Measuring Gravity-Fed Flow

Sept. 26, 2010

Key Considerations Gleaned from a Real-Life Application

David W. Spitzer, P.E.

There are certain types of applications that make me think twice. This is not because they are necessarily difficult or intricate or complicated, but rather because they are applications that I do not deal with on a regular basis, which increases the likelihood that I’ll overlook something and get into (big) trouble. The measurement of liquid flow in pipes that rely on gravity as their motive force is one such class of applications. So I’d like to give special consideration to this type of scenario here where I can fully flesh out the details.

Typical process plants use pumps to transport liquids from one vessel or location to another. On occasion, the process is designed to allow liquids to overflow or drain from a vessel. For example, assume a tank that is operated as a binary separator is fed in the middle. The feed flow contains approximately 90 percent heavy liquid that drains out the bottom while the remaining light liquid overflows at the top. The feed is pumped into the vessel and is manipulated with a control valve. Manipulation of the bottom flow can be used to control the interface between the heavy and light liquids. The cascade interface level-control strategy requires measurement of the bottom flowrate. Other process considerations dictate that measurement of the feed and overflow flowrates are required.

Noting that 90 percent of the feed flow is heavy liquid, the bottom flowrate will typically be approximately 90 percent of the feed flowrate. Therefore, the range of the feed flowmeter and bottom flowmeter will likely be the same. One might reason that the flowmeters will also be the same. However, the feed flow is pumped while the bottom flow is fed by gravity.

Typically, there is more pressure available from the pumped system as compared to the gravity system. If the overflow is two meters above the bottom outlet, the total pressure available due to gravity at the bottom outlet is 0.2 bar (three PSIG), with a specific gravity of 1.00. This contrasts with the pumped feed that may operate at a pressure of one bar or more. The amount of pressure drop available for the flowmeter can have a direct influence on the flowmeter size — even for the same flow streams flowing at (essentially) the same flowrate. In this application, the bottom flowmeter was one pipe size larger than the feed flowmeter because there was insufficient pressure available to push the bottom flow through the smaller flowmeter.

Flowmeter installation should also be considered when liquid is allowed to flow using gravity. In general, flowmeters should remain full of liquid in order to measure properly. The feed flowmeter located with flow rising in a vertical pipe should not pose a problem. However, a flowmeter located in a vertical line with downward flow could generate gas accumulation upstream of the control valve. Locating the control valve a considerable distance from the flowmeter should reduce this possibility, but installing a trap at the bottom of the tank would eliminate the problem. The trap piping would rise (say approximately one meter) after coming out of the bottom and then turn 180 degrees to flow downward. This configuration creates a vertical run that would be full of liquid where a flowmeter and (space permitting) the control valve could be located. A bypass valve at the bottom of the trap would allow the vessel to be drained for maintenance.

Check back next mont for a discussion of the overflow flowmeter in this application.

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.

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