Editor’s note: This article is the second part of a two-part series based on a presentation the author gave at the Fall 2009 Laboratory Design Conference in Chicago. Last month’s installment focused on the design of lab waste and vent systems.
Fig. 1. A reduced-pressure-type backflow preventer contains two check valves with a relief valve located in between them. Photo courtesy of Newcomb & Boyd
In the previous issue, we explored the inner workings of plumbing waste and vent systems for laboratories. We pulled back the curtain to reveal the levers that were being pulled and the switches that were being thrown in order to get laboratory waste safely from the point of use to the municipal sewer. But there is still more to the story of laboratory plumbing. In this second article of a two-part series, we deal with the water side of the equation, describing safety measures and practical concerns for laboratory water design. It will take brains, heart and courage, but let's throw open the curtain fully and see the world of laboratory water through the eyes of a plumbing engineer.
Preventing backflow: a crucial task
One of the biggest concerns for the plumbing engineer on any laboratory project is to protect the public water supply from being contaminated by the variety of hazardous chemicals used in today's laboratories. Chemicals flowing from the building water system into the municipal system ("backflow") could create a very dangerous public health situation. Consequently, most local municipalities require a high level of backflow prevention on a laboratory building through the use of a reduced-pressure type backflow preventer installed where the municipal water supply enters the building, typically in the main mechanical room on the ground floor.
A reduced-pressure-type backflow preventer contains two check valves with a relief valve located in between them (Fig. 1). A check valve is simply a "oneway" valve, having a spring-loaded stopper inside that will only open in one direction and will therefore only allow water to flow in one direction. Most of the time, check valves alone would be enough to prevent backflow; however, since it is possible that one or both check valves could fail, likely due to the presence of obstructions preventing the stoppers from fully closing, the relief valve acts as the final failsafe against backflow.
For backflow to happen, there must be higher water pressure on the building side of the backflow preventer than on the municipal side, since water always flows from higher pressure to lower pressure. Whenever pressure on the building side of the relief valve is greater than or equal to pressure on the street side of the relief valve, the relief valve will open, causing water to flow out of the piping system and into a floor drain or onto grade outside of the building. This prevents building water from flowing into the municipal water supply.
Fig. 2. Vacuum breakers at lab faucets prevent backsiphonage when pressure drops by closing off the faucet supply and venting the discharge side of the faucet to atmosphere. Photo courtesy of Newcomb & Boyd
There are other means of backflow prevention as well. Another common backflow preventer is a "double check" type. These are also typically located in a ground-floor mechanical room. This device is similar to the reduced pressure-type backflow preventer, except that the double check type does not have a relief valve; it is simply two check valves in line. If one check valve fails, the other check valve prevents backflow. If both check valves fail, nothing prevents backflow. For this reason, a double-check-type backflow preventer is less protective than a reduced-pressure type.
The highest level of protection possible is with an air gap. An air gap serving the entire building would be located either in a ground-floor mechanical room or in the building penthouse. An air gap simply means that there is no physical connection between the municipal water supply and the building water supply. Think of your bathtub at home. Water flows from the faucet into the tub and drain, but water cannot flow from the tub back into the faucet because the faucet is higher than the sides of the tub. In other words, water would flow out of the tub and onto the floor before reaching the faucet. The distance between the top of the tub and the bottom of the faucet is the air gap. Despite the high level of protection they offer, air gaps are rarely used in building water supplies, because their use drops building water pressure to atmospheric pressure, requiring the use of a building pump to supply the building with water pressure. Reduced pressure-type backflow preventers provide the best combination of protection and practicality.
In addition to protecting the municipal water supply from contaminants in the laboratory, it is important to protect the building users from ingesting these same contaminants through the building’s potable water supply. Contaminants can enter the water supply through backsiphonage from lab sinks and other fixtures and equipment that are open to atmospheric pressure. "Backsiphonage" is backflow caused when water supply pressure drops below atmospheric pressure, creating a vacuum that draws water from fixtures, such as laboratory sinks, into the building water piping system. While preventing backsiphonage, reduced-pressure-type backflow preventers would be impractical to install at each potential hazard; therefore, a vacuum breaker should be specified at each laboratory sink faucet and at any other place where backsiphonage may occur.
Laboratory sinks are particularly susceptible to backsiphonage because they are often provided with serrated nozzles and rubber hoses that reach into the liquid in the laboratory sink. In a condition where the faucet is turned on, the end of the hose is submerged, and the building water system pressure drops below atmospheric pressure, the liquid in the sink will siphon back into the faucet and into the building potable water supply. Vacuum breakers prevent backsiphonage when pressure drops by closing off the faucet supply and venting the discharge side of the faucet to atmosphere (Fig. 2).
Note that vacuum breakers do not prevent backflow due to backpressure. Backpressure occurs any time pressure is greater on the demand side of a device than on the supply side. Backpressure contrasts from backsiphonage in that backsiphonage only occurs when supply side pressure drops below atmospheric pressure. Backsiphonage can be thought of as a special case of backpressure. In a situation where general backpressure is a concern, a reduced pressure-type backflow preventer or an air gap, either of which prevents both backpressure and backsiphonage, should be installed.
