Editor’s note: This article is the first of a two-part series based on a presentation the author gave at the Fall 2009 Laboratory Design Conference in Chicago. Next month’s installment will focus on the design of lab water systems. For information about the upcoming Spring 2010 Laboratory Design Conference, April 19-21 in Raleigh, N.C., see http://www.rdmag.com/tags/conference .
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Polypropylene pipe with fusion connections. A images © Newcomb & Boyd
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"Pay no attention to that man behind the curtain!" This famous line from The Wizard of Oz describes the way people often think of their plumbing systems. People just want water to flow when they turn on the faucet and waste to go down the drain when they pull the plug. They don't know or don’t want to know what goes on behind the scenes to make those things happen. In this two-part series, we will reveal some of the inner workings of the plumbing system and the special considerations plumbing engineers have to think about when designing for laboratory spaces. This issue's article deals with the waste and vent side of the equation, describing basic waste and vent design and the unique challenges associated with laboratory spaces. So, exactly what is going on behind that curtain?
Understanding plumbing waste and vent systems begins with understanding what happens to waste after the plug is pulled in the sink. (Note that in this article, "waste" refers to any material that may be flowing from fixture drains into the piping system, from corrosive chemicals to clear water.) On its way to the municipal sewer, the waste must always flow through a p-trap. P-traps are almost always located directly underneath sinks and immediately downstream of floor drains. Their purpose is to keep gases from flowing from the sewer up through the fixture drain and into the occupied space.
As long as the p-trap stays full of liquid, sewer gases will flow up through the fixture drain and into the building. In order to keep liquid in the p-trap, every p-trap must be vented by a vent pipe. Without venting, a vacuum would be created in the waste piping system that would siphon the liquid out of the p-trap and allow sewer gases into the occupied space. It is worth mentioning that the previous description is a simplified version of drainage and venting meant to describe only the basics of waste and vent systems. In reality, the systems can be much more complex, and other involved factors are outside the scope of this article. In addition, the plumbing codes tightly control the design and installation of these systems.
In most installations, waste and vent piping is either cast iron or PVC (polyvinyl chloride). These materials are widely used and generally less expensive than more lab-friendly materials. Cast iron and PVC are rarely used for laboratory piping, however, because waste streams in laboratories are likely to have highly corrosive chemicals that may be at elevated temperatures, particularly if chemical reactions are possible within the piping system. While cast iron is a durable and robust material, it has low resistance to corrosion. On the other hand, PVC has generally good corrosion-resistance properties, but handles high temperatures poorly.
For these reasons, alternative piping materials must be used for laboratory waste and vent systems. Some of them are described below, but it is important to note that pipe materials must be selected based on the specific nature of each individual laboratory and the desire for future flexibility in a particular space. Each installation is unique, so general statements about corrosion resistance should not be taken at face value. In a given situation, a specific chemical may react very poorly with an otherwise inert material.
Piping material choices
One common laboratory waste and vent piping material is polypropylene. Polypropylene is a plastic widely used in a broad range of applications. For example, you may be storing last night's dinner leftovers in polypropylene containers right now. Propylene is used for laboratory waste and vent piping because of its good corrosion and heat resistance. The piping looks somewhat similar to PVC but is usually blue, turquoise, or brown in color. (PVC is usually white.)
While its physical properties make it a good choice, polypropylene has several disadvantages as well, most of them typical of other plastic pipe materials. Like most plastics, it is prohibited by code in return air plenums. This could be a hindrance if the HVAC system in the space below a particular laboratory space uses plenum return. Likewise, like all plastics, its thermal expansion characteristics cause the pipe to require more support than metal or glass pipe. Finally, again like all plastics, polypropylene pipes can be noisy when heavy flows run through them. One disadvantage unique to polypropylene is that its joints are made by thermal fusion, which requires a special machine and is labor-intensive.
An alternative to polypropylene is CPVC (chlorinated polyvinyl chloride), which looks nearly identical to PVC but is normally gray in color. CPVC has very similar corrosion resistance to that of PVC, but, unlike PVC, CPVC can handle high-temperature waste. CPVC is joined with cement that can have a long set time depending on the ambient conditions, but it is generally less labor intensive than polypropylene. Disadvantages are those typical for plastic pipe, except that CPVC is permitted by most codes in return air plenums.
If a particular situation calls for a non-plastic piping material, glass may be a good choice. Glass piping has very high resistance to corrosive chemicals. (After all, scientists perform experiments in glass, right?) It is also acceptable in return air plenums. However, glass piping is expensive, labor intensive, brittle and fragile.
There are other material alternatives as well, such as stainless steel, high-silicon iron and PVDF, that may be appropriate in a particular situation. The reader is encouraged to research these materials for further knowledge of this topic.
Sewer-system protection
In describing piping materials, we dealt with how to protect the piping systems inside the building from damage due to laboratory waste. In any lab plumbing design, it is also important to discuss how to protect the municipal sewer.
The responsibility to protect the municipal sewer from lab waste stems from both safety concerns and regulatory stipulations. Plumbing codes prohibit corrosive or otherwise harmful waste from flowing into the sewer without treatment. Treatment may be accomplished by neutralization or dilution, either outside the building or at the point of use.
Many laboratory buildings are equipped with exterior acid neutralization or dilution basins. Chemical waste enters the basin where it is either neutralized by limestone chips or mixed with less corrosive waste before flowing out of the basin and into the municipal sewer. Dilution depends on enough non-corrosive waste flowing into the basin to neutralize the corrosive waste that is present. Neutralization with limestone chips is a more failsafe method than dilution, but it also requires more maintenance because the limestone chips have to be replenished regularly.
An alternative to exterior neutralization or dilution is to install point-of-use tanks under lab sinks. This solution may be less expensive up-front than an exterior basin, but it creates the need for each individual tank to be maintained over the life of the building, and it takes up valuable space under the counter. For each situation, all of these factors must be weighed to make the right choice as to how to treat corrosive waste before it enters the public sewer. As with the other topics discussed in this article, this issue can be complex in reality, and this is intended only as a basic overview.
In conclusion, when designing the waste and vent system for a laboratory space, plumbing engineers must keep all of these issues in mind to create a space that is dependable and maintainable for everyone involved. Maybe you will always want to ignore the man behind the curtain, but I hope this article has helped shed some light on what is going on back there. In the next issue, we will pull back the curtain a little further and reveal the inner workings of the water side of laboratory plumbing.
Brett M. Gilbert, LEED AP, is an associate with Newcomb & Boyd Consultants and Engineers, an Atlanta-based consulting firm ( http://newcomb-boyd.com). Since joining Newcomb & Boyd, he has been responsible for mechanical system design on more than 30 projects, including laboratories at the Univ. of Miami and the Univ. of Georgia.
