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The purpose of check valves within Hydraulic Power Packs and Systems is to allow fluid to pass in one direction but to prevent it from travelling the other direction, or doing what is known as a reverse flow. The device is usually added to a pipe to prevent oil from flowing backwards. When necessary the valve will close so that all backward movement of fluid is stopped.
The hydraulic check valve has two ports. One is the inlet for the hydraulic fluid to enter and the other is an outlet. They will both operate in combination with the motor, cylinders and hydraulic pump. The valve controls the flow of fluid for the correct operation of equipment.
Hydraulic valves are available in a number of different designs. They may look like a poppet, a disc or one of the ball or plunger types. This will depend on where and how they are being used as to what style and size is used.
Most often you’ll find hydraulic check valves used in application such as braking systems, construction tools, lifting systems and other hydraulic systems. They are installed in systems where the backup of fluid could cause serious issues.
For example, if oil flowed backwards through a pipe, it could empty a hydraulic system back into the equipment reservoir. Even when the machine is turned off the hydraulic valve can prevent fluid from flowing through the system, keeping it full ready for the next time it is operated.
Dual Pilot Operated Check valves (abbreviated P.O.C), are check valves that can be opened by an external pilot pressure. Flow is blocked in one direction as per a standard in line check valve, but it can be opened when sufficient pressure from a pilot line is applied to the third port. The pressure required at the pilot port is normally only 1/3 of the pressure locked within the cylinder. This is determined by the Pilot Ratio (3:1 and 4.5:1) are normally available. They are regularly used with double acting cylinders to lock the system when pressure is switched off, either intentionally or by accident or failure. They can be fitted directly between ports on a ram or incorporated into a power manifold block or module. It is preferable to mount them directly to a ram with “hard” pipework as this increases the integrity of the device. If the pilot check is only required or desired on one side of a cylinder then it can be on the A or B sides, referred to pilot check on A or B.
Regular applications for pilot check valves are rear loading ramps on commercial vehicles. Balers and compactors where the load needs to be held while baling occurs. Security access bollards and blockers to stop the creeping down when the system is at rest. It is important top note that POC are not best suited to applications that have a load that that will over run when they are reversed.
Flow control valves regulate the flow of a fluid and take many forms:
Fixed orifice: Basically a hole in a tube or an insert that fits into the hydraulic line, restricting the amount of fluid that can pass through it for a given pressure.
Adjustable orifice: The size of the effective orifice is adjustable. Common forms are inline and barrel type where the body of the valve is twisted, needle valves for fine adjustment on low flow systems. When set the adjustment can be locked. These are regularly used on lifts or tipper applications where the load is uniform.
Pressure compensating: When a load such as a cantilever passes through an arc the system pressure can vary. This causes the speed of the cylinder to change leading to potentially undesired results. To overcome this pressure compensating valve account for changes in pressure and delivers broadly uniform flow to the hydraulic actuator. In a scissor lift a high pressure is required at the initial raise and decreases as the mechanical advantage increases. The reverse is true when lowering under gravity so a compensating flow control is suited here.
Reverse flow check: On a single acting power pack the pump and motor combination are optimized to give the desired lift speed of the hydraulic cylinder. The flow control valve has an integral bypass line that allows full flow in the out direction, through a built-in check valve. When lowering the full flow oil path is checked and forced to go through the flow restriction allowing controlled descent of the cylinder.
This consists of two valves in one block. When operating a double acting ram the extend and retract speeds will differ, due to the different fluid volumes. From our control valve full flow is permitted through in one direction whereas the other side is flow controlled and/or vice versa, in this way the different valve settings will optimize the actuator speeds. A common example of this valve configuration would be a rear door on a horsebox where the door will need to close much more slowly to prevent shock and noise.
A relief valve is an important control device in virtually every hydraulic system. They protect the overall system from generating a pressure that could cause mechanical failure. It is a mechanical valve that requires no external input other the applied pressure. When this excess pressure is relieved it re-seats to allow normal operation to resume. The most common type comprises a spring and plunger pushing onto a seat. If the pressure exceeds that of the spring force the oil is spilled to a volume usually the oil reservoir. The springs have adjustment ranges for example 20-100 bar and the valves can be housed in cartridge, module or designed directly into an aluminum or steel hydraulic manifold.
