Control valves control the flow of fluid by reducing or increasing the cross section of the passage. This article discusses about how does a control valve work.
Control valves can stop, increase or decrease the discharge of the fluid. These valves find their applications in hydraulic as well as pneumatic systems.
What is a control valve?
Control valves regulate the flow of fluid in the passage.
The applications may require different amount of discharge rate at different time. Valves are used to achieve this function.
How does a control valve control flow ?
The control valves regulate the pressure of the flowing fluid in order to change the discharge rates.
The cross section of the valve’s passage will reduce if one wants to reduce the flow rate and vice versa for increasing the flow rate.
Does a flow control valve reduce pressure?
In a high pressure system, the pressure can be reduced by using pressure reducing valves.
In pressure reducing valves, high pressure fluid enters the inlet and exits as low pressure fluid. It is done by restricting the flow downstream. Hence, we can say that a flow control valve can reduce pressure.
Which valve is best for flow control?
Many valves such as ball valves, globe valves, butterfly valves etc are used in industries out of which a globe valve is considered the best.
Globe valves are good at regulating the flow. They are used for throttling purposes. The opening and closing time of a globe valve is much shorter than other valves which makes this valve more desirable.
Can a control valve increase pressure?
Higher the flow rate through the valve, higher will be the pressure drop and vice versa.
While the pressure reduces downstream in pressure reducing valves, the pressure increases upstream.
Can ball valve be used for flow control?
Globe valves are the best type of valves for regulating the flow. Although ball valves can also be used to serve this purpose.
Opening and closing of ball in ball valve controls the flow. Ball is the main characteristic of ball valve. It can be closed, open or partially closed. Ball valves have an advantage over others because it is easier to tell whether ball valve is open or closed.
How do you choose a control valve?
A lot of characteristics are taken into account for choosing a control valve. Some considerations are noise, maintenance, cost and energy consumption. A good valve should serve most of the following-
- Ability to regulate the rate of flow or throttle.
- It should work with low turbulence.
- Short opening and closing time as it can be used in emergency.
- Tight seal that prevents leaks.
- Opening at a pre set pressure.
- It should be able to hold any corrosive materials to avoid rapid wearing.
What is a pressure regulator?
The type of valve that is used for controlling the pressure of the fluid to a desired value is called as pressure regulator.
Pressure is controlled by using a negative feedback from the controlled pressure. If the flow rate is high inside the valve then pressure drop will also be higher.
Image credits: Peter Southwood, Pressure regulators schematic, CC BY-SA 4.0
What are the types of pressure regulators?
There are mainly two types of pressure regulators used in industry. They are-
- Pressure reducing regulator– As the name suggests, it is a valve that reduces input pressure to a desired one at the output. It is a open valve that is installed in upstream.
- Pressure sustaining regulator– Unlike pressure reducing regulator, the pressure sustaining regulator maintains a set pressure at the input side by opening itself to allow the flow when it exceeds the set value.
How does a flow control valve affect pressure?
Generally, by controlling the pressure of fluid the flow of fluid can be controlled.
A higher flow rate through a restriction will result in greater pressure drop and similarly a lower flow rate will result in lower pressure drop. The pressure will be high upstream as the downstream pressure reduces.
What type of control is used to regulate the pressure of water?
Pressure of water can be regulated with the use of a certain type of valve. That valve is called as a water pressure regulator. It is also called as pressure-reducing valve.
It is a specialized plumbing valve which reduces the pressure coming into inlet. It is not needed everywhere but if the pressure of water from municipal corporation is high then a water regulator must be used in order to reduce the pressure.
How does a water pressure regulator work?
A water pressure regulator will look like a dome shaped device that would be situated near the mai shut off valve where the main water line enters the house.
It has a spring loaded mechanism for regulating the pressure of the water. A diaphragm is attahced at the end of spring which widens and narrows depending upon the amount of water coming inside the valve.
A water pressure regulator protects the inner valves of appliances as there will be less stress on them.
Mathematically,
F = Pi – Po + PoS + f
Where,
F is diaphragm spring force
f is poppet spring force
Pi is inlet pressure
Po is outlet pressure
s is poppet area
S is diaphragm area
What are the different types of pressure reducing regulators?
The pressure reducing regulators can be classified on the basis of number of stages through which water is passing. They are-
- Single stage regulator– As the name suggests, the water passes through a single stage. When operating with this type of valve, the outlet pressure starts climbing when the supply pressure gets low.
- Double stage regulator– In this type of regulator, the water passes through two stages. The first stage has a pre set value of pressure, so the inlet pressure gets converted to this pre set value. The next stage is variable that is user can control the pressure of the fluid by knob. This way the user can achieve the desired pressure just by rotating the knob.
The pressure gauges on the regulators show the magnitude of pressure of flowing fluid.
Installation tips on water pressure regulator
The best location of installing a water regulator is beside the main shutoff valve controlling the water coming into inlet.
This location is desirable because it makes it easy to quickly shut off the control valve and allows the regulator to protect all the pipes present in the system.
If the regulator needs to be replaced then it is easier to replace it with the same brand and model because it will have the examct dimensions as the previous one.
Maintenance of water pressure regulator
Like every mechanical component needs maintenance after some point of time, water pressure regulators also need maitenance for their smooth operation.
If water hammering takes place or other inconsistencies take place then it is a sign or symptom that pressure regulator is not working properly. It is always recommended to test the regulator at least once a year or whenever there is a slight noticeable change in regulator’s effectiveness. If the adjustment screw’s movement has no effect in changing the pressure then it can be concluded that it should be replaced.
Too much water pressure in the system can cause water hammer to occur in walls or sometimes burst pipes of the system.
What are the factors to consider when selecting a control valve?
Many factors are considered while selecting a control valve. If not considered, the system may work inefficiently. The major considerations are-
- A valve should have a good throttlability and should be able to control the rate of flow.
- The operation should not be turbulent.
- The valve should have a shorter opening and closing time.
- The valve should be able to tighten properly to avoid leaks through the system.
- It should be able to open at a pre set pressure.
- It should be able to handle abrasive fluids.
What are the three types of control valves used in hydraulic systems?
Hydraulic systems can be broadly classified into three types. These three types are- flow control valves, pressure control valves and directional control valves.
- Directional control valves-Directional control valves can start, stop or change the flow of direction of the flowing fluid. Most commonly used directional valves include directional poppet valves, directional spool valves and check and non return valves.
- Pressure control valves-Pressure control valve use throttling to regulate the flow of fluid. They switch at pre determined pressure setting. Examples of pressure control valves are pressure valves relief function, pressure valves fixed function, pressure valve reducing functions etc.
- Flow controlled valves-Flow control valves increase or decrease the area of throttling point to control the flow of fluid. Examples of flow controlled valves are pressure compensated valves, throttle valves etc.
What is CV value of control valve?
CV value of a control valve or Value flow co efficient of a control valve is defined as the ratio of capability of flow of a control at fully open conditions to the pressure drop taking place through the valve.
This particular value is significant as it is the only standardized selection method for control valve sizing which is why it is used throughout the industry.
Mathematically,
Where,
Q is the flow in gallons per minute
G is the specific gravity of fluid
Del P is differential pressure over valve.
Hi ….I am Abhishek Khambhata, have pursued B. Tech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth-year project was based on the performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like-minded people.