Considerations for pipe sizing

By Tom Attenweiler and Don Townley / Images by Lubrizol

Sizing a water distribution system for a commercial space requires consideration of a number of factors, from pressure and fixture count to pipe material.

Calculating all contributing factors is essential to meeting code requirements, as well as ensuring proper operation of the system.

Tabulating losses

The first step in pipe sizing is determining the water pressure coming into the building from the city; for commercial structures, there is pressure regulation at the water meter, which is where engineers and designers should start tabulating friction losses.

Ft Lauderdale HotelUnderstanding the friction losses contributing to a system enables the designer to translate those losses into an average pressure drop per 100 feet for the system. The pressure drop of a system is a fundamental parameter required for the calculation of the system velocity and, ultimately, the flow rate (aka, ‘demands’).


Corzan CPVC piping systems, shown here 10 years post-installation at a hotel in Fort Lauderdale, Florida, have been in use for 60 years in a range of commercial and industrial applications.


Plumbing codes have established guidance for the number of fixture units that can be serviced by a given flow rate. In addition to its dependence on pressure drop, flow rate is also dependent on the inner diameter of the pipe. Because of this relationship, the size of the pipe can be adjusted to achieve the necessary flow rate that is required to service the desired number of fixture units. However, the maximum number of fixture units capable of being serviced is dependent on the available pressure in the system.

Pressure coming into the system is typically limited by code to 80psi, though most systems average around 40–50psi. Factors such as altitude may come into play, as well. Generally, the pressure needs to remain around 15psi once it reaches each fixture.

Determining if the 15psi of pressure at the fixture level will be reached requires adding up the friction losses in the system. Manually or using a sizing calculator, input incoming pressure and then list known pressure losses. Every element in the water system, from the meter to a bend, results in pressure losses. Typical friction losses/gains include:

  • Water meters
  • Pressure reducing valves
  • Submeters
  • Elevation loss/gain.

Once you have tabulated the friction losses, you can determine the total pressure drop, which is reported on the building plans.

Selecting the pipe

Taking the knowns — friction losses, pressure drops, supply pressure, and the number of fixtures — code tables will indicate the flow rate (demand) needed to Ft Lauderdale Hotel2feed those fixture units. This flow rate enables the designer to determine what size pipe is required to achieve the proper flow rate to feed those fixtures.


Corzan CPVC piping systems, shown here 10 years post-installation at a hotel in Fort Lauderdale, Florida, have been in use for 60 years in a range of commercial and industrial applications.


The water pipe should be sized as outlined in section 6.10 and appendices A and C of the Uniform Plumbing Code. Some of the items that need to be considered are:

  1. Daily service pressure – Reference section A102.1
  2. Friction loss through the water meter – Reference section A102.2
  3. Supply demand – Reference section A 103.1
  4. The minimum required residual pressure at the highest fixture – Reference section A104.1
  5. The elevation of the highest fixture – Reference section A104.2
  6. Pressure loss through the building supply line – Reference section A105
  7. Pressure loss though branches and risers – Reference section A106
  8. Pressure loss though fittings – Reference section A107.3.

Material considerations

Among the other factors that can impact pressure loss calculations, are pipe materials. While CTS (copper tube size), PEX, CPVC, and copper all have the same outside diameters, the materials themselves require different wall thicknesses to meet standardised pressure ratings, with copper the thinnest, PEX the thickest, and CPVC in-between.

Therefore, for the same amount of water flowing, the velocity and pressure loss will be greatest with a PEX system due to its smaller internal diameter; a larger pipe size might be required to maintain the necessary residual pressure at the farthest fixture.

The flexibility of PEX may reduce the number of fittings versus CPVC, but the pressure drop through a CPVC fitting is less than a comparable PEX fitting. While CPVC and copper fittings surround the exterior of the pipe, PEX fittings are inserts, which creates orifices in the piping systems and restricts flow.

Copper can also experience some pressure drop over time as it corrodes, and the surface becomes rougher. This is calculated using the Hazen Williams C factor, which is a constant that applies to the pipe related to smoothness. Under the C factor, the higher the number, the smoother the pipe. At their origin, steel has a C factor of 120, and PEX, CPVC, and copper 150. However, copper’s number will decrease over time as the material scales and corrodes; CPVC’s number remains constant.

