Flow control valves come in all shapes, sizes, and designs. Their basic function, however, is the same—to control flow of air. Flow control valves for hydraulic systems (liquids under pressure) are of the same basic design. A typical example of a flow control valve is the simple water faucet installed in homes.
Globe valves and needle valves are standard designs used for flow control. Unidirectional flow control valves control the flow in one direction but permit free flow in the other direction. Pressure-compensated flow control valves are also manufactured. These valves control the amount of flow and will maintain a constant flow at different pressures. These valves are ideal for some applications but should be used only when required because of their higher cost.
The check valve is another type of flow control valve. The function of a check valve is to permit flow in only one direction. A very common function of flow control valves is to control the speed of cylinders and air motors. The speed of cylinders or air motors depends on the amount of air, which can be controlled by flow control valves.
Valves are classified by their intended use: flow control, pressure control, and direction control. Some valves have multiple functions that fall into more than one classification.
Flow Control Valves
Flow control valves are used to regulate the flow of fluids. Control of flow in hydraulic systems is critical because the rate of movement of fluid-powered machines or actuators depends on the rate of flow of the pressurized fluid. Some of the major types of flow control valves include:
Ball valves are shutoff valves that use a ball to stop or start the flow of fluid downstream of the valve. The ball, shown in Figure 7-1, performs the same function as the disc in other valves. As the valve handle is turned to open the valve, the ball rotates to a point where part or the entire hole that is machined through the ball is in line with the valve body inlet and outlet. This allows fluid flow to pass through the valve. When the ball is rotated so that the hole is perpendicular to the flow path, the flow stops.
Most ball valves are the quick-acting type. They require a 90-degree turn of the actuator lever to either fully open or completely close the valve. This feature, coupled with the turbulent flow generated when the ball opening is partially open, limits the use of ball valves as a flow control device. This type of valve is normally limited to strictly an “on–of” control function.
Gate valves are used when a straight-line flow of fluid and minimum flow restriction are needed. Gate valves use a sliding plate within the valve body to stop, limit, or permit full flow of fluids through the valve. The gate is usually wedge-shaped. When the valve is wide open, the gate is fully drawn into the valve bonnet. This leaves the flow passage through the valve fully open with no flow restrictions. Therefore, there is little or no pressure drop or flow restriction through the valve.
Gate valves are not suitable for throttling volume. The control of flow is difficult because of the valve's design and the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized by the manufacturer, gate valves should not be used for throttling.
Gate valves are classified as either rising-stem or non-rising-stem valves. The non-rising-stem valve is shown in Figure 7-2. The stem is threaded into the gate. As the handwheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will almost always have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate.
Valves with rising stems (Figure 7-3), are used when it is important to know by immediate inspection whether the valve is open or closed or when the threads exposed to the fluid could become damaged by fluid contamination. In this valve, the stem rises out of the valve bonnet when the valve is opened.
Globe valves are probably the most common valves in existence. The globe valve gets its name from the globular shape of the valve body. Other types of valves may also have globular bodies. Thus, it is the internal structure of the valve that defines the type of valve.
The inlet and outlet openings for globe valves are arranged in a way to satisfy the flow requirements. Figure 7-4 shows straight-, angle-, and cross-flow valves.
The part of the globe valve that controls flow is the disc, which is attached to the valve stem. Turning the valve stem in until the disc is seated into the valve seat closes the valve. This prevents fluid from flowing through the valve (Figure 7-5, view A). The edge of the disc and the seat are very accurately machined so that they form a tight seal when the valve is closed. When the valve is open (Figure 7-5, view B), the fluid flows through the space between the edge of the disc and the seat. Since the fluid flow is equal on all sides of the center of support when the valve is open, there is no unbalanced pressure on the disc that would cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disc in relation to the valve seat. This type of valve is commonly used as a fully open or fully closed valve, but it may be used as a throttling valve. However, since the seating surface is a relatively large area, it is not suitable for a throttling valve where fine adjustment is required.
The globe valve should never be jammed in the open position. After a valve is fully opened, the handwheel or actuating handle should be turned toward the closed position approximately one-half turn. Unless this is done, the valve is likely to seize in the open position, making it difficult, if not impossible, to close the valve. Many valves are damaged in this manner. Another reason for not leaving globe valves in the fully open position is that it is sometimes difficult to determine if the valve is open or closed. If the valve is jammed in the open position, the stem may be damaged or broken by someone who thinks the valve is closed.
It is important that globe valves be installed with the pressure against the face of the disc to keep the system pressure away from the stem packing when the valve is shut.
