It has become popular in our area to specify 300:1 turndown control valve solutions on HVAC projects. Why is this happening and is it really in the owners best interest?
In the bid and spec world that most HVAC projects are born from a consulting engineer reviews a job and writes specifications that ensure the building owner gets equipment that will work. The lowest price vendor that can provide a product that meets the engineers design specifications wins the job. If you are a vendor and your product has a unique feature such as 300:1 turndown valve solutions, your job is to convince the engineer that your product should be specified. Even if it is more expensive in the long run this product is what is best for the owner. If the engineer specifies your product this is also very good for you as a vendor because you have less competition bidding against you.
This process has lead to the 300:1 turndown valve solutions being specified on many projects in our area. In most cases I agree with value engineering that has the customer get a better product on bid day, but in the case of the 300:1 turndown control valve solution on a commercial HVAC project I believe in most cases an owner is paying for something he or she cannot use.
To explain this I reached out to Mark Harris of Belimo controls to explain turndown and range ability as it applies to HVAC control valves. Belimo is one of the few vendors that offers a 300:1 turndown valve so I thought he would provide a non self serving view point on the subject.
Here is what Mark had to say:”Is a control valve with higher rangeability better? The answer is yes; to a certain point.
There are a number of control valve products on the market claiming extremely high rangeabilities – 300:1 and even higher. The suggestion is that these valves provide better control under part load conditions; thus, justifying their increased cost. But are you wasting your money?
Let’s start off by defining our terms. The range ability of a valve body is defined as: “the ratio of the maximum controllable flow to the minimum controllable flow, expressed as 100:1, 50:1, etc”. It can be thought of as a measure of how finely the valve body can “slice” the maximum flow. For example, if the maximum controllable flow through a valve body is 100 GPM, then a range ability of 100:1 tells means the valve body can accurately control down to 1 GPM. The smallest “slice” that the valve body can control is 1/100th of the maximum. A similar valve body with a range ability of 25:1 can control down to 4 GPM, and so on.
Please note that I continuously refer to “valve body” in the paragraph above, and there is a good reason for that – range ability only refers to the valve body . Are you going to be using a bare valve body in a control application? I didn’t think so…
Range ability vs. Turndown
In order to predict the valve and actuator performance under installed conditions, the industry uses “turndown” or “turndown ratio”. It is a closely related term, and folks very often use it interchangeably with range ability; however, they are not the same thing! Turndown ratio, as applied to a valve, is defined as: “the maximum usable flow to the minimum controllable flow under installed conditions”. Turndown can also expressed as a ratio, like 100:1. It is easy to see why many people get confused!
But there are two key differences. First, turndown can be applied to other things besides valve bodies. It is perfectly acceptable to refer to the “turndown” of a system of components like a control valve that includes a valve body, linkage, and an actuator. You could even talk about the turndown of an entire chilled water system, including the chillers, pumps, piping, control valves, and all the other pieces and parts. But it would be nonsensical to talk about the “range ability” of a chilled water system. Furthermore, it is equally incorrect to talk about the range ability of a control valve assembly, including the actuator.
Second, as stated above, turndown talks about the valve under installed conditions. Range ability describes the valve body under laboratory conditions. Let’s look at an example to illustrate the difference. Let’s say you need a control valve for an application with a maximum flow of 87 GPM. In the catalog, you find that the best selection is the 100 GPM valve body with a 100:1 range ability. Upon installing the valve body, its turndown measures 87:1, not 100:1. The valve can still control down to 1 GPM, but the maximum flow as installed is less than it was in the lab.
This gets really tricky when we consider that the lab defines maximum flow through the valve body, “as the flow with the valve wide open at the rated maximum differential pressure”. When I think of maximum flow while sizing a valve, I think of design flow at the specified pressure drop, say 5 psi. Those two definitions of “maximum” probably are not close to one another.
But for the purpose of discussion, we will concentrate on the first difference…
Valve Turndown is Limited by the Weakest Link
Remember, we defined turndown as the pressure applied to the valve body, linkage, and actuator together. Surprisingly enough, turndown of a control valve “system” is limited by the one component with the lowest turndown or range ability. Consequently, if you install an actuator with a turndown of 100:1 on a valve body with a range ability of 25:1, what is the result? That’s right, the entire assembly is going to have a turndown of 25:1 (assuming that the application provides the maximum flow rating for the valve).
So now, let’s get back to our high range ability valve bodies. The problem is that the best actuators on the commercial market have a turndown of not much more than 100:1. You would have to utilize an industrial actuator costing more than the entire job to exceed that by much. What happens when an actuator with a 100:1 turndown is installed on a valve body with a range ability of 300:1? Yeah, the answer is obvious– you get a control valve assembly with a turndown of 100:1. What good does it do to have a valve body that can “slice” the flow into 1/300th’s when the actuator can only hit every third “slice”?
Expanding our definition of the control valve “system” a bit, you might also be limited by the turndown (resolution) of the analog output from your controller. This will be determined by, among other things, the number of bits the Digital-to-Analog Converter (DAC) uses. Bits, in this context, are the binary software switches used to control the AO. We can think of the resolution in terms of “levels” (or slices!); the AO will result in a number of levels equal to two raised to the number of bits. Therefor, a two-bit AO will result in 4 levels, a 3-bit AO will result in 8 levels, and so on. A 7-bit AO will result in128 levels, equivalent to a turndown of 128:1. An 8-bit AO will have a turndown of 256:1 and so on.
Clearly, the issue is a bit more complex than just “Higher Range ability=Better”. Now don’t get me wrong; high range ability valves are generally high quality products. In fact, Belimo sells a line of these valves on an OEM basis. Mainly because they are able to fill the application niche of extremely high pressures, temperatures, and high pressure steam than previously possible. The point is that these valves are best suited for a range ability higher than 100:1; if you do not need their other features for your application, you are probably wasting money.”