Averaging pitot tubes

Installing an averaging pitot tube is a simple process. However, problems can result that are difficult to correct later if a few basic factors are not considered during the mounting process. The first three steps to successful mounting are location, orientation and installation.

Some pitot tubes have hardware to support its far end. Tack weld such hardware with the pitot tube in place before performing final welds.

Step one: Location

First, determine if adequate straight run is available. Straight run refers to the amount of obstruction-free piping upstream and downstream of the averaging pitot tube. Elbows, control valves, changes in pipe size and other obstructions create flow disturbances that affect accuracy. Because the averaging pitot tube measures and averages the readings from multiple points along the flow profile (see Figure 1), its straight run requirements are less stringent than most other devices, but upstream and downstream disturbances must still be taken into consideration.

Figure 1. Examples of flow profiles

Most flow meter manufacturers publish straight run charts. However, the charts don’t tell where to locate the pitot tube when adequate straight run is not available. Intuition may not always be correct when deciding. For example, when the only option is to install it immediately downstream of an elbow, one may be tempted to install it as far away from the elbow as possible. While it’s true that upstream disturbances influence accuracy more than downstream disturbances, this is one case where installing the averaging pitot tube two diameters from the centerline of the elbow is best (see Figure 2).

Figure 2. Elbow mount installation

The velocity profile hugs the outside radius of the pipe immediately after an elbow in a predictable manner. At two pipe diameters from the centerline of a short-radius elbow, reasonable accuracy can be achieved if the instrument is mounted to the outside radius of the elbow and the flow coefficient is properly adjusted.

Notice how the pitot tube traverses the skew of the flow profile in Figure 2. If it were rotated 90 degrees from the position shown, it would not average the flow correctly. Another example of traversing the skew occurs downstream of a butterfly valve, with the instrument oriented 90 degrees from the valve axis.

Not all straight run rules are chiseled in stone. For example, the chart may require 24 pipe diameters after a valve. However, a fully open, full-throat gate or ball valve induces only a small flow disturbance. A modulating control valve or butterfly valve causes a much greater flow disturbance. Because there are so many combinations possible, consult the instrument manufacturer for a recommendation on where it should be installed and for an estimated accuracy. One pointer: always provide a sketch or diagram. Verbal descriptions are not always conveyed or interpreted accurately. In addition, there may be something in the diagram that seems irrelevant, such as a temperature sensor or pressure tap, but may affect the accuracy of the device.

Step two: Orientation

Consider horizontal piping runs first. For gas applications, mount the pitot tube in the upper 160-degree portion of the pipe to prevent condensate from collecting in the instrument lines or transmitter (see Figure 3). This is especially critical when the gas is saturated or operating at a temperature above ambient.

Figure 3. Horizontal pipe orientation

For liquid applications, the pitot tube should be mounted in the lower 160-degree portion to prevent air from collecting in the instrument lines or transmitter. Because steam applications require liquid legs to isolate the transmitter, the same consideration applies.

For vertical piping, mount the pitot tube in any location around the circumference of the pipe. For gas with a high moisture content, mount the instrument at a five-degree angle to allow drainage (see Figure 4).

Figure 4. Vertical pipe orientation

Indicate if the line is vertical when you order the device. The manufacturer will orient the connections parallel to the ground to eliminate low points for condensation build-up.

Non-standard orientation

Occasionally, it’s not practical to mount the pitot tube in the recommended orientation. For example, a buried water line does not allow access to the lower portion of the pipe; therefore, the instrument must be mounted to the top. In such cases, special mounting considerations prevent entrained air from collecting in the instrument lines or transmitter (see Figure 5).

Figure 5. Buried water pipe

Step three: Installation

An averaging pitot tube can be installed through various connections. The two most common methods are flanged and threaded. Regardless of the connection size or type, the size of the hole drilled in the pipe is critical to measurement accuracy.