Part 17(e)—Mechanical Design, Control Valves, Part 5
By Ed Butts, PE, CPI
We discussed the concepts associated with designing effective conveyance systems, usually referred to as valving, for pumping stations and water systems in the past several installments of The Water Works. In Part 5 of this subseries, we’ll wrap up the discussion with greater detail about the various types and applications of control valves used for these systems.
Control Valves 101
Another group of valves commonly used on pumping plants are automatic control valves. These valves, usually constructed using a globe valve, are most often used on the discharge side of pumps to control flow or pressure.
They utilize the principle of force differential from pressure applied from a larger surface area on the top of a diaphragm than the area of the valve disc. Although there are as many types of control valves as there are applications, we will limit our discussion to pilot controlled, hydraulically operated, diaphragm-type control valves for the purposes of this column.
This is not intended to ignore other types of valves such as actuator-controlled butterfly valves or ball valves used for fluid control, but rather to compare the type of control valve most often compared to variable frequency drives.
Diaphragm-type control valves operate on the principle of force differential. For any given valve, the surface area on the top of the diaphragm is much larger than the area of the valve disc or seat. This provides the necessary differential of surface area.
Control and regulation are accomplished by the application of water pressure to the control chamber which encloses the diaphragm. For example, a typical control valve (Figure 1) that has a disc area of 6 square inches may also have a diaphragm area of 10 square inches, which is 1.67 times the surface area of the disc.
Water pressure of 100 pounds per square inch (psi) applied at the inlet generates 600 pounds of force (6 square inches × 100 psi). This pushes on the disc and tries to open the valve. But this same pressure, when applied to the diaphragm, results in 1000 pounds (10 square inches × 100 psi) of force (or 1.67 times more force), pushing the disc down against the seat. In this case, this differential of force results in full closure of the valve.
For a single chamber valve (Figure 2a), pressure regulation or modulation is performed by systemically applying or removing varying amounts of water pressure in or out of the control chamber port, and thus onto or off the diaphragm.
The application or removal of this water pressure needed to accomplish the valve’s intended operation is performed using a series of small, hydraulic controls, typically called pilots. Pilots can be hydraulically or electrically commanded such as with a solenoid-controlled valve.
A closure spring is often used to assist with valve closure, particularly in cases with marginal force differential such as smaller valve sizes. The control pilots are designed to move water pressure in and out of the control chamber to maintain the required valve equilibrium or position, based on the specific valve function and setting. The need to cause a partial or full closure of the valve to lower downstream pressure, as needed for a pressure reducing or modulating valve, is provided by the varying application of water pressure onto and off the diaphragm.
The rate of water applied or removed from the diaphragm is a balancing act between the pressure applied to the diaphragm from the regulating pilot and speed control that forces downward movement versus the upwards force exerted on the disc. This type of modulating valve is illustrated in Figure 3.
Conversely, if the main valve needs to be opened (as for a pressure-relief application), partial water pressure is removed from the diaphragm, allowing the inlet force on the disc to overcome the force on the diaphragm, push up on the disc, and causes the main valve to open.
There are two types of single chamber valves commonly used. One uses a globe pattern consisting of the inlet and outlet configured directly across from each other. The other style uses an angle pattern configured for the inlet and outlet at a 90-degree orientation. The angle pattern is often advantageous for a specific service and installation with lower head losses at comparable flow rates, valve size, and installation space in certain applications.
Dual, or two-chamber, valves (Figure 2b) are used when hydraulically powered and precise control for opening and closing the valve is desired. This valve uses the same type of single diaphragm that now separates two control chambers, an upper and lower. This allows water to be placed onto or removed from the upper or lower chambers as needed to gain a hydraulic advantage.
In most cases, water is removed from one chamber at one rate while entering the other counteracting chamber at a different rate, providing a balanced and smooth valve function. Unlike single chamber valves, dual chamber valves are usually intended for fully open or closed travel and operation and not typically designed for throttling. Although the upper and lower chambers are separated by the diaphragm, the lower chamber is isolated from the valve’s flow region using stem O-rings placed to seal off water from entry or exit passage around the stem from or into the lower chamber.
