David W. Spitzer, P.E.

A liquid seal is formed in the impulse tubing in steam service in order to keep the hot steam from reaching the transmitter(s) and potentially causing damage. Steam pressure compensation is typically performed to calculate steam flow at actual operating conditions. This entails measuring steam pressure at the primary flow element. However, the transmitter that measures steam pressure is typically located below the primary flow element. What is the approximate pressure measurement error when the transmitter is located 10 meters (approximately 30 feet) below the flow element in an 80 bar steam header when the pressure measurement is not corrected for the pressure generated by the condensate in the seal?

A. 0.60 percent
B. 1.25 percent
C. 2.50 percent
D. 12.5 percent

For the purposes of this problem, let’s assume that bar is a gauge pressure unit. In addition, one atmosphere is 1.01325 bar absolute. However, one bar absolute will be used in this problem to simplify the calculations. In addition, one atmosphere of pressure corresponds to approximately 10 meters of water column.

The pressure associated with the seal is approximately one bar. Therefore, the effect on the pressure measurement is approximately 1.25 percent (1 / 80). However, the effect on absolute pressure is important for pressure compensation. The effect on the absolute pressure measurement is approximately 1.23 percent (1 / 81). It should be evident that the steam pressure is sufficiently high such that the effect of the errors on the gauge and absolute pressures are similar. Answer B is correct.

Of particular interest is the magnitude of the error. Not accounting for the pressure head of a 10 meter high seal does not seem like a big problem when the pressure is 80 bar. However, its effect on the pressure measurement is over 1 percent of the measured pressure. Further, its effect on a differential-pressure flowmeter will be approximately 0.6 percent. This error can cause the annual cost of the steam to be biased by over $100,000 in large steam headers.

Additional Complicating Factors
It is common for steam pressure to be measured in headers that are (say) 40 meters above grade. Many steam plants are designed such that the transmitter is located near the control room (say) 10 meters above grade. In such installations, the pressure measurement error will be approximately three times larger than calculated above.

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 and can be reached at 845 623-1830.