A hydraulic circuit may have multiple relief valves, one at the power pack end to protect the pump, another may be fitted onto a control valve circuit to relieve an induced load caused by external mechanical forces. If a hydraulic cylinder requires different relief valve settings on it full bore or annulus side then a dual relief valve module can be set to handle these needs. On the annulus side the area the oil is acting upon is smaller requiring higher pressures to exert the same force as the full bore side hence two relief valve settings are needed. One example of this is a hydropower generation sluice gate operation where something jammed in the gate such as log stops it closing.
Some terms associated with relief valve operation:
Overshoot: The pressure reading when a relief valve operates to bypass fluid. (It can be two times the actual setting.)
Hysteresis: The difference in pressure when a relief valve starts spilling some flow (cracking pressure) and when full flow is passing.
Stability: pressure fluctuation as the relief valve is bypassing at its set pressure.
Reseat pressure: The pressure a relief valve closes at after it has been operating.
Counterbalance valves are fundamentally a relief valve that is fitted in an application to generate back pressure in a system. They are normally used for ‘counterbalancing’ a load to stop it from running away during lowering. The valve is usually set at 30 percent higher than the pressure induced by the load.
Figure 1 Counterbalance valve circuit.
A built in check valve allows flow in the reverse direction (i.e. to by-pass the counterbalance valve when lifting the load). It should be noted that both sides of the valve will be subjected to full pressure, this is not possible on all relief valve designs. In Figure 1 the counterbalance valve has an integral check valve. When counterbalancing the return path must have a low back pressure to tank, as this will be additive to the valve setting.
What do you need to know to keep yourself and other people safe in the workplace?
Keeping a load safe is dependent upon ensuring that the pressure of fluid is correct. If the pressure of it becomes too high, then it will want to give that energy to other surroundings, and it’s only the soundness of the components that will prevent it from doing so. It will try to escape any way possible and this includes through weak seals, valves or other points of plumbing failure.
The metering devices such as valves, flow controls and counterbalances prevent the fluid from running away. To demonstrate this, a cylinder that is installed with rod down, and tension under load will often have a meter-out configuration to stop the load from taking control of the cylinder
Although this is safe, there is still a risk of the pressure on the rod-side intensifying. If the piston seals get blown by this, then the load will drop.
Some engineers will use a counterbalance to avoid metering out errors from occurring. Although a counterbalance valve is considered to be the same a pressure valve, it is what controls the speed of an actuator. It will control how fast the cylinder moves, even if there is a pressure intensification.
Another situation that can cause a catastrophic failure and even personal injury is a leak in the cylinder hose or tube. If fluid in the actuator exits through a broken conduit, it is no longer able to hold up a load. In the event of a conduit failure, the counterbalance valve will prevent the load from dropping. Another safety function that will hold a load is a pilot-operated check valve. Although it will hold a load indefinitely, it will not be as smooth with control of load induced movement.
The essence of a hydraulic system is pressure. It’s something that is required to make the system as powerful and effective as it is. However, there are many reasons why pressure can easily rise including load spikes, ‘water hammer’, intensification and even thermal expansion. If there isn’t enough control over pressure, then components can fail and seals can give way – leaving the machine to be unsafe. It’s for this reason that the hydraulic system has so many different types of pressure control valves.
Damage can be prevented by limiting pressure with relief valves. They can control the pressure in the main system or in isolated sub-circuits. In some systems, it’s necessary for sub-circuits to operate at different pressure to others. This can be achieved through the use of the pressure reducing valve which is able to limit pressure downstream of itself. It is also able to reduce pressure in situations where the fluid has become too heated and therefore has increased its pressure. In some systems there are a number of valves that will work to ensure that pressure is limited to a safe level in every part of the machine.
In summary, it’s important to control both the pressure and the flow in hydraulic systems. It enables safe operation of the system and movement of loads. Safe employees are everybody’s concern.
In a Hydraulic System, you are most likely aware that the main system pressure is maintained by the system relief valve or even another type of pressure setting device.