The most common applications for dual chamber valves are booster pump control valves that function with a normally closed position and deep well pump control valves designed for a normally open position. Booster pump control valves usually represent a viable on-off transitional alternative to variable frequency drives (VFDs) in virtually all pump applications where reliable and smooth transition of flow is the primary need, plus they provide the added benefit of exhausting air and the initial water on startup.
Control valves when properly applied are effective in pressure regulation as well as control of the hydraulic surges associated with the starting and stopping of pumps. Numerous options are available that can control the speed of opening or closing the valves as well as electrically operate the valve to open or close, using solenoid valves or hydraulically control the valve using various types of pilots.
When used with a pump with a relatively flat curve, control valves can even act as an energy-conserving device, although that is generally not their primary function. The most common application of single chamber control valves is their use as pressure reducing valves (PRVs), pressure-sustaining or relief valves (PSVs), solenoid control valves, and water-level control valves using float controlled pilots or altitude (water head) pilots. A detailed list is shown in Table 1.
Another type of inline control valve is the diaphragm valve (Figure 4). The main valve consists of a one-piece investment cast body and an elastomeric liner. The valve body is constructed with internal ribs and slots forming a reinforcing grillwork which surrounds the liner to provide support.
Installation of the liner is accomplished by inserting the liner through the inlet of the body, allowing the liner to seat against both the control chamber cavity and the seating surface of the body casting. The unique design of the control element (the liner) enables the valve to exhibit nearly infinite rangeability and high-pressure ratings, up to 700 psi.
The valve is capable of fully open or fully closed (check valve) throttling for pressure reducing and relief, solenoidcommanded operation, and use as a pump control valve.
The rolling action of the liner relative to the body seating surface and adjacent grillwork area provides the variable throttling opening in response to loading pressure change. Consequently, the liner face is not subjected to high velocity Bernoulli effects, which can develop unstable dynamic forces affecting the throttling position.
Additionally, because the liner is flexible, it will unseat at one point on the seating circumference first as the valve begins to open. These factors result in smooth and stable valve operation regardless of flow rate requirements. This valve type is also lighter and with a shorter laying length than a comparably sized globe-style control valve.
Selecting Control Valves
The decisions regarding valve selection must be made on a case-by-case basis. However, I have developed the following guidelines and recommendations regarding the application and pitfalls associated with control valves:
1. Control valves are typically less expensive to purchase, install, and maintain than VFDs. However, the associated piping changes and options needed to perform surge control and slow opening or closing greatly complicates the valve and can raise the cost to rival a VFD, especially in smaller horsepower.
2. To function properly, most control valves must be designed for a minimum velocity and pressure drop over the seat while in operation, especially at higher flow rates. Unfortunately, this loss of head increases the energy cost involved in pumping. To lessen this impact,
designers often mistakenly oversize a valve to match line size piping which may actually counteract the effectiveness of the valve.
In fact, more pressure regulation problems are associated with oversizing of a control valve than with undersizing. This is apparent for an application of 500 GPM with an inline pressure regulating valve. Although every hydraulic design manual would recommend use of a 6-inch valve for this flow, a 4-inch valve is actually appropriate to provide better and more finite control even though there is slightly over 6 psi of pressure drop through the valve at that flow rate. One possible solution to this while retaining a line size valve is the use of a reduced port control valve.
This recent innovation allows the use of a line size valve connection that functions with a reduced disc and port opening. This provides greater control over the flow rate while retaining the line size needed for fitting into the piping. When in doubt, always refer to the manufacturers’ recommendations and consult an experienced control valve representative or system designer before selecting this type of valve.
3. Pressure regulating control valves have a minimum and maximum recommended flow rate and pressure differential for proper operation that are a function of valve size and application. These flow rates are often stated as maximum velocities in feet per second (FPS) over the seat (such as 20 FPS for continuous service or 45 FPS for intermittent service).
The minimum flow rate is based on preventing continuous operation at an almost closed condition where the disc is barely off the seat. This condition, if allowed to occur continuously, can cause a severely high velocity or scouring across the seat and disc, resulting in cavitation across the valve seat as well as another condition referred to as hunting, where the valve tries to find a consistent operating position by rapidly opening and closing just off the seat.
