pressure port within the flowmeter body that could be used. Other flowmeters, such as orifice plates, have equations that allow the pressure transmitter to be installed at the downstream tap.

Temperature taps usually involve inserting a thermowell into the pipe and are typically located downstream of the flowmeter element. This is primarily for practical reasons because locating the tap upstream of the flowmeter would create turbulence that could affect the operation of the flowmeter. This would seemingly eliminate Answer B. However, a few flowmeters actually have a temperature sensor within the flowmeter body that could be used. On a finer note, it is my understanding that the downstream temperature (at lower pressure) is slightly cooler than the temperature within the flowmeter, so an accurate internal sensor would be preferable.

The correct choice for the application detailed here is Answer A.

Additional Complicating Factors
Pressure and temperature compensation was not considered in the process described above, despite the importance of the (reactor feed) flowmeter. Fortunately, pressure and temperature transmitters were installed for monitoring purposes. Not surprisingly, the pressure transmitter was located on the downstream pressure tap of the orifice plate, while the temperature measurement was located upstream of the flowmeter — even though both could have been located correctly at no additional cost.

Fortunately, orifice-plate equations were available for the downstream pressure measurement and the temperature transmitter was located far enough upstream so as to not affect the flowmeter velocity profile. You may not be so lucky. Install the transmitters in the correct locations — even when gas compensation may not seem to be warranted.

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

www.spitzerandboyes.com