Don't Let Heater Control Get You Steamed

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.