The purpose of pressure reducing values is to keep the secondary pressures correct in branches of hydraulic systems.
Most pressure reducing valves are open and 2 way, this allows the pressure to flow freely until they reach further downstream where there is a set pressure. They then shift to throttle the flow in the branch.
Forces from pressure downstream are what actuates pressure reducing valves. This is what will deliver the correct working pressure by enabling a pressure drop to occur in the main spool of the valve. The way that a press-reducing valve works is that it is not a device that is either on or off. In contrast, it delivers a continual adjustment to the pressure. Keep in mind that these types of valves are the most conducive to suffering from contamination when it comes to malfunctioning.
Pressure-reducing valves can go wrong in a number of ways. Again, pressure gauges will need to be installed in order to understand what’s going wrong with one. Once this has been done, you can look for:
· A low pressure at outlet port. If this drops below what it should be, the first action to take is to check the pilot head spool and seat. Check for wear and tear which may be affecting the drain flow. Too much drain flow through this area of the valve will result in reduced pressure and therefore affect performance.
· If you find that the valve will not retain a reduced pressure setting, and the pressure is exceeding it, then check whether the pilot drain line is blocked or affected by contaminants. This will increase pressure which will result in flow to the branch circuit. It’s also possible that the main spool is stuck open due to contaminants blocking it. Again, there could be scoring of either the main spool or bore.
· If you find that you cannot adjust the value to the low pressure setting, even after turning the adjustment knob, then check whether there is wear of the spool or bore. There may even be a broken spring in the pilot head, which will mean not enough force between spool to seat in the control head.
· If there is not enough pressure at the output port, check whether the main spool is stuck in the closed position. This will result in no pressure fluid being unable to flow to the branch. Contaminants could be to blame.
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When it comes to actuators, there are a few differences that you should know about if you’re in the engineering field. If you haven’t had a whole lot of experience, then here’s the lowdown.
Every mechanical movement system has a linear actuator in operation. It operates in a straight line but might be in the form of components, an assembly or a finished product. Used to perform the job of converting energy into movement or a force, an actuator might be powered by electricity, pressurised air or fluid.
Here’s what you need to know about the most common linear actuators, and what their pros and cons are.
How Actuators Work
Pneumatic line actuators. Consisting of a hollow cylinder with a piston inside, either a manual pump or an external compressor will move the piston. As pressure builds, a linear force is developed and the cylinder moves along the axis of the piston. It will then return to its original position using either fluid from the other side of it, or by a spring-back force.
Pneumatic actuators are really quite simple. They have bore sizes between ½ and 8 inches with a maximum pressure rating of 150 psi. So between 30 to 7,500lbs of force can be delivered. Steel versions can deliver forces between 50 to around 38,500 lb.
Because they don’t have any motors, and therefore produce no magnetic interference, they are often used in situations with extreme temperatures. You’ll find that they are very cost effective in addition to being lightweight and not needing of much when it comes to maintenance. Their components are durable as they aren’t under a lot of strain.
When it comes to disadvantages, they can suffer from pressure losses and therefore can be less efficient. Lower pressures equal slower speeds and lower forces. In addition, pressure must be made even if nothing is moving, so it’s necessary to continually run a compressor for them. For them to work most efficiently, they need to be sized for the job at hand and that makes them inflexible when it comes to other applications.
Hydraulic linear actuators. Working in a similar way to pneumatic actuators, this type of actuator is moved by incompressible liquid from a pump.
Being able to produce forces that are up to 25 times that of the pneumatic cylinder, they are considered to be rugged and well suited to high-force applications. The hydraulic actuator is able to hold force and torque without needing extra fluid or pressure sent through from the pump. Fluid is incompressible. This also makes it possible for hydraulic actuators to have both their motors and their pumps situated a distance away without suffering from loss of power.
The downside of the hydraulic actuator is that they can leak fluid and this can lead to them being inefficient. These leaks of hydraulic fluid can potentially damage components such as motors, fluid reservoir, heat exchanger etc…
Electric linear actuator. Powered by a torque converted from electrical energy connected to by a lead screw. As the screw rotates, either a ball or threaded lead nut will be driven along matching threads.
