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Imagine that you have a basic water pump with the discharge connected to a short piece of pipe with a pressure gage and a gate valve in it.
Suppose you shut the valve and turn the pump on. The pressure gage will reach some maximum and the flow will be zero since nothing can get past the valve.
If you now open the valve a little bit there will be some flow and the pressure gage reading will drop a bit. As you open the valve more and more the flow increases and the pressure drops.
When the valve is fully open you will get the maximum flow and the pressure will be zero.
If you plot these readings on a graph with flow in gallons per minute on the bottom ( x axis) and pressure measured in feet of water on the left ( y axis) you will have a pump curve.
This curve is determined at the foundry where the impeller is cast and nothing, within reason, will change it. It is used to tell what flow the pump will produce at any given head pressure.
Note that pressure is a measure of resistance to flow. It is the total resistance in a system that determines the flow output of the pump. Without any resistance, the pump delivers its maximum flow. Pumps are quite stupid and totally unaware of what you intend them to do. They deal only in what is real to them and they don't know anything except what is at their suction and what is at their discharge.
Pump curves are customarily marked in feet of head because any liquid pumped will be lifted to the same height. This is true whether it is oil, water or molten lead. ( Pressure and energy required are another thing altogether). For water applications,
( pressure in PSI) = feet x .433 or, ( head in feet) = (pressure in PSI) x 2.31
for example: 60 PSI = 138.6 ft
A centrifugal pump is basically a rotating shovel for liquid. Each rotation it expels a donut of liquid. The volume of the donut represents how many gallons per minute the pump delivers.
The liquid is thrown off the vane tips. At the center of the shaft there is no relative motion but the liquid there moves out to replace the liquid thrown off the tip. This creates a low pressure area at the shaft center, which is also the liquid inlet (pump suction). External pressure on the liquid supply, which may only be atmospheric pressure, forces more liquid into the pump suction.
The amount of velocity of the liquid as it leaves the pump determines how much head ( or pressure) the pump will develop. This is determined by the diameter of the vane and how many revolutions per minute it makes (shaft speed). h=V5/2g for you engineers.
Pumps are designed around a flow rate which determines how big the case must be to efficiently handle the quantity of water desired. This is indicated by the inlet and outlet pipe sizes but there can be considerable variation.
If it gets impractical to make an impeller large enough in diameter to get the head desired, two or more impellers ( stages) can be incorporated into one housing. This is very common in water well pumps where the pump must go down a hole. It is very hard to get a pump with a diameter larger than the hole to go in without the use of a hole stretcher. This device is large, dangerous, and illegal in most states. Ask your well driller.
A pump converts the energy used to turn its shaft into water energy. The efficiency with which it does so determines what it costs to move the liquid. It takes the same amount of energy to lift one gallon of water two feet as it does to lift two gallons one foot.
The formula for water is:
Horsepower = (Gallons per minute) x (Total dynamic head in feet)
3960 x pump efficiency
This is derived from the definition of horsepower and is always true. For liquids other than water, the specific gravity must be used but that is for another web page. 3960 is a conversion factor to make the units come out right. The word dynamic means the total head is figured when the liquid is in motion and so friction losses must be included.
The low pressure generated in the suction of a pump will lift water up into the pump. There are some limitations.
The basic limit is atmospheric pressure which is 33.9 feet. No pump can lift water up into it more than 33.9' even if you put a million horsepower motor on it. Once you get the water into the pump you can blast it to the moon if you want to put the energy in, but the suction side is limited. The pump is sucking on a big straw. If you run out of suction power the water will just sit there at what ever point you ran out.
The other limit is the amount of energy each individual pump needs to get the water into its suction area and turn up the blades. This is essentially an internal friction factor and it can get quite large, even much higher than atmospheric pressure. This is called Net Positive Suction Head required, or NPSH. If this requirement is not met, the water will form bubbles of water vapor ( not air). These bubbles will move with the liquid into the higher pressure areas of the pump and instantly collapse. When they do, the liquid around trying to rush into the void at infinite velocities. This erodes metal and makes a sound like pumping rocks. It causes vibrations, loss of capacity and severe impeller wear. It is a very bad thing for a pump.
The third limitation is liquid temperature. The warmer the liquid, the easier it is to go into the vapor phase and thus cavitate more easily. It is impossible to lift water that is boiling.
This is normally only a concern in boiler feed applications and industrial applications.
In a water pressure system a tank is primarily a hydraulic accumulator. Its main function is stop the pump going on and off excessively. Storage of water is secondary.
The tank can only serve its purpose if there is an air pad in the tank which has been compressed by the pump. If there is no air, the tank is just a fat piece of pipe. Pressure in the system drops immediately when any amount of water is used.
There are two basic types:
1. Captive air, or bladder tanks
2. Galvanized or epoxy coated non-bladder tanks
The bladder tanks have a rubber bag inside them which separates the air and water and is pre-pressurized. Since air under pressure dissolves in water, this means you don't have to add air or drain the tank. The pre-pressurizing gives you more useful storage in the same physical space. This makes them cheaper per gallon of draw down than the plain tanks, especially the larger ones. It also means they are lighter and easier to install. However, there are many manufacturers of wildly different quality levels, and all of the bladders will fail sooner or later.
The plain tanks probably have a longer life. They do have to be kept full of air. This can be done with a submersible pump by using an air charging system consisting of a down hole bleeder valve, a surface snifter and an excess air release on the tank. The snifter valves are cheap but prone to plugging.
All plain tanks can be drained and air let into them if you remember too do it. It is necessary to break the vacuum and actually let air in and not just drain the tank from the bottom or a water faucet.
