Part 17(c)—Mechanical Design, Valves, Part 3
By Ed Butts, PE, CPI
In the past two installments of The Water Works we introduced designing effective piping for pumping stations and water systems. In the next three parts of this series, we will continue with the various types of valves used for these systems.
Introduction to Valves
The application and location of valves in a water system is critical to the proper and safe function of the system. Valves serve various functions within a pumping and piping system such as isolation, flow regulation, safety relief, non-return (check), and special purposes—categorized in Figure 1 and detailed below in five main groups:
- Isolation (stopping and starting a fluid flow). Depending on whether a valve is open or closed, the system can pass the flow of process fluid unimpeded or halt the fluid flow entirely.
- Throttling the fluid flow. Some valves provide throttling of the fluid depending on the percentage of total opening. In other words, the less the valve opening, the higher the throttling rate will be.
- Controlling the direction of a fluid flow. Check or control valves decide the direction the fluid will go.
- Regulating a flow or pressure within the piping system. Some automatic control valves maintain the flow and pressure within the system by adjusting the opening and closing of the valve and providing a safety relief means for excessive pressure, flow bypass, or other safety purposes.
- Relieve air, pressure, or vacuum from the piping system and equipment. Air, pressure, and vacuum relief valves safeguard the process and pipe system from air entrapment, overpressure, and a vacuum.
In this installment, we will concentrate on isolation and regulating valves. In the next two installments, we will detail check, air release, and control valves. Regardless of the type of service, valves are equipped with materials, construction and body styles, end connections, and pressure ratings to coincide with the type of service and application.
The most common valve types used for potable and non-potable water for isolation or process shutoff include gate valves, knife valves, butterfly valves, ball valves, and plug valves.
A control valve that regulates the flow of fluid falls in the regulation category. Globe, needle, diaphragm, and pinch valves are often used as control valves; this group will be outlined later.
Typically, valves fall into one of three operating categories: linear, rotary, and quarter-turn.
Linear motion valves use a closure element that moves in a straight line and uses the element to start, stop, or throttle the flow. The element could be a disc or flexible material such as a diaphragm. Linear motion valves are slower in operation, but they provide a higher level of accuracy and stability in the closure element.
Rotary motion valves rotate a disc or swing it from a hinge pin that holds the disc. Swing check valves are examples of rotary motion valves. A 90° turn of the stem in quarter-turn valves fully open or fully close the valve. Because of this rapid turn, the operation of quarter-turn valves is much faster than linear motion valves. Some rotary motion valves are also known as quarter-turn valves.
The substance or media flowing through and in direct contact with a valve will often determine the type of material used. The material classification generally refers to the materials used for the valve body and the trim.
The valve body is comprised of the main part of the valve that encloses the other components and fluid, and may be produced from one material with the trim made of the same or different material. The internal components, or the valve trim, use materials specifically intended for service and exposure to the applied fluid, with velocity or pressure drop through the valve as the overriding factor.
Stainless steel, cast or ductile iron, and bronze are common for the disc. Steel, stainless steel, and various rubber compounds for the seat are the most common trim component materials.
Since valve trim is directly exposed to the flow media, it may be made of a different material than the valve body—most check valves are examples of this difference. By design, the trim is normally more resistant to corrosion and velocity (erosion) than the valve body. Therefore, corrosion and erosion are less likely to occur at the valve seating area.
Some valve materials are best suited for high temperature, while others are specifically used for corrosion or erosion resistance. In addition, some valve materials are selected when low cost is the primary factor.
Pressure drop or loss across a valve (between the inlet and outlet) is typically the most critical design factor. This becomes a critical factor if the valve is used for throttling or operates with a distinctive pressure drop across the valve.
Pressure drop through a valve is typically tested and reported in the Cv or the pressure drop in psi. The Cv refers to a valve’s flow coefficient for the capacity of valves. This allows different sizes, types, and manufacturers to be compared.
The flow coefficients are in general determined experimentally and express the flow capacity in units as: the rate in GPM (gallons per minute) that a valve will pass for an equivalent pressure drop of 1 lb/in2 (psi). Thus, a Cv flow coefficient of 10 indicates that a 1 psi pressure drop will occur with a 10 GPM flow of water through the valve.
Two other factors often used to compute the head loss through a valve is either the use of an appropriate K factor for use in the Darcy-Weisback formula or equivalent feet of pipe for use with the Hazen-Williams equation.
