Piping

Generally Type L copper, black steel and stainless steel are appropriate materials for the piping system. When using copper tubing, hard soldering is recommended for the collector loop. Care must be taken in using Teflon tape to seal threaded pipe joints when water /glycol is being circulated in the pipes. With proper dielectrics, black steel piping can be used on the collector side for use with glycol systems.

All piping shall be sloped at 1/4 in/ft (6.35 mm/0.30 m) of run back to the drain back tank. High points shall be kept to a minimum and combination automatic air vent valve/vacuum breakers shall be placed at all high points. Discharge of automatic air vents shall be piped back to the drain back tank and be provided with an in-line sight glass.

Piping should be designed for low pressure drop and the shortest routes used. All exposed piping should be well insulated with approved weather resistant insulation. Dielectric unions should be used at connections between dissimilar metals. Rubber or silicone hose used for connections must be of a high temperature type. The pipe ends should have ferrules to provide a good seal with the hose.

Pipe sizing should be in accordance with recognized methods. Figure 5.9 shows an example of a pipe layout with sizes for a collector field. The pipe sizes shown are metric sizes for copper tube. The equivalent nominal US copper pipe sizes are:

• 16u = 1/2 in.
• 19u = 5/8 in.
• 29u = 1 in.
• 35u = 1-1/4 in.
• 41u = 1-1/2 in.
• 54u = 2 in.

The piping system (valves, pumps, fittings, flanges, connections, and insulation) should be designed to withstand the special conditions caused by the extreme temperatures of stagnation, e.g., 320 °F (160 °C) plus, and frost, e.g., -5 °F (-21 °C); expansion of pipe-length; pressure (e.g., steam) and working fluid (e.g., corrosion). An expansion vessel of sufficient size should also be part of each closed piping network. The system should be designed to operate at a pressure less than 125 psig (861.75 kPa), which will allow the use of standard piping components (class 125). A discussion of system temperatures and pressures is found in Section 3.4.2 “Stagnation” (p 33).

Collector piping must be able to withstand the expansion and contractions of components caused by the changes in temperatures that could be experienced. These temperature changes typically occur daily, which is significantly higher number of cycles than experienced by a normal heating
system. The use of offset elbows, high pressure hoses, and expansion couplings should be considered rather than expansion loops unless they are placed horizontally due to drainage difficulties they would create.

Corrosion is a major concern in the solar hot water system. The two types of corrosion that cause the most galvanic damage and pitting corrosion. Solar energy systems generally contain a number of different metals such as aluminum, copper, brass, tin, and steel. This makes the solar system a prime candidate for galvanic corrosion. Heat transfer fluids can contain chemicals and heavy metal ions that would cause local or pit corrosion.

Galvanic corrosion is a type of corrosion caused by an electrochemical reaction between two or more different metals in contact with each other. A chemical reaction between the metals causes a small electrical current that erodes material from one of the metals. If the dissimilar metals are physically joined or if they are contacted by a common storage or heat-transfer fluid, the possibility of galvanic corrosion becomes much greater. Pitting corrosion is a highly localized form of corrosion resulting in deep penetration at only a few spots. This type of corrosion can take years to form, but can be very troublesome since it causes leaks that are difficult to locate.

Pit corrosion occurs when heavy metal ions such as iron or copper plate on a more anodic metal such as aluminum causing a small local galvanic cell can be formed. This corrosion spot or “pit” usually grows downward in the direction of gravity. Pits can occur on vertical surfaces, although this is not as frequent. The corrosion pits may require an extended period (months to years) to form, but once started they may penetrate the metal quite rapidly. Heavy metal ions can either come as a natural impurity in a water mixture heat transfer fluid or from corrosion of other metal parts of the solar system.

Pitting corrosion has the same mechanism (concentration cell) as crevice corrosion. Thus, it can also be aggravated by the presence of chloride or other chemicals that can be part of the water mixture or a contaminant from solder fluxes. Aluminum is very susceptible to pitting corrosion, while copper generally is not.

Several preventive measures will eliminate or at least minimize galvanic and pitting corrosion in collector systems that use an aqueous collector fluid. The best method to prevent galvanic corrosion is to avoid using dissimilar metals. Where this is not possible or practical, the corrosion can be greatly reduced by using nonmetallic connections between the dissimilar metals, thus isolating them. Galvanic protection in the form of a sacrificial anode is another method of protecting the solar system metals. Also, use of similar metals reduces the problems of fatigue failure caused by thermal expansion. Pitting corrosion is essentially eliminated if copper absorber plates are used in the solar collectors. Corrosion inhibitors can minimize pitting corrosion in aluminum absorbers.

When sacrificial anodes are to be used Their placement is important to obtain good protection and it depends on what is being protected, the anode material being used and the electrical conductivity of the heat transfer fluid.