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Valves are installed in piping systems to control the flow of fluids in a pipeline. There are many types of valves, and each has applications along with advantages and disadvantages. The following discussion covers the common valve types used in commercial and industrial construction and some of the valve components, such as the stem, seat, bonnet and end-connection options available from the various valve manufacturers.
The plumbing codes identify the standards approved for pipe materials and fittings but not specific valve standards, except for some temperature-control valves. The code requires the materials to be approved by National Sanitation Foundation for drinking water when that applies. Materials should be in accordance with the temperatures, pressures and material quality required for the application.
• Valve pressure ratings. Most valve manufacturers rate their valves in terms of saturated steam pressure or the nonwater hammer pressure for water, oil or gas. Valves typically have a valve pressure rating and other standards that they are tested and listed to cast into the body of the valve, indicating the standard for which they meet. These markings help identify the intended service application and pressure ratings.
The ratings usually appear on the side of the valve like this: “125 S,” possibly above another set of numbers that say “200 WOG.” The S stands for the saturated steam pressure rating for the valve, and WOG stands for the water, oil or gas pressure rating for the valve.
Valves tested by the Factory Mutual Global Insurance Co. carry an “FM” designation to show they have been tested and are listed for fire protection service. Another testing and listing agency is Underwriters Laboratories; valves meeting its criteria carry the “UL” symbol. Often in critical facilities, one of the major insurance carrier listings will be required to ensure a safe system and to minimize large losses that an insurance company might have to pay out.
• Valve end connection types. Usually, valves 2 1/2 inches and smaller in commercial and industrial or potable water applications are made of copper alloy (brass), stainless steel or one of many types of plastic. They can include screwed, flared, brazed, soldered, pressed or push-fit ends on the valves. Valves 3 inches and larger typically have flanged ends, but they also can have welded or grooved ends.
The valve end connection should be matched with the service that the valve will be installed. The end connection types can vary depending on the pressure or type of fluid or gas in the system and the compatibility of the fluid in the system. Recently, I have seen press-fit valve ends used without an adjacent union. This forces maintenance personnel to shut down the main or the building service entrance valve when a valve must be removed and replaced.
• Valve bonnets. Valve bonnets are the top part of the valve holding the seal or packing material between the bonnet and the valve stem. Valve bonnets are the same for gate, globe and angle valves. The valve packing material keeps the fluid from leaking out around the valve stem. Several valve bonnet styles are available:
1. Screwed bonnet. This is a simple design and probably the most common gate valve bonnet. It is typically only available on brass valves.
2. Union bonnet. A union bonnet holds up better for disassembling and reassembling the valve. Typically, union bonnets are only used in smaller brass-body valve sizes.
3. Flanged bonnet. A flanged bonnet is typically used in larger pipe sizes and for higher pressure and temperature applications. The flange and gasket materials for flanged bonnet valves need to be matched for the intended service.
The primary function of a gate valve is to serve as an isolation or shut-off valve in a piping system. Gate valves should be fully open or fully closed. When gate valves are fully open, they typically have the least resistance to the flow of all the valve classes.
The design of a gate valve does not lend itself to regulating or throttling flow. If the valve is partially open, it can create a chattering noise and vibration that subjects the valve seats to excessive wear. The gate on a gate valve extends past the waterway, so it is difficult to throttle with a gate valve because it does not open the waterway when the valve is turned for the first round or two.
Gate valves come in a variety of seats, body styles and discs. For domestic water service, the following are the most common choices:
1. For ordinary pressures and temperatures in commercial construction, valves 2 inches and smaller are typically bronze valves with bronze seats and discs.
2. For ordinary pressures and temperatures in commercial construction, valves 2 1/2 inches and larger are typically ductile- or cast-iron valves with bronze discs and seats.
3. For hard-to-hold fluids and gases, there are nonmetallic composition discs. Many materials are available such as steel, stainless steel and bronze or iron with inserted seats.
• Gate valve discs. Gate valves have several options for the type of disc used to seal off the flow of fluid in the piping system:
1. Solid wedge disc (single conventional disc);
2. Double disc (back to back);
3. Split wedge disc (spreading device);
4. Knife gate (a vertical sliding plate for large sewage valves).
• Bypass valves around gate valves. Small bypass or pressure equalization valves should be provided for larger gate valves where the pressure differential exceeds 200 pounds/square inch (psi) on valves sized 4 inches through 6 inches and 100 psi on valves 8 inches and larger. The pressure equalization line is needed because the pressure against one side of a closed valve disc can be so great it makes it difficult, if not impossible, to open a valve with high pressure pushing the disc against one side of the seat.
