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Water System Control Valve Fundamentals

“CV” is a term constantly used in selecting valves.  The CV factor is a standard used by all valve manufacturers.  CV stands for the “coefficient of flow” of a valve, and is further defined as the flow rate in gallons of 60°F water that will pass through a valve in one minute at one-pound pressure drop.  A CV of 1 then, means that one gallon of water passed through a valve in 1 minute at 1 pound DP.

When selecting a valve to control the flow of a specific media, the CV needs to be determined by solving the formula:          

                                                                                                                      (for a water valve)

The CV factor found is the CV needed to get full rated flow when the valve is wide open.

Manufacturers of valves determine the CV of a valve by actual test.  This test set up is illustrated in Figure 1.

Figure 1.

This testing procedure requires a constant pressure drop be maintained throughout the valve stroke, either by holding the water level in the tank, or by applying air pressure to a sealed water tank maintaining a constant pressure.  A stopwatch is used to time the flow into the measuring tank.

The cataloged printed CV for a valve is the CV found in the test when the valve was full open.  On special request, some valve manufacturers will make available the CV factors for various positions between full open and full closed.  This varying CV is seldom needed in valve selection, especially if one remembers that the DP across the valve was constant during the test, resulting in true linear flow.  Linear flow is defined as:  “flow when the DP across a valve remains constant for all positions of valve stem travel.”  Such a condition rarely, if ever, exists in an installed system. As a control valve closes to reduce flow, the DP across the valve increases, partially offsetting the intent to reduce flow.  When this happens, the valve loses its linear characteristics.

When water is circulated by a centrifugal pump, as a valve or valves in the system close, the pump head increases.  As noted, this increase in head pressure will offset the closing valve action relevant to flow.  How much the pressure increases depends on the pump curve.  Small residential pump applications are seldom critical, but on larger applications the system designer must carefully select the proper pump.  On very large applications where high head pumps are often used, and many valves, it becomes necessary to employ a bypass system to attempt to maintain a constant flow and therefore delivery pressure.  See Figure 2.

Figure 2.

When water is the heating transfer medium, problems dealing with coil characteristics must be taken into consideration, because the heat output of a coil is not linearly related to flow.  As an example, most hot water systems used to be designed for a 20°F temperature drop.  Water leaving a boiler at 200°F was assumed to return to the boiler at 180°F.  Even if this occurred, the water still had tremendous heat potential, actually having given up about 13% of its available heat (based on coil entering air temperature of 70°F).

Good pipe sizing, safety factors, etc., result in a system that may drop only 6 to 10°F.  Thus, only 5 to 7% of the total heat is extracted from the water.  This water resists efforts to reduce its heat output.  A reduction in flow results in a greater temperature drop as the water stays in the coil for a longer period.  This cancels the supposed effect of reducing flow.

Figure 3 illustrates what happens in a typical coil when the valve reduced flow.  Note that the valve does not do much until it nears the closed position.  When the flow is reduced to the point where the temperature of the water leaving the coil is equal to the entering air temperature, then the heat output will be directly related to flow.  This happens because the temperature drop can no longer increase.

Figure 3.

System designers found that by choosing higher temperature drops when designing systems, the potential of the system is lowered and the water reached entering air temperature sooner.  Temperature drops of up to 60°F are now widely used.  By doing this, the DP across the control valve will be high compared to the coil, and the adverse effects of changing DP’s will be almost eliminated.

By using high DP for a control valve, regulating pump pressure, designing around higher temperature drops, and using equal percentage port valves, a reasonable linear system heat output curve can be obtained.

How much a valve can reduce flow is called its “turndown ratio.”  A valve with a turndown ratio of 10 to 1 means flow can be reduced to 10% of its maximum flow.  Below that, any change in its stem position has little or no affect on its flow until shut off.

The ideal valve would be a throttling plug that would wedge into its seat for shut off.  See Figure 4.

Figure 4.

Unfortunately, a valve of this design is not practical.  It would require extreme force to loosen the plug.  Any temperature change would distort the seat due to expansion and contraction.  It would stick closed, or not be able to close tightly.

Valve manufacturers modified the ideal valve and made a throttling plug valve that is not affected by normal operating conditions.

Figure 5 shows the basic design of such a valve.  The clearance of .010” between the plug and seat solved the sticking problem and requirement of a huge powerful actuator.  However, the smaller the valve, the greater the effect this clearance has, because it represents a larger percentage of full opening.

Figure 5.

Water systems are being broken down into more and more individually controlled zones.  This results in the use of smaller valves and consequently turndown ratio becomes important.

A throttling plug valve on a test may be described as having a turndown ratio of 30 to 1.  However, when installed in a system, the higher DP across the valve, especially when nearing the closed position, results in an installed turndown ratio closer to 20 to 1 or 5% of maximum flow rate.  This is significant.  A system designed for a 20°F DT with a 5% flow will have a coil heat output of about 35%.  This becomes the minimum controllable condition.  Below that, the valve will shut off.  Two-position control usually results because the valve cannot proportion the heat output below 35%. System performance is unsatisfactory.

There are ways to solve these problems.  See Figure 6.

Figure 6.

Using a separate pump and three-way valve for each coil works well.  The flow is constant and the temperature of the water supply is varied.  Since heat output of a hot water coil is a linear function of the supply water temperature, this system works well.  It is costly.

Another way is to use face and bypass dampers with outdoor reset.  In mild weather, the desired coil temperature would be low.  On systems where the same coil handles hot and chilled water, face and bypass dampers should be used.  These systems will have oversized coils as far as heating is concerned.  A three-way valve system will not give good results, as the higher chilled water temperature under light load will result in the coil losing any latent effect.  Space temperature might remain okay, but humidity will be too high.  On these systems, face and bypass with outdoor reset for the hot water, and constant chilled water for cooling will give satisfactory performance.  These systems also are costly.

The HVAC industry has discovered the actuated ball valve as a very cost-effective solution to the problems of water systems.

The development of high-resolution low cost actuators, coupled to ball valves, solved the problems associated with turndown ratios.  Turndown ratios of 160 to 1 are common, even as high as 400 to 1.  Minimum flow is now no problem, as it is controllable (see Info-Tec 33 for full discussion of flow control with ball valves).

Of course, even the best-designed system can be ruined by poor installation.  The installing contractor needs to be aware of and follow good piping practice, such as the length of upstream and downstream piping into a control valve, proper piping support, etc.

In summary, water is an ideal controllable medium because its temperature and flow can be varied to suit a particular application.

The hydronics system designer is the most important person in the process of obtaining a good performing water system.

A well-designed system using flow control valves will be designed around:

   A high DT, about 60°F

   Use 50% or more DP for control valves

   Use pump regulation, if needed

   Outdoor reset the hot water

   Use control valves with high turndown ratios

   Use quality thermostats capable of positioning the control valves in small increments.  (A valve will function no better than the thermostat controlling it.)

When the designer takes all of these factors into consideration, a properly installed system using water as the heat exchange medium will give the ultimate in performance and comfort.

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