Maintaining Steam Pressure

 

Condensate must be returned, and a new steam heating circuit must be established before the steam loses too much pressure.  The pressure of saturated steam determines its temperature, and steam temperature is the primary driver in the system’s heat transfer capability.  There are two reasons why steam loses pressure as it flows through the circuit:

  1. Static pressure drop.  The collection of condensate in the jumpers forms a vertical water column that has an associated static pressure drop.  In order for the steam to bubble through the water column in the jumper, it must overcome the weight of the water.  In overcoming the column’s weight, the steam loses some of its pressure.  The total height of water column in all of the jumpers in the circuit determines the total static pressure drop.  (To visualize the water column which forms in the jumpers, watch the steam/condensate movie in the Steam Lab section.) 

  2. Dynamic pressure drop.  As with any fluid, steam flow through the circuit is impeded by friction in the pre-insulated tubing, jacketing elements, and jumpers.  Friction results in dynamic pressure drop in the steam.  The primary influence on dynamic pressure drop is the cross-sectional flow area of the pre-insulated tubing (connecting the supply manifold to the jacketing) and the jumpers.  Decreasing the flow area can have a significant impact on pressure drop.  For example, reducing the tubing/jumper diameter from 3/4” to 1/2” results in an 8X increase in dynamic pressure drop!

 

To ensure successful operation of the steam jacketing system, the factors of static and dynamic pressure drop must be carefully considered in designing the steam heating circuits.  Often, this is accomplished via the use of conservative plant standards which limit circuit lengths without consideration of steam flow rates and tubing sizes.  This approach, while effective in creating a functional system, can result in excessive infrastructure costs associated with many more circuits than are truly required.  By taking an engineered approach to routing the steam circuits, the number of circuits can be optimized, saving thousands of capital dollars on utilities.  Furthermore, a more optimized system requires less steam traps, which increases plant robustness and saves maintenance/repair dollars over the long term.  (This is particularly important since field surveys commonly report that one-third of a plant’s steam trap population does not operate properly.)