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Control Valves – Sizing &
It’s always a good idea to revisit an old topic, one that you might have learned about years ago, and reintroduce yourself to it. Especially with the way technology has a tendency to impact and influence our “old-school” design philosophies and engineering practices. For those who’ve been in the biz for as long as I have (or longer), consider this a simple refresher course on sizing control valves for coils. For those of you that are new to the industry, this will hopefully serve as an introduction to simple valve sizing, and encourage you to further your education by picking up some control valve catalogs and trying your hand at it.
The three types of control valves that we (mostly) encounter in the HVAC industry are globe valves, ball valves, and butterfly valves. I won’t get into detail on the mechanical construction of each type of valve, however I will give enough so as to convey a basic understanding of each and how they differ from one another.
The globe valve is the traditional control valve. Long before anyone got the idea of putting an actuator on a ball valve, the globe valve was fully adapted to electric actuation. Even these days, with less costly alternatives at their disposal, many in the business still prefer the globe valve as their control valve of choice. The body of a globe valve has a stem protruding through the top of it. Lateral (up and down) motion of the stem, and thus of the plug assembly inside the body, translates into variable flow through the valve body. An actuator is mechanically linked to the valve body, and operates the stem, moving it up and down.
Globe valves are generally available in the range of ½” to 6”. They are suitable for two-position and modulating control of both water and steam, for applications falling within this size range.
Ball valves have come into their own as a low cost alternative to the globe valve, at least for applications requiring smaller size valves. Inside the body of the ball valve is…a ball. With a hole through the center of it. And a stem fused to the top of it, that protrudes through the body of the valve. Rotation of the stem, and thus of the ball with the hole in it, translates into variable flow through the valve body. An actuator is mechanically linked to the valve body, and operates the stem, turning it back and forth.
Ball valves are available in the range of ½” to 3” (or larger!), and are generally limited to use with two-position and modulating water applications.
A butterfly valve is nothing more than an extremely short segment of pipe with a disk inside of it, the diameter of the disk being equal to the inside diameter of the pipe. The disk has a rubber seal around its perimeter, and is mounted on a shaft that protrudes through the wall of the pipe. Rotation of the shaft, and thus of the disk inside of the pipe, translates to variable flow through the valve assembly. An actuator is mechanically linked to the assembly, and operates the shaft.
Butterfly valves are normally available in the range of 2” up to 12” (or larger, if custom-built). They obviously take over where globe valves leave off, size-wise, but are also a cost-effective alternative to globe valves in the 2” to 6” range, given the right conditions.
Valve sizing is rocket science! No, it’s not. But it is pretty critical. It’s not critical in that it’s difficult to perform. It’s actually rather simple. It’s critical in that the individual making the valve selections must know what he/she is doing and how to go about it. For those not familiar with standard pipe and valve sizes, here they are: ½”, ¾”, 1”, 1-1/4”, 1-1/2”, 1-3/4”, 2”, 3”, 4”, 5”, 6”, 8”, 10”, 12”.
The first thing that needs to be considered when sizing a control valve is how it will be controlled. More specifically, is it a two-position application, or is it a proportional application? For a two-position application, all you need to know is the line size. The valve size should be the same. That’s it! Yeah, theoretically, we can get away with using a valve that’s perhaps a size smaller, and save a few bucks. But that savings is usually eaten up in fittings and the extra labor required for installation.
For proportional applications, sizing the control valve is more involved. An improperly sized control valve can cause an array of systematic problems. An undersized valve can result in insufficient capacity at full load conditions. With an oversized valve, stable control may not be able to be achieved, as exhibited by the control valve constantly “hunting”. To better understand the importance of “right-sizing” a control valve, we need to discuss three terms, and how they are related by a simple equation. We likely know of or have heard of the first two; flow rate (Gallons Per Minute, or GPM) and pressure (P). The third term is the valve coefficient, or Cv. The Cv value is a flow characteristic of the valve, and is directly proportional to the size of the valve and its ability to pass fluid through it. More specifically, Cv is defined as the flow rate (GPM) through the valve at a pressure drop (P) across the valve of 1 PSI (Pounds per Square Inch). The equation that relates these three values is as follows:
P = (GPM / Cv) squared
The pressure drop across a proportional control valve when fully open, for optimal control, should be equal to the pressure drop across the coil that it’s serving. Sometimes the coil’s pressure drop is not known or readily available. However, for typical HVAC applications, the pressure drop across a hot or chilled water coil is usually less than 4 PSI. Hence, a popular and commonly used rule of thumb exists that if the coil pressure drop is not known, select the valve so that the pressure drop across it when fully open is in the range of 3-5 PSI.
