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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.
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.
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.
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.
If you’ve been in the hydraulic industry for some time, there’s no doubt that at some point you will have seen cloudy oil. This is what happens when there is contamination with water above the oil’s level of saturation. The definition of a saturation level is how much water can dissolve in oil – for mineral hydraulic oil this will typically be around the 200 to 300 ppm at 68 F or 20 C.
As an aside, something worth knowing is that bearing life can be increased by almost 150% if water concentration is reduced to just 25 ppm.
The more water in the oil, the more issues you’re going to face. One of our engineers recently witnessed oil that so was incredibly cloudy because it had over 10,000 ppm of water in it which actually made it more than 1% water!
Here’s what happens when there is water in hydraulic fluid:
· Either depletes or reacts with additives to form by-products that can corrode some metals
· Clogs filters by reducing filterability
· Increases ability of air entrainment
· The likelihood of cavitation increases
· Lubricating film-strength is reduced leading to corrosion and wear vulnerability
It’s also possible to spawn bacteria with water present in oil.
Measuring and Removing Water
How can you measure how much oil and how much water you have in your hydraulic fluid?
The test that is considered to be the standard laboratory method is the Karl Fischer Volumetric Regent Method which others may know simply as the Karl Fischer test. Another method sometimes used is the FTIR or Fourier transform infrared spectroscopy test. However, this is a test that can only really be considered effective with oil and water mixes that are greater than 1000 ppm of water. If you’re serious about measuring water contamination, we recommend that you go with the Karl Fischer.
Now that you know that there are some very unpleasant side effects when there is water in your oil, what are you options with regards to removing it? If you’ve got a system that has only a small volume of oil, then you may opt to change the oil. This option will most likely prove to be the most cost effective approach. For larger oil volumes, it’s best to use filters built for water removal when there is small amounts of water involved.
Water removal filters come in two types, polymeric and coalescing. The former works by using chemicals that attract water. They absorb water drops and retain them permanently. Whereas coalescing filters collect the water and put it into a collector which is drained once in a while. Water that has been dissolved will not be collected by either filter types.
Another approach to collect water is the headspace dehumidification approach. This uses the reservoir’s headspace to circulate and dehumidify the air. Water will then migrate to the headspace where it is removed by a dehumidifier.
Headspace flush is another approach that is similar to the previous method, except that it is collected by a small flow of dry compressed air that is flushed through the headspace. The dry air will pick up the water.
One more approach is to use a variation on the headspace flush by using a hygroscopic breather and then connecting a vacuum pump. This approach is reliant on a spare port located on the top of the reservoir, as distant from the breather as possible. This method does not need a source of dry compressed air.
We are in the business of supplying mobile power packs for hydraulic systems. If you want to know more about our products, browse through our hydraulic unit product pages or call us for a no obligation chat.
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.
Global demand is not easing up when it comes to farming vital resources that are found in subsea environments. In fact, industries have now begun to expand their efforts and are putting more energy and effort into devising machines that are capable of delivering what is required.
With over 60% of the surface of the earth covered by water, it’s no secret that there are many resources that are awaiting exploration and development. This new frontier has a number of industries involved including oil and gas, natural science, mining, energy generation and infrastructure.
Hydraulic systems are incredibly useful to remote operated vehicles that are used underwater. They offer high power density and a reliability not found in other systems. It’s possible to use hydraulics in such a way that the vehicle is very compact and therefore it can be deployed and recovered easier.
Highly technical and complex systems need to be serviced and maintained by subsea remote operated vehicles. For example, equipment needs to be lowered and lifted to the seabed, emplaced systems need to be monitored, such as communication cables and petroleum wellheads.
Although some subsea hydraulic equipment is designed specifically for the task, in some cases, the equipment has been manufactured to a quality that can handle the high pressures and the corrosive conditions of the depths of the sea anyway and needs just a little customisation to perform at a reliable level. A major factor that is considered is the depth of the water and how that could impact the hydraulic system.
Here are other considerations that go into developing hydraulic systems for subsea operations:
Machines that operate at 1000ft below sea level are required to operate in salt water, but the water-pressure is not significantly high. Another factor that has to be catered for is that sunlight can reach up to 800ft into the water and could promote the growth of sea life over the surface of the equipment such as the cylinders and the rods.
Beyond 1000ft to as deep as 6,000ft, pressure becomes a major factor. Increasingly 14.5psi for every 10m of depth, it will be as high as 7250psi at 5000m. It’s at these depths that work is performed by subsea robots such as AUVs (autonomous underwater vehicles) and ROVs (remote operated vehicles).
Subsea vehicles aren’t typically in use for long periods of time. They will be used to accomplish tasks in electromechanically and electrohydraulic subsystems. Although they can operate beyond 100m of depth they typically won’t be submerged for long periods of time. However, they need to be ready when required and any downtime must be kept to a minimum.
Special design features may be required for components exposed to water pressures this high. For example, structural modifications may be required or pressure compensation.
These depths would normally be found a long way from shore, therefore would be operated by either ships, platforms or floating platforms. Water that is from 6000ft to 35,800ft is rarely entered unless it’s by subsea vehicles from the military or research. The conditions are so extreme, that every piece of equipment, including hoisting and tethering will need to be engineered to handle the weight and dimensions of systems at this water depth. In addition the size of the waves are larger, as are the forces brought on by maritime currents.
Ambient hydrostatic pressure is exposed to the hydraulic fluid using a pressure compensation system, with a flexible seal to prevent hydraulic fluid and seawater from making contact.
The benefits of hydraulic drives are brought into their own in these types of machines. Not only are they powerful and compact, rugged but precise, they are able to deliver power and be flexible for a wide range of tasks.
Engineers continue to work on how they can make the best of what hydraulic systems offer when it comes to subsea conditions.
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