Make the Most of Condensate

A few steps can yield sizable savings and improve exchanger performance.

By Steve Ashby, Armstrong International

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A steam trap is designed to drain condensate and vent noncondensable gases without allowing live steam to escape. Because its internal mechanism performs as a discharge control valve, the steam trap inherently operates comparably to a constant-level condensate system.

Similarly, with adequate steam pressure to overcome the condensate return backpressure, the heat exchanger will perform with optimum efficiency -- and if the supply-steam control valve closes due to reduced thermal requirements, the available condensate motive pressure decreases and potentially backs up and floods the vessel. Again, the heat exchanger loses performance and becomes subject to corrosion and structural damage.

The recurring theme is that positive pressure is required to ensure condensate drainage and non-condensable gas venting from heat exchangers. Unfortunately, the quick remedy to drain a flooded vessel is to open valves to the atmosphere, wasting valuable energy and creating a potential safety hazard.

Incorporating a pumping device will prevent heat exchanger flooding and ensure positive motive pressure to overcome any backpressure in the condensate return, keeping the heat exchanger operating at optimal efficiency and assuring structural integrity.

EQUIPMENT STALL
Stall is a condition in which heat transfer equipment can't drain condensate and becomes flooded due to insufficient system pressure.

Stall occurs primarily where the steam pressure is modulated to obtain a desired output (i.e., product temperature). The control requirements for heat exchangers (coils, shell-and-tube, etc.) can be segmented into two distinct modes: optimal performance and stall.

During optimal performance, the exchanger's operating steam pressure exceeds the backpressure present at the discharge of the steam trap. Therefore, a positive pressure differential exists across the trap, allowing for condensate to flow from the heat exchanger to the condensate return line.

In the stall mode, the exchanger's operating pressure is less than or equal to the backpressure present at the discharge of the steam trap. As a result, no pressure differential exists to force the condensate through the steam trap. Condensate begins to collect and flood the heat exchanger.

When exchangers become flooded due to stall, a variety of problems -- including water hammer, frozen coils, corrosion, poor temperature control, short equipment life, control valve hunting (system cycling) and reduced heat transfer -- can occur. Hammer and corrosion can lead to damage to the tube bundles/coils, causing product to contaminate the condensate; this also can create serious problems at the boiler.

Installing a mechanical pump in a closed-loop arrangement allows maintaining a dry heat exchanger regardless of the steam-system pressure, condensate rate or efficiency of the tube bundle. This eliminates tube bundle corrosion and potential tube failure; both could cause an upset condition and interrupt production. By ensuring condensate removal, you can take advantage of all the surface area available in the tube bundle. The heat exchanger can operate at the optimal performance level while utilizing the lowest, most-efficient steam pressure required.

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