3 words about cushion tanks on domestic water booster systems…DON’T DO IT. Instead, specify a variable speed system, save the owner the cost of the tank, the freight to ship the monster, the labor to put it in, the piping and PRV required to send the water back into the system and the headache and expense of repairing and maintaining the bladder inside the tank. I have only sold 4 tanks in the last 10 years for domestic water booster systems and these I sold at the insistence of the engineers who swore the systems would not work unless they had a tank. We put them in as requested only to return later to valve them off, remove them  and regain the valuable real estate they took up. Trust me I do this every day, the systems work without tanks, provided they are controlled with variable speed drives.

Variable speed systems don’t need tanks to shut down, or to cushion the system from hammer. I was in a lab in California for a week where they “Proved” you cannot shut down a variable speed system without the use of a tank. While they were correct in the lab, they needed to get out into the field more, because we don’t sell many systems which operate in a 20’ test loop. Buildings have plenty of storage and air pockets in the piping system. While tanks are a necessary evil for constant speed systems so they can keep the pumps off for longer periods of time. If you really want to save money, time, and electricity specify a GREEN variable speed pump system.

I had a customer tell me he did his homework comparing constant speed systems with variable speed systems for domestic high rise applications, and he didn’t see any benefit to the variable speed system.  He didn’t think he could get a 10% downturn in speed therefore it wasn’t worth it.  Let’s take a look at it. When you start off with a constant speed system you have to account for the worst case scenario, it’s only right. So if your system is rated 400 gpm for a 300’ building, what are your friction losses on the suction side? Most allow 10 psi through the meter and backflow preventer/double detector check. It might be 7psi but most figure 10 psi to be safe. If your street pressure is 45 psi, most figure 35 psi, for worst case scenario.

Let’s add it up, 45 at the street, minus 10 worst case, then minus 10 psi through the backflow and meter. We want 50 psi on the roof, plus 129 psi to overcome static going up, plus 9psi friction through the pipe, and 3 psi losses in friction though the pump system. We need a system 400 gpm @ 166 psi boost. 

Now let’s look at a realistic best case scenario, using a constant speed system, with a tank, and PRV’s to regulate pressure. You have only three people using water at 3 am, the pump starts, it winter time street pressure is 50 psi, you have no friction losses going up because nothing is going up, you have 1 psi losses through the meter and backflow, your constant speed system is dying because it wants to run all out full speed, that’s the way it works. The PRV’s are burning up all the excess energy because you don’t need all of the water the pumps have to give. The minimum run timer has locked the pumps into running for ten minutes so the motors don’t burn up prematurely, the solenoid valves are popping because the pumps have a minimum flow requirement of 10 gpm, the seals are leaking prematurely because the water is hot because it had nowhere to go, then the carbon got thermal shocked when the solenoids dumped. What a nightmare, did you do your homework correctly?  If you had a variable speed system, the pumps would run only enough to satisfy the system, yes take a look at the system friction losses now, with a variable speed system if suction pressure goes up, the pumps slow down, when the pumps slow down the friction losses through the meter and backflow preventer drop off, when 3 people are using water at 3 am there are no friction losses in the piping going up the building. But with a constant speed system, you can’t take these calculations back, you have to allow for worst case, and your pumps can’t react to changes in the system, only full steam ahead.

Variable speed systems can, and do allow for this. So we now have 50 psi coming into the building 2 psi losses through the meter/backflow/pump system. I only need 131 psi to maintain 50 psi on the roof instead of 166, do you know how much electricity that is? It’s a 35 psi difference, not to mention automatic built in soft start/ soft stop, which eliminates water hammer. You size the system the same way you would a constant speed system, just skip the tank, and the variable speed drives will do the rest.

Stay tuned for the math on how much electricity the above system was wasting.