This condition is more common on smaller valves with lower control chamber volume and can result in valve chatter and possible severe damage to the valve as well as possible pressure fluctuations in the system. Systems that must occasionally operate at or below the minimum flow rate are often equipped with either a smaller bypass regulating valve (for example, a pressure reducing valve station for flows between 50 to 1500 GPM), reduced port, or a modified disc/seat arrangement using a modified V-port arrangement.
The maximum flow rate is related to the maximum continuous or intermittent velocity allowed across the valve seat. Excessive velocity can not only result in extremely high friction loss but can cause a severe internal valve velocity, also resulting in premature failure. Many manufacturers now offer wye-pattern valves in addition to globe or angle styles. This valve pattern provides more of an inline design to the valve’s seat and disc where the water does not have to make a sharp 90-degree turn as it does with a globe- or angle-pattern valve. This valve style often provides a reasonable alternative to upsizing while maintaining the desired lower friction loss of the larger valve. All valve manufacturers offer published data to assist designers with specific applications.
4. Control valves usually use water-operated smaller pilot control valves to operate the main valve. Due to the relatively small openings and ports in these pilot valves, plugging due to sand, silt, or iron oxide is often a concern that can lead to valve malfunction, and in extreme cases result in serious pressure control problems.
For example, if extreme amounts of sand are present from a sandy water well, the sand traveling through the valve can not only result in possible malfunction from pilot plugging but cause a sandblasting effect on the valve disc and seat, which will often result in leakage when the valve is closed and premature failure of the valve internal components.
5. As was indicated, control valves are limited by minimum and maximum flow rates. However, many people do not know that most valves also have a limitation as to the maximum pressure differential allowed across the valve. This value varies from manufacturer to manufacturer and size to size, but basically a designer must be cautious in applying a valve where the inlet and outlet pressure will greatly differ.
For example, an inlet pressure of 150 psi and a desired outlet pressure of 30 psi will likely result in moderate cavitation across the valve seat and disc that could lead to premature failure if allowed to occur for an extended period of time. This is where proper sizing and combining the elements of flow rate with pressure differential must be employed.
All manufacturers offer published charts or can provide the information necessary to prevent this potential problem. An example of a cavitation chart that displays various operating hours until damage and severity based on pressure differential for most control valve applications is shown in Figure 5.
6. To ensure proper operation, the water pressure available to inline valve pilots must be at least 2-3 psi higher than the main valve downstream pressure. If the pressure is too low, the valve may stall or not function properly. In some cases of low pressure or extremely dirty pilot water supply, a separate or independent water supply with greater pressure is often used as the source of pilot water supply.
7. Due to their relatively small control chamber volume, control valves smaller than 2 inches are often difficult to use for consistent pressure regulation and have a tendency to cause line surges and often hunt for a stable operating location when used in this situation. Caution must be employed when using smaller valves for pressure regulation applications.
Examples of Dual Chamber Control Valves
Two of the most popular and successful types of inline, automatic dual chamber control valves use a globe or angle style of diaphragm valve that is equipped with various pilot controls to allow the main valve to properly function.
On well pump applications, a version of the dual chamber valve referred to as a deep well pump control valve (DWPCV) provides a smooth transition of flow into or from the system as well as expelling the initial air and water from the column or riser pipe, as in the case of a vertical turbine pump or submersible pump without a riser check valve. This type of system is illustrated in Figure 6.
In this case, an inline combination pressure reducing/check valve is provided between the system and the valve. A DWPCV operates as an offline blowoff valve that is fully open to atmosphere upon pump startup. Once the well pump is activated, the initial air and water from the well and riser pipe vents through this open valve to waste.
Following a predetermined blowoff period, an electrical signal is sent to the control solenoid valve, and the open main valve is commanded to close. As the valve closes, a pressure differential gradually increases between the upstream side of the PRV/check valve and downstream pressure, ultimately balancing and then overcoming the downstream pressure force exerted against the check valve, opening the valve.
The process reverses upon a shutdown command. A limit switch on the stem is set to disengage the pump motor once the main valve has fully opened, resulting in a full shutdown of flow. In lieu of a limit switch, some control valves operate using pressure switches to accomplish the same task. If properly designed, the system cannot restart until the DWPCV has fully reopened during a power failure. This prevents the pump restarting while the valve is closed.
