HVAC Special Valves Part 2 DPCV

DIFFERENTIAL PRESSURE
CONTROL VALVES (DPCV)
As the name suggests, the valve is designed to control differential pressure. It is installed
in a section of pipework and linked to another via an impulse (capillary) tube.
There are 2 designs of DPCVs, offering different functions;
1. Normally Open (NO) – closing on rising pressure
2. Normally Closed (NC) – opening on rising pressure
The Normally Open DPCV is used to control the differential pressure across a single
sub-circuit within a complete installation. As system pressures increase the DPCV closes
to absorb the additional available dynamic pressure. These are covered comprehensively
within this guide in the following sections.
The Normally Closed DPCV is used to create ‘bleed by-passes’ between flow and
return pipework. This type of DPCV opens as system pressures increase to allow flow.
Typically they are used to allow for minimum pump turndown and remove stagnant
pipework as 2 port control valves close allowing continued distribution of water
treatment. These are not covered in this guide.
DPCVs can be installed in either the flow or return pipework, provided that the correct
design of DPCV is selected to suit its installed position, ie a return mounted DPCV should
only be installed in the return pipework.
For ease and consistency, in this publication, DPCVs will be shown in return pipework,
which is the preferred installed position
Why are DPCVs required?
With the move from constant to variable volume system design,
the stability of the system is lost.
With a constant volume system, 3 or 4 port control valves are used to control the flow
and subsequently, the heat output of the terminal. Generally, a constant volume of water
is pumped around the system, being diverted through the by-pass by the control valve
if no heat output is required, giving constant pressure drops in the distribution pipework
and terminal circuits, with constant speed (flow volume) pumps being used.
The advantages of this type of system design meant easy sizing of control valves, with a
constant authority and system balancing. The disadvantages however, mean that water
is being pumped around the system even when there is no demand. Also, as water is
being diverted, ie returned ‘unused’, the required differential temperature (Δt) across the

boiler / chiller is compromised, therefore, reducing system efficiency.

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