Basics of Control Valves and Sizing

General Valve Terms and Definitions

Valve - A valve is a device that regulates, directs, or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways^[1].

Normally Open - A valve that opens as the system demand decreases. 

Normally Closed - A valve that closes as the system demand decreases.

Fail Open - An actuator that goes to the open position, to allow flow through the valve, when there is a power failure or loss of signal. 

Fail Close - An actuator that goes to the closed position, to cut off flow when there is a power failure or loss of signal.

Fail In Place - An actuator that remains in the last commanded position when there is a power failure or loss of signal.

2-Way Valves - 2-way valves have one input and one output. These are used in applications where flow in the system is variable. If the system has variable flow (i.e. VFD's) flow can be decreased as valves shut to maintain system pressure. One common application is on boiler isolation valves.

Figure 1: 2-Way Valve

3-Way Valves - Used when flow either needs to come from 2 locations and go to 1 (mixing valve) or flow needs to come from 1 location and go to 2 (diverting valve). These are used in applications where flow to the system needs to remain constant or in variable flow systems to maintain minimum flow. 

Figure 2: 3-Way Valve

Valve Connection Methods ^[2]

Valve connections are the components of a valve that connect to the plumbing and allow the valve to join with the piping system. There are six main types of connections used to install valves and other process components in piping systems, and each type has its pros and cons:

• Threaded
• Welded
• Flanged
• Compression
• Union
• Manifold-mount

The three most common connection types in the HVAC industry are threaded, compression, and flanged. 

Threaded Valves

Figure 3: Female Threaded Valve Connection

Threaded valve connections are one of the most common valve connection types. This valve style incorporates a screw-type design to integrate with the piping system. There are three primary configurations for threaded valve connections:

• Female x Female
• Male x Male
• Male x Female

Threaded valve connections provide a tight seal and streamlined connection between the valve and pipe. Valves will most commonly have female-threaded connections that fit over the end of a male-threaded pipe. The threads on both the valve and pipes must adhere to the same standard thread design to be compatible.

Valves that incorporate threaded connections are easy to install, maintain, and replace, are inexpensive, and are ideal for smaller applications. Threaded connections are typically preferred in applications that feature valves with a diameter of less than 4 inches. This is because connections with larger diameters are more difficult to seal and thus prone to leaks through the threads.

Even with smaller threaded valve connections, pipe tape or sealant between the male and female threads is recommended as a precaution. Both provide extra sealing, and sealant also acts as lubrication and prevents metal-to-metal contact and galling.

Flange Valve Connections

Figure 4: Flange Valve Connections

Flanges are solid metal plates with bolt holes that surround the edge of a valve connection. When a flanged valve is installed in a piping system, the plate can be bolted to another flange on the pipe, creating a secure connection. Flanged connections are often seen in valves larger than 4 inches in diameter and are common in industrial applications. Flanges are easy to install and remove for routine cleanings.

Compression Valve Connections

Figure 5: Traditional Compression Valve Connection

There are three types of compression connections:

Traditional Compression: A threaded nut and a soft metal cuff called a ferrule are slid over the end of a pipe, then the nut is tightened until it forces the ferrule into the valve socket. The ferrule is compressed between the valve and the compression nut, forming a leak-tight seal. Traditional compression connections are often found in residential plumbing.

Push-to-Connect: An O-ring inside the valve port that is slightly smaller than the outer diameter of the connecting pipe gets stretched around the pipe and compressed between it and the valve body. A grab ring with teeth digs into the pipe and prevents it from slipping out, but the pipe can be easily removed by pressing the release ring to retract the teeth. Push-to-connect solutions are often found in residential plumbing systems.

Barbed Hose Connections: Ideal for low-pressure systems, these connections are used to join a soft hose to a valve. The valve has a long end connection with barbs on the exterior, which bite into the inside of the hose as it’s stretched over the connection. The barbs hold the hose in place fairly well, but a hose clamp is recommended to strengthen the connection.

Control Valve Types

Globe Valves

Globe valves operate by using a plunger to allow or block flow through an opening in the valve body. For valve sizes from 1/2" – 2-1/2” threaded connections are used. For valve sizes from 3" - 6" flange connections are used.

Figure 6: 2-Way Globe Valve with Threaded Connections

Figure 7: 2-Way Globe Valve with Flange Connections

 Figure 8: 2-Way Normally Open (Left)  2-Way Normally Closed (Right)

Figure 9: 3-Way Mixing (Left) 3-Way Diverting (Right)

 Characterized Ball Valves

Ball valves operate by rotating a ball with a hole through it called a port. When the port aligns with the direction of flow, it is opened. When the port is perpendicular to the direction of flow, the valve is closed. The valve can be opened partially to modulate flow.

Ball valves only come in threaded connections and sizes 1/2” – 3”. 

Figure 10: 2-Way Valve with Threaded Connections

Butterfly Valves

Butterfly valves operate by rotating a plate to open or close the valve. The valve can be opened partially to modulate flow. 

