$150 VFD Commodities
Since 1993, we have been teaching the benefits of Cycle Stop Valves (CSV) over Variable Frequency Drives (VFD's), large pressure tanks, and water towers. Pressure tanks and water towers have become much more expensive but, VFD's have become a cheap commodity. A home owner or end user can purchase a VFD for a 2 HP for less than $200.00, and a VFD for a 100 HP for less than $3500.00. Like the electronic calculators of the 70's, technology and competition have reduced VFD's to a mere fraction of their previous cost.
With all the recent advances in VFD technology, nothing can change the fact that centrifugal impellers lose head by the square of the pump speed. Maintaining a constant pressure or head, limits the minimum pump RPM that will still produce the head or pressure required. Not being able to slow the RPM very much, means VFD's offer very little energy reduction for constant pressure applications. It has always been true that the natural linear reduction in pump horse power when using a CSV, is almost exactly the same as the exponential reduction in pump horse power when using a VFD. Energy reduction has never been a reason to choose VFD controls over CSV controls. Even though high cost is no longer an issue with VFD's, there are still many reasons why Cycle Stop Valves are superior when it comes to constant pressure pumping applications.
(Comparisons)
There are many advantages to the constant pressure technology of the CSV over large pressure tanks, water towers, and VFD controls. Reducing the RPM with a Variable Frequency Drive or VFD causes a loss of head by the square of the speed. Therefore, a VFD may only be able to reduce power consumption at low flow rates by a couple of percent, over restricting a full speed pump with a valve. The parasitic losses and loss of motor efficiency from a VFD, causes about 5% more power consumption at full flow than when using across the line controls. Using more energy at high flow and slightly less at low flow, the average power consumption when using a VFD is almost exactly the same as across the line with valve control.
VFD systems were introduced in 1968. Many states still have not approved VFD's for public water supplies, as the technical and electronic nature of the VFD is considered unreliable. Not only can there be technical and reliability issues but, many negative side effects are present with VFD controls. Harmonics, voltage spikes, resonance frequencies, bearing currents, and environmental concerns are only a few of the common problems associated with VFD controls.
The CSV has no electronics, which decreases cost, increases dependability, and lengthens the life of pump system components. The CSV was designed to mimic the performance of VFD controls, while eliminating the negative side effects.
It is true that when using water towers or large hydro pneumatic tanks, the pump is always running at it's best efficiency point or BEP. However, it has been documented that the difference in efficiency when using big tanks compared to CSV controls and a small tank, could take from 50 to 100 years to pay off the added expense of the big tank or tower. As the price of these big tanks and towers, as well as maintenance continues to increase, the pay out time also continues to increase.
Although the natural brake horse power of a pump makes it fairly efficient at low flow rates. It has also been documented that using different size pumps (small pump and large pump), can further increase efficiency in systems with large variations in flow rate. This makes it hard to justify the expense of large pressure tanks and water towers. The efficient and reliable control of the CSV, also makes it hard to justify the expense and problems associated with VFD controls.
(Hydraulic Differences)
Cycling on and off into a large pressure tank or water tower causes transient pressure waves and water hammer as the pump starts and stops at full capacity. These transient pressure waves travel throughout the distribution system from 3,000 to 8,000 feet per second. The water hammer from transient waves causes shockwaves of 10 times the normal line pressure. This can cause major line breaks and/or multiple tiny stress fractures throughout the piping system. These can be expensive and troublesome to repair, as well as wasting tremendous amount of our precious fresh water supply and energy. On average, major line breaks and multiple tiny leaks in the system account for 14% of our fresh water supply being wasted, due to transients and water hammer.
On smaller systems there are times when no water is being used. The CSV gives a mechanical soft start and soft stop. When water is demanded, the small pressure tank supplies water instantaneously. The CSV then lets the pump start at either 1 GPM or 5 GPM, and there is a smooth transition between water coming from the pressure tank, as the CSV quickly opens up to supply exactly the amount of water being used.
