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Actuator
Valve and actuator assembly installed in a system
Valves

Actuator

Resolution

Resolution is defined as the least interval between two adjacent discrete positions which can be distinguished from one another. Basically, what this is telling us is the number of discrete positions an actuator can achieve within a given control signal. It is usually expressed as a percent of span. For example, an actuator with 100 discrete positions across its control signal has a resolution of 100:1 or 1%. An actuator with 50 discrete positions has a resolution of 50:1 or 2%, and so on. The Flowrite and Powermite actuators all have a resolution of 100:1 or 1%. (This means they respond to every 0.1V change on a 0-10V control signal span). Also, the resolution may vary slightly depending on whether the actuator is moving forward or backward, but only one overall resolution is usually stated. The term resolution has no meaning with regard to a valve body only.

 
 

Valve and actuator assembly installed in a system

Turndown Ratio

Turndown Ratio is defined as the ratio of maximum usable flow to minimum controllable flow at a constant pressure drop. This is the ratio that takes both valve and actuator into account when installed in a system. The key here is maximum usable flow. This may differ from the maximum controllable flow used to calculate the rangeability. The maximum usable flow is the maximum flow that generally occurs in a given system. It will vary from application to application. Consulting engineers will attempt to select valves and actuators that provide the greatest turndown ratio for a given application. For example, if a valve has a maximum controllable flow of 50 GPM, maximum usable flow of 30 GPM, and a minimum controllable flow of 1 GPM, then the rangeability is 50:1 and the turndown ratio is 30:1 at a given pressure drop.

The net result then is this:

Rangeability: This is really only a way to compare how precisely manufactured are two similar styles of equal percentage valves. Comparing which will keep actual flow on the flow curve longer at the very low end.

Resolution: Does your customer want to purchase a valve and actuator combination with 100 discrete positions of control from Siemens, or one that offers only 80 discrete positions of control from Belimo?

Turndown Ratio: This is the key component, and yet the valve and actuator manufacturer has no control over it. This depends on the particular valve and actuator selected under system operating conditions.

 
 

Valves

Rangeability

Rangeability is a characteristic of the valve body only and applies only to equal percentage valves. The term has no meaning with regard to linear valves. Rangeability is defined as the ratio of maximum controllable flow to minimum controllable flow at a constant pressure drop. It is a relative term used to compare the control quality of any two valves. A rangeability of 100:1 tells us that the valve does "as good a job" of controlling when flow is 100 times the minimum controllable flow as it does at the minimum controllable flow. "As good a job" means that the flow follows the equal percentage curve plus or minus a 10% tolerance. The ratio gives us the minimum flow rate at which a valve's flow control remains on the equal percentage curve. For example, a valve with 100:1 rangeability and a Cv of 10 tells us that, at 1 PSI pressure drop across the valve, the minimum flow through the valve that still follows the equal percentage curve within 10% tolerance is 0.1 GPM. Below 0.1 GPM, the valve will still control, flow just won't follow the equal percentage curve.

Ball valves have an extremely high rangeability. The reason for this is the low end of their equal percentage flow curve very nearly approaches zero before flow leaves the curve. Therefore, it makes the rangeability appear very high since it represents the maximum controllable flow versus a very small minimum controllable flow. However, don't forget that all this means is that a ball valve can keep flow on the equal percentage curve at extremely low flow rates. This is unimportant unless you have vastly oversized the valve. Also, it is important to note that since the flow follows the equal percentage curve so precisely at the low end, a ball valve may have to rotate quite a bit (perhaps up to 10-20%) before flow changes even become noticeable. This may limit the effectiveness of the attached actuator.

Rangeability has nothing to do with the number of discrete flow control positions a valve can produce. This is a function of an actuator's resolution...