Don't lose your temper

August 1, 2005
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It’s always amazing to me how often people can make a great initial decision on a direction that needs to be taken and then blow it with a bad downstream call. So it is with tempered water systems for emergency equipment.

With the 2004 revision to ANSI’s Z358.1, the specific requirement for tempering emergency equipment outlet water temperatures to be between 60ºF and below 100ºF was clearly established. The requirement applies to all outlet water for the full 15-minute drench or irrigation cycle. Nowhere in the requirement does ANSI state that the temperature range does not have to be met if there is a water supply interruption or failure.

While a complete loss of input water would be impossible to overcome within the specification of the tempering system, there are a number of other significant failures or interruptions that can be addressed in system design. Redundancies in the system design and other measures can and should be taken to allow the system to function as well as possible, even during significant (non-catastrophic) water supply interruptions or fluctuations. The obvious regulatory direction established by ANSI Z358.1-2004 aside, common sense dictates that reasonable redundancies are prudent and in the best interest of one’s employees and others.

Types of systems

So once the wise decision is made to move forward with tempering emergency equipment water, which type of system provides the most safeguards?

It all starts with the main mixing valve, the heart of the tempering system. By their nature, the main mixing valves used in emergency equipment water tempering are complex designs. They usually measure outlet water temperature — internally or externally — and adjust the mixture of cold and hot input water to maintain the proper preset tolerances.

There are, however, limitations on the internal capabilities of the mixing valve. For instance, a valve that must adjust to a loss of hot water at its inlet side will sometimes have limited capabilities to allow a full flow of cold water in replacement. This often results in a lower by-pass flow capacity, usually below the threshold levels needed to properly supply downstream emergency equipment as specified in the ANSI Z358.1-2004 guidelines.

Specifiers should clearly ascertain the differences between the various mixing valve designs and capacities for themselves, as no guideline for mixing valves has ever been established by ANSI.

When the flow doesn’t go…

Let’s examine a variety of possible circumstances involving water interruption and/or fluctuation:

A cold water supply interruption: Candidly, this is a tough one to overcome. Without cold water, your emergency equipment, let alone the tempering system, is pretty much out of commission. And it’s important to note that you want the system to be completely out of commission in the event of a cold water failure. Design parameters for some of the mixing valves on the market allow for a small flow (up to .5 gpm) to continue, even when there is a complete cold water failure. Obviously, this could be a dangerous situation, because .5 gpm is sufficient to drive a minimal flow — of what could be scalding hot water — through some eyewashes.

Water temperature and pressure fluctuations: Fluctuations in supply line water temperature and pressure are quite common, especially in installations where the tempering system is located a distance from the hot water source. These fluctuations are relatively easy to handle, using a temperature sensing device that can be either internal or external to the main mixing valve. Compensation, after sensing a higher or lower than prescribed outlet temperature, is usually a matter of porting additional hot or cold water through the main mixing valve internally or externally directing it into the mixing valve’s outlet stream.

Loss of hot water supply to the inlet of the system: Any loss of hot water will obviously impact the system’s ability to provide tempering, but that should not be allowed to impact overall system flow during either a partial or complete hot water loss.

Simply stated, the system’s compensation during any by-pass operation should not result in a diminishing of flow volume or pressure to the emergency equipment. ANSI provides no relaxation of Z358.1-2004’s emergency equipment operating requirements because the system’s tempering capabilities have triggered a by-pass. A full-flow cold water by-pass is the only safeguard against diminished total system flow during a hot water supply deficiency. Yet, a number of main mixing valves on the market are rated showing significantly diminished flows during by-pass.

Specifiers are urged to pay close attention to the by-pass specs of any main mixing valve under consideration. The valve selected should be capable of adjusting the outlet water temperature based on temperature and pressure variations at its inlet. And the by-pass capacity must be sufficient to adequately drive the total number drench showers, eyewashes and combination units downstream of the tempering system.

Catastrophic failure of the main mixing valve: Without redundancies to accommodate a full flow by-pass — essentially supplying cold water around the main mixing valve — a valve failure may carry the unnecessary risk of diminished or cut off flows to the emergency equipment. Being an intricate piece of equipment, main mixing valves can develop internal operation and flow issues, especially in turbid or extremely hard water conditions. Those issues should not be allowed to risk the throughput of water to the emergency equipment. Once again, the only suitable safeguard for supplying water to the system during a main mixing valve failure is an external full-flow cold water by-pass.

So a properly designed system provides an appropriate hot water source and a circulation/recirculation system, as well as a robust main mixing valve, supported by as much external redundancy as possible. A system that features a main mixing valve assigned to handle all of the by-pass needs internally will likely fall short of the mark in terms of capacities during by-pass and system operation in the event of a catastrophic failure of the mixing valve itself. The best tempering package provides a system of valves and by-passes accommodating as many possible failure scenarios as it can.

As long as you’re taking the admirable first step toward tempering your emergency equipment water, why not do it right?

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