Fig. 3. Emergency showers, eye/face washes and combination units are covered by the American National Standards Institute/ International Safety Equipment Association (ANSI/ISEA) standard Z358.1. Photo courtesy of Newcomb & Boyd
On faucets that have vacuum breakers, you may notice that a small amount of water still flows out of the faucet after you have turned the faucet off. This is normal and shows that the vacuum breaker is operating as it should. When the faucet is turned off, the vacuum breaker closes and the water left in the spout downstream of the vacuum breaker gravity flows out of the spout. So don't turn the faucet handle hard when this happens, thinking that the faucet is not fully off. Doing so could damage the faucet over time. Instead, rest assured that your drinking water is safer because your vacuum breaker is doing its job!
Guarding against injuries
In addition to protecting the potable water supply, it is important to protect laboratory users from serious injury due to chemical spills. It is not difficult to imagine a situation where proper location and installation of an emergency fixture could make the difference between slight injury, serious injury and death (Fig. 3). Emergency showers, eye/face washes and combination units are covered by the American National Standards Institute/International Safety Equipment Association (ANSI/ISEA) standard Z358.1. Both nationally used plumbing codes (the International Plumbing Code and the Uniform Plumbing Code), as well as the Occupational Safety and Health Administration (OSHA), require compliance with ANSI/ISEA Z358.1, making it applicable virtually everywhere in the United States.
The standard details requirements for emergency equipment features, operation, location, access and many other criteria. For example, the standard requires that emergency equipment be in accessible and unobstructed locations that are reachable within 10 sec and include a highly visible sign letting users identify the emergency equipment location quickly in an emergency situation. While many of these requirements may be seen as a nuisance or an eyesore by some, we should all remember that if we are the individuals with chemicals in our faces or on our bodies, we will want the emergency equipment to be as easy to see and reach as possible.
ANSI/ISEA Z358.1 gives specific requirements for the water that is supplied to emergency fixtures. This water must be in a temperature range between 60°F and 100°F. The typical temperature for this water is 85°F. Maintaining this water temperature is usually the job of an emergency tempering valve, essentially a safer version of a standard thermostatic mixing valve, which mixes incoming hot and cold water to produce warm water (Fig. 4). This valve is typically located either near the water heater serving the space, or near the emergency fixture being served. In the event of loss of cold water pressure, the valve will shut off the hot water supply to the emergency fixture, to avoid scalding the user. If the hot water supply fails, the device will still deliver a full volume of cold water to the emergency fixture. These devices usually come factory-preset to output 85°F water.
In addition to temperature requirements, ANSI/ISEA Z358.1 requires the following minimum flow rate from each type of emergency fixture:
- Shower: 20 gpm (gal/min).
- Eye/face wash: 3 gpm.
- Eyewash: 0.4 gpm.
Each of these flow rates is required to be sustained continuously, at the temperature described above, for a minimum of 15 min. The combination of high flow rate, long duration, and elevated temperature create unique plumbing demands that plumbing engineers must address in their designs. Floor drains and hot water supply involve unique issues.
Floor drains and hot water supply
In the case of an emergency shower, the 20-gpm flow for 15 min means that 300 gal of water total (20 x 15) flows from the fixture during use. This large quantity of water is sometimes anticipated and handled during design by including a floor drain under the emergency shower.
Fig. 4. Maintaining water temperature in emergency fixtures is usually the job of an emergency tempering valve, which mixes incoming hot and cold water to produce warm water. Diagram: Newcomb & Boyd
Owners, laboratory designers and users should be aware, however, that a floor drain will not completely solve the problem of water on the floor produced by the emergency shower, because water splashes off the user, as it flows from the shower, and wets a wide area including the floor, walls, and surrounding equipment and furniture. While a floor drain will help in this situation, the floor would need to be sloped to the drain to ensure proper drainage of most or all of the water from the emergency shower. This is an added construction cost that may be seen as not outweighed by its benefit. Because of this, owners and laboratory designers usually opt to leave the floor level, either installing a floor drain or not, depending on the preferences of the particular people involved, and the needs and constraints for a particular situation.
When considering this high volume of water, the plumbing engineer must also think about how it affects the building hot water system. Hot water to emergency fixtures is usually supplied from a storage-type water heater, where hot water is stored at 140°F for use in the building. With a cold water temperature of 40°F (a commonly used worst-case scenario figure), usage temperature of 85°F, and total used volume of 300 gal as described above, 135 gal of 140°F water is required from the building hot water system when an emergency shower is used. This is calculated using the following formula: 300 gal x (85°F - 40°F)/(140°F - 40°F) = 135 gal.
Failure to provide this quantity of hot water would constitute violation of the requirements of ANSI/ISEA Z358.1. For this reason, a large water heater may be required for a small laboratory space.
When designing the water system for a laboratory space, plumbing engineers must keep all of these described issues in mind to create a space that is usable and safe for everyone involved. We have now investigated both the drainage and water sides of laboratory plumbing, and while there is much more that could be written on these topics, I hope these two articles have helped you gain more of a behind the-scenes understanding of plumbing systems and that this understanding will improve the way you design and use laboratory spaces in the future.
Brett M. Gilbert, LEED AP, is an associate with Newcomb & Boyd Consultants and Engineers, an Atlanta-based consulting firm (www.newcomb-boyd.com) . Since joining Newcomb & Boyd, he has been responsible for plumbing system design on more than 30 projects, including laboratories at the Univ. of Miami and the Univ. of Georgia. He is a member of the American Society of Plumbing Engineers.