IM-P157-38
CMGT Issue 4
42
Steam/Water Mixing Stations (MkII valves - 2002 onwards) Safety and Operation
Symptom
Cause and corrective action
Hot water outlet
temperature too low
Difference in supply pressures (see page 16)
When a mixing valve is supplied with a higher cold water pressure compared
to a lower steam pressure, in some instances it is possible that there will be
too much cold water for the steam to heat up. It is also possible that steam will
be prevented from entering the mixing valve, due to a higher cold water
pressure. A Spirax Sarco LRV pressure reducing valve would need to be
installed in the cold water supply line to reduce cold water pressure. This is
why it is recommended that the supply pressures are nominally equal.
Hot water temperature
too high
Temperature adjustment knob position
The mixing valve is supplied with the temperature adjustment
knob (
12
) set to
the highest setting, fully turned clockwise. To reduce the outlet temperature
the knob should be turned anticlockwise. However, if the temperature
adjustment knob is at full travel anticlockwise against the stop and the
temperature is still too high, remove the retaining screw (
10
) and adjustment
knob and reposition on the spindle to a similar position to its origional location.
The knob is now in a position that it can be turned anticlockwise to reduce the
temperature.
This operation may have to be repeated.
Cold water supply
Intermittent cold water supply
If the cold water supply pressure or flowrate fluctuates, the mixed hot water
temperature will also vary accordingly. This would occur where there are take-
offs prior to the mixing valve. If the supply cold water pressure or flowrate falls
the hot water temperature will rise. For sites known to have intermittent cold
water supplies a feedtank and booster pump should be considered. This is very
important in the U.K. where an air gap is required using a feedtank, after
changes in Water Bye Laws. If a booster pump is being used to supply an
adequate constant cold water supply pressure, then pressure switches must
not be used, since this can affect the operation of the steam/water mixing valve
station. Installation of a balanced line arrangement should be considered; (a
typical layout is shown in Figure 12).
Is cold water reaching the ball valve?
Check the pressure on the gauge prior to the ball valve. If this is zero or lower
than expected, check that all supply isolating valves are fully open; check
strainers are not blocked, (including steam/water mixing valve strainer); ensure
that the inlet check valve is installed with the direction of flow arrow in the right
direction. Ensure that the check valve cone is free to move.
Scale and dirt
Scale and dirt can hold the steam valve open. The mixing valve should be
cleaned and a maintenance kit fitted (see Section 5).
Fixed loading spring
Check that the correct fixed loading spring (
11
) is fitted. Replace with the higher
rated spring if necessary.
Steam pressure too high
If the steam supply pressure is 1.5 times greater than the highest fixed loading
spring the steam valve can leak. Replace with a higher rated spring. If this is
not high enough then the boiler steam pressure needs to be reduced with a
BRV or DP17 pressure reducing valve.
