June 99 piping

From: Subject: June 99 piping Date: Tue, 18 Nov 2003 11:44:12 -0800 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.fpweb.com/archive/June_99_piping.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165 June 99 piping

Plumb your plant air system with = plastic=20 pipe

By S. N. Robichaud

Ease of installation and greater economy certainly are big reasons = to=20 consider thermoplastic pipe for plumbing your compressed air = system.

For years, metal piping materials -- primarily copper and black iron = -- have=20 been the overwhelming favorite for plumbing compressed-air systems. = Recent=20 advances in materials technology, however, have made thermoplastic pipe = a safe=20 and economical alternative to traditional materials.

A big advantage of metal pipe, tubing, and fittings is that = installers are=20 familiar with them and the techniques for joining them. Black iron is=20 inexpensive, but installation is time-consuming and labor-intensive. = Moreover,=20 threaded joints often serve as a source of leakage. This leads to higher = operating costs because compressors must operate overtime to compensate = for the=20 leakage. Although connections between copper pipe and fittings are less = prone to=20 leakage, copper components are more expensive, and installation, again, = is labor=20 intensive -- especially when large diameters are involved.

But these aren't the only drawbacks to metal piping systems. Interior = corrosion can cause scaling and pitting on inside surfaces. As the = corrosion=20 products combine with moisture and other contaminants, they accumulate = on the=20 inner surfaces of the pipe and fittings, increasing their roughness. As = the ID=20 becomes rougher, pressure drop though the system increases. Again, this = ends up=20 costing money by reducing efficiency of the compressed air system. = Perhaps more=20 importantly, particles can dislodge and clog or damage end-of-line=20 equipment.

The good and bad of PVC

Because of these drawbacks, compressed air system users have been = seeking=20 alternatives to traditional metal pipe and tubing. Over the past five to = ten=20 years, industrial plastics have been developed that present an = attractive=20 alternative to metal piping.

PVC piping is relatively inexpensive, easy to install, lightweight, = and=20 corrosion resistant. However, PVC has one major drawback. It is brittle. = An=20 inadvertent impact could cause the piping to shatter, endangering = surrounding=20 personnel. Most PVC pipe manufacturers warn against using PVC for = compressed air=20 service due to potential liability from such failures. The Plastic = Piping=20 Institute, in their Recommendation B, states that plastic piping used = for=20 compressed air transport in above-ground systems should be protected in=20 shatter-proof encasements, unless otherwise recommended by the = manufacturer. In=20 many states, the Occupational Health and Safety Administration (OSHA) = has=20 stepped in and regulated against using brittle plastics such as PVC in = these=20 applications, and additional states are following suit.

The strictest standard in the country has been issued by California's = OSHA.=20 It includes five tests, as well as a requirement for comprehensive = marking of=20 the pipe and fittings. These tests include long-term hydrostatic, = short-term=20 burst, and three specialized impact tests -- all to ensure the safety = and=20 ductility of the system. The impact tests include striking frozen, = pressurized=20 pipe with both blunt and sharp strikers, using various forces, and = striking a=20 frozen pipe with a hemispherical striker, using various forces.

Manufacturers are required to present the results of these tests for = review=20 upon request. When specifying a thermoplastic system, for safety's sake = it is=20 important that your supplier meets Cal-OSHA regulations, regardless of = the state=20 in which the system will be installed.

An attractive alternative to PVC

New thermoplastic piping systems -- using high-density polyethylene = (HDPE),=20 for example -- overcome the brittleness problems associated with PVC. = They=20 efficiently and reliably deliver compressed air with lower material and=20 installation costs and longer service life than with metal systems. They = offer a=20 margin of safety missing from PVC.

These new thermoplastics are safe because they expand at the point of = failure, tearing open rather than fragmenting dangerously. They do not=20 accumulate scale on their ID, nor does pitting or corrosion occur, and = they are=20 unaffected by synthetic and mineral oils used in compressors.

The internal surface of thermoplastic piping typically has a = roughness=20 factor, C, of about 150 to 165. Metal piping systems, on the other hand, = start=20 out with an interior surface roughness factor of about 120. This factor = is=20 inversely proportional to friction head losses: as C increases, system = pressure=20 drop over a given length at a given flow decreases. This means that when = installed, the ID of thermoplastic pipe and fittings is smoother, so = lower=20 pressure drop occurs than with metal piping components. Moreover, = because it is=20 less prone to accumulating particulate contamination, and corrosion does = not=20 occur, the ID of thermoplastic piping systems remains cleaner and = smoother.

The substantially rougher surface of metal piping allows contaminants = to=20 collect in the millions of tiny pits and crevices on the ID of the pipe. = In=20 addition, moisture and other contaminants can react with the metal = itself and=20 produce corrosion products that also accumulate. Over time, these = contaminants=20 and corrosion particles continue to collect and build up to form scale. = As the=20 scale builds, it roughens the ID of the pipe and fittings, which = increases=20 pressure drops. Ultimately, the higher pressure drop increases the = demand on the=20 system's compressors, which increases operating cost. Moreover, pipe = scale=20 particles can dislodge and damage equipment when carried downstream. = Because=20 thermoplastics do not promote the formation of pipe scale -- even when = exposed=20 continuously to condensation -- these problems do not exist with = thermoplastic=20 piping systems.

Installation advantages

Thermoplastic systems also offer low cost and quick installation. = Heat-fusion=20 welding makes pipe joining quick, easy, and extremely reliable because = there are=20 no threads to leak. Unlike PVC, no glues or cements are used, so there = is no=20 cure time. Testing can be conducted immediately after installation. In = contrast,=20 some glues may require as long as 24 hours to fully cure before full = system=20 pressure can be applied. In addition, fusion welding can be performed in = any=20 environmental condition using simple, lightweight tools without prior=20 experience. These tools are available for low-cost rental or = purchase.

A cost comparison shows that black iron is less expensive than = standard=20 thermoplastics. However, in tool set up and installation comparisons the = plastic=20 system takes only half the tool set-up time and one third the time to = weld each=20 joint. Moreover, in many pipe sizes the weight of black iron is 10 times = that of=20 thermoplastic pipe, making iron components more difficult to maneuver, = support,=20 and install, especially in larger sizes.

Thermoplastics are even more cost effective when compared to copper. = The=20 price of thermoplastic pipe is less than copper, although the cost of = the=20 fittings is a little more. Overall, the total systems appear similar, = but,=20 again, the real savings appear in the installation time and maintenance = of the=20 system. Soldering of copper pipe is much more time consuming than the = socket=20 fusion method used with thermoplastics.

When done properly, soldering a joint can take from five to seven = minutes by=20 a well-trained and experienced plumber or mechanical contractor. If done = improperly, the joints can leak, especially in large-diameter systems. = On the=20 other hand, unlike soldering, socket fusion can be performed in less = than two=20 minutes by anyone with minimal training, and with less chance for error. = The=20 result is a strong, leak-free joint every time.

Considerations for thermoplastics

An important consideration when designing a thermoplastic compressed = air=20 system and selecting the appropriate thermoplastic material is thermal=20 expansion. Thermoplastics expand and contract from thermal changes more = than=20 metals do. This must be taken into account during system layout by = allowing for=20 expansion at corners or by building in expansion loops and offsets.

Another important consideration is pressure rating. Be sure to select = a=20 material and construction with an adequate margin of safety for the = pressures to=20 which your system will be exposed. In the case of a safety failure or a=20 temperature rise, a system should still perform at the operating = pressure.

Scott N. Robichaud, is assistant vice-president of engineering,=20 Asahi/America, Inc., Malden, Mass.

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