12-volt diaphragm pumps are semi-positive displacement pumps, unlike pressure washer pumps which are positive displacement. Semi-positive displacement pumps are still self-priming, but the smallest inlet air leak can cause priming issues and lead to cycling. Excessive cycling can wreak havoc on your electrical system and cause failure of the
• pressure switch• the relay (on high amp models)• or lead to a pump that will not prime at all.
Unlike leaks on the pressure side, inlet air leaks are not visible most of the time and are harder to find. In addition, the more complex you make your inlet feed system - adding things like 3-way ball valves or diverting to multiple tanks with bulkheads tapped into each tank – you increase the odds of developing an inlet air leak. The reason for this is not only do you have a 3-way ball valve, you also have 3 threaded connections, 3 hoses slipped over the barbs and 3 clamps that can come loose. Each of these connections increases your odds of having a problem. To reduce potential problems, we recommend using the “drop-stick method”. A drop-stick is a 12' hose that comes off the inlet of the pump connecting to a 45-degree elbow threaded onto a poly pipe with a check valve and filter at the bottom. You simply drop the stick into the chemical tank to pull your mix and when you want to rinse the system or change chemicals you turn the pump off and pull the stick from the current container and drop it into your next container. Then turn the pump back on and continue your job.
The duty cycle means the amount of time the pump motor can run before overheating. As the motor works it generates heat. Most manufactures recommend not exceeding an external case temperature of 180 degrees. We recommend not exceeding a case temperature of 140. The easiest way to measure this is by your hand. If you can hold your hand on the pump for 6 seconds, or longer, the motor is not to hot. If you cannot hold your hand on the motor for 6 seconds, turn the pump off and let it cool before operation continues. The addition of a cooling fan, like we use in our Pump-In-A-Box (PIB) system, will help extend the duty cycle of the motor. If you have a PIB and your pump does overheat release the trigger on the gun, leave the on/off switch in the on position. This will let the fan run without the motor running reducing the time the motor takes to cool. Some common causes for a reduced duty cycle are improper wiring, improper wire size, corroded connections, under rated switches, faulty or undercharged batteries and excessive cycling.
Cycling occurs when your pump rapidly turns on and off. However, to understand cycling you first need to know how the pump works. All demand diaphragm pumps use a pressure switch to turn the pump on and off. On higher amp draw pumps, like the Fat Boy for example, they also use a relay. The relay is used because the pressure switch is not capable of handling currents above 15 amps. So, the pressure switch sends it’s 12-volt output to the relay to turn the pump on instead of directly to the motor. Cycling should not occur. However, if the pump starts to cycle it should be kept to a maximum of 10 cycles per minute. If your pump is exceeding this typically the cause is a restriction on the outlet side of your system. The restriction could be too small of a nozzle, a gun or ball valve that is partially open, a kinked hose or an obstruction in the system. Regardless of the cause, when the pressure switch reaches 85 psi (preset at the factory) the micro switch contacts open and that shuts the pump off. The 12v system is designed to shut the pump off when the gun is closed. When you have a restriction in the outlet side of the pump pressure builds which opens the switch turning the pump off. As soon as pressure bleeds off - the pump turns back on causing a rapid on/off cycle. There are two reasons this is bad. First, the pressure switch and relay only have so many on/off “life cycles” and you use those quickly with rapid cycling. The second reason it is bad is called “in rush current”. When a motor first starts it pulls more current to get started and then levels out. This in rush current can cause heat to build up in the relay contacts. Therefore, when you look at the rating on a relay it has two current rates one is in rush and the other is constant current. When your pump cycles off/on rapidly you will build up so much heat in the contacts of the relay they will weld together, and your pump will not turn off. Our Fat Boy pump comes from the factory with a 40/30-amp relay that is more than enough if the Fat Boy wiring is setup properly and cycling issue are avoided. The use of a larger relay would just be a band-aid for a cycling or wiring issue that could lead to a worse problem. Adjusting the pressure switch to shut off at a higher psi can also decrease the cycling issue. We do not recommend this because the higher psi draws more amps and the motor will exceed it duty cycle faster. In addition, even though the poly head on the pump is a good choice for resistance to chlorine it can harden the poly creating a situation that a higher psi can crack the pump head.
When it comes to hose size there is a point of diminished return. Will you get more volume from a ¾” id hose than you would from a ½” hose? In theory the answer is yes. However, what you may not be considering is column pressure. Each foot of hose holds a certain amount of liquid. As the hose diameter increases so does the weight of the liquid in the hose. This weight causes column pressure. As the hose is elevated it creates back pressure on the pump, which increases the PSI causing the pump to work harder to push the heaver column of liquid. The same goes for the inlet. The pump can only generate a certain amount of vacuum to self-prime. Manufactures of these pumps recommend a maximum length of hose on the inlet of 12’ with an inside diameter of ½”. Increase the hose diameter and you increase the weight of the column decreasing the length that the pump can pull. Throw in a small air leak and the pump will never get primed. This theory does not apply if you are flood feeding the pump meaning you have the bulkhead fitting in the bottom of tank. When it comes to pumping chlorine solutions, we all know that it is best to pull from the top of the tank to avoid leaks at the bulkhead.