Understand Flow Control Valves
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Test Your Skills
The terms speed control and flow control are often used interchangeably to identify a valve with the primary function of limiting the velocity of an actuator. The process of limiting actuator velocity by restricting flow with an orifice introduces a pressure drop (ΔP) which may require an increase in supply pressure, depending on actuator type. Increasing the pressure also has an impact on the volumetric flow rate.
Flow controls commonly used in pneumatic systems are meter-out. The actuator speed is limited by restricting the air flow as it exits the component. This method will more effectively control overrunning loads. The meter-out flow control can al
so be installed on the exhaust port of the directional control valve. However, the load may jump if the actuator or piping has a large volume of de-pressurized air that will compress before being forced out the restriction. Meter-in flow controls will be used for components that do not have an exhaust port, such as a single acting spring return cylinder, for other applications where the load is constant, or if high back pressure would be detrimental to the component function.
Safety tip: Caution should be used when applying meter-out flow controls as load induced pressures could cause the rod end pressure to become intensified and this pressure could exceed the maximum pressure rating of the cylinder.
The simplest flow control is a fixed, non-adjustable orifice. One method is to drill a properly sized hole in a threaded plug or set screw, see Fig. 1-21. The plug or set screw is then installed into a component port below the conductor fitting. In other cases the orifice plug is screwed into a threaded opening in the fitting. The design intent is to insure that the adjustment is not tampered with or removed. This convenient method of adding an orifice is sometimes overlooked when servicing equipment. If the fitting is replaced or improperly installed on a different port, the resulting fault may consume significant troubleshooting time to identify the cause.
Needle valves are adjustable orifices that restrict flow (speed) in both directions (Fig. 1-22). Flow controls (Fig. 1-23) are adjustable orifices with an integral check valve to permit flow (speed) control in one direc-tion only and reverse flow with minimal restriction.
The most common flow control used is a needle valve integrated into a fitting. A bypass check valve will also be integrated in the fitting to permit free reverse flow. The direction of free flow should be indicated on the body of the fitting. Depending on the application and location, the fitting may be used for meter-in or meter-out functions. The adjusting knob may have a locking portion to prevent vibration from causing the adjustment to change.
If the flow control fitting is installed on the exhaust ports of the directional control valve, a reverse flow check valve is not required, but may be used because it is often included as an integral part of the flow control fitting. •
Test Your Skills
1. If you are lowering a vertical load, the best solution to control cylinder speed would be:
a. Meter-out.
b. Meter-in.
c. Pressure regulator.
d. Pressure compensated flow control upstream of the directional control valve.
e. Pilot operated check valve.
2. A speed control on the exhaust ports of a 5/3 directional control valve:
a. Requires both speed controls to be adjusted to the same setting.
b. Requires a reverse flow check valve only for the cap end of the cylinder.
c. Requires a reverse flow check valve only for the rod end of the cylinder.
d. Does not require a reverse flow check valve.
e. Requires a meter-in speed control.
See the Solutions
1. a, 2. d
The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #68, and was written by Greg.
When I am in an instrument and valve repair shop, I see many more control valves than instruments, particularly with the advances in sensor technology, transmitter intelligence, and asset management systems. Valves are mechanical devices and as such require more maintenance. Packings, seals, seats, and o-rings wear out. To ease maintenance, a control valve must be located so it can be readily and safely removed from the pipeline. However, there is much more to consider in locating a control valve.
With liquid streams, a change in control valve position causes an immediate change in liquid pressure and, in a full and pressurized pipeline, a pressure wave traveling at the speed of sound. In less than a second, the pressure imbalance from the wave provides a driving force that overcomes the liquid’s inertia and accelerates the liquid to a new velocity. Consequently, within a plant the location of a control valve does not appreciably affect the response of a process variable (PV) within the pipeline. However, if you are measuring PV response in a pipeline and the control valve is in a different pipeline, throttling a flow that is being added to the pipeline that is being measured, the distance of the valve from the measurement may cause a transportation delay. This piping and valve arrangement commonly occurs in the dilution and blending of streams.