Most valve bodies are constructed from plastic or bronze (≤ 4 inches), cast iron, ductile iron, forged or cast steel, stainless steel, or an alloy consisting of various metallurgies such as nickel and steel. Valves for water and wastewater service are available with various end connections including threaded, solvent weld, or soldered (typically used for PVC or bronze valves 4 inches and smaller), Class 125- and 250- pound flanged, wafer, and lug style (to fit between two sealing flanges), mechanical joint, solder or brazed, grooved, spigot, butt-weld, and specialty threaded or solvent-weld ends for PVC, HDPE, and other plastic pipe connections.
In addition, valves can be equipped with various types of operators from hand-wheels, 90° lever, gear operators, or automated valve operators used in above-ground applications to 2-inch-square operating nuts for direct valve operation or with gear operators and 2-inch operating nuts for underground service.
Valves are classified using nomenclature by the NPS (National Pipe Size) associated with the valve end connection, which also refers to the named size of the pipe. The NPS of the valve end connections must always match the NPS of the line in which it is to be installed unless size adjustment fittings or reduced ports are used.
Pressure ratings are highly dependent on the type of fluid, temperature, and service. In addition, different valve materials will possess different pressure ratings. Some materials can sustain more pressure than other materials at cold water temperatures and other materials are better suited for higher temperatures.
Historically, the valve industry has classified valves by primarily using a saturated steam rating with a secondary pressure rating applicable to water service known as WOG, which stands for water, oil, or gas rating. This rating, often with the steam pressure rating, is almost always embedded on the valve body.
Under this rating system, a 125-pound rated bronze valve may have a saturated steam rating of 125 psi with a secondary rating of 200 psi for WOG at cold temperatures. Common WOG equivalent pressure ratings include 125-pound, 150-pound, 250-pound, 300-pound, and 350 psi ratings.
It is always important to verify the pressure rating for the applicable type of service and fluid media for the intended valve.
Finally, as with all hydraulic devices, cavitation and the resulting damage is possible with valves. Cavitation potential is primarily related to the pressure differential that occurs across the valve. This can also be a factor of velocity where a high velocity across the valve seat or disc results in a substantial pressure drop. In cases where cavitation is either suspect or occurring, the use of special trim, hard surfacing, or special coatings may need to be considered.
Cavitation is a two-stage fluid flow phenomenon that is often a problem with high velocity liquid flow and can occur on pressurized and non-pressurized systems. Cavitation is primarily related to the pressure drop or pressure differential between an inlet and dischar
ge across a pump or valve and is made worse by velocity that impacts this pressure drop.
The first stage begins as noise and occurs in the liquid as the fluid passes at high velocity or incurs a significant pressure drop or differential through a pump’s suction, impeller, or valve trim and the pressure is reduced below the fluid’s vapor pressure that corresponds to the type of fluid, pressure differential, and temperature of the fluid under transfer. Small bubbles are formed during this pressure drop, which thereafter
implode or collapse as the fluid enters a region of higher pressure on the discharge side of the valve.
This noise is often compared to gravel being pumped, which rapidly increases to vibration, pitting, and resulting damage to valve internal components as the condition worsens.
Most valve manufacturers provide cavitation charts or reference data for their specific valve and trims. However, for estimating purposes, a pressure differential of 60 psi or greater for inlet pressures of 100 psi or lower, or 120 psi or more for inlet pressures of 200 psi or more, should be suspect for possible cavitation.
Selection and specification of valves for water service should be conducted by considering the factors shown in Table 1.
Gate valves in their various forms and functions are one of the most common peripheral components in a pumping or piping system. Isolation or shutoff valves are commonly used to isolate, segregate, or disconnect a piping system from a process or specific equipment in the piping system. Generally, they are considered as on-or-off valves, meaning they are designed to be fully open or fully closed while in service.
Gate valves comprise the majority of isolation valves for several reasons. They are low in head loss (generally less than 1 foot at rated flow); offer minimal internal restriction due to a full diameter passage when fully open; reasonably low in cost compared to other valves; possess a standard laying length with numerous connection possibilities; have a predictable number of opening and closure rotations per size; are easily disassembled and inspected with the valve body left inline; and are therefore usually field serviceable. Nor does their specific design generally permit dangerous rapid opening or closure to occur.
Gate valves can be divided into two main types: parallel and wedge-shaped.
Parallel gate valves use a flat gate set between two parallel seats. A popular type is the knife or slide-gate valve designed with a sharp edge on the bottom of the gate.
