Choosing the right control valve for your application

Choosing the right control valve for your application

If you‘re shopping for a control valve for your residential softener or filter applications, you‘ll find there are more choices than ever on the market. You can choose demand or time clock controls, mechanical or electronic timers, piston, rotary, diaphragm or flapper type valves, and so on.

How do you know which control is right for your needs? This article will look at the various aspects of residential control valves and attempt to answer that question.

Size and flow rate

The first question to consider is how large of a valve you need for the application. This doesn‘t mean physical size as much as the flow rate and pressure drop characteristics of the valve.

Service flow rate - This information tells you how the valve will affect available pressure in your customer‘s home.

Many manufacturers will provide a rated service flow in gallons per minute measured with a 15 psi (pounds per square inch) pressure drop across the valve. For example, a valve may claim a service flow rate of 10 gpm at a 15 psi drop.

This means if you had 40 psi coming into the valve and 10 gpm flowing through it in the service position, you would measure 40 – 15 = 25 psi at the outlet port.

Some manufacturers will provide a graph of service flow versus pressure drop, which is more useful than the single piece of information mentioned above.

A shorthand way of providing the information shown in the graph is to publish a measure called Cv (See Figure 1). Cv is the flow rate of a valve at a 1 psi pressure loss. This convenient term makes use of the fact that, for the typical valve, flow rate is proportional to the square root of the pressure drop:

Q = Cv · √ ∆P , where
Q is the flow rate in gallons per minute (gpm), and
∆P is the pressure drop in pounds per square inch (psi).

For example, if the manufacturer indicated their valve had a Cv of 2.6, and you wanted to know the flow rate at 15 psi drop, you would calculate:

Q = 2.6 · √ 15 = 2.6 · 3.87 = 10 gpm.

This simple formula allows you to determine the flow rate of the valve at any pressure drop. Remember to add the pressure drop through your tank, resin or filter bed, and riser pipe when calculating the total drop through your system.

Backwash flow rate - Most valves use a rubber or plastic flow control washer to limit the backwash rate and keep it fairly uniform over the full range of operating pressure. For softening applications, you will generally have no problem getting a high enough backwash flow from the valve you choose.

However, if your application involves backwashing a heavy filter medium like greensand, you may find that the control valve‘s backwashing capabilities are a limitation.

Always check that the valve can provide the backwash flow you need at the minimum operating pressure you expect to encounter.

Demand vs. time clock control

There are two basic methods for triggering regeneration of the system.

A time clock control allows the timer to start a regeneration after a fixed interval of days, for example, every other day, every third day, etc. In some cases, the timer allows you to choose the days of the week on which regeneration occurs -- every Tuesday and Friday for example.

Regeneration occurs at the preset interval regardless of whether any water was used or the bed was exhausted.

A demand-initiated control is able to regenerate only when needed, usually by monitoring water usage with a flow meter.

If the customer is on vacation and not using water, the system will not regenerate. A demand-initiated system will therefore be more efficient in salt and water use than a time clock system.

If a demand-initiated control is more efficient, when should a time clock control be used? The most likely application is as an entry-level system in your product line.

A time clock control will often use a mechanical timer, while most demand initiated controls are fully electronic (although mechanical and even non-electric demand systems are also available).

Time clock controls can also be useful in a problem water filter application where the need to keep the bed clean by frequent regeneration far exceeds the added value of saving water.

Valve types

All control valves have one or more moving elements driven by a motor, solenoid valve, or water pressure. The valve elements can be pistons, rotating discs, diaphragms, spring-loaded pins or flappers, to name the more common elements.

The choice of valve type, after considering all other aspects of the control, is really a matter of personal preference.

Controls will also be described by number of cycles, such as 5-cycle valve. The basic cycles for a softening system are:

· Service.

· Backwash.

· Brine draw or brine rinse - when the valve is actually drawing brine or other regenerant into the tank.

· Slow rinse - usually the same valve position as brine draw, but referring to the remainder of the rinse cycle after the brine tank has been emptied.

· Fast rinse - a final full-flow rinse, usually at the same flow rate as backwash, but in a downflow direction to settle the bed and rinse out the remaining salt.

· Brine refill - find out whether this is a timed cycle. Some controls leave the brine line pressurized at all times during service, and rely on the float shutoff in the brine tank to control the salt dosage.

Depending on how the manufacturer counts cycles, a control valve with all the above cycles might be called a 5-cycle, 6-cycle or more if it has intermediate positions like a soak time after brine refill, a pause after drawing brine, or secondary backwash or rinse cycles.

Find out what the cycles actually are when comparing valves from several manufacturers.

Electronic control features

A fully electronic control has a number of advantages over a mechanical timer. You‘ll want to evaluate these features in the electronic controls you consider:

· Unlike a mechanical timer, which loses time whenever power is out, an electronic control can usually maintain its clock for several minutes to several hours during a power outage. The circuit board will use a battery or a capacitor to store energy and keep the clock running. The display will be off during the power outage to conserve energy.

· The circuit board should maintain all of your installation and regeneration settings indefinitely even if power is out or the control is unplugged.

· The electronic control should provide diagnostic information useful for service. Typical information may include: number of days since the last regeneration, gallons used since the last regeneration, and number of regenerations in the last 30 days.

· Some controls provide smart features, such as the ability to adjust salt dosage based on water use (proportional brining) or to calculate the gallons treated between regenerations.

· The electronics should be resistant to line voltage fluctuations or radiated noise from nearby equipment such as a furnace blower or spark igniter. Look for a control that is CE compliant, which means that it meets European standards for resistance to electromagnetic and radio frequency interference, as well as line transients.

· The display should be easy to read, and the control should be easy to program. Evaluate how easy it is to operate the control without a manual.

It is also a good idea to limit the end-user‘s access only to changing the time of day or initiating an extra regeneration. It is helpful to have a lockout scheme or special code that will enable the installer/technician to initiate other functions such as changing the duration of cycles or setting capacities.

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