PVF FORUM: A Piping Primer
What is the best piping system? There are as many correct answers as there are types of piping systems. Basic factors that determine which system is the best for your facility start with the cost.
The initial cost is not always the deciding factor. The first consideration is the operating condition of pressure, temperature and corrosion that mandate material selection. How flexible is the system to expand or modify? The ability to use in-house staff for modification can be an important cost saver.
Cost of operational maintenance must be factored into the assessment as well as initial installation. Local, state and federal regulations and codes affect fabrication methods and approved materials. Relative weight of components materially affects the hanging costs of installation. Flow characteristics can influence cost of pumps and erosion of materials.
Process Piping & PlumbingLet’s consider the pros and cons of various piping systems according to the above factors. It is estimated that 90% of process piping is used at ambient temperatures below saturated steam (350°F). Much process piping and plumbing installations use threaded piping with steel pipe, brass pipe, plastic pipe, PEX and copper tube (not threaded). The lines are primarily in sizes 6 inches and smaller.
Flanged FittingsCast flanged fittings are selected for most large water supply and sewage piping systems. Large-size piping is normally connected by flanges to valves, pumps, vessels, etc., often requiring the use of flanged fittings. Because the clumsy flanged fittings cannot be hung flush to the wall, they require greater space and have other costly installation factors.
Additionally, the excessive weight of the flanged fittings and flanges converts directly into increased labor costs due to hanging. A flanged elbow with two connecting flanges weighs about four times as much as a weld ell connected to the pipe sections. The excess weight serves no useful purpose and places more strain on the support system in addition to the cost of hanging.
Flanged joints require regular maintenance to watch for gasket leaks and the need for tightening the bolts. Also, threaded pipe systems - particularly ones subjected to thermal expansion - have a tendency to seep or leak, and require regular maintenance. This is in contrast to the welded piping system, which is the only truly “leakproof” system and requires no routine maintenance for the life of the system.
Weld FittingsWhile weld fittings are efficient for small-size piping, they have many advantages in large-size piping. They have a unique flexibility that allows them to be close nested. They can be hung flush to walls and ceilings. This allows piping designers to tailor the systems to work in a limited space and conform to the physical parameters of the building, terrain or position of other equipment. Weld fittings can save tons of metal in a piping installation, along with reduced labor in hanging.
Entire sections of a welded piping system can be fabricated under ideal working conditions in a fab shop. The components can be assembled and erected in the field into a single unit with a few simple tie-in welds by qualified welders.
Weld fitting piping allows significant savings in pipe cost vs. threaded piping because the wall thickness of the pipe is reduced when cutting threads, thereby requiring the use of heavier wall pipe (more expensive) to handle the same pressure as a thinner wall pipe in a welded system. This is a significant saving, particularly when using stainless piping.
Allowable working pressure of most piping is limited by the sealing capability of the joint, whether it is via threads, flanges, gaskets, bolting, solder or mechanical. Welded piping has the highest pressure-temperature capabilities (when properly code welded), limited only by the maximum characteristics of the piping material.
Threaded FittingsA distinct advantage in using threaded fittings is that most facilities have portable pipe-threading machines in their maintenance department that allow in-house staff to make modifications or perform maintenance. This precludes the necessity of bringing in outside piping contractors with their expensive qualified code welders. Another advantage of threaded piping is it allows for easy access to clean and service lines, pumps, filters, etc., for normal maintenance.
Threadless FittingsSignificant technological improvements in solder-less connectors with copper tubing, such as the Press-Fit method, have made copper tubing easier to work with. It requires less skill to make up solder-less connections.
The rapidly growing use of PEX tubing with the improved connectors also is ideal for applications where the pressure-temperature requirements are approved for those materials. Since 2000, PEX plumbing systems have experienced double-digit growth and now command 25% of the market, mostly at the expense of copper plumbing systems.
Grooved CouplingsThere has been rapid growth of grooved piping systems for low-pressure applications. While grooved couplings have been used for years in temporary water systems and irrigation, now lower pressure, lighter weight grooved fittings and couplings are being utilized in many other applications. They are especially popular in fire protection systems.
Grooved piping systems have great flexibility to make modifications or expansions by maintenance or in-house staff. This system is ideal for efficient dismantling and reconnecting for servicing and cleanout.
Stainless weld fittings are also available in grooved ends to use with thin wall (Schedules 5S & 10S) pipe for corrosive applications. This permits significant cost savings below standard weight (Schedule 40) stainless pipe, as well as much lower cost of rolling the pipe grooves vs. expensive heliarc welding for the connections.
Welding OutletsIntegrally reinforced forged outlet fittings (Class 3000) in socket weld, threaded and butt-weld ends have been in use for many years to provide branch connections for high-pressure and code jobs. An ever-increasing market has developed for lower pressure applications not requiring heavy forgings.
Many low-pressure outlets are made from pipe or tubing as are merchant steel couplings. These outlets create a branch without the expense of adding a tee into the fabrication. Fire sprinkler installations incorporate many low-pressure welding outlets in their design.