Wedge-shaped gate valves use two inclined seats and a slightly mismatched inclined gate; the gate and body are fitted with metal sealing rings by threading or plastic deformation to prevent loosening in service. A continuous seal is provided behind the rings to prevent corrosion. The valve is closed by appropriate rotation of the stem which drives the gate down between the sealing rings. Sealing is affected by the mating of the sealing rings. Sometimes the sealing surfaces can become damaged from the impact of foreign debris being lodged between the sealing
Generally, the traditional type of standard gate valve (Figure 2a) (AWWA C500 solid wedge gate valve with a metal seat in 2-inch to 1
08-inch sizes and in various pressure ratings and end connections) has historically been used for decades as an isolation valve on the suction or discharge side of a water pumping system. As this type of valve is equipped with a metal disc, guide, and seat, corrosion or wear can degrade any one of the components, lowering the sealing capability of the valve after years of service or repeated wear.
The conical wedge design and angular sealing devices of a metal seated wedge disc require a slight depression in the valve bottom to ensure a tight closure. Thus, sand and small pebbles often settle or become embedded in this bore. In addition, misalignment between the disc and guide can occur, causing difficulty in opening or closing the valve disc. Therefore, regular exercising of the valve is needed to maintain clear guide channels and full travel.
Although, an AWWA C500 gate valve is a trouble-free and low maintenance piping component that seldom needs attention for the most part, they can pose problems on occasion—sometimes serious ones for a pumping plant.
The most common problems occurring with a typical wedge-type gate valve is with leaking stem packing or the disc separating from the stem and either falling into the flow stream during operation or failing to open or close properly when required.
Obviously, having the disc fall into the flow path during pump operation can result in disastrous consequences for the pump and the suction or discharge system. Usually, a disc falling into the flow path at any given time is due to either corrosion or wear (i.e., failure) of the threads on the stem or disc. This most often occurs when the valve is called upon to operate continuously in a slightly or mostly closed condition.
The constant undulating impact of fluid against the disc and pressure differential across the valve disc can cause constant vibration and fluttering of the disc, which over time can result in an ultimate outcome known as a dropped disc or gate. This constant wear on the threads can result in eventual loss of thread strength on either or both of the disc or stem in a relatively short time, depending on the sustained
position of the disc in the flow stream and pressure differential across the disc.
The amount of damage that might be inflicted to the pump depends greatly on which side of the pump—suction or discharge—the defective valve is located and the characteristics of the system itself.
A totally dropped disc on the suction side will immediately interrupt normal flow into the pump. Without a method of sensing suction pressure or motor load, this condition can cause the pump to run without water until steam, followed by severe heat, finally results in the complete seizure of the pump.
Conversely, a dropped disc on the discharge side will result in a shutoff or deadhead operating condition, which can result in extreme cases in the rupturing of the pump case and piping, or with the suction side, eventual seizing of the pump due to liquid flashing.
The result to the system is highly dependent on the amount of reliance and importance on the specific unit. If the affected unit is the only operating one in the process, then an immediate failure as is described can have an additional detrimental effect on the system itself.
A sudden failure in cases of open tank suction supply can result in serious flooding on the suction side (the result of a sudden interruption in normal flow), or loss of process water on the discharge side, usually resulting in a complete and unexpected shutdown of the process.
In either case, the outcome is not a pleasurable one, nor one that is easily predicted with maintenance alone. Although this type of event is relatively rare, I have personally seen it happen frequently enough to warrant mention here.
The best troubleshooting technique for this situation is not to get into it in the first place. Standard C500 wedge-type gate valves certainly have their place in pumping systems but using them as throttling valves is definitely not one of them. Always remember it basically requires up to 75%-90% of closure of a gate valve before any appreciable throttling begins. This means throttling won’t even start until the valve is around 75% closed. From that point to full closure, the increase in backpressure and decrease in flow becomes dramatic and rapid. With this built-in limitation, it is obvious why gate valves are not desirable as throttling valves.
Another type of gate valve receiving more recognition and use over the past 30 years, the resilient seat or wedge gate valve (Figure 2b) has features that make them superior to standard gate valves in many cases. The design preserves the sealing capability as the disc is combined with the sealing mechanism and the valve bodies are sprayed with an effective interior coating that directly seals against the disc.
Resilient seated gate valves contain an encapsulated rubber-coated ductile iron gate instead of a wedge gate disc. The valve is closed by appropriate rotation of the stem which drives the gate against the fusion-bonded epoxy or nyloncoated internal sealing surfaces of the valve body. Sealing is achieved by the compression of the rubber liner on the gate against the valve body.
In resilient seated gate valves, maximum torque of the stem is associated with closing the valve to overcome the effects of friction and compression of the rubber coating and not with the force needed to overcome internal friction as the disc travels through the guide surfaces.
Properly operated and closed, these valves can create a drip-tight seal. As opposed to a metal seated gate valve with a depressed guide in the bottom of the valve, a resilient seated gate valve has a fully open valve bottom allowing free passage for sand and pebbles through the valve (illustrated in Figure 3).
