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Tempering Your Emergency Shower Water
PEOPLE's reaction to being hit with cold water has been used in comedy routines for as long as anyone can remember. Providing it's not you being doused, it can be really funny. But cold water, when used in emergency showers and eyewashes, is no joke. While a cold shower might be invigorating, it can be downright dangerous to run untempered or cold flushing fluid through industrial emergency equipment.
Emergency showers and eyewashes minimize the effects of exposure to hazardous materials by drenching the affected areas of the body with significant amounts of water. This serves the dual purpose of removing any remaining hazardous materials and minimizing further injury.
Through the years, the largest challenge faced by designers and specifying engineers has been providing ready access to emergency showers and eyewashes. The basic design objective is to place emergency assets as close to potential accident sites as possible. This challenge becomes much more difficult in larger, more complex plants. More recent design configurations usually feature dispersed location of eyewashes and showers with centralized activation monitoring.
Another, more recent design challenge focuses on the effects of exposure to the high volumes of water used in eyewashes and drench showers. Safety protocols call for at least 15 minutes of immediate and constant eyewash or shower use in response to exposure to hazardous materials. Exposure to domestic water temperatures in many areas of the country, when combined with the increased cooling effects of moving water passing over the body, easily can lead to hypothermia. Concern for workers' safety has driven many companies to upgrade their emergency equipment to include tempered water. The use of "tepid" water easily allows for the full 15 minutes of eyewash or shower exposure required, without risking hypothermia.
Currently, ANSI has no precise definition of tepid water. The American Society of Plumbing Engineers is starting a study to determine an adequate temperature range to be used as the definition of "tepid." Until this study is done, manufacturers and specifiers have chosen 80 degrees F as a base point for delivery temperature. As with all installations, a medical advisor should be consulted to determine the optimal delivery temperature.
System Components
Tempered water blending systems generally consist of three major components: a hot water storage tank, a heater, and a blending valve. Additionally, a booster pump is often specified where water pressure might be too low to run multiple pieces of emergency equipment at peak demand.
While the foregoing components do define the additional items needed to upgrade an existing emergency equipment system to a tempered water system, opinions differ as to exactly what constitutes a complete system. Most emergency equipment manufacturers agree the specific requirements of the showers or eyewashes that comprise the system need to be taken into account when sizing the componentry for the tempered water. That means the definition of the complete system includes both the tempering components and the specific emergency equipment in use or anticipated. An ANSI-compliant shower may not actually be compliant if the upstream tempering components are sized improperly. ANSI compliance is sought and granted based on specific pressure and flow ranges, which can be affected by tempering components placed in front of the otherwise compliant emergency equipment.
Tepid Water for Emergency Equipment
The heart of the tempered water delivery system is the mixing valve, which ensures the safety equipment safely receives water at the required temperature. Safety equipment manufacturers are in the best position to design appropriate mixing valves because we understand our equipment and our specific flow rate requirements. Knowing what flows and potential peak demands are expected is critical in properly designing a good system.
For instance, consider a facility that has several pieces of safety equipment and needs tepid water delivered to all of them. First, we must determine how many of those devices may be required to run at once because 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 not only helps your safety equipment manufacturer to properly size the mixing valve, it also allows it to determine the size of the required hot water source.
Once the mixing valve capacity has been established, we need to know the associated pressure drop at the highest demand. The most common oversight is underestimating the pressure required to drive the safety equipment after it has passed through the mixing valve. The mixing valve can have a significant pressure drop because the flow requirement can be 30 gpm and higher. Also, safety equipment is designed so 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, we can determine what supply pressure is required. We might find there is insufficient pressure at the facility to run the system. If this is the case, we must either add a booster pump to the supply or limit the number of showers and/or eyewashes on the system. Other options are to look at increasing the pipe size one or two sizes so the friction losses are less. Furthermore, many safety equipment manufacturers offer components that will reduce the flow at the showerhead without sacrificing the required flow patterns.
When you have the essential data to size the system, you need to ask the safety equipment manufacturer about the safety redundancies of its valve. This is very important. You need to ensure 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 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 need to ask whether this bypass can handle the maximum flow of the system design. If the bypass is less, there is a good possibility an eyewash will not flow water if it is needed concurrently with a shower because the shower will take as much as it can receive. The offered mixing valve also must 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 because there may be enough flow to operate an eyewash, and temperatures in excess of 100 degrees F have been determined to be harmful to the eyes.
These safety redundancies are essential to the safe operation of the mixing valve and should be external to the main mixing valve so their performance is not jeopardized by the overall performance of the valve.
Absolute Certainty
Tempering water for emergency equipment requires more functionality than is built into most mixing valves. That's because most valves are individual valves, not valve systems, and they have been designed by valve manufacturers--not by emergency equipment manufacturers who know their equipment's requirements. Knowing the emergency equipment's operating requirements from the start allows us to see whether the tempered water system under consideration meets all of the necessary requirements.
Simply stated, there are two things to look for in a mixing valve system:
1. The mixing valve system must have external bypass to compensate for hot water loss or thermostat failure.
2. The mixing valve system must provide for 100 percent hot water shut-off in the event of cold water supply failure.
In designing emergency equipment systems, we sincerely hope they are never used. But, in the event they are needed, we must make certain the system will deliver the maximum volume needed, safely and adequately. There also must be safeguards and redundancies built into the system to maintain an acceptable minimum operation in the event of any possible failure or contingency. The design process should progress from a complete understanding of your shower and eyewash requirements to building the upstream tempering elements with those requirements in mind.
The goal is to deliver tempered water through your emergency equipment in one integrated, engineered solution, regardless of whether you are specifying a totally new system or adding tempering capabilities to an existing emergency equipment installation.
This article originally appeared in the October 2003 issue of Occupational Health & Safety.