There is a common misconception that condensate return lines are solidly filled with only condensate. However, flashing condensate lines can be primarily filled with flash steam that has formed due to the sudden pressure drop of high temperature condensate discharging into the return. There is thus a two-phase flow condition (flash steam and condensate) in typical condensate lines that can create violent water hammer in the header when gravity drainage is not possible.

Changes in elevation in two-phase condensate return lines are especially prone to creating severe shock from the acceleration of slugs caused by wave action within the piping. The force of these slugs can intensify as pipe size increases in diameter, due to the larger magnitude of condensate mass present.

To demonstrate how water hammer can occur at the bottom of vertical risers and suggest a possible piping arrangement that may reduce or eliminate this water hammer, TLV Consulting · Engineering · Services constructed the clear piping system shown in the below video.

The video is a demonstration of two scenarios:

• Water hammer occurring in a flashing condensate transport system which includes at least one vertical riser, a commonly seen, but not recommended, non-gravity-drainage installation.
• A condensate transport system incorporating a drop-down loop seal (or "DDLS"; a drop introduced in the piping immediately before the riser) to help mitigate some effects of water hammer caused by the flow obstruction of a vertical riser.

In the top piping arrangement, significant wave action can be seen along the horizontal run. Waves are caused by high velocity steam (represented by compressed air) moving over the surface of the liquid. When a wave grows large enough, it momentarily seals the cross section of the horizontal pipe, building a pressure wall behind it. This causes a water slug to accelerate down the line, potentially causing serious damage to piping, valves, gaskets or fittings. Additionally, in a flashing condensate line the vapor space is occupied by low energy flash steam which can collapse and generate hammer as condensate rushes in to fill the void.

The second, lower piping arrangement introduces a non-ideal solution to this problem: a drop-down loop seal. The ideal solution is always to incorporate gravity drainage in the flashing condensate return system design. However, when gravity drainage was not accomplished in the original design, a drop-down loop seal may help reduce water hammer. Such a configuration should be evaluated on a case-by-case basis by a professional engineer to ensure suitability and safe operation for each specific application.

As the video shows, the drop-down loop seal reduces the condensate level in the pipe and minimizes the length of pipe where surging can occur. Thus, the loop seal piping arrangement can reduce surging and lower the probability of incurring dangerous water slugs. The result of such a configuration can be smoother condensate flow with lessened water hammer.

Although horizontal piping is used in the drop-down loop shown to demonstrate how the interaction of vapor and liquid in the loop can promote upward flow, it is anticipated that joining two long radius elbows at the bottom of the loop with no horizontal section between them could further reduce hammer.

It should be noted that strategies for eliminating water hammer differ depending on the cause of the hammering. When water hammer occurs, plants should always ask a professional engineer to analyze the cause of the water hammer and develop an appropriate countermeasure.