In the world of mechanical and plumbing engineering, piping layouts can range from the basics to the overly complicated. Even in the most basic sites, piping considerations for emergency eyewash and shower equipment tend to become an afterthought, often overshadowed by more operation-critical components.
Some layouts may only require additional considerations for a few simple eye/face wash stations and potentially a shower unit here and there. On large sites with increased risks, the risk assessments may require provisions for hundreds of shower, eye/face wash and combination units (shower and eye/face wash together) to be integrated into the design.
To add to the complexity of these systems, since 2009, ANSI requires all emergency shower and eye/face wash systems to supply tepid water, which is defined as 60 F to 100 F (16 C to 38 C). It requires the addition of tempering equipment to the site design and brings with it additional piping runs and loops as well as the necessary pumps and valves to correctly supply tepid water to the equipment.
While most emergency equipment tempering units are designed by the equipment supplier to be as single-point-connection as possible to simplify both layout design and installation, there are design parameters that must be considered by the plumbing engineer. A qualified safety equipment manufacturer can work with the engineer to advise on design and requirements. Ultimately, however, items such as appropriate piping size and arrangement to provide adequate pressure and flow to the equipment, air eliminators, pressure regulator, etc., all fall under the purview of the plumbing engineer.
Now that emergency equipment tempering requirements have been in place for almost a decade, most plumbing engineers know what is necessary to properly create a design to meet the needs of the system. However, there is one aspect often overlooked regarding emergency equipment systems more than any other — water hammer. Most plumbing engineers are very familiar with water hammer in commercial and industrial applications and how to properly design for water hammer arrestors, but they may not understand how vital they are for emergency shower and eye/face wash piping.
Safety showers and eye/face wash stations are designed and installed as a precautionary measure. Everyone involved from the manufacturer to on-site operators hope the equipment never needs to be used for their intended purpose. If they are, it means someone has been exposed to a dangerous situation and is at considerable risk for injury and possibly death.
Facility managers must ensure the proper maintenance and testing of these systems to keep the equipment ready for response in these situations. ANSI/ISEA Z358.1 not only dictates requirements for emergency equipment such as flow rates and run times, it also requires that the equipment is flow-tested on an annual basis (full 15-minute run time) and operation is tested on a weekly basis.
This testing is often not considered during the design phase of the emergency equipment. At the design stage, the piping system is considered a stagnant system that should only see substantial flow on the rare occasion there is an onsite emergency requiring either a shower or eye/face wash station to be activated.
Because water hammer arrestors are typically only designed into piping systems that involve multiple faucets, fixtures and pumps, it is not as obvious that emergency equipment piping requires them, especially if the engineer is not familiar with the mandatory weekly and annual maintenance operations. To reduce both system cost and complexity, it is standard for emergency shower and eye/face wash systems to be designed for one central tempered water skid to service numerous stations either through a dead leg or recirculation loop layout.
Finding Pressure Spikes
As previously mentioned, emergency shower and eye/face wash systems are designed to the ANSI/ISEA standard that requires minimum flow rates of 20 gallons/minute (75.7 liters/minute) for drench showers and 3 gpm (11.5 lpm) for eye/face washes. These are minimum flow rate requirements but, in practice, variances in manufacturers’ equipment designs can exceed the required rate. These high flow rates mean piping flow velocity can be high as well.
The recommendation is that piping is sized so velocity is 5 feet/second and not to exceed 10 ft/s. Piping size is a major consideration in plumbing design, particularly when it comes to keeping the cost of the project down; therefore the recommended 5 ft/s may not always be feasible. For most cases, we can split the difference and assume the average pipe velocities are about 7.5 ft/s. If we review a few cases using the basic equation for finding water hammer pressure spikes, we expose the apparent need for water hammer arrestors in these systems.
The basic equation for finding the pressure spike due to water hammer is:
dp = 0.070 dv x l / t
dp = increase in pressure due to water hammer (psi)
dv = change in flow velocity (ft/s)
l = length of pipe upstream of the closing valve (ft)
t = closing time of valve in system (seconds)
Let’s assume the average pipe velocity of 7.5ft/s. Take a small, one-shower dead leg system with a 50-foot pipe run and use a valve closing time of 0.5 seconds. The typical safety shower and eye/face wash station need between 30 and 90 psi water pressure to operate correctly, with the average pressure running about 60 psi. Observing maintenance personnel doing shower inspections, a 0.5 second valve closing time seemed to be about normal, so we can assume that here as well.
dp = (0.070 x (7.5-0.0)) x (50) / (0.5) = 52.5 psi
While this particular example is not a huge pressure wave, it certainly should be considered. However, when piping runs get longer, the pressures increase and can begin to damage equipment within the line. The water hammer pressure spike increases linearly as the length of the piping run increases.
In this example, going from 50 feet of piping to 200 feet is a fourfold increase up to 210 psi above operating water pressure. This is enough pressure to potentially cause damage to components such as check valves and other piping components if water hammer isn’t considered and mitigated during the design phase.
As safety shower and eye/face wash systems become larger and more complex, including new equipment designs, the risk of water hammer significantly increases. It is now commonplace to see larger tempering system installations which supply tepid water to dozens of showers and eye/face washes. Each of the units must be tested weekly to ensure proper functionality, meaning each time they are activated, the system will see the water hammer pressure spike.
For a system with a 1,500-foot run of pipe, operation at the farthest unit could create a pressure spike of 1,575 psi. Even if pipe velocity is lowered to 4 ft/s, the pressure rise will still be 840 psi, a damaging rate. And with the weekly testing, the system will potentially see dozens of the water hammer pressure spikes each week. Even if a one-time pressure occurrence isn’t enough to cause concern, the repetitive cycling can be enough to cause component damage and failure over time.
These larger tempering systems almost always contain multiple valves, check valves, booster and recirculation pumps, and other equipment that can both add to the water hammer issue as well as be susceptible to damage. In addition to this type of standard equipment within a system, we see new equipment incorporated as technology allows and new demands are created.
Components such as instantaneous in-line water tempering units, UV lighting for water purification and increased instrumentation for equipment monitoring all add a highly increased level of risk for component failure from water hammer.
Although emergency shower and eye/face wash equipment are often an afterthought, there are many considerations during the design and installation phase to protect the equipment and ensure proper functionality during testing and, more importantly, in the event of an emergency.
Tyler Brower is a sales engineer at Haws Integrated. Brower earned a B.S. in Mechanical Engineering from the University of Nevada-Reno, and is a member of the American Society of Mechanical Engineers. Visit www.hawsco.com.