07/08 Model TWBS.SH Page 2 of 6
I. WHAT IS THE HAWS TWBS.SH?
The HAWS TWBS.SH is a patented tempered water mixing system (US Patent # 5350112) for use with
emergency drench shower and eyewash stations where cold and hot water is thermostatically blended to
provide a discharge of tempered water at a selected temperature. The TWBS.SH is a thermo-mechanical
system with thermally activated and pressure-activated safety features, which do not require electricity for
operation. These safety features utilize back-up control valves that ensure a substantial water flow
without the risk of scalding, even in the event of main mixing valve failure. Additionally, a temperature limit
control valve detects the outflow temperature and helps to reduce the discharge temperature should
excessive temperature conditions occur.
II. HOW DOES THE TWBS.SH OPERATE?
The heart of the WTWBS.SH system is the thermostatically controlled mixing valve. It includes an integral
strainer and check valves, and has an adjustable temperature control, which is typically set at 80°F. The
main mixing valve includes a connection to a cold water supply and a hot water supply, with the ability to
provide a discharge outflow of water at a selected/blended temperature level. On the hot water inlet side
of this mixing valve we include a high limit temperature control valve, which senses the temperature of the
discharge flow. When the discharge flow temperature exceeds our factory pre-set limit of 85°F, this
control valve closes and/or modulates, thus reducing the hot water flow being sent to the mixing valve.
On the cold-water inlet side, we include a flow control bypass valve, which is responsive to the pressure
differential between the cold water supply and the discharge of the tempered water. Should the main
mixing valve fail to produce the required mixed flow through the discharge line, the bypass valve senses
this and opens up, allowing the incoming cold water to bypass the system and proceed directly to the
emergency shower and/or eyewash. These two back-up components permit the temperature level of
tempered water to be maintained without undesirable temperature reductions/fluctuations experienced in
most failures. They also permit the flow of cold water should the mixing valve fail altogether.
III. PERFORMANCE FEATURES OF THE HAWS TWBS.SH SYSTEM
Essentially, the HAWS TWBS.SH provides fail-safe protection against scalding or blocked flow. There are
three types of failures that can occur with any mixing valve system. They are as follows:
1) The mixing valve can stick in an open position and provide an unregulated flow of hot
water.
2) The mixing valve can stick in a position, which allows an unregulated flow of cold water.
3) The mixing valve can fail so as to provide an insufficient and/or blocked flow of either hot
or cold water.
The unique HAWS TWBS.SH answers the above three failure modes in the following manner:
Fail-safe Performance Mode # 1 Too much Hot Water
The primary safety in the HAWS TWBS.SH is the high limits hot shut-off valve, or the temperature limit
control valve, which senses outlet temperatures and modulates the incoming hot water to maintain an
outlet temperature below 85°F. This high limit shut-off valve is non-adjustable (tamperproof) and provides
a regulated reduction in the temperature level of the discharge flow. This high temperature limit valve
provides the first backup for regulating the temperature of the discharge flow in the event of mixing valve
malfunction (i.e., too much hot water).
Fail-safe Performance Mode # 2 Unregulated Full Flow of Cold Water
The HAWS TWBS.SH answers this condition in two ways. When improper mixing valve operation occurs
and produces a restricted flow (i.e., a pressure differential between the cold water supply and the
discharge tempered water outflow), the flow limit control valve opens to permit the direct flow of cold
water past the main mixing valve and into the system. Additionally, the temperature control valve will
sense a reduction in the temperature of the discharging tempered water and will open to permit an
increased flow of hot water into the system. In this case, the HAWS TWBS.SH can help to produce warm
water even when the pressure relief bypass valve is operating. (The high temperature valve opens so as
to assist in maintaining a degree of tempered water out).
