Understanding the Flow of Water in Pipes

Water flows through pipes due to the pressure created by gravity and the water supply system. This pressure pushes water through the pipes, driving it towards your faucet or appliance.

The average household water pressure is around 30-80 pounds per square inch (PSI), which is strong enough to push water through the pipes. However, this pressure can vary depending on the location and the water supply system.

In a typical household, water flows through a network of pipes, including main lines, branch lines, and service lines. These pipes are usually made of materials like copper, PVC, or PEX, each with its own strengths and weaknesses.

Flow meters measure water flow in pipelines by detecting the velocity of the water. This can be done using various technologies, such as propeller or turbine meters, which measure the rotational speed of a propeller or turbine placed in the pipe.

Some flow meters are more accurate than others, like electro-magnetic meters, which generate a magnetic field and measure flow based on distortion of the field. This method is one of the more accurate and has no moving parts.

The type of flow meter used can affect the installation process, with some meters requiring more effort to install than others. For example, transit time meters can be installed on the outside of a pipe, but must be protected from the elements.

Flow meters can be categorized by measuring method, including mechanical and electronic meters.

Mechanical meters are among the least expensive options, but they're strongly affected by flow velocity distribution and prone to mechanical wear.

Electronic meters, on the other hand, are one of the more accurate methods and require no moving parts.

Some common types of mechanical meters include propeller and turbine meters, paddle wheel meters, and ultrasonic transit time meters.

Propeller and turbine meters are commonly used in irrigation, while paddle wheel meters are also used in this application.

Ultrasonic transit time meters can be installed on the outside of a pipe, but they must be protected from the elements.

Electronic meters, such as electro-magnetic meters, are based on flow rate, not velocity, and may require less straight pipe than other methods.

Here's a summary of some common flow meter types:

Determine WsFu by identifying the number of each plumbing fixture connected to a pipe, as the plumbing code establishes the WSFU value for each fixture type.

The WSFU value is determined by the governing plumbing code, which assigns a specific value to each fixture type.

A pipe's piping layout dictates the amount of each fixture type it feeds, which in turn affects the WSFu.

In essence, WSFu is a crucial measurement that requires a clear understanding of the plumbing fixtures and the governing code.

One pound per square inch is equal to 2.309 feet head. A 1-foot head is equal to a pressure of 0.433 pound per square inch.

All formulas for finding the amount of water that will flow through a pipe in a given time are approximate, giving results within 5 or 10 per cent of actual flows if applied to pipe lines carefully laid and in fair condition. This is a crucial consideration when designing pipe systems.

The formula to find the equivalent length of a right-angle bend in feet is given by L = (4d) / 3, where d is the diameter of the pipe in inches. This formula is derived from experiments that show a right-angle bend should have a radius of about three times the diameter of the pipe.

To find the coefficient C for a given pipe diameter, you can refer to the table below:

The table lists the coefficient C for different pipe diameters in feet and inches.

One pound per square inch is equal to 2.309 feet head. A 1-foot head is equal to a pressure of 0.433 pound per square inch.

All formulas for finding the amount of water that will flow through a pipe in a given time are approximate. They will give results within 5 or 10 per cent of actual flows if applied to pipe lines carefully laid and in fair condition.

The formula to find the equivalent length of a right-angle bend is L = ( 4 d ) / 3, where d is the diameter of the pipe in inches.

To find the equivalent length of a right-angle bend, you need to know the diameter of the pipe. For example, if the diameter of the pipe is 0.5 inches, the equivalent length would be L = ( 4 * 0.5 ) / 3 = 0.67 feet.

The coefficient C is used in the formula to find the approximate mean volume of water that will flow through a pipe in a given time. The coefficient depends on the diameter of the pipe and can be found in the accompanying table.

Here is a table of the coefficient C for different diameters of pipes:

The formula to find the approximate mean volume of water that will flow through a pipe in a given time is Vfps = V · 0.7854, where V is the approximate mean velocity in feet per second.

To convert WSFU to GPM, you need to consider the type of plumbing fixtures connected to the pipe. The WSFU value is determined by the number of each fixture type, and the governing plumbing code establishes the WSFU value for each fixture type.

For tank type water closets, a fill time of around 20 seconds is acceptable in residences or areas with infrequent use. However, in public spaces with frequent use, this fill time is not acceptable.

Predominantly flush valve type fixtures require a much higher minimum pressure than tank type fixtures. In fact, a pipe that feeds predominantly flush valve type fixtures will have a greater volumetric flow rate requirement than a pipe that feeds predominantly tank type fixtures.

