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HVAC Valve Sizing In The “Real World”

A control valve is the single most important element in any fluid handling system. Selecting the proper valve will have great effect on the efficiency of a system. There are volumes of information on the science of valve sizing and fluid mechanics. To properly size a control valve, a design engineer would take into consideration the entire system; pumps, piping geometry factors, Reynolds numbers, correction factors, specific heat factors, velocity of approach, pressure recovery, etc., etc. In the “real world” of valve sizing, we just don’t have all of that information available. Valve sizing in the real world is not an exact science, but an art.

The basic plan in sizing a valve is to determine the CV required and to match it with a valve having that CV. CV is the flow coefficient of a valve. CV is defined as “the flow rate in gallons of 60°F water that will pass through the valve in one minute at a one pound pressure drop”. Valve manufacturers test their valves and list their CV factors in their valve specifications. The CV of a valve is a standard, and knowing the correct CV allows us to compare valves of different manufacturers.

The formula for finding CV is: CV = GPM / square root of DP

By looking at the formula, it is obvious we need to know the maximum flow rate required and the pressure drop the valve takes. If we don’t know these parts of the formula, it is impossible to find a CV. Anything we do to pick a valve without knowing flow rate and pressure drop will result in guessing at what is required and the valve selected may or may not work as required.

There will be other things we need to know to select a valve, but CV is the focal point of valve sizing. It is the first, most important step in valve selection.

Flow rates are expressed in standard units. For fluids, this is gallons per minute (GPM). For steam, it is pounds per hour of saturated steam (in our “real world” valve sizing, we will assume all steam is saturated steam), and for air and other gases, cubic feet per minute (CFM).

The flow rate must be specified to determine the proper valve size. There is no way we can intelligently make a guess at a flow rate. A flow rate may be calculated from other information such as a valve to control flow to a steam coil rated 80,000 BTU/Hr. Using a 1000 BTU in a pound of steam the flow rate is 80 Lbs/Hr. The other part of the CV formula, the DP across the valve is as important as flow rate. All valves impose a pressure drop on the flow. The DP has nothing to do with the static pressure. If a gauge is installed on the inlet side of a valve and a gauge on the outlet side, and flow established, the inlet gauge will always read higher than the outlet gauge. The difference is the DP across the valve.

In Figure 1, with the valve wide open, the inlet gauge reads 100 psig, the outlet gauge 90 psig. The outlet gauge’s reading subtracted from the inlet gauge’s reading is 10 psig. 10 psig is the DP.

Figure 1.

With a flow rate and a DP known, we can then find the all-important CV factor. If the flow media is water, we can solve the formula. Better yet, many manufacturers print charts from which one can find the CV factor required. Attached are charts for the two most common flow media we deal with, water and steam. (Other CV charts for air, oil, etc, are available.)

If the flow rate and the DP are unknown, some “rules of thumb” can aid in determining a CV.

Control valves in the HVAC industry fall into one of four categories:

1. Two position (on-off) valves.

2. Proportional control of water, varying the amount of flow.

3. Proportional control of water, varying the temperature of the flow.

4. Proportional control of steam.

1. For two position valves for both water and steam, if the inlet pressure is known, use a DP of 10% of available pressure to calculate a CV factor. If the inlet pressure is not known, line size the valve. In the HVAC field, line sized valves will generally have appropriate CV Factors. For instance, most 1-1/2 IPS globe style valves have CV’s around 25.

Example for a water valve: Flow rate given = 30 GPM. Inlet pressure 20 psig. 10% of 20 psig = 2 psig. Enter the water valve-sizing table at DP=2. Go down the DP=2 column until you find a number closest to 30, the GPM flow rate. In this case, you arrive exactly between 25, a CV of 17.4, and 35 a CV of 25. When using two position valves, low-pressure drop across the valve is desirable, so select the larger CV of 25 to keep pressure drop low. Seldom will you find a valve to exactly match the calculated CV. In this case, most globe style valve bodies will be 1-1/2" IPS, a ball valve 3/4" IPS.

Caution: if any valve selected, not just for two position water but also for proportional water or steam valves, is one or two sizes smaller than line size (i.e. 3" IPS line size, valve 2" IPS) a piping geometry factor (symbol FP) will reduce the effective CV. This has to be taken into consideration. Some manufacturers have FP charts for their valves; most don’t. Therefore, we apply another of our rule of thumb calculations. Use a divisor of .7 to correct the CV. The formula is:

Originally calculated CV = .7X, with X being the new CV. Solve for X.

Example: calculated CV is 17. 17=.7X. X = 24.28CV.

This resizing may result in a valve that will be line size, so be careful. You could end up back with the originally selected valve. In all cases, avoid down sizing by more than two pipe sizes.

Note the CV chart for steam valves. Note the two pressures for DP under each inlet pressure. It is clearly marked for two-position and proportional control. As an example: a two-position valve for a flow rate of 200 lbs. per hour and 15 lbs. inlet pressure. At the 15 Lb. inlet pressure box, enter the chart at the 1.5* column and go down to the closest number representing a flow rate of 200 Lbs/Hr. You’ll find 205. Go left to the CV of 10.5. As has been mentioned before, for HVAC steam valves, we are going to assume saturated steam of 100% quality. No correction factors will be applied for superheated steam or steam at less than 100% quality.

2. For two-way proportional control of water, varying the amount of flow, a high-pressure drop is desirable. The valve should provide the greatest resistance to flow in a control loop. If the valve has less resistance than say a coil it is to control, the limiting factor to flow would then be the coil, not the valve. Consequently, until the valve throttled to a position so that it has greater resistance than the coil, the valve would have no control.