On larger systems the flow never completely stops, it is just varied from a little to a lot, according to demand. One of the first laws of physics we learn, is that a body in motion wants to stay in motion, and a body at rest wants to stay at rest. A constant pressure system varies the flow rate to keep the water in motion, which completely eliminates transients and stress on the piping system.
The pressure spikes and water hammer from pumps cycling into large hydro tanks and water towers can be easily seen as a heart beat on pressure recording charts. Constant pressure controls vary the pump output to match the demand. This eliminates heart beat pressure spikes by keeping the pump from cycling on and off. Even electronic soft starts and soft stops do not eliminate water hammer. Head is increased by the square of the pump speed, so the first 90% of pump speed does not produce flow. Only the last 10% of pump speed produces enough pressure to push open the check valve and initiate flow. With a soft starter, the last 10% of pump speed happens too fast to prevent water hammer. The same problem occurs with electronic soft stops as well.
(Reaction Time)
Cycle Stop Valves react to changes in pressure and flow instantaneously. A slight increase in pressure directly and instantly changes the position of the valve inside the CSV. This means that a slight increase in pressure is instantly met with a decrease in flow rate, and a slight decrease in pressure is instantly met with an increase in flow rate. The non-closing feature of the Cycle Stop Valve allows for extremely fast valve travel. Since the CSV can never completely close, slamming the valve shut does not cause a dead end in the pipe line for transient pressure waves to bounce off of. The non-closing or "notched" seat in the CSV allows for some flow, even when the valve is in the fully closed position. The small flow through the notched seat equalizes pressure, and eliminates the negative pressure wave that normally follows a valve closure. Before the valve opens, some flow is already flowing through the notch, so quickly opening a CSV does not cause a surge of pressure that would normally initiate a transient pressure wave in other fully closing type valves. Response time of the pump control system is crucial in eliminating transient pressure waves in a pipeline. The non-closing feature of the CSV is also crucial, allowing for valve response times that are almost immediate.
The same increase in pressure that causes an immediate reaction in the CSV, causes a much slower reaction by a VFD. The increased pressure pushes on the pressure transducer. The pressure transducer decreases the 4 to 20 milliamp signal to the VFD. The VFD decreases the frequency to the motor. Then the motor starts slowing down to produce less flow. Even if all this happens in less than a second, transient pressure waves traveling between 3,000 and 8,000 feet per second, have ample time to wreck havoc on the pipeline. While response time can be adjusted in many VFD's, the shorter the programmed response time, the more "hunting" or pressure bouncing is seen in the pipe line. The slightest delay in the response of the pump control such as a VFD, can exacerbate transient pressure waves, which cause destructive water hammer.
(Electronic Differences)
The squirrel cage induction motor and centrifugal pump are two of the longest lasting and most dependable pieces of equipment in use today. Running on the standard sinusoidal power, there is a slow rate of voltage rise, and no voltage spikes to the motor. Reducing the flow rate with a CSV causes a natural reduction of amperage which de-rates the motor. De-rating the motor produces less heat, and requires less flow for cooling purposes. The pump and motor never go through destructive critical speeds or resonance frequencies, when the unit is always spinning at full RPM. The CSV allows the pump to produce full flow and pressure when needed, and simply works the pump back to a minimum safe flow, when less flow is required by the user.
