Last month, in Part 1 of "Just Right!", we discussed the advent of a specific outlet water temperature range for emergency drench showers and eyewashes. Heretofore uncontrolled, ANSI clarified the Z358.1 standard in 2004 to include upper and lower acceptable temperature limits. Sustained outlet temperatures, per the standard, must be no lower than 60 degrees F and below 100 degrees F during a full 15-minute use cycle for either an emergency shower or eyewash. This can be a tall order in applications where ambient temperatures and other industrial process-related factors can drive "uncontrolled" temperatures to extremes. Temperatures easily can range from sufficiently cold to cause hypothermia to sufficiently hot to cause scalding. And, in between, there are those areas that are cold or hot enough to force an injury victim to cease using the emergency equipment due to discomfort. That leaves the temperature band, in the middle, that is "Just Right!"
Last month we also dealt with warm-water temperature countermeasures, including both the use of chillers and re-circulation loops to assure that outlet water temperatures meet the guidelines. In those instances, the approach is relatively straightforward. The same is not true when you are dealing with input water that is below the low-temperature boundary. The equipment and redundancies necessary to safely address raising input water temperatures are much more complex.
As a general rule, tempered water blending systems consist of three major components: a hot water storage tank, a heater and a blending valve. A fourth component, a booster pump, is sometimes specified where water pressure might be too low to run multiple pieces of emergency equipment at a combined peak demand. Turnkey matched and balanced tempered water blending systems are available in both skid-mounted and Enclosed Emergency Environment (E3) booths.
The specific requirements that the showers or eyewashes impose on the system need to be taken into account when sizing the componentry for the tempered water system. This is an important point. By considering the tempering componentry and the demands of the showers and eyewashes within the system, the specifier can build a complete, matched system. An ANSI-compliant shower may not actually be compliant if the upstream tempering components are sized improperly. That's because ANSI compliance is sought and granted based on specific pressure and flow ranges.
Heart of the System
When your objective is to warm cold input water, the heart of the tempered water delivery system is the mixing valve. (See photo #1) It ensures that the emergency equipment safely receives water at the required temperature, by monitoring output temperatures and adjusting cold and warm input streams accordingly. Emergency equipment manufacturers are in the best position to design appropriate mixing valves, since they understand the equipment and the specific flow rate requirements.
Knowing what flows and potential peak demands are expected is critical in properly designing a good system. For example, consider a facility that has several pieces of emergency equipment and needs tepid water delivered to all of them. First, you must determine how many of those devices may be required to run concurrently, as the mixing valve must be capable of handling the highest demand of multiple uses, as well as the lower demand associated with a single eyewash. Determining flow requirements will not only help the emergency equipment manufacturer to properly size the mixing valve, it also will allow them to determine the size of the required hot water source.
Once the mixing valve capacity is calculated, you will need to know the associated pressure drop at the high demand. The most common oversights are underestimating the pressure required to drive the emergency equipment after it has passed through the mixing valve and the flowing pressure available. The mixing valve can have a rather large pressure drop as the flow requirement can be 30 gpm and higher. Also, emergency equipment is designed so that it will produce the required flow patterns at a minimum pressure of 30 psi. Adding this minimum pressure requirement to the pressure drop of the valve and the associated piping, you can determine what supply pressure is required. You might find that there is insufficient pressure at the facility to run the system. If this is the case, you will have to either add a booster pump to the supply or limit the number of showers and/or eyewashes on the system. Another option is to increase the pipe one or two sizes so that the friction losses are less.
Now that you have the essential data to size the system, you will need to ask the emergency equipment manufacturer about the safety redundancies of their valve. This is very important. You need to ensure that the mixing valve can offer a full flow bypass of cold water. In the event there is a loss of hot water at the valve, or the cold-water inlet at the valve becomes restricted, it is essential that there is a means to offer a full flow of cold water to the equipment. If you are considering a mixing valve stated to have an internal cold water bypass, you will need to ask if this bypass can handle the maximum flow of the system design. If the bypass is less, there is a good possibility that an eyewash, for example, will not flow water if it is needed concurrently with a shower, as the shower will take as much water as it can receive. The offered mixing valve must also have a positive means of hot water shut-off in the event of a cold-water failure. If there is a sudden loss of pressure on the cold side, the mixing valve must shut off completely and not allow any hot water to pass. You do not want a valve to pass only hot water, as there may be enough flow to operate an eyewash. As mentioned earlier, temperatures in excess of 100 degrees F are harmful to the eyes. These safety redundancies are essential in the safe operation of the mixing valve, and should be external to the main mixing valve so that their performance is not jeopardized by the performance of the mixing valve itself.
As we've seen, providing the proper temperature water to emergency showers and eyewashes can be a challenging undertaking. That's because circumstances in the plant, including ambient temperature fluctuations, during different seasons or with different industrial processes, all have an impact on outlet temperatures. The situation is further complicated by the ongoing possibility of multiple concurrent casualties, which could drive the use of more equipment and a vastly different scenario regarding water flows and pressures. So, a temperature that's just right might require a different set of equipment in a different configuration from one plant to another. The good news is that there are avenues for obtaining assistance in specifying these custom-tailored solutions.
Casey Hayes is the engineering manager at Haws Corp. (www.hawsco.com), located in Sparks, Nev. He can be reached at (775) 353-8320 or casey [email protected]. Haws Corp. designs, manufactures and distributes drinking fountains and emergency equipment that are ranked No. 1 in quality by specifiers in both product categories.