Pump Control Strategies

Overview:

The purpose of pump controls is to protect the pump motor from excessive wear and tear leading to pre-mature failure.   Heat is generated when electric motors operate.  The pump motors require additional power upon start up to accelerate the pump and begin moving the water.  This additional power (cold cranking amps) generates extra heat.  Most motors associated with pumps for small water systems are designed to dissipate the additional heat associated with up to six starts per hour.  If a pump starts more than six times per hour, the additional heat generated will begin to shorten the life of the motor.

The life of a pump should be though of more in terms of the number of starts rather than the number of years or the total length of run time.  Indeed, one would expect a pump motor that runs continuously to last longer than one that starts and stops regularly.

Large Hydropneumatic Tank

One common pump control strategy on existing systems employs a large hydropneumatic tank with a pressure switch.  The pressure switch turns the motor on at the “cut-in” pressure (say, 40 psi) and turns the pump off at the “cut-out” pressure (say, 60 psi).  Pressure switches usually operate on a 20 psi differential.

The hydropneumatic tank acts as a large buffer.  The large, steel vessel is filled approximately 50% with water and 50% air.  As the pump is running, the air inside the hydropneumatic tank is compressed and the volume of air decreases as the pressure increases.  Then, when the pump is off, the air in the tank expands as there is demand for water and “pushes” water out of the hydropneumatic tank until the pressure drops to the point where the cut-in pressure is reached and the pump is re-activated.

There is a direct interface between the water and air.  Therefore, over time, air dissolves into the water.  Additional air must be periodically added to the hydropneumatic tank to maintain the ~50% air to water ratio.  This is typically accomplished automatically with an air compressor.

Advantages of hydropneumatic tanks include the fact that they typically have a very long life, are easy to troubleshoot (especially when equipped with a sight tube), and are a long-standing, well-established technology.  The primary disadvantage is that current regulations have made new hydropneumatic tanks cost prohibitive for most small systems.

Bladder Tanks

Another method for controlling pump cycling is through the use of bladder tanks.  Bladder tanks operate on the same principal as hydropneumatic tanks with two distinct differences:  1) They are smaller in size and 2) They have a rubber bladder that provides a barrier between the air-water interface.

Bladder tanks operate in conjunction with the same 20 psi differential pressure switch.

The advantage of bladder tanks is that they are individually relatively affordable.  They also do not require an air make-up system because the air is “captive” inside the bladder (or in some cases, the water is captive inside the bladder).

The disadvantages of bladder tanks is that they have a relatively short life (avg. 10 years), are difficult to troubleshoot, and collectively can become very expensive.  In order to determine if the bladder had ruptured and the tank failed, it must be drained down and the pre-charge pressure on the tank.  Failed bladder tanks often go for months, or years without being identified as bad.  Failed bladder tanks no longer provide the pump protection needed and also contain stagnant water, which is an excellent habitat for microbial growth.

In general, we see bladder tanks as a viable option if four or less are required.  If more than four bladder tanks are necessary, then the operational costs of regularly identifying and replacing failed tanks makes them a less attractive alternative.

Cycle Stop Valve

A cycle-stop valve (or more accurately a variable pressure flow control valve) is a mechanical device that restricts flow as a function of downstream pressure.  As the pressure downstream of the valve decreases, a diaphragm operated valve opens and allows more flow through the valve.  Conversely as the downstream pressure increases, the valve closes and restricts flow.  Most of these valves still allow a minimum flow though, even when fully closed.

As with the hydropneumatic and bladder tanks, this valve operates with a 20 psi differential pressure switch.  It also requires a single bladder tank to provide a small degree of cushion.  The bladder tank and pressure switch are located downstream of the cycle stop valve.  As the bladder tank fills and the downstream pressure begins to approach the “cut-out” pressure of the pump, the valve closes and allows only minimal flow through the valve, filling the bladder tank slowly and lengthening the cycle.

Power consumption in pumps is a function of amperage.  The amp draw is directly proportional to water flow.  Therefore, electricity is not “wasted” by the restriction, except for the fact that the pump is likely operating at a less efficient point in the curve when pumping at a lower rate.  Typically, the power savings associated with fewer starts more than offsets the fact that the pump operates for more time at a less efficient point on its pump curve.

The advantages of the cycle stop valve are that it is less expensive to install, requires little maintenance, and typically has a long life.  The primary disadvantage is that it relies on a single bladder tank.  This tank must be monitored carefully, or the pump could short cycle as the tank fails and the air in the bladder slowly dissolves into the water.

Variable Frequency Drive

Another option is the use of a Variable Frequency Drive (VFD).  This control method uses a pressure transducer which sends an electrical signal to the controller.  The controller then varies the frequency of the power to the pump which changes the rotational speed of the pump motor.  Therefore, as the demand for water increases, the speed of the pump also increases to supply more water and maintain a constant pressure.  A small bladder tank is required as a cushion for the controller.

The primary advantage of the VFD system is that it is able to provide a constant pressure.  The mechanical equipment is minimal, reducing the necessary footprint in the pumphouse.  One of the disadvantages is that the control system can be a little complicated, although most well drillers and pump installers are becoming comfortable with them.  They also have the same dependence on a single bladder tank, although, in theory, the controller has built-in electronic pump protection to prevent damage to the controller and pump motor.  This system requires the pump to start and stop much more often; however, as a VFD, the pump has a “soft start”, which does not generate the heat of a traditional “hard start”.

This option requires a three-phase pump motor to operate.  If only single phase power is available, there are controllers that convert the incoming single phase power to three phase power which is compatible with the pump motor.

Reservoir

The last pump control method is the use of float switches in a reservoir.  The pump fills an above-ground water tank open to the atmosphere.  As the level in the water tank reaches the “full” line, a float switch turns the pump off.  As the water is used and the level of water in the tank drops, the float switch turns on at the designated level.

This is an extremely effective method of pump control and has the advantage of providing water during a power outage (assuming gravity feed); however, unless storage is already required, or natural elevation differences are adequate to provide the pressures needed by the system, then a reservoir is probably not a cost effective method of control.