Ideally, the pressure drop for proportioning two-way water valves should be half the difference between the supply and return mains at the valve location. In the “real world” of valve sizing, this will almost never be known. So, our rules of thumb for these valves:

A. If the DP across the coil is known, the valve DP should be the same. If DP across the valve is unknown, use 50% of available inlet pressure. If neither the DP across the coil or the valve is known, use 5 psig as the minimum DP.

B. If the design DT of the system is known, DP can be based on design DT. If the design DT is 60°F or more, use 50% of inlet pressure, 40°F, 66%, and 20°F 75%.

C. If the pump head is known, using 50% of that head can be used for DP. Proportional control and varying flow can also be done using three-way valves. Due to three-way valve peculiarities, they will be discussed in the next Info-Tec.

3. Proportional control of water, varying the temperature, usually uses a three-way valve and therefore, will be discussed in the next Info-Tec.

4. Proportional control of steam requires high-pressure drops. Because of the nature of steam and its heating abilities, it needs high DP for good control. The steam pressure of the system will have to be known.

Low-pressure steam is 15 psig or less. Anything greater than 15-psig steam is considered high pressure.

The DP for 15-psig steam or less is 80% of inlet pressure. Note that on the steam valve-sizing table, the higher DP shown is 80% of inlet pressure up to 15 psig.

Usually, you will end up with a valve one or more sizes smaller than line size, even after correcting the CV with an FP factor.

Over 15 psig use 42% of absolute inlet pressure to get DP. Absolute pressure is gauge pressure plus 15 lbs.

Example: 20 psig inlet pressure is 35 psia. Multiply by .42 and your DP is 14.7 lbs. or, rounded off, 15 lbs. Care should be taken selecting pressure drops for high-pressure steam valves. Note on the steam valve chart the proportioning DP for 20 psig and up is the DP based on the absolute inlet pressure. Never use a DP more than 50% of absolute inlet pressure for steam valves. This is critical pressure drop. Maximum flow through a steam valve occurs when DP equals 53% of absolute inlet pressure. A further increase will not increase the flow rate.

Thermodynamic reactions take place that limit the flow and ruin the valve. Steam trim, usually stainless steel or other hard trim, should always be selected for steam valves.

For all valves, one should check the close off pressure rating.

All manufacturers print the close-off ratings of the various valve actuator and linkage combinations for their valves. On water vales, the close off rating need only exceed the DP by a few pounds. On steam valves, the close off rating should exceed the inlet pressure rating, since when the valve is closed, the steam downstream could completely condense and cause the DP to be full line pressure. The valve must remain closed against this pressure.

In review, these are the steps that should be taken to select a control valve:

1. Calculate the required CV.

2. Check for correction factor (FP) and adjust CV.

3. Check for proper close-off rating.

4. Make sure the valve is suitable for the system pressure rating.

5. Make sure the valve has the correct trim for the flow media.

The attached “Engineering Data” sheet may help you in calculating flow rates when the customer can’t supply that information, but can give you some idea how the valve is to be used.

While it rarely occurs in HVAC applications, some mention should be made relative to cavitation. Cavitation is a two-stage phenomenon, which can greatly shorten the life of the valve trim in a control valve. Whenever a given quantity of liquid passes through a restricted area such as an orifice or a valve port the velocity of the fluid increases. As the velocity increases, the static pressure decreases. If this velocity continues to increase, the pressure at the orifice will decrease below the vapor pressure of the liquid, and vapor bubbles will form in the liquid. This is the first stage of cavitation.

As the liquid moves downstream, the velocity decreases with a resultant increase in pressure. If the downstream pressure is maintained above the vapor pressure of the liquid, the voids or cavities will collapse or implode. This is the second stage of cavitation.

The second stage of cavitation is detrimental to valves. Because of the tremendous pressures created by these implosions (sometimes as high as 100,000 PSI), tiny shock waves are generated in the liquid. If these shock waves strike the solid portions of the valve, they act as hammer blows on these surfaces. Repeated implosions on a minute surface will eventually cause fatigue of the metal surface and chip a portion of this surface off.

Low degrees of cavitation are tolerable in a control valve. Minimum damage to the valve trim and little variation in flow occur at these levels. However, there is a point where the increasing cavitation becomes very detrimental to the valve trim and possibly even the valve body. Also at this point, the cavitation is beginning to choke the flow through the valve. At some point, the flow rate will stay the same, regardless of increases in pressure drop.

The point at which cavitation becomes damaging can be expressed by the following:

DP (Allowable) = 0.5 (P1-PV)

P1 = Absolute Inlet Pres. PSIA

DP = Maximum Allowable Pres. Drop

PV = Absolute Vapor Pres. PSIA

(Note we are using absolute pressures.)

This formula is to estimate the maximum allowable pressure drop across a valve. Vapor pressures for water at various temperatures can be found in the table below. The pressure drop calculated by this method should be considered the maximum upper limit when choosing a valve for a given application. The pressure drop that is referred to here is the drop appearing across the valve in the near closed position. For example, if a valve is controlling 180°F water at a pressure of 20 psig, the maximum pressure drop would be: DP = 0.5 [(20 + 14.7) - 7.5] = 13.6 psi

Table 1. Pressure of Water

As was previously shown, the recommended pressure drop for this valve would probably be 5 to 10 psi. Cavitation then would not be a problem in this application. If you suspect cavitation might cause a problem, use the cavitation formula to check. If cavitation becomes a problem, resize the valve using the maximum DP found in the cavitation calculation.

Figure 2.

Table 2. Water Valve Sizing Tables

Table 3. Steam Valve Sizing Table (1 of 2)

Table 4. Steam Valve Sizing Tables (2 of 2)

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