When do you slope a process line? When is self-draining required? How does self-draining contrast with self-venting? How do these differ from no pockets? Such issues seem to confuse many engineers. So, let's delve into line geometry.
Convenience and cost of piping runs set the overall geometry, except when special process requirements impose specific layouts. Piping designers rarely consider process needs, except for line sizing based on pressure drop or velocity.
At a minimum, the line geometry must account for startup and shutdown.
Filling a liquid line requires venting gas or air out. Draining such a line demands getting the liquid out of low points. For piping designers, the easiest approach is to add high-point vents and low-point drains. However, for some specific services, you many want to avoid extra valves and connections.
Liquid lines also may suffer from vapor pockets. These may block lines, preventing liquid flow. Under other conditions, a sudden shift of a vapor pocket can create slug flow — damaging equipment and creating operating or safety problems.
Many process vapor lines may accumulate condensation. Liquid also may get into lines due to upsets and entrainment from towers or separator drums. The accumulation may not be apparent until serious unit upsets occur. So, rather than depending upon subtle signals and operator draining of low points, configuring piping to prevent accumulations may make sense.
As mentioned, basic pipe layouts often include high-point vents and low-point drains to deal with startup, shutdown and operating upsets. Experienced piping designers ensure the drains are easily accessible. Figure 1 shows lines with both vapor pockets and liquid pockets.
If process requirements mandate, the piping will feature a self-draining, self-venting, no-pocket or sloped layout (Figure 1).
Self-draining lines may have a high point, so liquid can flow in either direction away from that point. Self-draining lines are relatively common in many plants. The intent is to prevent liquid slug formation and to keep liquid in vapor lines from increasing pressure drop.
Self-venting lines may have a low point, so vapor can flow in either direction away from that point. These are less common than self-draining lines because they still require a liquid drain.