Jul-2010

Optimising steam systems: part II

A second article on steam system optimisation addresses steam at point of use and good engineering practice to remove and return condensate

Ian Fleming
Spirax Sarco

Article Summary

Use of steam in the oil and petrochemical sector can be classified under:
• Primary process requirements; for instance, a reboiler on a distillation column
• Secondary process requirements such as steam tracing
• Emergency requirements, including snuffing in the case of fire or turbine rupture
• Utility requirements such as turbogenerators.

Steam in process heating
Steam is used to vapourise, preheat and heat a process. Regardless of the heat exchanger used, whether it is a kettle-type reboiler (shell and tube), plate-type heat exchanger or heating coils in a tank, the principles of operation and the end result on the steam side are very similar.
When using steam, the rate of heat transfer in a heat exchanger can be defined (in its simplest form) by a basic equation:

Q = U*A*(Ts - Tp)

Where:
Q  = Heat transfer rate, W
U  = Heat transfer coefficient, W/m2 °C
A  = Heat transfer surface area, m2
Ts  = Steam saturation temperature, °C
Tp = Mean process temperature, °C

The heat transfer coefficient, U, decreases during operation due to fouling, so heat exchange manufacturers add in a fouling factor to ensure that the reboiler, for example, meets the process load for the required service period. The fouling factor is the additional heat transfer surface area (A) of the heat exchanger, which can be anything from 10–50% greater than the area required for a clean surface (depending on the process). The importance of the fouling factor on steam loads and condensate removal will become apparent later in this article.

The steam saturation temperature (Ts) is determined by the steam pressure within the heat exchanger and is defined by the steam saturation curve (see Figure 1). The control valve therefore achieves the correct process temperature by limiting the steam flow (and consequently the steam’s pressure/ temperature) entering the heat exchanger, replacing the steam that has condensed as it gives up its enthalpy of evaporation to the process fluid. As demand increases, the control valve opens, increasing the steam pressure and temperature, leading to a greater heat transfer rate.

If demand reduces, the control valve throttles and the opposite occurs, lowering the steam pressure and temperature on the primary side. In addition, if the heat exchanger is new or has just been cleaned, the additional fouling factor may lead to a significantly greater heat transfer surface area (A) than is required for the actual duty, resulting in a lower steam temperature requirement. It is not unusual to find heat exchangers operating with steam pressures just above atmospheric conditions or even at sub-atmospheric pressures, regardless of the steam pressure upstream of the control valve. Understanding this helps to explain the root cause of many of the problems arising with heat exchangers and how they can be overcome.

Heat exchanger stall
Stall occurs when the steam pressure in the heat exchanger drops below the back pressure (condensate line pressure) acting on the steam trap. This prevents the flow of condensate through the steam trap, which in turn causes the condensate to back up. Although this sounds unusual, it is a fairly common situation, particularly on temperature-controlled equipment.

Typical symptoms indicating that a heat exchanger is suffering from stall include: 
• Cold or cool steam traps draining the heat exchanger, due to a back up of condensate in the heat exchanger
• Corrosion within the heat exchanger, due to waterlogged condensate standing in the steam space. Many operators believe this is normal and accept it as a fact of life. (It is worth mentioning that corrosion is also a sign of poor water treatment, which should also be investigated)
• Unstable control or cyclic temperatures of the process fluid: as the heat exchanger stalls, it begins to flood, reducing the heat transfer surface area and heat transfer rate. The control valve opens to meet the demand and, in so doing, the steam pressure rises, which in turn overcomes the stall conditions and the condensate is rapidly removed from the heat exchanger. With this sudden increase in available effective heat transfer surface area, the process starts to go over-temperature. The control valve closes and the cycle repeats itself
• Mechanical stress and cracking in the heat exchanger can be caused by the difference in temperature between steam at the top of the heat exchanger and cool condensate at the bottom
• Water hammer can lead to premature failure of the heat exchanger or surrounding pipework. The heat exchanger makes cracking, banging or thumping noises as hot steam bubbles, surrounded by cooler condensate, implode as they condense. Since steam has a considerably higher specific volume compared to water, when the steam collapses condensate is accelerated into the resulting vacuum. As the void is filled, water impacts the centre, sending shockwaves out in all directions
• Water and energy losses are caused by the steam trap bypass valves being left open and condensate being dumped in an attempt to achieve the required process conditions.

Stall can occur when:
• The process temperature is less than 100°C (implying steam temperature on the primary side could be lower than 100°C and therefore below atmospheric pressure)
• The heat exchanger is oversized (which may be necessary to allow for fouling)
• Heat exchanger loads vary, resulting in the control valve having to throttle on low load conditions
• Back pressure is present on the condensate line due to lift, failed open steam traps pressurising the line or if the line is undersized for the condensate loads and flash steam generated.