Another challenge is oversizing a pipe system. At times, sizing tables do not offer the necessary granularity to accurately size a system. When this is the case, it is best practice to select a larger pipe size to ensure that required demand is met with excess capacity. The result is greater expense and wasted water, which is a growing concern in many regions around the world.

You can use a small pipe and push water through very quickly, but it’s not necessarily feasible. First is the potential for erosion in copper pipes, which are generally limited to five linear feet per second for hot water and 8–10 feet per second for hot and cold, depending on the size of the pipe. Second is the phenomenon of water hammer. For devices with a quick-closing valve, such as a washing machine or an ice maker, water goes from seven or so feet per second to zero in less than a second. This creates a pressure wave in the pipe.

In general, the peak pressure wave should not exceed one-and-a-half times the maximum operating pressure of the pipe that is experiencing the pressure wave. The pipe should be properly sized to avoid exceeding this limit. In some regions, water hammer arrestors are required by code to absorb the pressure wave before it travels down the pipe.

For these devices to be effective, they must be placed as close to the quick-closing valve as possible. Even if the pressure wave does not exceed one-and-a-half times the maximum operating pressure, the wave can still create unacceptable noise in the system. Pressure arrestors can be used to dampen this noise.

Sizing tool output 20171215

Understanding CPVC

CPVC is ideally suited for commercial applications, including schools, office buildings, retail, and hospitals, as well as industrial applications, including chemical processing, manufacturing, mineral processing, wastewater treatment, power generation, and marine applications.

CPVC is self-extinguishing and has relatively low smoke generation. CPVC has a much higher limiting oxygen index (LOI) value than many other common materials of construction, and thus will not support combustion under normal atmospheric conditions.

Performance-wise, CPVC is characterised as follows:

  • Durable: CPVC plumbing systems will not pit, scale, or corrode like metallic systems — regardless of water quality.
  • Cost-efficient: A CPVC system costs less and is faster and easier to install than traditional metal systems. No torches required. Pipe and fitting are solvent-welded quickly and firmly, and in the long term, the CPVC system offers additional savings because it is highly energy efficient.
  • Selection: Pipes and fittings up to 24 inches meet the needs of nearly any commercial project.

CPVC piping systems can be installed using one of two methods: chemical joining or mechanical joining. The chemical joining process, called solvent welding, eliminates the need to use a separate primer. Installers cut the pipe, clean the pipe and fittings, apply solvent cement inside the fitting socket, assemble the joint, and verify proper installation. The science of solvent-welding, which forms a chemical bond, ensures that a properly installed fitting is the strongest part of the system.

Mechanical joining options consist of cutting and grooving the pipe, then connecting pipe sections with specialty fittings. Mechanical joining options are especially ideal for alterations or repairs, as they eliminate drying time and shorten system downtime.

Expansion loops 

Expansion Loops

Another key component to designing pipe systems is factoring in expansion loops. Like all building materials, CPVC pipe materials will expand when heated and contract when cooled, and engineers must factor this into the system design.

CPVC will expand about one inch per 50 feet of length when subjected to a 50-degree F temperature increase. Linear expansion does not vary with pipe size. Expansion is mainly a concern on hot-water lines; however, expansion allowances for hot or cold water pipe installed in unconditioned spaces should account for the temperature difference between the installed temperature and the service temperature.

Expansion loop requirements for CPVC are not much different than those of properly designed copper systems. Generally, the effects of expansion can be controlled with changes in direction; an offset or loop may be required on a long straight run. One properly sized expansion loop (see illustration) is all that is required in any single straight run, but two or more smaller expansion loops, properly sized, can be utilised in a single straight run of pipe. Be sure to hang pipe with smooth straps that will not restrict movement, and remember that the pipe must be free to move for the expansion loops to work.

Table 1 shows calculated loop (offset) lengths with ΔT of approximately 80 degrees F (90 degrees F to 170 degrees F) based on diameter and length of two types of CPVC pipe.

Tables1 2 Lubrizol

 

Per IAPMO IS 20, expansion loops are not required in vertical risers, provided the temperature change does not exceed 120 degrees F. Vertical piping must be supported at each floor as specified by the design engineer to allow for expansion/contraction, and piping should have a mid-story guide. Specify only hangers and straps that do not distort, cut, or abrade the piping.

No matter the material or the project type, consult with your pipe manufacturer for special considerations and installation/engineering requirements.

 

Tom Attenweiler is a product engineer for Lubrizol, while Don Townley is global codes and approvals manager for Lubrizol.


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