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Control Valve Minimum allowable Pressure Drop
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(Chemical)
(OP)
30 May 22 14:06Hi there,
A problem I've come across a few times and had a few different opinions so I thought I would asked here...
Typical rule of thumb for allowable control valve pressure drop is 30% of the total frictional losses or a minimum of 0.7 bar. How critical is the minimum value... I have a system with quite a low total pressure drop so I can achieve the 30% criteria (using the typical line sizing velocity/DP criteria) but not the 0.7 bar. Is the minimum of 0.7 bar critical? In this case I have a flashing liquid (1 barg) upstream going to a tank at 0.1 barg using a 200m line with a 10m drop. It will be used very infrequently so trying to minimise investment without having to rely on an operator to control it locally by hand.
(Chemical)
30 May 22 15:25Are you controlling a level in the receiving tank or the flow rate of the flashing liquid?
If a level, can you just do a deadband control with an actuated ball valve? It's cheaper than a control valve.
Good Luck,
Latexman
(Chemical)
(OP)
30 May 22 16:12Hi Latexman,
I don't think that would work for me. The LCV is controlling the level in the upstream vessel and there's only a few mins holdup at peak flow. The upstream vessel is filled from elsewhere with a flow that ramps up over time (10 hrs) and then stops. This is repeated a couple of times a year.
To add detail, the reason I am looking at this is the associated gas blowby case. The smaller line size would help the gas blow by case but not sure if it would be to the detriment of the control valve being able to control adequately. The increased DP for the smaller line size does not appear to impact the LCV Cv since it is choking but helps a lot with reducing flow for gas blow by flow.
(Mechanical)
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30 May 22 17:52I think that the rule of 30% control valve pressure drop, is applicable to liquids, but in this case, as you say, there is flashing in the valve and in the downstream line 200m long.
If the line and the valve completely open, has capacity to discharge the peak flow, the design is correct.
Keep in mind that the valve has choking conditions and the valve flow is limited to the critical flow. The downstream line must have enough size to discharge this critical flow.
(Chemical)
(OP)
30 May 22 20:31Thanks Casflo.
Yes, I think that is a good point. The 0.7 bar min is probably based on not having too big a control valve but in this case due to the choking it doesn't matter since it does not affect the control valve Cv until you get to a very low DP, in this case ~0.1 bar. Making my downstream line size larger does nothing to the Cv unless it causes the control valve DP to be less than 0.1 bar.
(Chemical)
30 May 22 20:39It depends entirely on how sensitive you need you control system to be. Sometimes on/off control is adequate. For other application you you may need to control flow to +/-1% of setpoint.
(Petroleum)
31 May 22 05:19Rule 30% and dP 10 psi rule are both just rules of thumb and have a lot of exceptions.
30% related to pressure rise of head curve of a conventional centrifugal pump so in your case % can be calculated based on actual pump head curve and sensitivness required.
dP 10 psi related to sensitiveness of a conventional globe control valve. So as Compositepro said - dP can be decreased depending on sensitiveness or construction of control valve. For example I used control valves (with expensive special internal devices) with dP 1-2 psi at normal point for both gas and liquid services with no problems reported.
If flashing is a problem you can located a control valve just downstream of a pump (or any another source of pressure) in a point of hydraulic profile where total pressure is max so way it will be affected by high outlet pressure caused by flow friction. In this case flashing will occur only at start of pumping when flow friction is 0.
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Directional Control Valves (DCVs) form the backbone of hydraulic systems, wielding the power to regulate fluid flow and channel it precisely to desired locations. In this comprehensive guide, we embark on a journey to explore the fundamental aspects, working principles, and applications of these crucial valves, enabling industries to harness the potential of fluid power.
A directional control valve is a mechanical device that controls the direction of fluid flow within hydraulic systems. These valves act as gatekeepers, determining which actuators or hydraulic cylinders receive pressurized fluid and in which direction it flows. The significance of a directional control valve lies in its ability to provide precise and efficient control over hydraulic systems. By regulating fluid flow, these valves allow machinery to execute diverse tasks with accuracy, whether it's the smooth movement of heavy construction equipment or the intricate operations of aerospace systems. The proper functioning of DCVs is vital for optimizing performance, reducing energy consumption, and ensuring safety in hydraulic applications.