The WSFU to GPM conversion depends on whether the connected fixtures are predominantly flush valve type or tank type. Table 2 shows the conversion difference between these two types, with predominantly flush valve type fixtures requiring a much higher flow rate.

A group of plumbing fixtures is considered predominantly flush valve if it has more than one flush valve for every ten tank type water closets. If there is a branch run that serves no flush valve type water closets, then that branch can use the predominantly tank type WSFU to GPM conversion.

Flow in a pipe isn't always uniform, with water flowing slower next to the pipe wall due to friction.

The velocity pattern in a pipe can be distorted by fittings or pumps, making it harder to measure flow accurately.

In pipes that are flowing completely full, the velocity pattern is consistent and can be relied upon by many flow meter technologies.

Partial flow can occur when a pipe has an open discharge or when air is trapped in a pipeline, creating a very different velocity pattern than full pipe flow.

Recording Types can differ among flow meters, with multiple options available for many meters. For example, a meter may only have a manually readable dial, or it may have datalogging capacity and remote access.

Some flow meters have a manually readable dial, like a needle and totalizer, which records measurements manually. This type of meter is often the least expensive option.

Electronic display/totalizers are similar but use a digital display instead of a needle. They're found in propeller/turbine, paddle wheel, and magmeter meters.

The 4-20 mA output type is a common option, generating an electrical current output ranging from 4 to 20 mA. This type of output can be added to many meters to adapt to external dataloggers.

Pulse output is another option, generating an electrical output with pulses that can be DC (on/off) or AC. Many flow meters offer this type of output.

Digital output is also available, outputting a digital signal that can be read by many types of dataloggers. This type of output may be more expensive than other electrical signal options.

Some flow meters have built-in datalogging, eliminating the need for an external logger. This type of meter is less common and often requires data to be downloaded with a computer.

To ensure proper conditions for accurate flow measurement, it's essential to install flow meters in a way that allows for proper velocity distribution in the pipe. This can be achieved by having a sufficiently long section of straight, full-flowing pipe without fittings or valves.

The 10-5 Rule is a general guideline that requires at least ten times the inside diameter of the pipe upstream of the flow measurement device and at least five diameters downstream of the device. For example, a flow meter installed in a 10-inch pipeline needs at least 102 inches of straight pipe upstream and 51 inches downstream.

Researchers have found that some flow meters can function with less straight pipe length, but this is specific to certain models and types of meters. For instance, a study in Nebraska found that some spool-type magmeters can work with as little as two pipe inside diameters of straight pipe upstream and one inside diameter downstream.

In some cases, straightening vanes can be used to reduce the necessary straight pipe length, making it possible to install flow meters in pipes with limited space. However, this is typically necessary for specific types of meters, such as propeller meters.

In some cases, the difference between two things can be quite subtle. Piping is primarily used as a fluid carrier and is measured by inside diameter (ID).

Copper pipes, for instance, have an outside diameter that's roughly 0.625 inches, even if they're only ½” nominal. This is because piping is measured by ID.

The inside diameter of a ½” nominal copper pipe is roughly ½”, which is quite different from the outside diameter. This difference is important to remember.

In contrast, tubing is primarily used for structural purposes and is measured by outer diameter (OD). A ½” copper tube has an outer diameter of 0.545 inches.

This difference in measurement can make a big difference in how piping and tubing are used. In domestic water piping systems, copper tubes are used and not copper pipes.

It's worth noting that the difference between piping and tubing is not just about measurement, but also about their intended use.

There are several types of pipes used to transport water, including PVC pipes, which are commonly used for residential plumbing due to their durability and resistance to corrosion.

Galvanized pipes are another type, which are coated with a layer of zinc to protect against rust.

Copper pipes are also used, and are known for their high heat conductivity, making them ideal for hot water systems.

PEX pipes are flexible and resistant to freezing, making them a great choice for outdoor use.

Copper pipes are durable and resistant to corrosion, but they can be expensive and difficult to install.

PEX pipes are also resistant to scaling and mineral buildup, which can clog other types of pipes.

Copper pipes are often used in high-pressure applications, such as in commercial buildings.

Galvanized pipes are coated with a layer of zinc to protect them from corrosion, but they can still be prone to leaks.

PEX pipes are relatively easy to install and can be glued together using a specialized tool.

Copper pipes are often used in potable water systems due to their resistance to contamination.

The Press Connection (ProPress) method of joining copper tubing has gained popularity among contractors due to its ease of installation.

This system was unveiled in 1999 and is on its way to becoming the favored joining method for many.

Pre-engineered copper fittings are used in this system, ranging from ½” to 4” in size.

The ProPress system is designed for efficiency, making it a great choice for contractors who value speed and accuracy.