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Control Valves vs. Variable Frequency Drives:
Factual Answers to Ed Butts' Article

Ed Butts' article (Control Valves vs. Variable Frequency Drives) in the 2003 October issue of Water Well Journal (WWJ) does not have any comparisons between valves and drives. There are no comparisons between pressure regulation, horse power requirements, control reaction speed, installation and operation requirements, or how pumps and motors are affected by valves and drives. The only comparison discussed in this article is between valves and old fashioned pressure tank systems. Even this comparison is not accurate as Ed describes the pressure tank only system to be "one of the most reliable methods of water system control" on "typical water systems". The problem being that Ed's view of a "typical water system" is one in which "flow variations are not a primary operating concern". Most houses with there own pump system have other uses for water besides just human consumption . Typical home pump systems now have heat pumps, drip systems, irrigation systems, or at least a garden hose and a tractor sprinkler. Any of these things need a constant flow for an extended period of time which rarely matches the pumps output. Ed says that "for his money you can't find a more reliable method than a bladder tank only control for systems up to 100 GPM". Let's take a better look at this type system.

First, why would anyone use a pump system with over 10 GPM flow for a house that has no demands other than human consumption? Home water systems with pumps larger than 10 GPM even up to 100 GPM must have uses for water other than domestic consumption or they would not need this large of a pump. Lets use the 100 GPM pump system as an example. When using enough tanks to get a two minute run time, which Ed describes as the best, we would need about six 119 gallon tanks. With a draw down of 200 gallons and a continuous demand of 50 GPM the pump would run four minutes and be off for four minutes. In a 24 hour day (1440 minutes) that would be 180 cycles per day. A submersible pump this size has a limit of 100 cycles per day. A draw down of 350 gallons (at least ten tanks) would be required to keep cycles at less than 100 starts per day. And even at less than 100 cycles per day the pump and system will not survive for long. That is 100 times per day the bladder in the tank has stretched up and back down. 100 times per day the pump has kicked on and off. 100 times per day the pressure switch and other controls have had to engage and disengage. The customer has had to purchase ten of the biggest and most expensive tanks available, construct a well house large enough to house them, and the system will still cycle 100 times per day when the average use is 50 GPM. This system will be expensive to install and maintain as the motor, controls, and tanks will be destroyed from cycling on a regular basis.

With pressure tank only systems the pump is always running at BEP, or it is off. Systems requiring multiple flow variations and continuous uses are more typical than not in modern homes. With modern typical situations, large or multiple pressure tank only systems are expensive up front, and can cause cycling that keeps you continually spending thousands of dollars replacing equipment on the pretense of saving a few bucks a month in energy. With luck you save enough on energy to pay for the pressure tanks you used before cycling causes you to replace the equipment and start over.

Pump energy consumption is a factor of the brake horse power curve. Pumps with a steep brake horse power curve can have a flat or a steep performance curve. A steep brake horse power curve means that the pump is pulling full load, at maximum flow and will use only about 50% of load at minimum flow. When using a control valve care should be taken to pick a pump that has a steep horse power curve and a flow and head curve that fits the requirements. With a small correction for the power factor, volts times amps equals watts. (V x A = W) Therefore if your voltage stays the same as it should, a 50% decrease in amperage equals about a 50% decrease in power consumption. Excess pressure is a free byproduct of horse power because as pump pressure increases the flow rate and power consumed decreases.

Pressure tanks being used for storing water are worthless. Even with 200 gallons of draw down a commercial system will be out of water before they know the power is off. A system with a 100 GPM pump is capable of using 144,000 gallons per day. Even if the tanks where full when the power went off this 200 gallons could be gone in two to six minutes. Six minutes is not much time when a commercial operation is counting on water. Millions of acre feet of water are stored in the aquifer or the reservoir from which we are pumping. A back up generator or a standby pump are the only way of having real emergency water available, not pressure tanks.

A control valve system starts out way ahead because it saves the user many thousands of dollars up front by using one small tank instead of multiple bladder tanks, large hydro tanks, or even a water tower. Control valves nearly always reduce energy consumption but, even an increase in energy consumption can be more than offset if a control valve increases the life of the motor and other equipment by eliminating the destructive cycling.

Ed's version of "control valves 101" is describing a valve that has not changed in forty years. Most of the problems associated with the type valve described has been completely eliminated with newer non-closing type valves from Cycle Stop Valves. The simple non-closing feature stops "hunting" and chatter while eliminating the need for speed controls. Water hammer and "hunting" cannot occur in non-closing valves so the pilot systems have been greatly enlarged to speed up valve reaction speed tremendously. Able to quickly react to changes in flow rates, these larger pilots have no screens to maintain and allow considerable debris to pass straight through. Differential pressures must stay within the valves limits but, "scouring" the seat which causes premature failure in fully closing valves, has little effect on a valve that was never designed to close anyway.

Minimum flows needed to cool submersible motors are much less when the motor is not running on full service factor load. When the pump is restricted with a valve the horse power begins to drop 30, 40, even 50% of load. With the load reduced by 30% a 30 HP motor can pump 140 degree hot water. (See Franklins Hot Water Applications) If by reducing the load by 30% this motor can pump hot water, how much cool water does it need to remain cool? The answer is very little. Since motor companies make products that compete with control valves they do not want you to know this. We have had to test motor cooling for ourselves. We have submersible motors as large 250 HP that have been running several years with flows as low as 5 GPM. With 5 GPM of cool water as a minimum for 5 HP and larger motors we have never destroyed a submersible motor in the twelve years we have been in business.

While it is true that pilot operated control valves smaller than 2" may have control problems, direct acting valves do not. Direct acting valves in 1" and 1.25" work great for pump control and are an inexpensive solution to problems associated with the old pressure tank only systems.

Ed's theories about control valves and pressure tanks are at least a decade behind the times. His article doesn't have any side by side comparisons of control valves to variable speed pumps.

Sincerely,
Cary Austin
Cycle Stop Valves Inc.

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