The roots of directional control valve technology can be traced back to ancient civilizations that harnessed the power of water to operate watermills and irrigation systems. However, it was in the 20th century that significant advancements in hydraulics and control systems spurred the development of modern DCVs. The adoption of hydraulic machinery in various industries, such as manufacturing, construction, and transportation, propelled the demand for more sophisticated and precise control over fluid flow, leading to the evolution of today's highly efficient directional control valves.
Spool valves are among the most common types of DCVs, characterized by a cylindrical spool that slides within a valve body. The position of the spool determines the fluid pathway and, consequently, the direction of fluid flow. There are several configurations of spool valves, including:
Poppet valves utilize a disc or cone-shaped poppet to control fluid flow. They can be either direct-acting or pilot-operated, depending on the method of actuation. Poppet valves offer advantages in high-pressure applications and exhibit minimal leakage, making them suitable for critical systems.
Rotary valves utilize rotary motion to control fluid flow, typically achieved through a rotating spool or cylinder. They offer precise control over flow paths and are commonly used in specialized applications.
A directional control valve can be actuated through different methods, each offering unique benefits for specific applications:
Directional Control Valves are symbolically represented in hydraulic diagrams, where specific symbols denote different valve positions. The most common valve positions include:
Single Solenoid vs. Double Solenoid Valves: Directional Control Valves can be classified as single solenoid or double solenoid, based on the number of solenoids used to control their positions. Single solenoid valves have one solenoid for actuation, while double solenoid valves employ two solenoids—one for each direction.
Series Circuit Design: In series circuit configurations, multiple directional control valves are connected in sequence to control fluid flow step-by-step, allowing precise control over actuator movements.
Parallel Circuit Design: Parallel circuit configurations involve multiple directional control valves arranged in parallel, offering simultaneous control over multiple actuators, increasing system efficiency.
Combination Circuit Design: Combination circuit designs combine series and parallel configurations, providing versatility and flexibility in controlling fluid flow for complex applications.
When selecting the appropriate directional control valve for a specific application, several factors need to be taken into account:
Safety is paramount when dealing with hydraulic systems and directional control valves. Implementing proper safety measures is crucial to prevent accidents and ensure the well-being of operators and maintenance personnel. Some key safety precautions include:
Lockout/Tagout Procedures: Isolating and securing the hydraulic system during maintenance or repair to prevent unintended activation. Performing proper LOTO procedures ensures the safety of workers.
Personal Protective Equipment (PPE): Wearing appropriate PPE, such as gloves and eye protection, when handling hydraulic components.
Avoiding Hydraulic Hazards: Being aware of potential hydraulic hazards, such as high-pressure fluid leaks or sudden movements of actuators.
Directional Control Valves find applications in a wide range of industries, facilitating essential processes and operations:
Industrial Automation: In industrial automation, DCVs control the movement of robotic arms, conveyor belts, and other automated equipment, enabling efficient production and assembly processes.
Construction and Heavy Machinery: Directional Control Valves play a pivotal role in heavy machinery, such as excavators and loaders, providing precise control over hydraulic cylinders for digging, lifting, and maneuvering.
Agricultural Equipment: In agricultural machinery, DCVs regulate the movement of tractor attachments, ensuring precision in plowing, harvesting, and other farming operations.
1. How do I identify a directional control valve?
To identify a directional control valve, look for standardized valve symbols, check for labels or markings, observe the valve type, count the ports and positions, note the actuation method, and consult manuals or seek expert advice if needed.
2. How do proportional directional control valves differ from on/off directional control valves?
Proportional directional control valves provide variable control over fluid flow, allowing for precise regulation of speed and position, whereas on/off directional control valves only offer binary control, i.e., fully open or fully closed. Proportional valves are commonly used in applications requiring smooth and proportional control, such as in precision automation and motion control systems.
3. Can directional control valves be used in conjunction with other types of valves in hydraulic circuits?
Yes, directional control valves can be used in combination with other types of valves, such as pressure control valves, flow control valves, and check valves, to create complex hydraulic circuits. This allows for more sophisticated and efficient control over fluid flow, pressure, and direction in various hydraulic systems.
4. How can I ensure the proper maintenance and longevity of directional control valves?
To ensure optimal performance and longevity of directional control valves, regular maintenance is crucial. This includes periodic cleaning, inspection for wear and tear, checking for leaks, and replacing worn-out seals and O-rings. Following manufacturer's guidelines and adhering to proper installation procedures are also essential for extending the life of the valves.
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