Flow control stations typically include a flow meter, flow control valve, and valves to isolate the station during maintenance activities. Flow control stations are sometimes equipped with a remote terminal unit (RTU), which communicates with a Supervisory Control and Data Acquisition (SCADA) system, to monitor and control the station remotely. (For more information about SCADA systems, see Chapter 6 and Appendix E.)
The transient pressures that result from the operation of flow control valves depend on the design of the flow control station, particularly:
• The time period of the valve position change • The valve type and its hydraulic characteristics
• The system hydraulic characteristics (for example, head loss in the piping relative to head loss through the valve)
When considering valve position change, it is important to consider that the reduction in flow due to valve closure is not proportional to the valve travel distance (stroke). In fact, with most valves (including hydrants), most of the change in velocity occurs when the valve is barely open. It is at this time that a quick turn of the valve can lead to a significant water hammer event. For example, if it takes 20 turns to close a valve and the initial velocity through the valve is 16 ft/s (5 m/s), the velocity may change to 6.6 ft/s (2 m/s) over the first 19 turns. The velocity is then reduced from 6.6 ft/s to zero over the last turn (known as the “effective stroke” of the valve). The change of velocity over the last interval having a duration equal to the characteristic time (2L/a) determines the magnitude of the transient.
One of the most important considerations when selecting the flow control valve type is cavitation. Cavitation occurs when the minimum pressure at critical points within the valve reaches the vapor pressure (pv) of the liquid, and vapor bubbles form. If the
differential pressure across the valve is excessive, or if the pressure downstream of the valve is minimal, cavitation can occur during the steady-state flow condition. Cavita- tion can damage the valve and cause excessive noise, especially if an inappropriate valve is selected. Control valves specifically designed to minimize the potential for cavitation should be selected for these cases.
Depending on its severity, cavitation can also affect the hydraulic capacity of the valve. When the flow stream expands immediately downstream of the valve, the pres- sure increases, causing the vapor bubbles to collapse. This dynamic vaporization and
collapse phenomenon causes noise and vibration and can erode the interior of the valve.
To completely eliminate valve cavitation, the head loss across the valve must be reduced, or the downstream pressure must be increased. However, these requirements may not be feasible for a particular valve station. Limited cavitation during critical flow conditions is acceptable. To avoid excessive maintenance and repairs, valve materials that are resistant to cavitation, such as stainless steel, should be specified in these cases. If the required head loss across the valve cannot be reduced, and the downstream pressure cannot be increased to reduce valve cavitation, a valve specially designed for these extreme hydraulic conditions, such as a multijet valve, should be used.
The last key design consideration concerns system hydraulic characteristics. The fric- tional losses in the piping system can cause an attenuation of the wave front and pack- ing near the valve, which in turn influences the transient heads that are produced and propagated through the system. The friction of the piping system also impacts the valve position necessary to produce the desired flow rate.
Figure 13.24 compares the hydraulic characteristics of various flow control valves, including a Howell Bunger valve, which is designed for free discharge energy dissipa- tion. The graph shows how the discharge coefficient for different types of valves changes with the size of the opening as a percentage of the fully open position. The discharge coefficients can be used in calculations to determine flow rate as a function of valve position.
Automatic Control Valves. Valves that do not require an external source of
energy to operate are referred to as automatic control valves. Automatic control valves open and close based on system pressure. The valve body usually has a globe or angle pattern, and the internal operator is connected to a diaphragm or a piston sit- uated in the valve body. Hydraulic pilot controls and piping use the system pressure to control valve position by directing water to either side of the diaphragm or piston operator. Depending on the selection and arrangement of the hydraulic pilot controls, the valve can be designed to perform specific functions, such as maintaining a con- stant downstream or upstream pressure or maintaining a constant flow rate.
For an automatic control valve to operate properly, it must be installed at a location within the hydraulic system that has adequate pressure to overcome the weight and friction associated with the internal valve operator. Valve location is typically not an issue because it does not take much pressure spread over the area of the diaphragm or piston operator to produce a force sufficient to overcome this internal operator resis- tance. Periodic valve inspections are necessary to ensure that nothing that obstructs operation has become lodged in the valve seat and that strainers or filters in the con- trol piping are kept clean.
The inherent design of automatic control valves limits their ability to quickly respond to rapid transients. Upon sensing the system pressure change due to a transient, the hydraulic pilot controls direct control water to the internal valve operator. The time required for the direction of the water limits the response time of automatic control valves, usually in proportion to valve size, as larger valves require greater volumes of
control water. However, some automatic control valves can be equipped with fast-act- ing features to stop reverse flow.
Figure 13.24 Flow control valve characteristics
For control valve stations that are required to break a considerable amount of head across the valve, significant turbulence can occur in the valve control piping if the pip- ing is not adequately designed. This turbulence can cause an oscillatory phenomenon that inhibits the valve’s ability to maintain a steady-flow condition in the system. Also, cavitation can occur in the control piping, causing excessive wear and prema- ture failure of the control pilot valves.
An automatic control valve can be designed to operate as a pressure relief valve that discharges water from the system if a maximum system pressure is exceeded. How- ever, because of its limited response time, the effectiveness of this type of pressure relief valve in controlling system transients is also limited.
Check Valves. There are several types of check valves available for the preven-
tion of reverse flow in a hydraulic system. Required check valves should be carefully selected to ensure that their operational characteristics (such as closing time) are suf-
ficient for the transient flow reversals that can occur in the system. Some transient flow reversal conditions can occur very rapidly; thus, if a check valve cannot respond quickly enough, it may slam closed and cause the valve or piping restraints to fail. Check valves that have moving discs and parts of significant mass have a higher iner- tia and therefore tend to close more slowly upon flow reversal. Check valves with lighter checking mechanisms have less inertia and therefore close more quickly. External counterweights present on some check valves (such as swing check valves) are intended to assist the valve in closing following stoppage of flow. However, for systems that experience very rapid transient flow reversal, the additional inertia of the counterweight can slow the closing time of the valve. Spring-loaded check valves can be used to reduce closing time, but these valves have higher head loss characteristics and can induce an oscillatory phenomenon during some flow conditions.
It is important that the modeler understands the closing characteristics of the check valves being used. For example, ball check valves tend to close slowly, swing check valves close somewhat faster (unless they are adjusted otherwise), and nozzle check valves have the shortest closing times. Modeling the transient event with different types of check valves can indicate whether a more expensive nozzle-type valve is worthwhile.
In summary, transient analysis is needed to evaluate water column deceleration at check valve installations in order to understand how quickly the water column will reverse and thus ensure that a check valve with an adequate closing time is selected to prevent flow reversal.