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What are the most common valves and their functions in HVAC & Hydronic systems?

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521 views asked Dec 30, 2015 by Devan Talebi

2 Answers

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Gate Valves. Gate valves are commonly used today as shutoff valves in steam pipes and underground water pipes. Formerly, they were common in indoor hydronic systems, but they have largely been replaced by ball and butterfly valves. Partially, this is because of cost, but also gate valves have a reputation for eventually sticking fully open or fully closed.

 

 

Ball Valves. Improvements in elastomer technology have made ball and butterfly valves very reliable. Ball valves are probably the most common valve type in most hydronic systems. They are inexpensive in sizes up to approximately 3 in. (76 mm). In large sizes, the amount of metal used in the ball makes ball valves more expensive. Ball valves are available as manual shutoff valves, or with motorized operators for control valves. Specifying full port valves is appropriate when system total head is a concern. This is because free area within the valve assembly is maintained when in the full open position, minimizing head loss. Either soldered or screwed connections are acceptable. But if soldered connections are used, the valve must be rated for soldering at the temperature that is needed for the solder specified without the ball removed. Often, handle or lever stem extensions are required, so the handle is outside of the insulation.

Butterfly Valves. Butterfly valves are the most common valve type for 2½ in. (64 mm) and larger sizes. This is because large butterfly valves are less expensive than other large valves, and because butterfly valves are reliable. Manual butterfly valves use a 10-position locking lever for operation. Above approximately 6 in. (152 mm), however, a gear operator is used to reduce the manual force (strength) that is needed to open and close the valve.

Butterfly valves are generally applied in large piping applications. Their installation is different than for ball valves because large piping commonly uses flanged connections. One option is wafer-style valves with no threaded bolt holes. This option is the least expensive but limits the system maintenance flexibility. Another option is fully lugged butterfly valves that have the same number of lugs as pipe flanges. When rated for the system maximum pressure, and with bolts extending to the midline of the valve, disassembling of piping on one side of the valve without draining the entire system is possible without additional flanges. Semi-lugged valves have lugs, but not as many as the pipe flanges. They normally will require draining of the system for disassembly unless the maximum system pressure is low. Semi- lugged valves are less expensive than fully lugged valves.

 

Globe Valves. Globe valves are generally more expensive than ball or butterfly valves, and they have higher pressure drop for the same connection size because of the tortuous path water takes through the globe valves. Although less common today due to the introduction of ball and butterfly valves, the most common uses are as small control valves in water systems and for steam control valves.

Three-Way Valves. Three-way valves can either be mixing valves or diverting valves that have one inlet and two outlets. Mixing valves are more common because the operators required are smaller and because it is usually possible to pipe systems so a mixing valve can be used instead of a diverting valve. Older hydronic systems typically used three-way control valves to allow a constant flow of water in the main pipes, while modulating the amount of water that was routed through devices based on demand. This design wasted significant pumping energy because the systems operated at maxi- mum flow and head continuously.

Check Valves.  Check valves allow flow in one direction and pre- vent flow from reversing. Check valves are installed in hydronic systems to protect equipment that can be damaged by reverse flow, and to prevent reverse flow upon system shutdown (typically seen at pump discharge and steam traps). The three types of check valves commonly used in hydronic systems are swing, double disc, and silent.

Swing check valves use gravity to close a swinging disc. They can be installed in horizontal pipes or vertical pipes with upward flow. They cannot be installed in pipes that flow downward. They are most commonly specified in applications up to 2 in. (51 mm) pipe size. One disadvantage of swing check valves is that they close slowly and can “hammer” if flow reverses.

Double disc check valves have a center post, two pivoting discs, and a spring coiled around the center post. Double disc check valves are commonly used in 2½ in. (64 mm) pipes and larger. They have the lowest pressure drop of the common check valve types, and close fast enough to prevent water hammer in nearly all applications.

Silent check valves are also known as center-guided check valves and are commonly used in applications in reverses. The main disadvantage of silent check valves is their high pressure drop.

Balancing Valves. Balancing valves are used to impose artificial head in all pipe routes besides the critical one to prevent short circuiting (excessive flow through lower pressure drop paths that results in insufficient flow through the highest pressure drop path). These valves are usually located on the return side (outlet) of the device as because this subjects their elastomers to lesser extremes of temperature and pressure, extending their lives. The most common type of balancing valve consists of a variable orifice and two pressure taps to measure the pressure differential across the valve. The flow rate is determined by measuring the pressure drop and noting the opening position of the variable orifice. Then a chart is used to find the flow rate.

Another type of balancing valve uses a flow sensor to measure the flow rate, plus some type of throttling valve, impeller trimming, or a variable speed drive to limit the maximum flow rate. This approach is generally more expensive, but has lower pressure drop and reduces pumping energy.

Automatic flow-limiting valves are preferred by some designers. They consist of a spring and variable orifice to limit the flow rate to the maximum intended for that flow path. They are commonly applied on heat pumps.

One apprehension with this type of control device is that there is often no way to measure flow other than to assume that the automatic flow limiting valve is operating properly. There is also no way to adjust the maximum flow rate without replacing the automatic balancing valve. The advantage is that no manual balancing is required if they operate properly and are not fouled by debris in the piping.

answered Dec 30, 2015 by Devan Talebi
0 votes
Ball valve using commonly at homes and at industrial level
answered Jan 13, 2016 by Qasim Tariq

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