Tech Topics: Constant-pressure Systems
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
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
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