Needle valves are similar in design and operation to globe valves. Instead of a disc, a needle valve has a long tapered point at the end of the valve stem. Figure 7-6 shows a cross-sectional view of a needle valve.
The long taper of the valve element permits a much smaller seating surface area than that of the globe valve. Therefore, the needle valve is more suitable as a throttling valve. Needle valves are used to control flow into delicate gauges, which might be damaged by sudden surges of fluid flow under pressure.
Needle valves are also used to control the end of a work cycle, where it is desirable for motion to be brought slowly to a halt, and at other points where precise adjustments of flow rate are necessary and where a small rate of flow is desired.
Although many of the needle valves used in fluid power systems are the manually operated types (Figure 7-6), modifications of this type of valve are often used as variable restrictors. This valve is constructed without a handwheel and is adjusted to provide a specific rate of flow. This rate of flow will provide a desired time of operation for a particular subsystem. Since this type of valve can be adjusted to conform to the requirements of a particular system, it can be used in a variety of systems. Figure 7-7 illustrates a needle valve that was modified as a variable restrictor.
The purpose of a flow control valve is to maintain the desired portion of the flow to each flowmeter individually and allocate a portion of the flow rate to meet scheduling requirements for the month, week, day, or hour.
For orifice, turbine, and rotary displacement flowmeter applications, the flow control valve(s) should be installed in each flowmeter assembly upstream of the exiting DB&B valve. Alternatively, a station flow control valve arrangement may be installed downstream of the outlet header to provide the same functionality. For multipath ultrasonic flowmeter applications, a station flow control valve arrangement should be installed downstream of the outlet header. This requirement is designed to minimize the ultrasonic noise effects from the control valves on the ultrasonic flowmeters.
The flow control valve(s) should have a fail-in-place design.
Control valve ramping logic should be installed to prevent damage to equipment and inaccurate measurement of the facility.
The cooling water piping and flow-control valves can create noise of variable and unpredictable frequencies and at unpredictable times; therefore as much of the system as possible is kept outside the cell and the pipework within the cell is run under floor when possible, an example of such transition tubes is shown in Fig. 18.2. The distance between the temperature control devices and the engine usually means that the thermal inertia of the system is too high to have good transient response but, as with the ventilation air flow, this is rarely critical to the type of recordings being made during NVH testing. However, pumped circulation of the coolant fluid is often needed to reduce the temperature control time lag. Sound transmission through steel pipes needs to be minimized by decoupling with nonmetallic sections.
If an anechoic cell is connected to fluid fuel systems, they have to be fitted with all the isolation valves and interlocks described for other cells in Chapter 7, Energy Storage; in the case of acoustic anechoic cells, the valves are often fitted below the bottom row of the lining wedges. Given that the duration of running in most ICE testing within an anechoic cell is a matter of minutes rather than hours, the use of a small “outboard engine” type of fuel tank may be rigged with the engine, obviating the complication of plumbed in fuel lines.
A choke valve is a type of flow control valve used on Christmas trees and manifolds to reduce the pressure and flow of the fluid coming out of the well. Choke valves allow fluid flow through a very small opening to kill the reservoir pressure while regulating the well pressure. Different types of choke valves such as fixed or positive choke valves have been explained in this chapter. Different types of internal design such as needle and seat, multiple flow orifice, fixed bean, plug and cage are explained in this chapter. There are many operational problems that are associated with choke valves such as erosion, cavitation, and corrosion which are explained. Choke valves could have retrievable operation design by diver or remote-operated vehicle (ROV). The actuation and operation of choke valves is explained at the end of this chapter.
Figure 6.41 shows one method of how a flow control valve can be used to control the level of a liquid in a container. Because there may be surface turbulence as a result of liquid entering the container or stirring of the liquid or perhaps boiling, such high frequency ‘noise’ in the system is often filtered out by the use of a stilling well, as shown in Figure 6.41. However, it must be recognised that the stilling well constitutes a U-tube in which low frequency oscillations of the liquid level can occur. Vertical movement of the displacer results in a signal being passed to the controller. This may be achieved by the movement causing the slider of a potentiometer to move across its track.
220.127.116.11.2 Three-Element Control with Scoop-Tube Actuator for Control of DP across Valves
When the three-element control for the drum level is accomplished by the flow control valves, another control loop controls the differential pressure across the feed control valves to enable them to operate smoothly. Controllability is also improved for a control valve when subjected to a DP with a predictable range. In some plants this DP is maintained at a fixed value by adjusting the BFP speed through hydraulic coupling vis-à-vis the scoop-tube actuator. Figure VIII/7.3 may be referred to for a schematic representation of implementing the control loop in a typical 250 MWTPS having drum-type boiler.
