A-4
Appendix A Model 224 Differential Pressure Unit
F igure A.2—Model 224 DPU components
required to actuate process instruments, such as recorders, indicators, trans-
mitters, controllers, and switches.
If the bellows are subjected to a pressure greater than the range of the DPU,
they move their normal travel range and a small amount of overtravel until
the overrange valve seals against the valve seat. As the valve closes, the fi ll
liquid is trapped in the bellows. Since the fi ll fl uid is essentially noncompress-
ible, the bellows are fully supported and will not be damaged regardless of the
overpressure (up to the full rated pressure of the instrument) applied to them.
Also, since dual overrange valves are used, full protection against overrange
is provided in either direction.
An additional convolution in the high-pressure bellows provides for expan-
sion or contraction of the fi ll fl uid relative to ambient temperature changes.
This extra convolution acts as an accumulator, permitting the fi ll fl uid to
change volume without materially aff ecting the pressure within the bellows or
changing the physical relationship of the two bellows.
DPU Install ation
General
Inspect the instrument after unpacking it to detect any damage that may have
occurred during shipment.
IMPORTANT: The DPU was checked for accuracy at the factory. Do not change any of
the settings during examination or accuracy could be aff ected.
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