Product(s):	SewerGEMS, CivilStorm, SewerCAD
Version(s):	CONNECT Edition, V8i
Area:	Modeling

Problem

What is a virtual pressure pipe? What does it mean when you choose "true" for "Is Virtual?" for a pressure pipe and how virtual pipes are handled by each solver?

Solution

A virtual pipe basically means that the flow that comes into one end instantly goes to the other, with no hydraulic effects. It's essentially a way to connect elements without impacting the hydraulics. 

For example, you may have a pump station with multiple pumps in parallel; in this case, you can simplify the hydraulics by making the pipes adjacent to those pumps virtual. This will eliminate the complexities involved with the manifold that would otherwise be created just downstream of the pump station. (which the dynamic wave calculation engine can sometimes have trouble with) 

How each solver of storm-sewer products handle the virtual pipes whether that may be gravity pipe or pressure pipe is mentioned below. 

• Implicit - Virtual Pipes handled as expected.
• GVF-Convex - Virtual Pipes handled as expected.
• Explicit - Virtual Pipes must be used as the link on the Upstream end of a Pump.
• GVF-Rational - Virtual Pipes not supported in GVF-Rational. Virtual Pipes are converted to Small Physical Pipes.

Pump suction pipe with SWMM solver

When computing a model with the Explicit (SWMM) solver in SewerGEMS or CivilStorm, you might encounter these User Notifications:

"The upstream link of the pump is non-virtual. The SWMM engine will treat this link as virtual."

"When using the explicit solver, it is preferable that pumps should be connected immediately upstream and downstream to short, virtual links."

These notifications are informational and related to the short pressure pipes that are often used adjacent to pump nodes. The first notification above refers to the pressure pipe between the wetwell and the pump. The SWMM solver needs to treat this as a virtual pipe so that the hydraulic impact of that pipe will be ignored. This is typically an acceptable assumption since this pipe is typically short. Most likely any impact this would have on hydraulic results would be very minimal and less than the margin of error introduced by other inaccuracies between the real system and the hydraulic model data input. 

The second notification is referring the downstream pressure pipe but is a suggestion (it does not automatically change it to virtual). It appears to be in reference to a typical case of multiple pumps in parallel where you would typically have a common downstream junction with another short pipe, like this:

Treating both the upstream and downstream short pipes as virtual tends to work better with the Explicit solver, hence the user notifications. Since they are typically quite short, they should not have a notable impact on hydraulic results. If you have a case where you have a single pump and there is a long pressure pipe on the downstream side, you can simply ignore the user notification. (again, only the suction-side pipe will automatically be treated as virtual). If you experience problems with the pump results in such a case, you can try inserting a junction on the downstream side of the pump by using the pressure junction layout tool to split the pipe, creating a short pressure pipe that you can try changing to be virtual.