Perspectives: Plant InSites

Ease Measurement of Column Internal Flow

Opting for a pump instead of gravity to return liquid to a tower often makes sense

By Andrew Sloley, Contributing Editor

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.

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

More from this perspective...

Title

Heat Exchangers: Is Mist a Must?

Water sprays may boost performance of air-fin exchangers.

01/02/2013

Is Your Vessel Foundation Really Strong Enough?

Certain situations can lead to exceeding design limits.

12/03/2012

Backflush Away Fouling

Flow reversal can remove fiber buildup in heat exchangers.

11/06/2012

Properly Position Interlock Valves

The obvious location may not be the most cost effective

10/16/2012

Piping Geometry: Grasp Line Layout

Draining, venting, fouling and other considerations affect piping geometry.

09/06/2012

Consider Compact Reboilers

Several types of heat exchangers can save space

07/24/2012

Select the Right Reboiler

Understand the strengths and weaknesses of various options.

06/04/2012

Get Your Head Around Velocity Head

Confusion about what the term means can spell trouble.

05/02/2012

Old Doesn't Mean Outdated

Some equations have stood the test of time.

04/04/2012

Vacuum System Fix Fizzles

Use of scavenged equipment leads to problems.

03/07/2012

More Exchanger Area Can Pose Pitfalls

This "obvious solution" for heat-transfer problems may not work.

02/01/2012

Do Liquid Injection Right

A variety of factors influence performance.

01/05/2012

Let the Buyer Beware

Understand common dangers when developing specifications and evaluating bids.

11/21/2011

Tackle Tray Trade-Offs

Find the right compromise between process and mechanical requirements.

10/26/2011

Watch Out For Dead-Legs

Lack of flow doesn't mean lack of problems.

09/27/2011

Distill Your Internals Choice

Some general guidelines can ease the selection process.

09/01/2011

Take a Different Look at Centrifugal Pumps

An unconventional assessment can provide insights for effective control.

07/25/2011

Pick the Proper Centrifugal Pump

Consider the impact of startup conditions and prospective operating points.

07/11/2011

Don't Calculate Pressure Drops in a Vacuum

Consider flow regime boundaries and the limitations of methods.

06/08/2011

Find the Real Maximum Pressure Of Vessels

Always consider static head when assessing pressure vessels.

04/21/2011