If impurities settle in this space as the valve is operated or closed, the rubber surface will crush or close around the impurities while the valve is closed. A high-quality rubber compound is soft enough to absorb these impurities, yet strong enough to withstand their applied pressure and then wash the impurities through and out of the valve when the valve is opened again.
Common materials used for the valve body are ductile iron and the resilient seats include Buna-N, Viton, Neoprene, Teflon, Hypalon, and EPDM.
Selection of the seat material must consider the application and fluid. This means the rubber surface will retain its original shape, securing a drip-tight sealing. This valve style offers a smooth and unobstructed waterway, ductile iron construction for higher pressure applications than cast iron, and is easy to service or replace the disc/seal.
Stem sealing is performed by use of an O-ring rather than the traditional packing type. Therefore, maintenance is greatly reduced. In addition to popular end connections such as flanged, mechanical joint (MJ), and spigot, these valves can be fitted with an end connection that incorporates an O-ring sealed compression connection and integral pipe restraint in one ready-to-use package or restrained MJ connection.
Resilient wedge or seat gate valves are available in sizes of 2 inches through 12 inches, complying with AWWA Standard C509 and 4-inch to 54-inch sizes complying with AWWA Standard C515.
Many water purveyors now specify the exclusive use of resilient seated gate valves for new or replacement equipment in their pumping and distribution systems over metal seated valves.
Wedge-type gate valves have traditionally been constructed using a threaded rising stem or non-rising stem that attaches to a threaded wedge gate that slides between a guide seat on both sides of the wedge. As a closed valve is opened, usually in a counter-clockwise rotation, the nut rotates around the threaded stem, extracting the wedge and causing it to rise within the valve body.
Eventually, the wedge (gate) is fully removed from the flow passage, allowing unimpeded flow through the valve.
Most rising stem valves are equipped with an outside stem and yoke (OS&Y) assembly that can be used to observe and monitor vertical stem travel. This is advantageous and mandatory for fire protection as an alarm signal can be sent to notify personnel in the event of a closed valve.
Many gate valves are offered with a factory-mounted indicator on the upper end of the stem to indicate the valve position, but this type of gate valve is only suitable for above-ground installation.
Non-rising stems are threaded into the gate and rotate with the wedge, rising and lowering inside the valve. They take up less vertical space since the stem is kept within the valve body. Gate valves with non-rising stems are suitable for both above- and below-ground installations. The difference in function between a rising and non-rising stem valve is shown in Figure 4.
An additional type of shutoff valve, metal or resilient seated knife gate valves (Figure 5a) range in size from 2 inches to 24 inches for ductile iron or stainless steel body construction up to 150 psi cold water pressure rating.
Knife valves, like slide gates, are also available in sizes up to 120 inches for low-head service. They offer the advantage of short laying length for use in tight piping situations. Knife gates are similar in action to a conventional gate valve although the gate itself consists of
a straight-sided rather than wedged gate, that slides between a seat on one side and a guide on the other.
The biggest difference between knife gate valves and standard gate valves is that gate valves are manufactured to ANSI standards, while knife gate valves comply with TAPPI standards.
Standard gate valves are bi-directional in flow, widely used in fluid applications, and provided with metal or resilient seats.
Another difference between a knife gate valve and an ANSI gate valve is within the packing gland area. A standard gate valve has a V-ring or packing set that seals the shaft that is attached to the gate. Knife gate valves have a packing gland area that seals around the gate.
A knife gate valve has a thin profile in comparison to an ANSI gate valve. Knife gate valves are predominately uni-directional (one-way) although some options provide bi-directional flow and feature a lugged or wafer body without flanges. The knife gate valve seats are available in resilient to the metal seated versions.
As often as the two terms are intermingled, a knife gate and a slide gate are not the same valve. A knife gate valve is a component that uses a blade to cut through light or heavy liquids, whereas a slide gate valve is a component using a slide plate in order to better manipulate or control the flow of water or dry bulk material.
Slide and canal gate valves are available in various types of metals, including aluminum (Figure 5b) for low head, stainless steel, and cast iron with the selection of the material generally dependent on the head of shutoff.
They are low-head valves and intended to be only used as end-of-line valves, meaning they are mostly used at the end of drainage lines, low-head surface irrigation, tailwater collection, etc.
Slide gates are often placed on the end of gravity drainage lines to control or divert the flow of water. The gates are made in rectangular, square, or round patterns. Round gates are often placed at the open end of piped canals, and thus, are often called canal gates (Figure 5c).
