High-pressure steam traps, condensate

Operators stop steam hammer by dumping either the hot condensate (from a high-pressure steam trap) or the colder condensate (from a low-pressure steam trap) to the sewer. Either way, the condensate is no longer recycled to boiler feedwater. The sort of design required to collect condensate without steam hammer is illustrated in Fig. 12.8. Basically, hot, high-pressure condensate is collected in a... 

High-pressure condensate forms at the same temperature as the high-pressure steam from which it condenses, as the enthalpy of evaporation (latent heat) is transferred from it. When this condensate is discharged through a steam trap to a lower pressure the energy it contains is greater than it can hold while remaining as liquid water. The excess... 

On the downstream side of the steam condensate control valve or steam trap, divert the condensate into an empty 55-gal. drum. Note the time it takes for the drum to fill. Add 5% (for low-pressure steam) to 15% (for high-pressure steam) to the observed pounds per hour of flow. This accounts for condensate flashing as it drops to ambient pressure. For instance, a reboiler having a duty of 25 MM BTU/hr will fill a 55-gal. drum in about one minute. 

Operators who have problems with loss of reboiler capacity often attribute these problems to condensate backup. This is usually true. To drop the level of water out of channel head, either the steam trap or the condensate drum is bypassed by putting the condensate to the sewer. Sometimes the float of the trap is sticking, but mostly the difficulty is an erratically high pressure in the condensate collection... 

One pound of steam at Opsig occupies 1,600 times the volume of a pound of water at atmospheric conditions. This ratio drops proportionately as the pressure increases. When the steam collapses, water is accelerated into the resulting vacuum from all directions. This happens when a steam trap discharges relatively high-pressure flashing condensate into a pump discharge line. 

Water hammer (also known as hydraulic shock) occurs in two distinct ways when the flow of liquid in a pipeline is suddenly stopped, for example, by quickly closing a valve [13], and when slugs of liquid in a gas line are set into motion by movement of gas or condensation of vapor. The latter occurs when condensate is allowed to accumulate in a steam main, because the traps are too few or out of order or in the wrong place. High-pressure mains have been ruptured, as in the following incident. 

It follows that condensate at the outlet side of the steam trap can only be at atmospheric pressure, and if it is to be lifted to a high-level return line, or into a return line... 

On a single-evaporator flooded system, a float valve can be fitted which will pass any drained liquid from the condenser direct to the evaporator. The action is the same as that of a steam trap. The float chamber is at condenser pressure and the control is termed a high-pressure float (Figure 8.3). 

The downstream piping must be adequately sized to effectively handle this volume. An undersized condensate returnline results in a high flash-steam velocity, which may cause waterhammer (due to wave formation), hydrodynamic noise, premature erosion, and high backpressure. The latter condition reduces the available working differential pressure and, hence, the condensate removal capability of the steam trap. In fact, with some traps, excessive backpressure causes partial or full failure. 

A steam heater, as shown in Figure 3.10(A), can provide responsive temperature control because the entire steam side is at the condensing temperature and has a high heat transfer coefficient. The pressure on the steam side is determined by the temperature that provides heat transfer equal to the heat released by the condensing steam. The steam trap provides condensate level control within the trap in order to provide a seal for the condensing steam. 

To overcome this problem, a submerged condensate pot is often installed instead of the steam trap (Fig. 17.le) as described earlier (item 5 above). An alternative remedy is replacing the steam trap by a level condensate pot (Fig. 17.1/). By varying the level control set point, the surface in the reboiler can be adjusted so that the reboiler operates at a pressure high enough to ensure condensate removal at all times without a pump. Note that the bottom of this drum is located below the bottom of the condensing side of the reboiler (189) otherwise, "dry reboiler operation at high rates will not be possible, and reboiler capacity will be reduced. 

Steam pressure used for heating and corresponding saturation temperature. In case of condensate discharged at high pressure it may flash into low pressure steam in the condensate exit line and create water hammer/back pressure on the trap. 

---