Pipe Invert Elevation

David
Etukudo

Pipe invert elevation is the vertical distance between the inside-bottom of a pipe and a given benchmark. Moreover, the term ‘invert’ stems from the resemblance of the pipe bottom to an upside-down arch popular in architecture. In this article, you will learn the implication of pipe invert elevation, how to calculate it, and its applications in industry.

Pipe geometry showing pipe invert and obvert
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Implication in Piping Design

In practice, professionals often refer to the pipe invert level as the floor level. Its counterpart, the obvert, refers to the ceiling level. When designing and implementing NPS piping that relies on gravity flow, the pipe invert elevation is critical. Engineers and architects need to ensure that flow remains positive throughout the system. As a result, the invert level at the outlet should always be lower than that at the inlet. Otherwise, it leads to poor drainage, clogs, and flooding in such systems. In addition, it is this elevation that determines the slope of a pipe in draining systems. An example of the importance of pipe invert elevation is clear in the case of the invert of a street sewer connection and the basement of a house. If the sewer invert connection is not low enough, then it eliminates the possibility of having a toilet in the basement.

Similarly, the pipe invert elevation is important in force flow systems – systems that use a pump to drive the flow. It is not as critical as in gravity flow applications. In any case, the pipe invert elevation determines the amount of pressure a pump must overcome, thus, the power of the pump. Having a much lower elevation will minimize the pumping requirement and ultimately, the cost of the piping design.

How to Calculate Pipe Invert Elevation

Calculating a pipe invert elevation can be a basic task, depending on the application. However, the task requires care when deciding which measurements to use. For example, there are some pipe sections with male and female ends also known as spigot and bell ends respectively. The divergent bell end fits into the spigot. The bell end offers the lowest point in the pipe where water runs and is the recommended invert level. Similarly, corrugated pipes often have ridges grooved into them during fabrication. This means water must fill the groove before achieving steady flow along the pipe. As a result, the bottom of the groove is the invert level.

Parameters used to calculate a buried pipe invert elevation.
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After sorting out the appropriate point for the invert level, the pipe invert elevation for a buried pipe can be calculated as follows:

    \[ pipe \: invert\: elevation=depth\: of\: cover+wall\: thickness+internal\: diameter \]

The ground surface is the benchmark, and the pipe is parallel to the surface. In a situation where the pipe slopes at a gradient the pipe invert elevation include this vertical distance (known as “fall”):

    \[ fall=gradient\times distance \]

Applications in Industry

It should now be apparent that the pipe invert elevation is clearly an important concept. Especially when dealing with drainages and other gravity flow applications.

Pipe Invert Elevation in Sewer Design

Sewer pipes are large conduits for evacuating wastewater in cities. They serve as the central drainage for homes and other facilities, so it is important to get their invert elevations spot on. Generally, the benchmark that engineers adopt for the invert elevation of sewage pipes is the top of the street. Thus, contractors executing the installation of these pipes will ensure this elevation requirement is met throughout the city. Because of the varying topography occurring in most cities, professionals are meticulous in the sewage system design. Following this, they keep the sewage pipes well below the road surface and keep the drainage channels from buildings well above its connecting sewage line.

Courtesy: Kevin Andre on Unsplash

Furthermore, engineers locate manholes at intervals along the sewage system. One of their functions is to provide access for the measuring of the sewage pipe invert elevation. Therefore, builders can properly locate the central drainage point of a building above the invert of the sewage pipe invert. When taking the invert measurement, the professional should:

  • Ensure the tape measure, or an alternative device, seats on the middle of the pipe where the sewer pipe is tallest.
  • The tape measure height should be read against the top of the road or hole.

Pipe Invert Elevation in Overhead Tank Design

Apart from drainage applications, the pipe invert elevation concept is useful when supplying water or other fluids from an overhead tank. The overhead tank design is such that a motorized pump draws water into it from a sump. While on the supply side, the delivery of water does not involve the use of a pump. Hence, gravity provides the driving force.

As a result, the tank’s discharge pipe invert should have an elevation to deliver water at sufficient pressure to the point of use. To achieve this, a plumber/engineer should note the invert level of the highest supply pipe in the apartment, and the elevation between this level and the overhead tank’s invert. Then, the pressure (p), that the elevation (h) offers, is given as a function of water density (ρ) and gravity (g).

    \[ p=\rho gh \]

This pressure should be above the set delivery threshold considering all losses.

Overhead tank working on the principle of pipe invert elevation
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