These valves shouldn’t be used against high heads unless the gate, seat, and opening mechanism has been designed for the force and to unseat the valve against this force. For example, a 4-foot-diameter gate against 10 feet of water head differential will require a minimum of 7800 pounds of strength just to resist the force against the gate.
They should also not be used to regulate flow because whenever fluid is forced against a partially closed gate, a vibration takes place within the valve, gradually eroding the disc and seat. Consequently, knife and slide gate valves should only be used in a completely closed or opened position. In addition, these valves are designed to
Valves intended specifically for throttling or flow-regulating service—globe, plug, or eccentric valves, and to a lesser degree, butterfly or ball valves—are much better for this application and less prone to sudden or progressive failure than gate valves.
Newer styles of gate valves such as the resilient seat type, while not necessarily recommended, are also better suited as isolation and shutoff valves than the older style wedge or twin disc type.
Butterfly and plug valves are both quarter-turn rotational motion valves, meaning the entire travel of the valve used to stop, regulate, and start flow is limited to a 90° travel.
A butterfly valve has a disc mounted on a rotating shaft and when the valve is fully closed, the disc completely blocks the line. When the butterfly valve is fully open, the disc is at a right angle to the flow of gas or liquid. They provide bottle-tight shutoff and are not as
prone to hanging up during opening or closing due to debris or misalignment in the valve disc guide. This has historically been a problem with gate valves in the past due to sand or silt often becoming trapped in the guides of older valves.
Butterfly valves are available in numerous valve body materials, although ductile iron is the most popular. Seat materials include most rubber compounds. End connections include wafer (Figure 6a), flanged (Figure 6b), grooved, and threaded ends for smaller valves. Construction and material details for AWWA butterfly valves are illustrated in Figure 6c.
Ball or plug valves are other styles of quarter-turn valves that are effective for throttling as well as isolation service.
Ball valves are available in both reduced and full port sizes and used in numerous material types, including plastic (PVC and CPVC), bronze, brass, and stainless steel. PVC and CPVC ball valves (Figure 7a) in 1-inch through 2-inch pipe sizes are commonly used for water and chemical applications and are typically available in Schedule 40 or 80 construction.
Connections are also diverse and include true-union (Figures 7b and 7c), threaded, or socket style for PVC valves, threaded or soldered for brass and bronze ball valves, and threaded and flanged for stainless steel valves.
Although many ball valves are constructed using one-piece assemblies, two-piece valves are also available allowing disassembly and repair of the valve’s internal components. Figures 8a and 8b are examples of two-piece ball valves in bronze and stainless steel bodies.
Plug valves (Figure 9a) are more commonly used for throttling service in sizes of 1 inch and larger. The closure element of a plug valve is a cylindrical (full port) or tapered (eccentric) shaped plug with an integral flow port.
As with a ball valve, a plug valve allows straight-through flow in the open position and shuts off flow when the plug is rotated 90°. However, plug valves are available in much larger sizes than ball valves.
Eccentric plug valves are similar to butterfly valves as a portion of the device remains in the flow stream, even when the valve is fully open. The operation of an eccentric cam type of plug valve is shown in Figure 9b. In all cases, when using a manually adjustable valve for throttling, a valve with some method of locking down or securing the valve disc should be used to prevent gradual opening or closing of the valve during operation due to the constant impact of fluid against the disc.
In my humble opinion, a wafer butterfly valve as was shown in Figure 6a is often the most efficient and cost-effective valve to use for typical water service on both sides of a pump, suction and discharge.
The reasons are numerous and include from an installation standpoint, a gate valve is more difficult to handle, requires more time to install, and costs more.
A gate valve requires two gaskets and two bolt/nut sets, one for each flange end. The butterfly valve requires no gaskets since the valve seat provides a tight seal against the flange faces. Plus, only one bolt/nut set is needed with a true wafer style (lug patterns require tw
o bolt sets) because the face-to-face dimensions are much shorter than the end-to-end dimensions on the gate valve.
Gate valves weigh more and have longer end-to-end dimensions than comparable butterfly valves as a butterfly valve has typically less weight and is shorter in laying length than other types of valves. This saves capital costs and does not add appreciably to the weight needed to offer support by pipe or valve stands.
Butterfly valves are equipped with valves position indicators, providing an operator with valve position at a glance. They are acceptable throttling valves (although not the best for this application) and are not subject to the dropping disc problem, and do not usually present air introduction pro
blems on the suction side of a pump when the pump is under a suction lift. Stem packing on gate and globe valves are subject to drawing air during this type of operation (especially older, dried out packing).
This concludes this installment on valves for water works service. The next edition of The Water Works is in April and we’ll expand upon this discussion with an overview of check and air/vacuum release valves.
Until then, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.