This is part two of our jet pump series. Last month, we said that a jet pump was a centrifugal pump with a jet assembly (also called an injector), either integrally attached as in the case of shallow-well jets, or remotely attached as in the case of deep-well jets. We further explained that a shallow-well jet pump pulls the water up to the surface, and is limited by atmospheric pressure to a depth of about 25 feet. Deep-well jets – with the jet assembly located down in the well below the water level – push the water to the surface and can pump from as deep as 200 feet.
The operation of a jet pump requires a certain amount of water to be circulated back through the jet assembly as drive water. In a shallow-well jet pump, the amount of water diverted is governed by the internal construction of the pump, and is not adjustable because the narrow range of pumping conditions can be met without having to adjust the amount of drive water. The performance of a deep-well jet pump, on the other hand, can be optimized to specific pumping conditions through the use of a control valve to create the right amount of back pressure, and this article will explain their function and how to adjust them.
Types of Control Valves
Control valves can be as simple as a manually adjustable flow restricting valve built into or attached to the outlet of the pump, or as complex as a pilot or manually operated spring-loaded diaphragm valve like a pressure regulator, attached to the outlet of the pump (see Figure 2). Used manually, diaphragm valves operate just like any manually operated valve to restrict the pump’s discharge to create backpressure to force drive water to the jet. Their advantage over a simple gate or ball valve is that they can be adjusted more precisely and hold their settings better. The disadvantage of manually adjusted backpressure valves is that they are set to a specific pumping level, and if the level changes, the pump’s performance will be diminished.Some jet pump manufacturers offer an automatic backpressure valve that has a small line connected to it from the suction side of the pump. Vacuum regulates the valve and thus the output of the pump, to provide optimum performance as the water level in the well goes up and down. When the water level is high, the vacuum on the suction side is low, and the valve is wide open. As the water level drops, the vacuum increases, and the valve begins to close to circulate more drive water down to the jet to provide the additional lift needed to pump from a deeper level. Automatic backpressure valves are factory-set, and need no field adjustment.
How to Set Manual Control Valves
Before setting the valve, the pump must be primed, and the system purged of air. To do this, manually fill the pump and piping down to the jet with water, and turn on the pump with the control valve open. You should see pressure at the outlet of the pump, indicating it is primed. Then, begin to close the control valve to purge air from the rest of the system.Method one, using a pressure gauge – Some manufacturers recommend adjusting the control valve to provide a specific amount of backpressure between the pump and the control valve, usually 20 psi. To set it this way, run the pump until the well has stabilized and the pumping level established. Once there, set the control valve to give you 20 psi on the outlet side of the pump before the valve.
Method two, using an amp meter – With the pump pumping at the stabilized pumping level, use an amp probe to set the control valve to give you an amp reading equal to the name plate amps of the pump. Don’t use service factor amps because you could overload the motor if the pumping level rises in the well.
Method three, using your ears – This is a little less orthodox, but one used by many old-time jet pump installers. Open a faucet near the pump so there is no backpressure from the tank, and so you can observe and estimate the discharge flow rate. Slowly open the control valve until the flow rate appears to maximize. If you continue to open the valve, you will reach a point where the pump starts to rattle, as if it is full of marbles, due to cavitation. If you continue to open the valve beyond this point, the pump will loose prime, and you’ll have to start over. Back off one turn on the backpressure-adjusting screw, and tighten the lock nut. You’re done.
You can check your setting with a pressure gauge on the discharge and a bucket to measure flow. Watching the pressure gauge, restrict the discharge flow until the gauge reads 30 psi. Using the bucket and a stop watch, measure the flow rate at this pressure, and verify that it agrees with the manufacturer’s performance tables for the pump/injector package you have at 30 psi. If not, play with the control valve adjustment to maximize the flow.
Pumping from a Slow-producing Well
Because air entering a jet pump system causes it to stop pumping and necessitates re-priming the system, it is important to take precautions to ensure the water level in the well does not drop below the inlet of the foot valve. To prevent air from entering the foot valve in a slow-producing well, try the following. Because a pump cannot lift water more that 34 feet maximum due to the limitations of atmospheric pressure, installing a 34-foot section of suction pipe below the injector will preclude the possibility of the water level being drawn down below the foot valve. As the well draws down, increasing the suction head on the injector, its pumping capacity slows down until it matches that of the well. With the foot valve set at 34 feet below the injector, even in a well with no production, air cannot enter the foot valve because the injector is not be able to lift the last foot of water to empty the well.Summing Up
Shallow-well jet pumps have the jet assembly built into or attached to the suction port of the pump, and are designed to pull water out of a shallow well (less than 25 feet in depth). Deep-well jet pumps have the jet assembly down in the well so they can push the water up out of the well, and are not confined by the 25-foot limit. They can pump water from as deep as 200 feet. Convertible jet pumps are designed with the injector assembly removable and small enough to fit into 4-inch and larger well casing. They can be used as either a shallow-well jet pump with the injector assembly attached to the pump, or as a deep-well jet pump with the injector assembly down in the well, but cannot pump from as deep as a dedicated deep well jet – 130 feet is about as deep as they go.Properly sized, installed and adjusted, a jet pump can be a real workhorse, and provide years of trouble-free service. Millions of people around the world depend on jets for their water.
Hopefully, this series has given you a better understanding of the inner workings of these thoroughbreds. Next month, we learn how to determine the pressure and flow requirements of a pumped water system. ’Til then ….
ND
