COMMUNICATION TIME – PIPE PERIOD

While wave speed is a differential approach to pressure wave transmission, communication time expands the concept of wave speed to a system approach. Communication time, also known as pipe period, can help predict how a system will react to an inciting transient event.

How is communication time defined?

Communication time relies on two system parameters, the wave speed (a) and the pipe length (L). The communication time describes the time it takes for a signal at one end of the pipe (most often a change in flow rate or change in pressure) to travel to the other end of the pipe, reflect, then travel back to its point of origin. This is shown mathematically in Equation 1 where L is the pipe length and a is the wave speed.

A system cannot react to an upstream or downstream change until the signal point has seen the result of its own reflected wave.

What length of pipe is used to determine the system’s communication time?

Figuring out which length to apply largely depends on what part of the system is being analyzed. For a simple example with a valve between two reservoirs, the length of pipe from the valve to the upstream reservoir is likely the most important length. This upstream pipe would govern the surge created when the valve closes. As the valve closes and changes the system (generally by reducing flow and causing more significant loss through the valve), that signal must travel back upstream to the reservoir to inform a new intermediate operating condition. The ability for a system to adapt to this new intermediate operating condition is discussed further in Instantaneous Transients where communication time can inform the severity of a water hammer event.

On the downstream side of a valve, the concern might instead be cracking open a vacuum breaker valve after valve closure. In this case, the length to analyze is between the closing valve and the vacuum breaker. Half of the communication time can inform elements like remote sensing relief valves by providing the time for the signal to travel from the point of origin to a point of interest.

For branching systems, the communication time can vary and introduces the potential for interacting wavefronts. Often a conservative estimate for the system’s communication time is based on the longest stretch of pipe. Since the relationship between the time of a transient and the system’s communication time impact’s the severity of a pressure surge, using a longer communication time is generally more conservative.

Communication time analysis can be further complicated by partial reflections at pumps, area changes, or other fittings. These components can cause partial reflections where some of the wave passes through the component while a portion reflects. These reflected waves would shorten the distance the signal must travel before returning to its point of origin. These partial waves will have unique communication times and potentially cause wave interference with the partial wave that was transmitted.

If the component can interact with the signal wave (as with check valves, relief valves, and vacuum breaker valves), the component will impact how the wave travels and therefore impact its communication time.

How does communication time predict transient events and interactions?

Wave speed is essentially a speed limit for signals to travel through a piping system. Analogous to echolocation or using thunder and lightning to determine distance (where the wave speed and travel time is known), communication time is useful for determining the timing of events when distance and wave speed is known.

As mentioned, communication time can inform how transient events are approached to limit surge or how to structure a system to react to a moving wavefront. Adjusting pipe lengths or adding a dampener can take advantage of the additive nature of wave interaction to mitigate a water hammer event for branching systems with many reflections and interactions to consider. Communication time can simplify the avoidance of constructive wave interference or aid in the creation of deconstructive wave interference to a system’s advantage.

Communication Time alone cannot inform mitigation design

It is often difficult to consider all possible wave interactions by hand solely using communication time. This is where engineering software can easily consider interactions with components, the variable wave speed of different pipe materials and diameters, and interacting wavefronts.

Below in Figure 1 is an example animation of transient analysis software demonstrating the impact of a dead-end branch, an area change, an opening relief valve, and a closing check valve on how a wave travels through a system.

Figure 1: Animation demonstrating various communication times caused by different pipe components

With the top system as a baseline, it is clear from the animation that communication time can be impacted in different ways depending on the type of components. These components can severely impact the peak pressure surge of the system once wave interference and component interaction are considered.

Visualizing results and concepts using software provides an engineer more context to design and effectively mitigate a water hammer event, especially in complex systems.