Cycle Stop Valves, Inc.

Excessive use of energy is an ailment that we desperately want to cure. Those afflicted with this syndrome are so eager for a cure that they can be easily mislead with promising treatments. Many articles claim that the variable speed drive is the magic pill that cures excessive energy consumption. As with any medication, you should carefully read the fine print. You may find that variable speed drives used with pumps do not save energy. Drives may also cause side effects that can far outweigh any possible benefits.

One recent article explains why variable speed pumps do not save energy. This article talks about the fact that losing head by the square of the speed can have more of an impact on pumping applications than reducing horse power by the cube of the speed. Another recent article says that variable speed drives save so much energy that it is worth putting up with all the technical problems and side effects. A third article claims that variable speed drives have already saved so much energy that they have cut CO2 emissions by up to 400 million tons compared to pumps using throttling valves as control. Because of misleading articles, many people have become victims of hype and hysteria, myths and legends, or just false information. Doing the math ourselves is not difficult and is apparently the only way to uncover the truth.

With a simple pump curve, you can compare throttling with a valve to variable speed control. If you are stuck with an existing, oversized, or badly designed pump, a variable speed drive may be able to save some energy in low flow conditions. With compressors, fans, conveyor belts, positive displacement pumps, and other applications, variable speed drives might be worth putting up with, and can possibly save energy. If a system requires the pressure to increase as flow increases, real power savings could be had from increasing the pipe size, which will allow you to reduce the pump size. If you are able to show energy savings using variable speed to hold a constant pressure or TDH on a centrifugal pump, someone should have done the math before the pump was chosen. Any pump with a centrifugal impeller that has a good brake horse power curve will save as much if not more energy when controlled with a throttling valve than when controlled with a variable speed drive. Throttling valves do not burn energy as many drive manufacturers would like for you to believe. As back pressure on the pump increases from throttling with a valve, the horse power or power consumption decreases. This is a "counter intuitive" property of a centrifugal impeller. Many people falsely believe that choking back a pump will make it work harder. Choking a pump actually makes it work easier as the power consumption is proportional to the flow rate and inversely proportional to the back pressure.

Be wary of drive manufacturers or so called "smart pumps and controls" who will try to tell you that valves waste or burn energy. Some will even compare valve control to driving a car with one foot on the gas and one foot on the brake. These people obviously do not understand how pumps work. Since there are many other good uses for variable speed controls besides centrifugal pumps, they should not have to resort to false information to sell drives.

Most pump curves include a brake horse power curve. With these horse power curves you can easily see that as flow decreases, horse power also decreases. If a horse power curve is not available you can do the math with the following horse power calculation that has been used for generations.

With this calculation or the horse power curve you will see that when throttling a pump, even when efficiency drops to 5%, horse power can still decrease by about 50% of the maximum horse power.

To do the math with the variable speed controller you will need to use the Affinity Law. However, you can make a huge mistake in focusing on the power savings by the cube of the speed, when the most important part of the Affinity Law is that head is reduced by the square of the pump speed. First you must determine the minimum RPM that the pump can spin and still produce the head required before you can use the Affinity Law. The following is a simple calculation to help you determine the minimum possible pump speed. Once the minimum possible pump speed is determined, then the cube of that number will give you the Minimum Possible Horse Power using a variable speed drive.

Total Dynamic Head or (TDH) determines the minimum possible speed. Divide the (TDH) by the pumps Shut Off Head (SOH). The answer will be a percentage of head loss possible or Possible Head Reduction (PHR). Minimum Possible Speed or (MPS) squared equals the Possible Head Reduction (PHR). (MPS) cubed equals the minimum percentage of Horse Power or Minimum Possible Horse Power (MPHP).

Another common mistake is not to give the centrifugal pump credit for it's natural brake horse power characteristics. All of the articles you have read that show a pump saving large amounts of energy using variable speed compared to a throttling valve have made these mistakes. The fact that we are using a pump means we must need to produce a certain amount of head. An example would be pumping over a mountain that is 231 feet high. The flow rate could vary from a little to a lot but we always need 231 feet TDH to get the flow over the mountain. Using the calculation above for a pump that has a Shut Off Head (SOH) of 285 feet, we can only reduce the RPM by 9% and still produce enough head to get the flow over the 231 foot mountain. Slowing a pump by only 9% reduces the speed of a 3550 RPM pump to only 3280 RPM. Any slower than 3280 RPM and the pump will not produce enough head to get any water to the top of the mountain. According to the Affinity Law, reducing the speed by 9% only reduces the power required by 25%. The Affinity Law would then reduce the power consumption of a 100 HP pump to 75 HP, not 1 HP as many drive companies would like for you to believe. Using a throttling valve will maintain the head at 231 feet as required while throttling the full speed pump back to low flow reduces the 100 HP load to 42 HP. Unless we can change the TDH required, which would mean changing the height of the mountain, variable speed control for this pump will only save a tiny amount of energy over the same pump simply throttled with a valve.