Butterfly valves only come in flange connections and sizes 2-1/2” – 12”. 

Figure 11: 2-Way Butterfly (Left) and 3-Way Butterfly (Right)

Pressure Independent Control Valves (PICVs)

PICVs are a type of valve found in hydronic systems that provide heating and cooling in buildings. These valves are several different valves conveniently combined into one unit. This saves on design and installation time as well as improving the efficiency of the system. They have two main functions which are to control the amount of liquid flowing through a pipe and to automatically adjust and compensate for pressure fluctuations in the system to maintain stable and reliable control. For more information about PICVs and how they work please visit The Engineering Mindset (external link).

PICVs only come in threaded connections and in sizes 1/2" and 3/4".   

Figure 12: PICV Older Belimo Design

Figure 13: PICV Newer Belimo Design

Electronic Pressure Independent Control Valves (EPIV)

The EPIV is a pressure-independent control valve that incorporates a flow meter and a 2-way control valve. The actuator has an algorithm that modulates the control valve to maintain the flow regardless of variations in system differential pressure. In addition, the EPIV provides feedback to the BMS system. Depending on the system requirement, this feedback can be configured to be either True Flow or Valve Position^[3].

EPIVs come in threaded connections, in sizes 1/2" – 2” and in flange connections in sizes from 2-1/2” – 6”.    

Figure 14: EPIV with Threaded Connections

Control Valve Characteristics

Heat Transfer

Heat transfer through a coil is not linear. 

Figure 15: Heating Coil Characteristic

Figure 16: Cooling Coil Characteristic

For a 2-position valve, this is not an issue (flow is either all or nothing). However, for the best modulating control, we want a linear heat transfer throughout the modulating range of the valve. The control valve should have an equal percentage plug (i.e. nonlinear).  

Figure 17: Valve Plug Options

Figure 18: Valve Plug Flow Rates

Theoretically, this will result in a linear heat transfer. In practice this results in a reasonably close approximation of the ideal.  

Figure 19: Theoretical Heat Transfer by Plug Type

Figure 20: Actual Heat Transfer by Plug Type

To make a ball valve equal percentage, a characterizing disc is added to the valve.

Figure 21: Cutaway of a Characterized Control Valve

Figure 22: Flow through a Characterized Control Valve

Butterfly valves are only linear in a certain range. Because of this butterfly valves should be limited to 60% - 70% angular travel.

Figure 23: Typical Butterfly Valve Flow Characteristic

Valve Authority

Valve authority relates to the shifting of the equal percentage curve based on varying system pressure. Valves should be sized for the maximum allowable pressure drop across the valve and the system pressure should remain as constant as possible. Pressure-independent control valves are designed to keep the flow rate linear even with varying pressures.

Figure 24: Control Valve Characteristic and Authority

Cavitation

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. 

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. 

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. 

Figure 25: A Cavitated Valve

Valves usually need to be downsized, relative to the pipe size, to obtain the proper pressure drop across the valve. As a general rule of thumb, the size should not be reduced more than 2 pipe sizes to prevent cavitation.

Control Valve Sizing

Pressure-Dependent Water Valves

To size pressure-independent control valves the following information is needed:

Q – Maximum water flow through the valve/coil (gpm)

ΔP – Maximum pressure drop allowed across the valve (psi). Most engineers specify 10 ft. head (4.33 psi)

Cv – Valve flow coefficient. The amount of 60F water in gpm that will flow through the valve with a pressure differential of 1 psi.

Pipe Size – The size of the pipe that the valve will be installed in.

Steps for sizing

1. Calculate the ideal Cv of the valve using the following formula:
2. Select a valve with the closest available Cv that is within two sizes of the pipe diameter.
3. Calculate the actual pressure drop across the valve using the following formula
4. Select a valve actuator with the proper failure position, control signal, and close-off pressure.
   1. H.W. Coil valves should fail open.
   2. C.W. Coil valves should fail in last position or closed.

Pressure Independent Water Valves

PICVs are sized/selected by flow.

Control Valve Schedule

Figure 26: Examples of Control Valve Schedules

Belimo Select Pro

Since we mainly buy Belimo valves through Columbus Temperature Controls, use the Belimo SelectPro sizing tool. There is an online version and a downloadable version which can be found here Belimo Sizing Tool.

Figure 27: Belimo Sizing Tool

References and Citations

This article has been adapted from documents and presentations by Brian Bowers

[1] https://en.wikipedia.org/wiki/Valve

[2] https://www.geminivalve.com/category/valve-education/

[3]https://www.belimo.com/mam/americas/technical_documents/Tech%20docs/belimo_epiv-ultrasonic_technical-documentation_en-us.pdf

Image Credits

Figure 1: https://www.sauter-controls.com/

Figure 2: https://omegavalves.com/

Figure 3: https://www.supplyhouse.com/

Figure 4: http://www.tech-threads.com/

Figure 5: https://blog.boshart.com/