With the best precision control possible, the electrical actuator operates in a smooth and quiet fashion. They can also be reprogrammed and networked in a considerably short period of time. Without any fluid leaks, they are also less of an environmental hazard.
On the negative, each electrical actuator costs far more than that of either a pneumatic or a hydraulic actuator of similar power. They are also not suited to all areas either such as hazardous or flammable areas. There is also a danger that they can overheat due to having a continuously running motor.
Whichever motor is used is going to be reflected in terms of how much thrust, force and speed limits are required. If any of these need to be changed, then it will be necessary to acquire another motor.
As you can see for yourself, actuators come in 3 flavours, and it will depend upon your individual circumstances with regards to which one will give you what you need with the least disadvantages.
Do you know how long any hydraulic pump should last? In this industry, using past experience might not always deliver the answers you were hoping for, and are likely to give you answers that are actually no better than guessing.
Disappointingly, there is no dependable approach to determine how long your hydraulic pump will last. Using historical data is perhaps something that will give you the best indicator, but if it’s a new pump and you have no data – that’s where the guessing game beings. Fortunately, there are a number of factors that determine how long any pump will last and using these can give you an estimate that is more informed.
For example, let’s consider your hydraulic system. The type of application it is will make a different to the pump life and so will the temperature. Using pumps that are graded as ‘industrial grade’ will deliver a better lifespan than those that are not. Using auxiliary information can also help. For example, an axial piston design pump has less heavily loaded shaft bearings and therefore are not at a great risk of premature failure.
Of course, roller type bearings in this type of piston design can fail prematurely due to brinelling. That’s why it’s better to use shell-type bearings as they are more like a bushing than a bearing.
Another major consideration is the type and grade of oil being used. If it’s ‘special purpose’ and is fire resistant then it won’t always have a positive influence on the service life. However, it will run cool which could help with its lifespan as there will be less temperature related lubrication issues.
Keeping a high level of oil cleanliness will also work well in extending the life of any hydraulic component.
Another point to ponder is how hard the pump is working. This is about how fast it’s spinning and under what pressure –how much of each hour is the pump under load? If they are under load for 55 minutes of every hour, then that’s going to be a 90% duty cycle, which is a lot to maintain compared to being under load for say 42 minutes of every hour. Under ideal conditions such as a duty cycle of 70% or less, 1200 rpm spinning with clean oil, you can hope an industrial grade hydraulic pump would last 20,000 hours or more. However, if you’ve got a 90% load with special purpose oil and 1800 rpm then you are more likely to get something in the arena of 10,000 hours of service life.
Running To Failure
There’s no doubt that these are only informed estimates using the information that we have about the pump and how it’s being used. Of course, if there are any hidden design flaws then the lifespan of the pump could be drastically compromised. For example, if there are pressure spikes that are caused by rapid valve shifts, then over time this could lead to a pump failure.
To continue to run a hydraulic pump until it fails is not a good idea. Its failure could cause consequential damage to other components. The cost of the rebuild of the pump will increase. Changing a pump before its life expires should be managed, whilst historical data is collected.
So if it’s looking like 20,000 hours is a strong lifespan possibility for any pump, then it’s wise to pull it out at 12,000 hours. It can be inspected and put back into service until say 15,000 hours. Then run to 17,500 hours and if all is well, then run until 20,000 hours. Getting too greedy will put the pump into the correct timeframe for a failure, so it’s not wise to push it too far.
Using this approach can provide information to make informed decisions on realistic expectations for component lifespan without putting the hydraulic system at great risk.
It’s no secret that water can cause incredible damage to any hydraulic system.
If you’ve been a hydraulic system engineer for some time, you may have already seen a system that has cloudy oil in it. Cloudy oil is the result of having so much water in oil that it is above the saturation level. Most often the saturation level will be at 200 to 300 ppm at a temperature of 68°F or 20°C.
Going the other way on the scale, by reducing water in a system, you can increase the life of it by a significant amount. For example, by ensuring that it’s at a level that is lower than 100 ppm, the life of a bearing could be increased by 150%. (According to Timken Bearing Company in their Stauff Contamination Control Program).
