Piping design: which pipe for what purpose? (Part 2)

By Fiona Ingham

In the July edition, we looked at the pros and cons of HDPE and copper piping. In this edition, we consider PVC and PEX piping.

POLYVINYL CHLORIDE (PVC)

PVC is known in the industry as being affordable and easy to install. According to Renier Snyman, product manager at DPI Plastics, the main advantages that PVC pipes offer are that they are light, strong and easy to handle. They are also corrosion resistant and offer a minimum service life of 50 years. PVC pipes are maintenance free, and therefore do not require upkeep after installation. PVC is resistant to combustion as it is self‑extinguishing. PVC is highly resistant to a wide range of chemicals, including strong acids, and is therefore used in drainage applications and in laboratories. PVC has a low friction coefficient and resists scale build‑up, therefore results in low pumping costs and long‑term energy savings. Its thin wall thickness results in bigger inner diameters and therefore lower friction, compared to other pipes of the same diameter and pressure class. PVC can be joined by solvent welding or integral rubber ring socket, which is both cheap and reliable. Neither of these methods need electricity on site.

On the downside, PVC pipes are only available up to 630mm diameter and 25 bar maximum operating pressure in South Africa. PVC pipes containing integral sockets have to be restrained under pressure and cannot be laid above ground. They are particularly light sensitive — only white PVC soil and vent pipes are UV stabilised for continuous use in direct sunlight. Pipes can be painted with water‑based paint to protect them against sunlight. PVC underground pipes need to be protected from sunlight when stored on site. Most types of PVC pipes need to be installed in a trench, and backfill and compaction need to be done correctly.

Their best applications are in potable water mains in civil and irrigation, gravity sewer pipe systems and underground mining pipes.

PVC pipes are mainly used below ground, with the exception of soil, waste and vent pipes that can be used above the ground.

Obviously, the performance of PVC pipes is compromised with incorrect installation. “Installers usually make the mistake to not clean or lubricate the socket and spigot before installation. Also, not making sure they keep the pipes straight when pushed together during joining is an issue. Stones and sharp objects come into contact with the pipe during installation,” says Snyman.

“Insufficient backfill and compaction during installation negatively affects performance, as does incorrectly testing pipes during installation — both can lead to premature failures. Not working clean and quickly during solvent cement joining is another common installation issue, as is applying too little or the incorrect type of solvent cement,” Snyman continues.

However, the price of PVC is perhaps where it leads the pack. “In its size range PVC is highly competitive. Approximately 95% of installed sewer pipes up to 500mm are PVC,” Snyman says.

Owner of AGE Plumbing Design Technicians, Graham Elsey, agrees on the pricing of PVC. “PVC’s greatest advantage is that it is affordable. It is perfect for domestic uses, builds such as small ablution blocks and light commercial uses.”

Cross-linked polyethylene (PEX)

Cross-linked polyethylene or PEX is a form ofpolyethylenewith cross‑links. According to Simon Hough, managing director of Cape Town‑based Afripex, flexible PEX tube is manufactured by extrusion, and both shipped and stored on spools. Rigid plastic or metal piping must be cut to some practical length for shipping and storage. This leads to several advantages, including lower shipping and handling costs due to decreased weight and improved storage options.

“PEX plumbing installations require fewer fittings than rigid piping,” says Hough. “The flexible tubing can turn 90 degree corners, without the need for elbow fittings.” PEX tubing unrolled from spools can be installed in long runs, without the need for coupling fittings. Attaching PEX tubing to fittings does not require soldering or heat. There are also no health risks involved, as the tubing does not need to be joined with lead‑based solder and acid fluxes.

PEX is also safer to install, since a welding torch is not needed to make connections. PEX resists the scale build‑up commonly found with copper pipe. PEX does not pit or corrode when exposed to acidic water. PEX is much more resistant to freeze-breakage than copper or rigid plastic pipe. PEX tubing does not transfer heat as readily as copper, and so conserves energy. Water flows more quietly through PEX tube, and the characteristic "water hammer" noise of copper pipe systems is virtually eliminated.


“Installers make several kinds of mistakes: they cut the pipe using a brick, hacksaw or a spade, instead of a pipe cutter.”


PEX plumbing installations cost less because PEX is not as expensive as copper pipe. In addition, less time is spent running pipe and installing fittings than with rigid pipe systems. Installing fewer fittings reduces the likelihood of expensive callbacks. It is far easier for an installer to get it right than when using other systems. The risk of theft from warehouses or during installation is minimised, because it holds a low recycle reward. Contamination risk is reduced because PEX doesn’t contain aluminium. It has excellent pressure and temperature ratings.

The main disadvantages are that PEX is hard to come by; it requires a specific tool and installers do not understand how to install it. “Most systems are not UV stable, with the exception of UltraPEX,” says Hough. Designers are also reluctant to specify it, as they want to stick to what they know.

Another disadvantage is that the industry suffers from a lack of understanding about how good PEX is. PEX is not form stable: you have to fasten it to a wall to keep its shape. PEX also expands more than metallic pipes, and this fact needs to be considered when installing it.

“PEX’s best uses are in housing, commercial and industrial plumbing, from small to large contracts. It is excellent for chilled water reticulation systems in hospitals, hotels, shopping malls and gas reticulation systems,” says Hough. Underfloor heating and cooling systems use huge volumes of PEX Monolayer. PEX is also an exceptional electrical insulator and is used extensively in cabling. It has a wide range of industrial applications, for example, transporting explosives, thyristor cooling and blood dialysis.

PEX can be used both above and below ground, as well as internally. UltraPex, which is UV stable, can be used externally where it will be exposed to direct sunlight.

Installation horrors

“Installers make several kinds of mistakes: they cut the pipe using a brick, hacksaw or a spade, instead of using a pipe cutter. They damage the fittings on site and then try to use them. They forget to put collars on the pipe or they mix incompatible systems. They use flux as a lubricant or they try to weld PEX — this is not possible as PEX is a thermoset type plastic. Installers try to use PVC glue to bond, which is also not compatible. Another mistake they make is to size the system incorrectly or to misread the gauge when pressure testing: they push pressure to 60 bar instead of 6 bar,” Hough continues.

In conclusion, PEX as a system offers the most compelling cost benefit and commercial advantage — and polymer pipes in general are overtaking copper pipes. PEX has the lion’s share of the market globally when cost benefit is required.

Another advantage is that PEX piping — when used as a monolayer together with fittings that use the expansion for the material and the shrink back — is the most secure of all fitting systems. “PEX systems require the least amount of skill, and they are the most forgiving when done incorrectly. Also, the fitting systems on PEX Monolayer will accept a huge margin of error compared to all other known systems,” Hough says.

In the next issue of Plumbing Africa, we will examine other piping materials.

 

 

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