In this control loop strategy, the DP across the feed valves is measured with sufficient redundancy, and, after necessary voting, the selected raw DP signal is taken as a measured or process variable for further action in the control loop. The error signal is formed after receiving the desired or set value of DP, which is not a fixed value as in other types of power plants, as stated earlier. This particular power plant has a variable set point for the DP after due characterization of the selected (high) position transmitters of the first-stage attemperation spray valves. All of these position signals are passed through a high selection algorithm.
There is another fixed input of the high selector which ensures the minimum value of this selector output as the 80%–90% (adjustable) position of the attemperation spray water valves. After characterization as per the predictable relation of the flow control valve DP with respect to the spray valve position, the output is again passed through a high/low selector switch, which limits the derived set point not to go beyond the maximum and minimum DP values across the flow control valves. The error signal thus obtained drives the PID controller, whose output is used for position adjustment of the scoop-tube actuator for varying the speed of the BFPs.
The control loop strategy maintains the DP across the feed valves to such an extent that the upstream pressure of the flow control valve, which is incidentally the upstream pressure of all the spray control valves as well, slides to the minimum required value (for better efficiency), which is sufficient to push an adequate amount of spray water with the spray valves almost fully open. It is quite obvious that wider opening of a control valve demand less DP across it with the flow being same. With the downstream pressure of the spray valves being dictated by the boiler load, the upstream pressure is now the minimum pressure required for necessary flow of spray water.
This minimum pressure calls for the minimum discharge pressure of the BFP required at that point of operation, thus minimizing pumping loss. The flow control valve readjusts its opening to suit this DP to maintain the feedwater flow to keep the drum level at a constant value. This control strategy thus ensures minimum but adequate upstream spray water pressure to keep the spray control valves operating in a satisfactory manner without running the BFPs at an unnecessary high discharge pressure. For a single-loop control strategy, the scoop-tube actuator(s) would remain in a fixed position irrespective of whether the control mode is auto or manual.
With one BFP running, the standby BFP (for a 2 × 100% configuration) should have its scoop tube in auto follow-up mode so that if the running pump is tripped, the second pump should start forwarding feed to the boiler without much loss of time. The maximum allowable delay time in starting the second pump is dictated by the boiler-drum storage capacity and the permissible time limit required to come to a very low drum level from normal drum level. For a 3 × 50% BFP configuration, the standby BFP has its scoop-tube actuator in auto follow-up mode to track the maximum position of the two running BFPs’ scoop-tube actuators.
For TDBFPs, there needs to be a motor-operated BFP mainly to act as a fallback. The hydraulic coupling scoop tube always tracks the TDBFPs so that in case of a trip of any TDBFP, a motorized BFP can immediately meet the flow requirement. Also, the discharge valve is kept open and the BFP motor must be rated in such a way for loaded start.
Interlocked operations of this particular loop are based on whether the BFP is running at a safe pressure against the flow delivered at a particular point in time. Here the characterized BFP discharge header pressure represents the limit of safe discharge flow at a particular pump speed. When the actual flow is more than the safe flow, a binary signal is generated in the high/low signal detector (high in this case) and forces the scoop-tube actuator to raise the speed of the BFP so that the operating point is shifted so that the safe flow is more than the actual flow at a higher discharge pressure. The interlocked signal is also generated at the same time to prevent the flow control valve from further opening and thus increasing the feed flow to an unsafe region of operation. For 3 × 50% BFP, individual pump suction flow is to be measured and compared to safe flow from the characteristic curve at corresponding discharge pressure.
The manifold consists of two 6-inch headers, assorted tees, elbows, flow control valves, and flange hubs as shown in Figure 20-3. The manifold is modeled in AutoPipe and analyzed with the various combinations of temperature and pressure that may be experienced during operation conditions.
The allowable stresses of manifold piping in different operating conditions are determined as per ASME B31.8. Table 20-3 shows the maximum stresses of manifold piping determined in the AutoPipe analysis for different operating conditions. The allowable stresses as per ASME B31.8 are also included for comparison.