A VFD uses standard sinusoidal power to charge a capacitor bank. The VFD then pulses this DC current to simulate AC power. The quick DC pulses causes a rapid rate of voltage rise that is extremely hard on motor windings. This pulsing also causes the voltage to overshoot, and can cause voltage spikes to the motor that are 400% higher than the fundamental voltage. The frequencies generated by the VFD causes "harmonics" or "dirty power". This "dirty power" is overlain on the fundamental power, which causes excess heat and less efficiency from the motor. The odd order harmonics are not canceled out, and end up going back into the electrical grid. This can cause other equipment on the same electric grid to be less efficient, and can disturb any other equipment effected by radio signals. Line and load filters, as well as VFD's with an active front end can help alleviate harmonics but, they cannot completely eliminate them. Filters also generate heat which requires additional cooling and waste energy. The "stray voltage" created by VFD's also negatively effects cattle and other animals. This has been proven to cause reduction in milk production, mastitis of the udder, and stillborn calves, among other things. Although less studied and documented, there is no reason to believe that these same problems would not affect humans as well.
(Mechanical Differences)
Every component in a pump and motor has a resonance frequency. Varying the speed from zero to full speed with a VFD causes every component to vibrate, as the unit goes through the resonance frequency of every particular component. The VFD itself also has heat losses which reduces efficiency. When a VFD manufacturer says it's VFD is 96% efficient, that means it is losing 4% of the available power before it ever gets to the motor. These VFD "parasitic losses" cause a pump controlled by a VFD, to use more power than across the line controls, when running at maximum capacity. The VFD also causes an electrical current to run on the motor rotor and discharge through the ball bearings. These currents cause electrical fluting of the bearings, which looks like welding damage and causes premature failure of the bearing races.
The CSV never varies the RPM of the motor. Therefore the unit never runs at any critical speeds or resonance frequencies. With across the line controls, 100% of the power gets to the motor. There are no harmonics generated to cause "dirty power", bearing currents, or all the problems they associated with VFD controls.
(Differences in motor cooling)
With pressure tanks and water towers the pump is always running at full flow, and minimum flow required for motor cooling is not a problem. With VFD controls, the water flow rate past a submersible motor, and the air flow rate through an above ground type motor, are critical in maintaining proper cooling for the motor. With VFD controls the motor actually requires more cooling as the harmonic frequencies generate additional heat. When a motor is slowed down to produce less flow, the VFD requires less motor horse power from a larger motor. The VFD controlled motor still requires the maximum amount of cooling flow. With an air cooled motor the fan is now spinning slower as well, and in many applications an additional fan must be used to provide adequate cooling for the motor. With a VFD controlling a submersible motor, the standard ½ foot per second flow, as with motors running at maximum service factor amps, must be maintained to properly cool the motor. In the case of a 6" motor installed in an 8" casing, 45 GPM minimum flow must be maintained at all times. This means a VFD can vary the flow from a 300 GPM pump to as low as 45 GPM. When flow rates are less than 45 GPM, additional equipment and controls such as large pressure tanks must be implemented to safely handle the demand.
At low flow rates, the CSV reduces the amp draw of the motor, yet the fan in an air cooled motor is still spinning at full RPM. This provides more than adequate cooling for the motor at low flow rates. With a submersible motor, the CSV derates the motor by decreasing the amp draw. A derated submersible motor can be adequately cooled even when pumping hot water, so very little cool well water is required for proper motor cooling. The same 300 GPM pump and motor that requires a minimum of 45 GPM to be properly cooled when controlled by a VFD, only requires 5 GPM for proper cooling when controlled with a CSV.
(Conclusions)
For years many pump installers have enjoyed the high profit margins and short life expectancy associated with selling expensive VFD's or constant pressure pumps. Now that any home owner can purchase a VFD on the Internet for $150, the days of high profit margins are over. Some pump companies have integrated VFD controls into the construction of their motors. Pumps with integrated VFD's will now cost several times more than the market allows. Being able to compete in this economy will now mean offering products that do more, last longer, and cost less. Frivolous spending on gadgets that cost more and do less is a thing of the past.
Now more than ever simplicity, precise control, and dependability make the Cycle Stop Valve the best choice in pump control technology. Cycle Stop Valves, Inc. is dedicated to a distributor and dealer network that allows the installer to make the desired profit margin, while still offering the end user the ultimate in constant pressure performance at an affordable price.
