During
the last several years, our industry has seen a lot of hype about
constant-pressure systems. Some of the information has come from the
manufacturers of pump systems with variable-speed motors, and some from
manufacturers of constant-pressure valves. Both types of systems address an
issue that has been a part of pumped residential water systems from the
beginning – household pressure varies as the pump cycles on and off because we
use pressure switches to control the pump. They typically have a 20-psi spread
between the turn-on pressure and the turn-off pressure, which not only shortens
the life of the pump system, but also can result in shower pressure that varies
from a gush to a dribble to a gush during the course of the
shower.
Variable-speed systems can provide constant water pressure over a fairly broad
range of flow rates by electronically changing the speed of the motor as the
demand changes to keep the system pressure constant. The advantages to the
end-users include the elimination of annoying pressure fluctuations in their
homes, the possible use of a smaller pressure tank if space is a problem, and
if the pump is oversized for the application, they may reduce the amount of
electricity used by the pump because the pump motor will be running at a
reduced speed much of the time. This is due to what is called the third
affinity law, which states “The amount of energy consumed by a pump motor
varies by the cube of its speed.”
However, the variable-speed systems do not come without baggage. Some are
noisy, both audibly and electrically, the latter possibly affecting a
neighbor’s TV reception, cordless phones and radios. Reliability, though
improving, hasn’t been what it should be, and if they do break, “repair” often
means replace, since the problem area usually involves the electronic
circuitry. Simple fixes like cleaning the bugs out of the pressure switch or
filing down the points don’t hack it with these systems. You will need a
different set of spares on your service truck, and perhaps a different service
man. And finally, if the pump is sized such that it runs at close to full speed
most of the time, the power losses in the VFD may result in more energy per
gallon being used. Even with these shortcomings, some dealers swear by these
systems, so if you are the adventuresome type, jump in. Just keep your eyes
open.
An alternative method of providing your customers with constant pressure while
extending the life of their pump by is to add a constant-pressure valve (CPV)
to a conventional water system (see Figure 1). These valves provide a constant
pressure over a wide range of flow rates. The pressure is held constant by the
use of a spring- or pilot-operated diaphragm assembly, which senses the
pressure on the load side, and modulates the opening of the valve as the demand
varies.
Constant-pressure valves are plumbed between the pump and the tank/pressure
switch. The maximum pressure on the pump side of the valve will be whatever the
pump provides at deadhead (maximum pressure), less the pressure loss due to the
elevation above the pump, so make sure the piping and any valves and fittings
on that side of the constant pressure valve can take the pressure
Constant-pressure valves control the system pressure on the downstream side of
the valve, i.e. to the tank, the pressure switch and all points of usage in the
house. Some CPVs are factory-set, and are not adjustable, and the others are
field-adjustable, the latter having an adjustment screw to raise or lower the
system pressure. What makes constant pressure valves work in a pumped water
system is a small bypass that allows a trickle of water to bypass the valve
assembly when the household demand stops. The following example shows how they
work.
Remembering that the pressure switch is on the downstream side of the CPV,
let’s run through a typical system cycle. Imagine a system with the CPV set at
50 psi and a 40/60-pressure switch. The pump is off, and the tank is at 60 psi.
When someone turns on the shower, the first few gallons come from the tank as
the system pressure drops from 60 psi to the pump turn-on pressure of 40 psi.
This allows the use of the total amount of drawdown in the tank, and prevents
the pump from having to start for such things as filling an icemaker or
brushing teeth. When the pump does turn on, and hopefully it has more capacity
than the demand, the demand is met, and the pressure tank will begin to refill.
Once the pressure reaches 50 psi, the valve will begin to modulate, maintaining
50 psi as the demand varies. Finally, when the household demand stops, the
valve closes, and the bypass lets enough water through to cool the pump motor
and slowly fill the tank. The downstream pressure will slowly increase to 60
psi, at which time the pressure switch turns off the pump, and we are ready for
another cycle. See Figure 2, the pressure vs. time graph for a pictorial
representation of a constant-pressure valve cycle. During the time when there
is no household demand, and the tank is filling through the bypass, the pump
side of the CPV is at deadhead pressure of the pump. Without a CPV, the
pressure switch limits the system pressure to the pressure switch shut-off
pressure.
During this pump-on cycle, the demand could vary considerably, say from 1 gpm
to 20 gpm, as the usage goes from brushing teeth to showers to watering
gardens. The pump stays on the whole time, as long as there is more than about
3⁄4 gpm demand (varies with manufacturers). Pump and tank cycles are minimized,
which can increase the life of the pump, motor, pressure tank, switches and
relays. Also, reducing the pump’s flow with a valve can reduce the amp draw –
much like what happens when reducing the pump’s speed with a VFD.
Constant-pressure/anti-cycling valves can be a worthwhile addition to any
pumped water system to reduce cycling and provide uniform system pressure. They
are particularly useful in systems, both residential and commercial, where
there is a large variation in demand. They can save money by minimizing the
number of starts and stops, which are costly, both in terms of power consumed,
and in motor and tank life. They also can reduce the size of the pressure tank
required.
As to which type of system offers the best efficiency, both sides claim a win.
Though it may not be the final word, an article on the subject in the February
2005 World Pump magazine says that, depending on the operating conditions of
the system and in particular the system backpressure, variable-speed systems
actually could use more energy than constant pressure valves. And, some experts
contend that running a pump at full speed at its best efficiency point uses
less energy per gallon than either a VFD or a CPV. We likely will see more
studies on this issue in the future.
The bottom line: Whether you opt for the variable-speed drive route or the
constant-pressure valve method, offering constant “city-like” pressure to your
customers can make you a hero in their eyes, and provide some differentiation
between you and your competitors, which ultimately will add to your bottom
line.
We have talked a lot during the last couple of years about pump curves. Next
month, we will explore well curves. ’Til then ….
ND
Tech Topics: Constant-pressure Systems
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