Table 20-3. Analysis Results of Stress in Manifold Piping
Under subsonic flow conditions, the rate of gas flow through a down-hole flow control valve can be expressed as
QUS = flow rate in volume (Mscf/day)
Ain = choke port area (in2)
zup = upstream gas compressibility factor
Tup = upstream gas temperature (°R)
γg = specific gravity of the gas relative to air
In calculating down-hole control valve flow performance, it is sometimes more convenient to calculate the flow rates in mass flux (lbm/sec) to avoid having to insert a P-v-T correction. In the unit of mass rate (lbm/sec), Equation (8.10) can be expressed as
The gas velocities (ft/sec) at upstream and downstream loci can be calculated separately by the equations:
Water jet machining system consists of water reservoir, hydraulic pump, intensifier, accumulator, control valves, flow regulators, high pressure tubing, nozzles, nozzle head motion control unit, catchers, workpiece holding and feeding unit. Figure 3.3.2 shows the schematic diagram of water jet machining system.
WJM pumping unit
The WJM pumping unit supplies high pressure water to the nozzle from water reservoir. The hydraulic oil pump is driven by an electric motor and supplies high pressure oil from oil reservoir to drive a reciprocating plunger pump termed an intensifier. The intensifier delivers ultra-high pressure water to the accumulator by increasing the pressure of water received from low pressure water reservoir through water filtration unit. Accumulator is a reservoir of high pressure water which eliminates the pressure fluctuation of water coming from intensifier. Accumulator provides the continuous supply of high pressure water to the nozzle through control valves and high pressure water tubing.
Nozzle converts high pressure water into high velocity water jet. The water jet should have minimum lateral expansion and it should remain coherent over the maximum nozzle tip distance. Nozzle should produce better coherent jet to reduce noise level of water jet. Nozzle can produce long coherent jet of length 500–600 times the nozzle diameter with the addition of long chain polymer such as polyethylene oxide to the working fluid due to increase in viscosity of the flowing fluid. Nozzle diameter of 0.05 mm can produce a coherent jet of 25 mm long while a nozzle diameter of 0.35 mm can produce a coherent jet of 175 mm length . The velocity of water jet as high as 900 m/s. The materials of nozzle are of stainless steel, tungsten carbide, diamond and synthetic sapphire, etc. The life of nozzle typically varies from 100 to 500 h. The life of nozzle depends on the nozzle materials, the velocity of fluid flow and the presence of foreign particles in the flowing fluid. Synthetic sapphire is most commonly used nozzle materials for better quality cutting. The inner surface of the nozzle should be very smooth to reduce the level of turbulence of water jet at the exit of the nozzle. The diameter of the nozzle can be varied depending on the application for which the water jet is being used. The nozzle hole diameter typically varies from 0.05 to 0.5 mm. A nozzle has a passage of conical entry of 6 to 20 degrees included angle followed by cylindrical exit having length of 2–4 times end diameter of nozzle opening. For better quality WJM cutting, long exponentially tapered nozzles are also used. The nozzle can be fixed or movable as per requirement. The nozzle can be moved up and down to adjust nozzle tip distance and also to accommodate the thickness of the workpiece. The nozzle tip distance can be typically varies from 2.5 to 50 mm during water jet machining. For CNC WJM machine water nozzle head motion is controlled by CNC X-Y-Z axes drive control system for profile cutting operations.
For high pressure operation, the mixture of glycerine and water in the ratio 1:1 is used as working fluid. The chances of leakage of fluid are reduced due to the increase of viscosity of mixture of glycerine and water. The presence of glycerine in mixture reduces the compressibility of the working fluid. The materials of the tubing is generally stainless steel to withstand the high stress developed due to high pressure fluid flow. Pure water is commonly used as working fluid as it is cheap and it can be recycled after filtration. Alcohol can be used for cutting meat. For cutting frozen foods the cooking oil can be used . The abrasives such as alumina, zirconia, silicon carbides, glass beads, etc. can be mixed in water jet to improve the machining ability in hybrid abrasive water jet machining process.
In order to reduce the noise level in WJM, the catcher catches the used water jet. After cutting the workpiece, water jet enters into the catcher. A catcher is of long tube or reservoir type. The catcher serves as reservoir for the purpose of collecting the used water jet and also the machining debris entrained in the water jet. The most common type of catcher is the tube type which consists of a 300–600 mm long tube that is attached to a draining hose and the length of the tube is sufficient to allow the water jet to break up completely before it reaches the bottom of the tube . For movable water jet and stationary workpiece, slot type or reservoir type catcher is commonly used, but to reduce noise level it is less efficient. For disposal of waste materials and storage of water a separate catcher basin is used in WJM system.
Figure 3.3.3 shows the photographic view of water jet machining system with single water jet. Figure 3.3.4 shows the photographic view of water jet machining system with multiple water jets for higher productivity.