Perspectives: Plant InSites

Don't Let Heater Control Get You Steamed

By By Andrew Sloley

Process engineers often neglect the utility side of many of their services. However, ignoring steam-side control of steam heaters frequently creates serious problems. For instance, we'll look at one case where inadequate control compromised operations and product quality. Here, effective control of the extreme process variability required combining two traditional methods for steam-side control when process outlet conditions set duty requirements (Figure).

On the left in the figure is a classic cascade control configuration. Steam enters the process heater based on pressure control on the steam side. A process temperature controller resets the pressure control. Varying steam-side pressure changes temperature difference in the exchanger, which, in turn, alters overall heat transfer.

This configuration, often referred to as a steam chest design, makes control response more linear, and offers many benefits, including:

  • fast response to duty demand -- steam pressure increases are nearly instantaneous and steam pressure decreases are fast; and
  • heat-transfer at minimum skin temperatures in the heater, which is an important advantage for temperature-sensitive systems.

    It imposes extra costs compared to other configurations, however, because of its use of an external condensate receiver. In addition, maintaining controllable operation at the upper end of the required duty range often requires extra surface area for the exchanger. This enables the steam control valve to isolate the process from steam-system pressure variations.

    To avoid those extra costs, many engineers opt instead for designs, like the one on the right in the figure, that hold a condensate level in the exchanger. Here, the process temperature (or duty) requirement resets a liquid level controller that varies the flooded surface area in the exchanger. Less flooded area allows for more heat transfer due to higher heat-transfer coefficients for condensing service.

    Compared to the steam-chest design, flooded-area control provides:

  • lower installed cost;
  • constant pressure for condensate disposition;
  • fast-to-medium response to duty demand -- level drops are very fast but level rises may take a considerable time;
  • skin temperatures at the maximum;
  • potential difficult level control spans in small exchangers; and
  • highly non-linear control of duty versus level when the level is close to the top and bottom of a horizontal exchanger.


    Control Alternatives
     

    Effective control required combining two traditional approaches. The improved method is shown in orange.

    Solving multiple problems

    The condensate receiver sent fluid to a shared medium-pressure condensate system that included a flash recovery receiver for recovery of low-pressure (40 psig) steam.

    However, problems occurred even at plant startup. Depending upon reactor duty and temperature requirements, the pressure demand on the heater could drop below the medium-pressure condensate header pressure. Under these conditions, the condensate would backflow to the receiver. As the receiver filled, the control valve would continue to open, worsening the situation. Eventually, condensate would enter the exchanger and flood the entire heater, turning it into a hot water heater with a very low water rate.

    At different process loads, one of two problems would occur:

    1. Duty available from subcooling the condensate could not meet the temperature requirement. The steam control valve would rapidly open, blowing the condensate from the system. Heat transfer would rise rapidly and the process temperature would cycle.

    2. Condensate subcooling barely met duty. Nevertheless, the control valve on condensate continued to "hunt" for a response. Operator action and continual retuning of control valves was necessary to achieve the minimum required performance, and persistent product-quality upsets continued.The solution

    Some instrumentation, shown in orange on the figure, was added to allow the unit to operate in either steam chest or flooded mode. A simple control switch that moved the system between the two modes established stable control. When duty demand drop resulted in steam pressure below 100 psig, the steam side switched from steam chest to flooded operation. The flooded operation had a fixed pressure control of 100 psig on the steam supply to the heater. When duty demand rise resulted in a flooding level beyond 75% of the exchanger level, the steam side switched from flooded to steam-chest operation. Establishing these switch points and the speed of changeover for stable operation required plant testing and loop tuning. This system has an extremely wide duty control range and has accommodated ranges from 4% to 115% of the original design duty without problems.In this particular case, a highly variable process required reactor feed heated to a fixed inlet temperature. The feed was heat sensitive. High temperatures caused exchanger fouling and also the formation of color bodies that would put the product off-specification. For this reason, the steam side of the heater employed a steam chest design.

More from this perspective...

Title

Don’t let parallel pumps cross you up

Small differences with parallel pumps can lead to big difficulties, according to Andrew Sloley in this month's Plant InSites column.

03/28/2007

Keep others from making “classic” mistakes

A heat exchanger problem provides a textbook example of design limitations, according to Contributing Editor Andrew Sloley.

04/24/2007

Teaming two solutions can be counterproductive

Sometimes things just don’t add up as expected. Andrew Sloley discusses how sometimes two solutions aren't always better than one solution.

06/06/2007

Properly protect centrifugal pumps

Consider various factors when selecting how to guard against low flow, Andrew Sloley says in this month's Plant Insites column.

07/11/2007

Don’t look at precondensers in a vacuum

Consider the impact of the downstream vacuum ejector on precondenser performance, advises Andrew Sloley, in this month's Plant InSites column.

08/20/2007

Correct those disturbing lapses

Keeping liquid in a reflux drum still can prevent problems, says Andrew Sloley, in this month's Plant InSites column.

08/31/2007

Do some heavy thinking about light loads

Cutting steam demand can get you into hot water, advises Contributing Editor Andrew W. Sloley, contributing editor, in this month's Plant InSites column.

10/01/2007

Carefully check vendor references

Determine the differences between a cited installation and yours, Andrew Sloley, contributing editor, says in this month's Plant InSites column.

10/30/2007

Keep parallel pumps in line

Mind the curves when using different size pumps.

02/13/2008

Don’t stack the deck against yourself

Always consider future expansions before finalizing layouts

03/14/2008

Plant InSites: Cast a cold eye on columns

Subcooling can complicate and compromise tower performance

05/23/2008

Understand the cold facts about subcooling

Systems sometimes can surmount major layout errors

06/04/2008

Use water to check vessel integrity

Hydrostatic testing offers advantages but requires care

06/25/2008

Right Vacuum Control Choice Takes the Pressure Off

Choosing the proper recycle stream for regulation is crucial

08/07/2008

Plant InSites: Look Beyond the Lore

Don’t rely on recollections about how a unit had performed

10/10/2008

Full Vacuum Rating Isn't an Empty Benefit

It often can provide important but underappreciated advantages for vessels

10/16/2008

Stripper Bares All

Analysis reveals how operating practices compounded design mistakes

11/21/2008

Steam Systems: Simple Solutions Can Prompt Complex Problems

Steam systems are especially susceptible to developing difficulties

12/23/2008

Troubleshooting: Take to the Field

Troubleshooting demands getting into the plant to inspect units.

01/30/2009

Take Control Down a Level

Sometimes you can design out the need for instrumentation.

02/23/2009