A bypass valve helps to equalize the pressure across the valve and should be at least 1/2 inch for 4-inch valves and 3/4 inch for 5-inch or larger gate valves with a significant pressure differential. Bypass valves allow the operator to equalize the pressure on both sides of the valve; it makes his job much easier, along with extending the life of the gate valve by reducing wear and tear.
• Gate valve stem type. Gate valves have an option for the stem operation type.
1. Rising stem with outside screw and yoke (OS&Y). This valve offers a visual indication of whether the valve is open or closed. The hand wheel on an OS&Y valve does not rise, but the stem does. Indicating-type valves are required for fire service lines so firefighters can visually observe a valve to determine if it is open or closed. This makes an OS&Y valve good for fire protection service because it has a rising stem.
Rising stem valves need adequate clearance for the valve stem in the fully open position. When planning space for valves in mechanical rooms, consider how far the stem will extend on an OS&Y valve. These valves are also recommended for services where high-temperature corrosives and solids might cause damage to inside valve stem threads.
2. Rising stem, inside screw. This valve type needs adequate clearance for the hand wheel and the stem as they rise. When the valve is opened, both the hand wheel and the stem are fully extended. The valve disc or wedge rises on the end of the stem.
3. Nonrising stem, inside screw. The nonrising stem valve uses female threads inside the valve disc; the disc rises on the stem’s threads, but the stem does not rise. Minimum clearance is required for a nonrising stem valve when it is in the open position.
A globe valve is named for the shape of its body. The body is more rounded because of the seat pattern and waterway path inside the valve body.
It has much more resistance to flow than a gate valve when wide open. The flow of water enters the valve flowing horizontally, changes direction to go through the horizontal valve seat in a vertical flow pattern, and then changes back to a horizontal flow at the valve exit.
The globe valve is used to regulate flow and as a throttling device. It is often used in bypass piping around pressure-reducing valves and other apparatus. The effects of seat erosion (wire drawing) are less than the gate valve because all contact with the seat and the disc ends when flow begins.
The operator can gauge the rate of flow by the number of turns of the wheel. If the valve is turned 10% of the number of turns from fully closed to fully open, it will be 10% opened off the valve seat. Compare that to a gate valve; at 10% of the turns, the valve may barely allow water to leak because the gate valve disc passes the edge of the valve seat. The percentage it passes depends on wear and disc type. The disc of the globe valve travels a relatively short distance between fully open and fully closed, so it takes fewer turns to open and close.
• Globe valve discs and seats.
1. Conventional disc: Flat with beveled edges.
2. Plug disc: Similar to conventional disc.
3. Composition disc: More tapered, fits not into but over the opening.
An angle valve is similar to the globe valve for the inlet and valve seat arrangement, except the outlet is 90 degrees to the inlet. It can cut down on installation costs and time by acting as an elbow and a valve. However, it is less resistive to flow than the globe valve because the water does not change directions as many times in the valve body.
Angle valves are common in fire sprinkler and standpipe systems. They are typically used for system main drains and hose valves. Fire hose valves require hose threads matching the local fire department hoses on the outlet; they should be listed and approved for fire service.
The ball valve gets its name from the drilled ball that swivels on its vertical axis and is operated by a handle.
Its advantages are its straight-through flow, minimum turbulence, low torque, tight closure and compactness. Commercial-grade ball valves typically include a reduced port and can cause some resistance to flow. If pressure drop or flow is important, a full port valve should be specified.
Most medical gas valves are ball valves and should be full-port, three-piece ball valves. Medical gas ball valves typically come with pipe extensions to prevent the heat from brazing operations from damaging the valve seats.
The quarter turn of the handle from fully open to fully closed makes it a quick-closing valve. A water hammer arrestor should be used when installing this valve in systems with potentially high flow velocities.
Reliability, ease of maintenance and durability makes the ball valve popular in industrial, chemical and gas transmission applications.
The butterfly valve gets its name from the rotating disc in the center of the valve, similar to a butterfly flapping its wings when opened and closed. Butterfly valves are often used in place of a gate valve to save money in an installation. They are not suited for slurry lines or process lines where solids are piped in solution. When the valve is in the fully open position, the butterfly disc is sideways in the middle of the pipe, so when a full port valve is required, a butterfly valve may not be acceptable.
