ASHRAE Journal - September 2019 - 30

TECHNICAL FEATURE

Building Return Pressure (psig)

pressure blockage identification map. Buildings F and
FIGURE 5 Return pressure blockage identification map.
G are downstream from Building A, while Building E is
located upstream. Each building has a different demand
Bldg. G
flow and pump pressure requirement because of the
8 in.
building type, geographical location, and elevation.
Bldg. E
The "Return Pressure Blockage" simulation has two sce8 in.
narios: with and without PSV installed at Building A. The
Bldg. A
8 in.
building return pressure values only rely on primary
10 in.
8 in.
return pressure, and not primary supply pressure.
Figure 6 shows the building return pressure versus
10 in.
10 in.
the primary loop return pressure with PSV installed
at Building A. Whenever primary return pressure
Bldg. F
CHWR
12 in.
increases, the nearby return pressures at each juncCHWS
tion also increase. The different return building presBldg. w/PSV
Bldg. w/o PSV
sure values are all higher than their associated primary
return pressure values at each junction, which is related
to their physical location on the hydronic
FIGURE 6 Varying primary return pressure effect upon nearby buildings.
loop with respect to the plants. Building E
80
has a higher return pressure than Building
p Building A
Building E
Building F
Building G
p
75
A because it is upstream of Building A, and
p
the higher pressure value is consumed to
70
p
overcome pipe friction loss between two
65
buildings: A and E. There was no reduced
p
60
or completely blocked flow identified on
p
55
Building A's return flow pipe. Also Building
50
A does not block any flow from Buildings
50
55
60
65
70
F and G because water flow and pressure
Primary Return Pressure (psig)
converges at each junction. Therefore, it
was determined that the installation of the PSV had no
valve, not remotely. A wire sensor connects the controlimpact on the surrounding buildings.
ler to the direct digital control (DDC) to provide PSV
position data (percent open). The valve has an automatic
PSV Operation & Performance
modulation feature that reads the upstream pressure
Sequences of Operation
of the valve and then adjusts the position of the valve
CHW and LTHW distribution systems at the university
according to the pressure set point. The PSV operation is
consist of variable hydronic flow, primary pumping, and also directly connected to building pump control as an
building pumping as shown in Figure 2. The hydronic
equation (1):
system operation for plants and buildings is heavBuilding Pump Speed
ily related to ambient temperature and the university
1/ 2
 Ppri. supply − Ppri. return + ∆Pbldg . + Safety Factor 
(1)
schedule. Based on primary and building pressure pro=
4


files, along with flow profiles and pump affinity laws,
Pbldg . pump head


the sequences of PSV operation have been developed
where
to make sure the top coil receives sufficient pressure to
P_(pri.supply) = primary supply pressure
maintain the PSV upstream pressure at its PSV set point.
P_(pri.return) = primary return pressure downstream of
There are two types of PSV control, one is an automatic
remote control and another is a standalone control. The the PSV
∆P_(bldg.) = pressure loss when building pump delivers
PSV at the university has standalone control, so its preswater throughout the building, safety factor is for the top
sure set point has to be physically set or adjusted on the
30

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ASHRAE Journal - September 2019

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