Ease Measurement of Column Internal Flow

Sept. 17, 2014
Opting for a pump instead of gravity to return liquid to a tower often makes sense

Last month, we looked at using a gravity-flow loop to measure liquid flow inside a column (“Get Some Inside Information”). An often-attractive alternative uses a pump to return the total liquid draw taken from the column. The important engineering fundamentals of this system lie in the pump and control logic.

Figure 1 shows two possible configurations for measuring internal liquid flow in a column using a pump. The upper drawing represents the conventional configuration, i.e., a fixed-speed pump with a flow controller; the lower one depicts a setup with the pump controlled by a variable frequency drive (VFD). The conventional configuration may include a low-flow bypass if rates below the lower operating limit of the pump are expected. Most applications will benefit from a more-modern configuration with level controlled by a VFD.


Figure 1. Either a valve (top) or a variable frequency drive (bottom) can control the amount of liquid pumped back to the tower.

Total frictional losses in most systems will be low. Typically such losses —from the flow instrument, piping and the return liquid distributor inside the tower — account for at least half of the total system loss. The static head loss between the draw point and return point is relatively small. This makes a good application for a VFD on the pump.A pump should not run dry. So, control of the upstream level is necessary to protect the pump. This requires adding a level controller span on the collector tray; for effective control the tray must provide sufficient level range, which usually results in high liquid inventory inside the tower. The high liquid level on the tray would make even a small gap on a bolted tray leak a large amount of liquid —undermining the accuracy of measurements because the leaking liquid doesn’t go through the flow meter. So, the tray must be fully welded for leak-free operation. The control configurations shown assume a centrifugal pump, which means discharge pressure and flow rate are closely linked. Use of a positive displacement pump would require different control configurations.

The pumped system has both advantages and disadvantages compared to a gravity-flow system.

Advantages include:
• The distance between the draw and return points isn’t critical.
• Pressure drop across the flow instrument isn’t a major issue.
• As long as sufficient pipe length for flow conditioning is available, there’s more flexibility in picking instrument locations.
• In vacuum systems, the pump supplies pressure for a sampling station (as shown in the bottom drawing in Figure 1).
• The pump also provides pressure drop for closed sampling stations, if needed.

Disadvantages include:
• The system costs more. The installation requires pump(s), foundation(s), more instrumentation and more-complex control.
• Auxiliary equipment, such as strainers, may be needed to protect the return-liquid distributor inside the tower.
• The critical distance H1 remains. It is necessary for providing sufficient net positive suction head to the pump.
•The pump and associated valves create new locations for potential leaks.
• Unit startup may be more difficult. This often is true when the draw is from a tower with trays and a full collector tray isn’t used to ensure liquid flow to the pump.

Different process requirements may favor either the pumped or the gravity-flow system. However, pumped systems tend to work more often. That’s not due to any magic. Rather, because a pump raises costs, everyone pays greater attention to system fundamentals and layout. As a result, more pumped systems get done right. However, keeping track of the fundamentals and making sure they’re applied correctly always should be a concern.

ANDREW SLOLEY is a Chemical Processing contributing editor. You can e-mail him at ASloley @putman.net

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