There are some really bad control valve locations that show no understanding of the negative impact of deadtime by process and mechanical design engineers (Tip #70). One of the worst is where there are several vessels between the control valve and the measurement. The residence time of the smaller vessels in series with the larger vessel becomes deadtime. Also bad is gravity flow. Now a change in control valve position starts a wave traveling slowly down the partially filled pipeline. For small flows in vertical runs, the flow is a falling film causing a large and unpredictable deadtime. The worst case of deadtime resulting from valve location is encountered in pH control. A control valve often throttles the reagent flow to a dip tube. The dip tube has a minimum size for structural integrity and normal mixing rules put the dip tube down near the impeller. Unfortunately, this creates a dip tube volume of 2 gallons. If the reagent flow is 1 gallon per hour, when the control valve opens, it takes 2 hours for the reagent to flush the process fluid out of the dip tube. When the control valve closes, the reagent will continue to migrate into the process for several hours. The solution is to have the control valve add the reagent to a high flow recirculation line, thereby reducing the injection delay to seconds.
To prevent flashing, control valves should be located so the fluid pressure in the vena contracta (that is, the narrowest opening in the flow path) does not drop below the vapor pressure of the fluid. If flashing cannot be avoided, the valve type and trim design should be selected to prevent cavitation in the valve or downstream equipment. Stan Weiner, my coauthor of the Control Talk column, recommended the flashing control valve be installed directly on an inlet nozzle near the top of a vessel so the collapsing of bubbles would occur in the vessel vapor space when cavitation could not be prevented.
Concept: Control valves do not require much in the way of straight runs and do not introduce appreciable delays within plants when they are in the same pipeline as the measurement. (Long-distance oil and gas pipelines are another story.) Valves adding flows to process equipment can cause large injection delays or bypass mixing in the equipment. Valve location on streams to equipment should not introduce excessive deadtime or noise. Valve location, type, and trim should minimize flashing and cavitation and provide safe and easy access for removal and repair. On-off valve locations should minimize the volume to the destination when flow is stopped to prevent totalization errors in charges.
Details: Control valves should be at floor level or accessible from platforms. Block, flush, and drain valves should be installed to enable them to be safely removed. Control valves should be located on the same equipment or pipeline as the measurement and downstream of flow measurements. Reagent control valves should be moved from dip tube to recirculation line injection to eliminate injection delays for pH control. On-off valves close to the point of injection should be added to provide isolation and to shut off the flow. For pH and reactor control, the volume between the on-off valve and the nozzle should be minimized by flanging the on-off valve to the nozzle or nozzle block valve for low reagent and reactant flows and high process sensitivity. Gravity flow piping is not recommended because of variable head and velocities, but if it is used, the control valve should be as close to the nozzle of the destination as possible.
Watch-outs: The location of the nozzle and dip tube entry points into a vessel must not result in the flow being injected close to an exit nozzle; thereby short circuiting inlet flow to outlet flow and bypassing the mixing in the vessel. Throttle valves should not be used as isolation valves, and isolation valves should not be used as throttle valves (see Tip #83). If an on-off valve for batch control is not close to the flowmeter, the pipeline inventory between the on-off valve and flowmeter can cause the charge to be significantly different than the batch setpoint. To minimize excess charge, the on-off valve should stroke as fast as necessary when the “close” command is given.
Exceptions: For Coriolis meters in liquid service with no possibility of flashing, the control valve can be located upstream of the flowmeter because a Coriolis meter is not sensitive to velocity profile.
Insight: Control valves can cause damage to piping from cavitation and poor control from injection delay and short circuiting.
Rule of thumb: Locate control valves to be maintainable, provide fast injection into mixing zones, and prevent flashing and cavitation.
About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.
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Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.
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