Constant Pressure Systems
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 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 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. One of the affinity laws states, “The amount of energy consumed 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 affecting a neighbor's TV reception. Reliability, though improving, hasn't been what it should be and if the systems 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.
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 minus 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 constant pressure valves are factory-set and are not adjustable, and the others are field-adjustable, these 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 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 CP valve, let's run through a typical system cycle. Imagine a system with the CP 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 pressure reaches 50 psi, the valve will begin to modulate, maintaining 50 psi as the demand varies. Finally, when the household 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 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 orifice, the pump side of the CP valve is at deadhead pressure. Without a CP valve, the pressure switch limits the system pressure at the pressure switch shut-off pressure.
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.
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 back pressure, variable speed systems actually could use more energy than constant pressure valves.
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 differentiation between you and your competitors, which will add to your bottom line.
Next month, we will look more closely at variable speed pumping systems. 'Til then ….