|David W. Spitzer, P.E|
In last month’s Applications Corner, I mentioned there are certain types of applications that make me think twice. Since I don’t work with these applications on a regular basis, I’m always concerned that I’ll overlook something and get into (big) trouble. One such application is the measurement of liquid flow in pipes that rely on gravity as their motive force.
The specific example I cited last month was a tank operated as a binary separator that was fed in the middle where the feed flow contained heavy liquid that drained out the bottom while the light liquid overflowed at the top. Sizing and installation issues involved with the feed and bottom flowmeters were discussed. Now let’s take a look at the overflow flowmeter.
Overflow piping is typically installed using a horizontal nozzle near the top of the vessel with a 90-degree elbow to allow the liquid to drain downward to its destination. In this configuration, liquid will overflow the tank when liquid in the tank reaches the lower invert of the overflow nozzle. While the tank remains full of liquid up to the overflow nozzle, the vertical overflow pipe does not remain full. This should be taken into consideration because the flowmeter should remain full of liquid in order to measure properly. Failure to keep the flowmeter full will typically result in flow measurements that are far from correct.
One solution to this problem is to control the tank level so the liquid level covers the overflow nozzle. The disadvantage to this approach is that an additional tank level control loop (level transmitter, controller, and control valve) is necessary to maintain the higher tank level. Aside from the expense, the tank can overfill if the control valve remains in its closed position.
Another solution is to install a trap below the overflow nozzle that drops, turns 180 degrees to flow upward, and then turns 180 degrees to flow downward. This configuration creates a vertical run that would be full of liquid where a flowmeter could be located. Static hydraulic analysis reveals the pipe would be full of liquid in the pipe where the flow is downward or upward, so either location would be suitable for installation of the flowmeter. However, the liquid flow in the downward leg of the trap can be likened to that of a waterfall so foam and bubbles can form and affect a flowmeter installed in the downward flowing part of the trap. Therefore, it is recommended that the flowmeter be installed after the foam and bubbles disengage, such as in the upward flowing part of the trap. A bypass valve at the bottom of the trap would allow the vessel to be drained for maintenance.
In most applications, process and piping considerations strongly influence flowmeter installations. However, some applications can benefit significantly when the person installing the flowmeter takes the lead.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience with 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 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. He can be reached 845 623-1830.