Another version of a dual chamber valve, referred to as a booster pump control valve (BPCV) shown in Figure 7, is equipped with an electric four-way solenoid valve that routes pressurized control water onto the lower chamber to force the main valve open while slowly venting the upper chamber during a run command, placing the pump online.
Depending on which chamber is pressurized or depressurized results in a force difference that commands the BPCV to either open or close. It also tends to provide a smooth and uniform opening and closing speed, particularly when equipped with inline needle valves used as speed control adjustments. In this case, the same type of limit switch is set to disengage the pump motor once the main valve has fully closed, resulting in a full shutdown of flow. Normally, a BPCV is also equipped with the necessary pilot controls to provide check valve closure as well, usually providing non-slam closure upon an instantaneous power failure condition.
Examples of Single Chamber Control Valves
Single chamber control valves are the most common of the two types and found in numerous water supply, irrigation, and industrial applications. Three of the most common applications involve their use as pressure reducing, relief, or altitude valves.
Although they perform completely different functions, pressure reducing and relief valves possess similar names and are two of the most common control valves in use. A pressure reducing valve is used to lower a higher inlet pressure to a lower outlet pressure (Figure 8). It is commonly used in water systems to reduce pressure from one zone to another lower zone.
Multiple valve installations are commonly performed using two or three parallel valves with one smaller valve, usually 2 inches to 4 inches, designed for the lower flows, and larger valves generally sized at 8 inches, 10 inches, or 12 inches, intended for peak demands and fire flows.
A pressure sustaining or relief valve is designed to exhaust excess pressure from the upstream side to the outlet side. As a pressure relief valve, it generally exhausts upstream water to atmosphere and is intended to relieve excessive system pressure to avoid damage to piping and components.
As a pressure sustaining valve, the same type of valve discharges into a lower pressure to maintain a constant back pressure against the upstream pressure—therefore, it is often called a back pressure valve.
A special type of single chamber control valve, called a surge control or anticipator valve, senses the lowered (negative) pressure developed in the pipeline during a sudden stoppage of flow and partially opens the main valve to permit a controlled release of the excess pressure when the higher-pressure wave returns but before flow reversal occurs.
A special pilot control then slowly closes the valve to prevent a second surge event. In this fashion, the valve opens by anticipating the high-pressure or rebound wave by opening from the initial low-pressure wave. This valve is also equipped with a high-pressure pilot to open the valve upon a high-pressure value—in essence, making the valve a pressure relief valve as well. A schematic of this type of valve is shown in Figure 9.
However, the application of all these devices requires a special degree of engineering judgment and should not be used haphazardly since a misapplication can cause a secondary surge problem worse than the original issue.
Pressure relief and surge valves are considered as offline valves. Check and pressure reducing valves are considered as inline valves. In other words, the normal flow delivered by the pump with an offline valve does not travel through the valve itself, and the valve is normally placed offset from the discharge line using a tee. This eliminates any concern of added head loss to the pump at normal flow, so the valve is planned and sized for relieving the surge or high pressure only. This is why a 4-inch valve, capable of up to 800 GPM of continuous flow for an inline application, can handle up to 1800 GPM of intermittent flow for offset or offline relief service.
The final type of common application for a single chamber valve is an altitude valve. Altitude valves are inline valves and primarily used for reservoir filling and water level control from a water system with higher pressure or head. They are designed to open upon a preset low water level to refill the reservoir and close when the high water level is reached.
Altitude valves are commonly used in one-way flow service where water is directed into the reservoir for filling only with discharge water to the system provided through a separate pipe (shown in the left image of Figure 10) or two-way flow that allows water to bidirectionally travel through the valve for reservoir filling, and in reverse, provide stored water back to the system from the reservoir (shown in the right image of Figure 10). An example of a typical one-way altitude valve schematic is shown in Figure 11.
Each one of the systems outlined requires special design and application considerations and techniques when applying and designing a system for a control valve since some applications simply work better than others. This includes the type and number of pilot controls for a specific valve as some work better, more rapidly, and are more sensitive to low head when the pilot system consists of a large pilot spring such as wastewater applications, while others require the pressurized hydraulic pilot system. A designer must carefully weigh all relevant source and system variables.
This concludes this outline on control valves and wraps up the series on mechanical equipment. In the next installment of The Water Works, we will begin a discussion on electrical system design and equipment specifically related to water system applications.
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 email@example.com.