Not Too Hot, Not Too Cold

February 20, 2007
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How long would you shower in 40 or 50 degree water? A minute would feel like an eternity. Even if our health depended on it, most of us would stop well before rinsing for a full 15 minutes.

Before the revised edition of the American National Standard for Emergency Eyewash and Shower Equipment – ANSI Z358.1-2004, that’s exactly what often happened. A worker splashed with a hazardous chemical would run to use a drench shower, activate it and then stop showering after a minute to keep from going into shock.

The same type of situation can happen when water is supplied at higher temperatures. The user could be scalded, and again stop short of using the shower for an extended period of time.

Water temperature plays an important role in ensuring someone will drench long enough to remove all hazardous chemicals. Emergency equipment — both new and existing systems — must have a tepid flow of water.

Tepid water requirements

ANSI Z358.1-2004 suggests minimum performance and use requirements for emergency equipment such as drench showers, eye and face washes, and drench hoses. In this latest version, ANSI defines the upper and lower limits of recommended water temperatures to help ensure that users are able to flush affected areas for the full 15 minutes required by the standard.

Tepid is defined by ANSI as “moderately warm; lukewarm.” The appendix to the standard further explains that the lower temperature limit should be about 60°F, and that water over 100°F could enhance interaction of the chemical with the skin and be harmful to the eyes.

The best way to control incoming water temperatures within a high degree of accuracy is to install a thermostatic mixing valve (TMV) designed for emergency fixtures.

Emergency applications

Thermostatic mixing valves blend hot and cold incoming water together to achieve the appropriate outlet temperature within a preset temperature range. (These temperatures should be set and adjusted by the installer according to the site conditions.) The blended water is then fed to the emergency drench shower or eyewash.

To maintain a constant temperature, incoming water is routed over an internal thermal element called a thermostat. The thermostat continually measures the temperature of the mixed water flowing through the TMV. If the temperature differs from the valve’s set temperature, the thermostat will react and move a mechanism that modulates one or both of the inlet ports until the valve returns to the set temperature, thus ensuring that each emergency fixture is provided with tepid water.

There is a difference between standard thermostatic valves and thermostatic mixing valves designed for emergency fixtures. Standard thermostatic mixing valves shut off the flow of water should there be a loss to either the hot or cold water supply. This is fine for typical domestic situations where a supply of only hot or only cold water would shock someone taking a shower.

An emergency application is different — it’s better for someone to shower in cold water than no water at all. For this reason, emergency valves are designed with a cold-water bypass. Should the thermostat fail or the valve lose its hot water source for any reason, the cold water would still be allowed to flow through the valve to the emergency fixture. If the cold supply is lost, the valve will shut off and prevent the hot water from flowing through the valve to the emergency fixture. This protects the user from the potential dangers of a hot flushing fluid being supplied to the eyewash or shower.

Sizing the TMV

To select the appropriate valve, particularly for emergency showers that use a high volume of water, it’s critical to size the valve for each particular application. One criterion is the maximum flow demand on the valve. Another is the minimum incoming flow rate required to maintain the appropriate set temperature. If the valve will feed more than one fixture, it should be sized to provide the appropriate amount of water to all eyewashes and drench showers simultaneously.

According to ANSI, the minimum flow rate for a drench shower is 20 gallons per minute at 30 psi. Eye/face wash units require a lower volume of water with a minimum of three gallons per minute at 30 psi. Smaller emergency fixtures, such as eyewash units, require just 0.4 gallons per minute at 30 psi. With a combination drench shower and eye/face wash, a valve capable of supplying tepid water at a rate as low as three gallons per minute or as high as the combined flow rate of the drench shower and the eye/face wash (minimum of 23 gallons per minute at 30 psi but maybe more) would be required.

In addition to some of the more typical calculations, note that line pressure will affect the size of the valve required. At higher pressures, a thermostatic mixing valve will typically be able to supply a larger volume of water. The size of the hot water tanks is another factor; without sufficient incoming hot water, the TMV cannot achieve the desired temperature.

Best performance

Reliability is obviously important in selecting an emergency TMV. The TMV must deliver accurate temperature control and hold temperatures within a few degrees. Fixed cold-water bypass ports ensure an uninterrupted flow of cold water when hot water is lost, without the need of a moving mechanism. A thermometer allows maintenance staff to easily read the outlet temperature. Inlet strainers and check stops prevent the cross flow of hot and cold water and keep dirt and debris from entering the valve for better long-term performance.

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