By Rick Mohammed
Control valves appear in many industries, from nuclear, oil and gas, and pharmaceutical to name a few. Control valves have adapted to manage the flow of liquid, pulp, and even sewage. For the purpose of this article, we will focus on control valves used in hydronic applications.
Types of valves
There are three main types of control valves you will typically find in any hydronic system. They are known as characterized ball valves, globe valves, and butterfly valves. In addition, there are pressure independent control valves and six-way valves that are based on the basic ball or globe valve design.
CHARACTERIZED BALL VALVE
The characterized ball valve was introduced to the HVAC market in 1999 and it is an innovation of the standard ball valve used for isolation or shut off purposes. The ball valve operates in a quarter turn motion to open and close the valve. When the ball is in the closed position it has an ANSI class VI rating defined as bubble tight. This means there is no leakage of flow when the valve is fully closed.
Introducing a characterized disc in front of the ball, as shown in Figure 1, transformed the shut off ball valve into a game changing modulating control valve which can be used in most hydronic applications. There are versions of the characterized ball valve that can be used for 15 psi steam applications too.
Before the characterized ball valve gained popularity, the globe valve was the standard control valve used on coils in air handling units, VAV reheat coils, and perimeter rads. Globe valves operate in a linear motion to open, close and modulate the flow. The plug, as shown in Figure 2, moves up and down and is driven by an actuator. For applications with high inlet water pressure, the actuator size gets bigger and adds cost to the overall valve and actuator assembly. As a result, the close off pressure of a globe valve must be considered during the selection process.
Globe valves can be used for water applications and steam as high as 50 psi. They are available with an ANSI class leakage rating of Class III or IV. This means when the globe valve is fully closed it will pass 0.1 per cent or 0.01 per cent of the rated flow. If a globe valve is too large for an application it can remain at a minimum position during low load periods for an extended amount of time. This creates a high pressure drop and high velocity through the valve causing excessive wear on the plug, and eventually the valve will leak when closed.
What are ANSI Leakage Rates?
There are six classes rating the amount of leakage a valve has when fully closed. Class I being dust tight and Class VI being bubble tight.
Valves come with an ANSI Class body pressure rating. This is the maximum pressure a pipe or valve material can handle based on temperature of the medium.
The BFV (butterfly valve) works in a quarter turn motion like a ball valve. Unlike a characterized ball valve and a globe valve, a BFV offers a large opening for applications that require high flow in a short time. The face-to-face dimension is very small compared to other valve types making it a compact control valve. They can be used for modulating duty between the 0⁰ to 60⁰ range of the stroke. Between 60⁰ to 90⁰ of the stroke there is little change in flow. Like the ball valve, BFVs provide a Class VI close off. A close off pressure of 200 psi is commonly offered but to save cost a 50 psi close off is available when pressures are lower.
BFVs come with an EPDM seat also known as a soft seat as shown in Figure 3. EPDM stands for Ethylene Propylene Diene Terpolymer. It has good resistance to heat and cold, and its elasticity property helps provide a tight seal when the valve closes. When selecting a BFV the velocity of the fluid should be considered. The velocity limits are 12 FPS for standard BFVs and 32 FPS for high performance BFVs. Exceeding these velocities will erode the EPDM seat material overtime causing leaks.
What is an ANSI flange connection?
It is a standard that defines the bolt circle, number of bolts and bolt size for a flange.
Try selecting a BFV with an actuator that closes on torque instead of position. Over time the position setting may change as the EPDM seat wears.
Performance in dirty water
Characterized ball valves are versatile because they come with a high close off pressure rating and they can be used in any hydronic application. Unlike globe valves, the characterized ball valve doesn’t rely on the actuator to achieve its close off pressure rating. One relatively small actuator mounted to a. 1/2 in. valve will provide 200 psi close off, and 100 psi close off on a four-inch valve.
Another advantage of the characterized ball valve is its ability to operate with dirty water. The quarter turn action cuts away at the debris found in many hydronic systems. Other factors to consider when selecting a control valve are the body pressure rating, the close off pressure rating, actuator type, height of the valve and actuator assembly, open/close or modulating duty, NEMA 4 enclosure for the actuator if outdoors, and a Canadian Registration Number (CRN) to name a few.
A flow characteristic curve is created when a valve travels from fully closed to fully open. There are two main types, linear and equal percentage. It is important to select the correct flow characteristic for the application to establish good control of the flow. A linear flow characteristic will give you 30 per cent flow when the valve is 30 per cent open, and 50 per cent flow when it is 50 per cent open as shown in Figure 4. This is suitable for perimeter rads that have a linear heat output response. For devices that have coils such as an air handling unit, reheat coil, or fan coil unit a valve with an equal percentage flow characteristic is preferred as shown in Figure 5. When the valve is at 50 per cent open the flow is approximately 12 per cent. The equal percentage flow curve is created by the shape of the characterized disc in the ball valve and by the shape of the plug in a globe valve.
When we compare the equal percentage flow curve to a typical curve created by the power output of a coil, we can see they are a mirror image of each other.
These two curves are shown in Figure 6 with the resulting linear line in the middle. The linear line represents the resulting relationship between flow and coil output power. When the valve is at 50 per cent open, the coil cooling or heating power is also at 50 per cent. This is why we always choose an equal percentage control valve when controlling flow to a coil.
More to come
An incorrectly applied control valve can cause problems with your heating or cooling system and occupant comfort. Correctly sizing a control valve, as well as balancing of the system flow, are areas worth discussing in detail. In upcoming issues, Part II of this series will focus on sizing and selection of control valves, followed by Part III, hydronic system balancing and the effects of low Delta T.