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3.7 Protection from dirt
The valve should be protected by a pipeline strainer with 100 mesh screen. The strainer
should be fitted on its side to prevent the accumulation of water. The strainer screen should
be examined and cleaned at regular intervals.
3.8 Pressure sensing
For applications that require closer control, improved stability or maximum capacity condition
the internal balance pipe should be replaced by an external pressure sensing pipe (supplied
by others) as follows:
Remove the internal balance pipe assembly.
The resulting BSP tapping in the side of the body should be blanked using the plug provided
in the linen bag attached to the valve (which also contains the fitting instructions). The other
BSP tapping in the side of the pilot valve chamber, should be used to fit the external
pressure sensing pipe. This is suitable for the fitting of 6 mm O/D pipe. If suitable pipe is not
available the compression fitting can be removed and ¼" nominal bore steel pipe screwed
directly into the pilot valve chamber.
The pressure sensing pipe should be connected into the top of the reduced pressure main
at a point where in either direction there is a length of straight pipe uninterrupted by fittings
for at least 1 m or 15 pipe diameters whichever is the greater. It should be arranged with a
positive fall so that any condensate can drain away from the DP27. Where the size of the
reduced pressure main makes it difficult to maintain a fall when entering the top of the main,
the pressure sensing pipe may be connected in the side of the main.
3.9 Pressure gauges
It is essential to fit a pressure gauge on the upstream and downstream side so that the valve
can be properly set and monitored.
3.10 Continuous duty
For continuous duty applications where a constant steam supply is essential then a parallel
stand-by PRV station is recommended to allow for planned maintenance, see Figure 5,
page 13. Alternatively a bypass line can be used (see Figures 8 and 9, page 17). It is
important that the bypass should have a similar capacity to the pressure reducing valve
which is achieved by choosing an appropriately sized valve or by using a reduced orifice
in the line.
The handwheel should be padlocked to prevent unauthorised use, and when in use should
be under constant manual supervision.
The bypass may be arranged above or to the side of the main assembly but never below it.
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