If oil is cloudy, then it will have at least 200 ppm of water in it. Of course, the greater the level of water in the oil, the more issues you’ll have in terms of performance and reliability. We once had a look at a system for a client and it had more than 10,000 ppm of water which is more than 1%.
Here’s a checklist of why you don’t want water in your hydraulic fluid:
· It decreases the presence of some additives
· It reacts with some additives to make corrosive products that will attack metals
· Clogs filters
· Increases cavitation
· Increases air entrapment
· Reduces lubrication
Although if you do your own research, you may come across information that states that having 0.1% of water in your system is perfectly acceptable, according to the Timken Bearing Company report, it is far better to have as little as 0.01% of water in your hydraulic fluid as it will increase life expectancy of bearings in their case, but components etc. in ours.
If that isn’t enough information to convince you, then take note that 500 ppm of water and over can even create micro-biological contamination if you have the following elements also present:
· Food: i.e. nitrogen, carbon or phosphorous from the oil
· Oxygen: there is usually between 7 and 10% of air in hydraulic oil
· Temperature: bacterial growth can occur between 24°C and 49°C
· Low flow: the reservoir is a great place for breeding to take place
· Particles: these will help to transport and colonizing
Although you will need each of the above present to help keep bacterial growth going, water is what is behind the success of it. Therefore, keeping your oil dry is critical in stopping growth.
There are many factors involved in how much subsea hydraulic related applications are able to grow and increase in usefulness in the marine related industries. For example, key considerations are how long they can be kept in use without there needing to be maintenance performed or costly repairs and engineering undertaken. How safe are they for both the ocean and for people? How can the harsh environment be overcome for industries to achieve their goals?
Some maintenance for subsea work is of course something that cannot be avoided. For example, the high external water pressure, corrosion, powerful currents and operating machinery by remote control all come at a price. With clever design and careful planning, it’s possible to keep costs to a minimum.
Pressure compensation and seals
Something that can affect performance of any system is external pressure. Pressure compensation can be used to enable better underwater operation. Used as a means to keep pressure constant between the reservoir and the seawater, it helps to ensure that seals can still operate as they are usually designed to operate for flow travelling in just one direction, and to handle pressure drop for just one way.
The majority of components that are designed for hydraulic systems are land or surface based. They will have been built to cater for the environment without any specific issues such as high pressure. These components therefore cannot withstand the pressure found in deep water or even pressure drops that are severe.
One of the solutions to handling and supporting pressure-sensitive components in their operation is to seal them inside a protective chamber. However this can be difficult and costly to implement. The chamber would need to be of rigid construction with heavy-duty rugged seals installed that could handle the high external pressure. Pressure compensation is another method that is often seen as being more effective. It’s used by applying a pressure that is equal and opposite to that of the pressure found outside the component.
Piston rods and reservoirs
Plasma arc welding is used to apply high velocity oxygen fuel (HVOF) gun and cobalt-alloy coatings to piston rods that will be used in subsea deep water conditions as part of a hydraulic cylinder.
When it comes to reservoirs, they will often be replaced by sealed reservoirs. They will contain a flexible medium separator to ensure that the pressure of the external environment will also be in the reservoir, just as can be found in normal surface systems. However, the difference being that the oil and the seawater do not mix as they are prevented from doing so.
This ingenious system then makes it possible to use any component that is used on the land, underwater, as long as any areas are filled with fluid as opposed to the air that would normally be in them if they were operating on the surface. They will then need to be connected to the reservoir to maintain the balance of pressure.
Corrosion is a subsea challenge
Another area of challenge is that of corrosion. The study of keeping seals and seawater working together is known as tribology. It’s essential for subsea hydraulic system designers to be familiar with the concepts involved – keeping seawater out and hydraulic fluid inside a system. For large hydraulic cylinders, keeping the integrity of the piston rod in full operation, even after being exposed to extreme environmental conditions is critical for securing a long-term operation of the system.
Another area of concern is ensuring that all subsea application machinery is designed to a level that does not hurt the ocean environment or people.
Overall, the challenges of designing subsea equipment are multiple due to the harsh environment of the ocean, the reliability required for operators in addition to safety. As industries opt to travel deeper into the ocean, we can only see the challenges increasing.
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