Constant Pressure Systems
During the last several years, much has been written about constant pressure systems. Most of the information has come from the manufacturers of pump systems with variable speed motors. These 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 pressure constant. The advantages to the end users include the elimination of annoying pressure fluctuations in their homes and an increase of motor life by reducing the number of damaging on/off cycles. Additionally, these systems may allow the use of a smaller pressure tank if space is a problem. And finally, they likely will reduce the amount of electricity used by the pump because of the affinity law, which states, "The amount of horsepower drawn by a pump motor varies by the cube of its speed."
However, variable speed systems do not come without baggage. Some are noisy, both audibly and electrically, the latter possibly effecting a neighbor's television reception. Reliability hasn't been what it should have been, 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. 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 is to add a constant pressure valve to a conventional water system. These valves provide a constant pressure over a wide range of flow rates. The pressure is held constant by the use of a spring-loaded diaphragm assembly, which senses the pressure on the load side and modulates the opening in the main flow orifice as the demand varies.
Constant pressure valves are plumbed between the pump and the tank/pressure switch. The pressure on the pump side of the valve will be whatever the pump provides at deadhead, less the pressure loss due to the elevation above the pump. 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 constant pressure valves 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 orifice that allows a trickle of water to bypass the spring loaded diaphragm 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 constant pressure valve, let's run through a typical system cycle. Imagine a system with the constant pressure valve 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. When the pump turns on - and hopefully it has more capacity than the demand - the pressure tank will begin to refill. Once the downstream pressure reaches 50 psi, the valve will begin to modulate - maintaining 50 psi as the demand varies. Finally, when everyone in the house is through using water, the demand stops, the spring-loaded diaphragm closes, but the bypass orifice lets enough water through to slowly fill the tank. The downstream pressure will slowly increase to 60 psi, at which time the pressure switch turns the pump off and we are ready for another cycle. See the pressure vs. time graph for a pictorial representation of a constant pressure valve cycle.
During this pump-on cycle, the demand could have varied from 1 gpm to 20 gpm as the usage went from brushing of teeth to showers to watering of gardens. The pump stayed on the whole time - as long as there was more than a certain gpm demand (varies with manufacturers). Life shortening pump and tank cycles were minimized, as was the amount of inrush current needed for multiple pump starts.
Constant pressure/anti-cycling valves can be a worthwhile addition to any pumped water system when trying 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.
Whether you opt for the variable speed pump 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 differention between you and your competitors - putting a few more coins in your cords.