Torque arrestors are recommended for installations that use PVC drop pipe for three reasons. The first is that most pumps rotate in a direction that will cause the drop pipe to unscrew. A torque arrestor keeps the pump snug in the well casing reducing the possibility that the pump starting torque will result in any right hand thread loosening. It is attached to the drop pipe right at the pump, then it is expanded until it fits snugly in the well casing.
The second reason a torque arrestor is used is to keep the pump centered in the well. Not all wells are straight, a pump that is running up against the well casing may experience motor cooling problems and hydraulic imbalances. A pump hanging on plastic pipe will tend to move around and collide with the well casing which can result in abrasion to the pump and motor housings, damaged wire or damaged well casing.
The third and most important reason is that fatigue from repeated start-up torque will occur in the PVC and can cause the pipe to break.
Air charging systems require an air snifter valve, a bleeder orifice and an air release valve.
An air release valve is required on air charging systems to maintain the correct water to air ratio in the tank. In an air charging system, excessive air is pumped into the tank on each cycle. There are usually two check valves installed in one of these types of systems, one on the pump and one on the surface pipe. When the pump stops, both check valves close. The water in the pipe between the air snifter valve and the orifice plug bleeds back through the orifice and down into the well. When the pump starts again, the air that replaced the water in the pipe between the snifter and bleeder orifice is pushed into the tank.
Excessive air will continue to lower the water level in the tank, as the water level is lowered, the air ejector float is also lowered which opens the release valve and lets excessive air out until the release valve setting is reached. When the pump starts again, the water level rises and raises the air ejector float which closes the releases valve before the water level reaches the valve.
In most domestic pumping systems there is a pressure switch that controls when the pump starts and stops. That pressure switch is typically connected to the system with a pipe nipple. It is important to protect that connecting nipple from freezing because if it does freeze while the pump is running, the switch will never see the system pressure reach the shut off point and therefore will not turn the pump off. Dangerously high pressures can be developed depending on the head a specific pump is capable of achieving. Pressure tanks will detonate if the pressure in them gets too high. An inexpensive way to protect pipes from freezing is with pipe insulation which is usually sold at hardware stores. Burlap or fiberglass insulation are also commonly used surface pipe insulators .
Slightly more than half of all submersibles sold are two wire and Franklin Electric says the failure rates are the same or slightly better for two wire motors.
Local preferences govern, if you choose the type not common in your area all your neighbors will make fun of you.
We will supply either, two wire pumps are easy to install.
A low well is one that produces less water than normal required demand rates. Usually, a well that yields less than 10 GPM is considered low yield although in some poor water areas, that would be a good well.
The difference between immediate needs and well yield is made up by pumping from storage. This can be an above ground non-pressure tank, an above ground pressure tank, or the water in the well casing.
Wells that are just marginally low are most likely to be buffered by pressure tanks. Pressure tanks are more expensive per gallon stored, but you need one anyway, and in a close case it is simpler and cheaper to increase pressure tanks.
In extreme low yield wells, less than 2 GPMs or so, an above ground reservoir of some type is required. Water usage is very spotty. It peaks in morning and evening hours for
washing and cooking. The well gives a sustained 24 hour yield which can be pumped up and stored and used at peak times by means of a booster pump and small pressure tank. A 3/4 HP jet pump and a captive air tank and a check valve so water canít backflow from the pressure tank to the storage tank are all that is required.
The storage tank can be an above ground steel or poly tank, or a below ground cistern. This system uses smaller submersible pumps since they do not have to develop pressure other than that required to lift the water to the surface plus the top of the tank. A level switch in the tank controls the well pump.
The third method is using the well itself as the storage tank. This works well if the end user understands what is going on. It is also the most common cause of submersible pump mis-applications and consequent short life and excessive costs.
The pump will be designed to work within a flow range and a pressure range. Pumps have an operating range of 200' to 300'. This means that this is all the water you can use without getting out of the range the pump was designed for.
The manufacturers use up all the horsepower available for each condition set. A 5 G.P.M. design pump will not necessarily be suited for a 5 G.P.M. well. The pump will be designed to pump from a deep depth to use up the horsepower. The essential stupidity of the pump prevents you from explaining the situation to it. The water comes up inside the pump to the exact same level as it does outside the pump without the pump running. If the water level is high when the pump starts, as it would be most mornings, the pump may well go into an up thrust condition which shortens the life of the motor and the pump. It is also inefficient from a power standpoint.
A typical case is a 500' well yielding 3 G.P.M. with water standing at 30'. A pump is chosen that can lift water from 500'. This pump runs off itís performance curve at 200' and so at the 30' startup it up thrusts and pumps inefficiently. In addition, the water flow in the well may all come from above thus reduce the motors ability to cool and shortening itís life. ( Franklin motors in sizes below 2 HP are able to operate satisfactorily this way but will last longer if cooled with water flow from below.) It is also true that it costs more to pump water the greater the distance you have to lift it to the surface.
All pumps in low yield wells can benefit from protection by a Franklin Pumptec which will shut the pump off if it runs dry. No pump can withstand prolonged running dry without damage.
My conclusion is that the proper way to use the well as a reservoir is to compare the storage capacity of the well in 200' with your expected daily use.( A six inch well stores about 300 gallons in 200'. The average usage of a family of four is 200 gallons per day plus irrigation needs.) Select a pump that will work well in this range and live within these limits. Protect the pump with a Pumptec. The smallest pump that will meet these requirements is the correct pump from installation cost, operating cost ,and durability standpoints.
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