Butterfly valves can have screwed or soldered ends in smaller brass or stainless-steel body sizes. In smaller pipe sizes, ball valves are still more common than butterfly valves because ball valves offer less resistance to flow and the costs are similar.
Most butterfly valves are used in 3-inch and larger piping with flanged connections. Butterfly valves with wafer lugs allow long bolts between two flanges to be used to hold the valve body in between two flanges.
Swing Check Valves
Swing check valves are typically used for flow-directional valves. They are not backflow preventers, although they operate in a similar fashion; backflow preventers are testable and check valves are not testable. Check valves simply prevent reversal of flow. Because seat fouling can occur, without a way to test for fouling, it is not recognized as an adequate backflow prevention device.
A swing check valve may remain open; upon a significant flow reversal, it may cause water hammer in the piping system. Spring-loaded check valves help to prevent slamming from a reversal of flow. Swing check valves are often referred to as horizontal swing check valves. These valves must be installed in the horizontal or up-feed position only. Installing them upside down or in a down-feed position would cause the valve flapper to hang open by gravity.
If a check valve is required in an unusual location, a spring-loaded valve could be installed, or the piping could be rerouted to accommodate the manufacturer’s recommended installation position.
Lift Check Valves
A lift check valve’s body is similar to a globe valve, but the disc has no stem. Gravity holds the disc closed, and the flow is up and lifts the disc off the seat. The lift check valve disk includes guides to keep the disk straight in the valve body. Lift check valves are primarily used for gases and air, but they can be used for clean liquids.
Water Pressure-Reducing Valves
There are two types of pressure-reducing valve (PRV) body designs: direct-acting and pilot-operated. The direct-acting type has a globe valve body design with an adjustable spring and stem to hold or apply pressure down against the valve seat. The water pressure pushes up from below and forces the valve open.
Tightening or loosening the nut on top of the direct-acting PRV adjusts the pressure on the spring on the top of the valve and can adjust the outlet pressure setting of the PRV. All PRVs should have a pressure gauge on the inlet and outlet pipes inside the isolation valves.
The pilot-operated PRV is typically a larger valve and has a tube with a small direct-acting PRV in the control line of the valve that exerts pressure on the top of a diaphragm and assists the spring with operating the valve. The pilot line allows pressure on the diaphragm to assist the spring, which forces the valve closed or resists flow, causing a controlled pressure drop across the valve seat.
Tightening or loosening the nut on the pilot-line PRV adjusts the pressure on the spring on the top of the valve, which adjusts the outlet pressure setting of the PRV.
I have seen many high-rise and tall buildings experience PRV failures when the plumbing system relies on them. All valves fail over time; the failure rate depends on the pressure differential across the valve. Higher pressures will cause wire drawing or velocity erosion failure of the valve seats.
When PRVs fail, pipe or fittings burst or plumbing fixtures violently fracture in these tall buildings; they experience flooding and extreme costs for cleanup and repairs. The taller the building, the more extreme the pressure differentials will be and the more often the PRVs fail.
Since PRVs will fail, a preventative maintenance plan must be in place to periodically remove valves from service and inspect or replace valve seats and valve stems. PRVs cause a massive waste of energy!
I have always taught plumbing designers, contractors, inspectors and engineers in my high-rise plumbing design classes how to design a high-rise, tall or mega-tall building without using PRVs. Good plumbing design eliminates or minimizes the use of PRVs by using pressure zones with dedicated express risers.
A design using dedicated pressure-zone booster-pump packages for high-rises and tall buildings can save at least 75% on the power used to run a large, single booster-pump package that boosts the pressure and volume for the entire building with multiple pressure zones. The PRV design boosts the pressure, then scrubs off the energy with wasteful and maintenance-prone PRVs that fail on a regular basis.
I have always said that using one booster-pump package and PRVs in a high-rise building is like driving down the street with your foot pressed all the way to the floor on the gas pedal and controlling your speed with the brake pedal. That design wastes tons of energy, uses the maximum amount of energy, wears out pump impellers and erodes PRV seats.
When designing mega-tall buildings that get beyond normally available pressure ratings on piping, valves and fittings, the designer should consider the use of break tanks for the water risers because of the extreme pressures on the piping system. These pressures can create problems for PRVs — problems acquiring more expensive, higher pressure-rated pipe and fittings that are suitable for potable water.