Generally then, all that really needs to be known is the flow rate. The above equation can be manipulated, fixing the pressure drop at 4 PSI and inputting the GPM, to come up with a suitable valve Cv rating. Valves come in discreet sizes though, and a valve may not be available for the Cv rating that you calculate. So then you select a valve with a Cv that’s close to what you’ve come up with, and plug it into the formula, solving for pressure. If your result is between 3 and 5 PSI, then you’re done!
What if your result is not within the rule of thumb? What if it’s too low? Or too high? Now it becomes a judgment call. If selection of a valve, using this approach, does not fall within the 3-5 PSI rule of thumb, then it’s time for some insight. Does the system pump have plenty of power, even to overcome the pressure drop of a valve selected as “kinda high”? Is precise control of critical concern, where oversizing may be less desirable than undersizing?
These are concerns that typically must be dealt with on a job-to-job basis. Realistically speaking, the body of a proportionally controlled “globe” valve will almost always be a size or two less than the pipe size. This is another rule of thumb that serves as a “check” when making the valve selection, and helps in determining the “appropriate” selection. Rocket science it ain’t, but we do need to be aware and make a concerted effort when sizing control valves.
|Tip of the Month:
On plan & spec projects, always refer to the specification section on control valves for the specifics on valve sizing and selection. Whereas the information given here is theoretically sound, the consultant may, and often will, have different or more stringent guidelines that will prevail. Submit a control valve schedule for engineer’s review, and you will see just how serious the consulting engineers are about their written specifications!
As far as ball valves go, recent developments in technology (recent meaning the last 10-15 years) have brought the traditional ball valve to the forefront of the industry, and thus it has become the valve of choice for many. One reason for the shift in paradigm is that the manufacturers have gotten creative with the standard full port ball valve, and now offer ball valves with differing port sizes (not to mention specialized configurations) given the same connection size. For instance, a ½” valve body is offered in several different port sizes, and thus several different Cv values. Same for a ¾” and 1” valve body. And even for the rest of the range of ball valve sizes. This gives the valve selector some flexibility, and opportunity, to apply ball valves in proportional control applications. Where in the past a required Cv of 10 would mean a ½” ball valve, now a valve with that same Cv value might be available in a ¾” size, or even a 1” size!
It is important to note that control valve sizing as described herein applies to sizing valves for coils, and doesn’t necessarily extend to other control valve applications. Rules that apply in sizing control valves for applications other than coil control are not included here, and must be found elsewhere. Coils make up the majority of control valve applications in our business, and so the material presented here should go a long way in control valve sizing and selection. Yet there are other control valve applications out there, and the designer must be weary of other guidelines that govern valve sizing, for these less common applications. Such applications include hot or chilled water system (pressure) bypass control, hot water system (temperature) reset control, and cooling tower condenser water (temperature) bypass control, just to name a few.
To wrap up the whole topic of valve sizing, we have to at least make mention of steam valve sizing. Okay. For two-position control, the steam valve should be line size, and for proportional control, the valve will be one or two sizes smaller than line size. Enough said? Well, there is a formula for sizing proportional steam valves as there is for sizing proportional water valves. It’s more complicated, though. Generally, tables are available that will allow you to find the required Cv for a steam application, given that the steam pressure and the steam capacity are known. This is the simplest way to size proportional steam valves; find the required Cv from the table, and then find a valve whose Cv rating most closely matches. For steam, it’s most recommended to “err on the high side”; if the required Cv falls between two choices, pick the larger of the two.
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