Maintaining the Total Dynamic Head required is the most important function of a pump. The curves in many articles show the head drop from 10 to 7, or 200 feet to 140 feet. This is a typical case of not being able to see the forest for all the trees in the way. Even the so called experts forget that head is lost by the square of the speed. This limits the effectiveness of drives and makes the energy calculations in most articles completely incorrect.

With a variable speed drive there are many negative side effects. Pulsing DC voltage creates high voltage spikes in the motor. EDM currents build up in the rotor and discharge through the ball bearings causing damage from electrical fluting. Certain critical speeds cause pump and motor vibration. Harmonics are created and fed back into the electrical grid. Radios, cell phones, and other electronics can be negatively affected by RF interference created by the drive. Three percent of energy is also wasted due to losses in the drive itself. Considerable technical assistance may be required. I have even seen in hot climates where the air conditioning system needed to keep the drive cool, uses as much energy as the pumps themselves. Drives are computers with a microprocessor base. Parts needed to repair a five year old drive are no more available than parts for any other five year old computer. Many articles state that the expense and side effects or the "myths and legends" are worth putting up with because of the energy savings. However, if there are very little or NO energy savings, all of these side effects can be eliminated by using other simpler and less expensive controls such as a throttling valve. Even if energy savings can be shown using a variable speed at low flow, at maximum flow losses in the drive itself adds three percent more energy needed than if the pump were running on an across the line panel. If any energy savings are present, they can be so small that it takes many years to get a payback instead of the few months that we were promised.

(See following curve)

Example from curve:

231' TDH, 1200 GPM 6 hours per day, 100 GPM 6 hours per day at $0.12 per KW hour, 100 HP drive cost $10,000.00

KW to run pump with valve:

1 KW per HP X 100 HP X 6 hours = 600 KWhrs.
1 KW per HP X 42 HP X 6 hours = 252 KWhrs.
Total 852 KWhrs.

KW to run pump with drive:

1 KW per HP X 103 HP X 6 hours = 618 KWhrs.
1 KW per HP X 38 HP X 6 hours = 228 KWhrs.
Total 846 KWhrs.

Cost of running pump with valve control:

852 X .12 = $102.24 per day

Cost of running pump with drive control:

846 X .12 = $101.52 per day

Cost difference of running valve and drive = $0.72 per day

Drive cost $10,000.00 divided by $0.72 = 13,888 days or 38 years to pay off drive if the pump is used everyday of the year. If the pump is only used for six months out of the year it will double the pay out time of the drive to 76 years. If you are paying less than twelve cents a kilowatt, it will take even longer to pay off a drive. When it takes longer than 10 years to get a payback, the drive itself may not last long enough to ever recuperate it's own expense.

Another argument you will hear is that variable speed drives eliminate end rush current compared to across the line starting. While this is true, the only time this matters is when the pump is started. When a valve is throttling the flow from a pump to match the usage, cycling on and off is eliminated and end rush currents do not come into play. On smaller systems that do occasionally require a zero flow rate, the starting and stopping is also minimal and end rush current is not important. Throttling valves can also be used with soft start panels when needed to reduce end rush currents. The older style reduced voltage soft starters will still eliminate end rush currents. Auto transformer soft starters will do away with the negative side effects that come with the new style electronic soft starters and variable speed drives.

Many people also believe that throttling is bad for pumps. The truth is, pumps that cannot handle throttling have been built for planned obsolescence and are cheaply designed. Pumps with slightly stronger shafts and bearings can easily handle throttling. As long as a small cooling bypass flow is maintained, especially when pumping fairly cool water, there is no heat buildup from throttling. Contrary to what some people were taught, recent studies also show that vibration in some pumps actually decreases with throttling. Modern constant pressure valves have also eliminated screens, shafts, and speed controls. This makes a simple, dependable throttling valve even more dependable.

The multitude of technical aspects of using a variable speed drive can be baffling. A simple pump system is a dependable pump system. Don't allow sales tactics that falsely claim energy savings talk you into spending more money and having more headaches by using drives. Do the math yourself. Talk to people who have experience with both variable speed drives and throttling valves. Take advantage of the natural characteristics of a centrifugal pump that causes horse power to decrease as flow decreases. A variable speed drive can be a complicated way of trying to trick a pump into doing something that a pump already does naturally. Anyone who truly understands centrifugal pumps and their relationship to variable speed would never claim energy savings compared to throttling valves. Those that do understand are trying to complicate the issue to keep you from finding out what they don't want you to know.

Cycle Stop Valves, Inc.
10221 CR 6900 Lubbock, Texas 79407 Send Email	800-652-0207 806-885-4445 Fax: 806-885-1994

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