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Sequence valves have an important role to play in hydraulic systems. They control flue flow to both the primary and the secondary circuits. They also control the sequence that particular events occur and maintain the pressure of the primary line. It’s important to remember that the pressure of the sequence valve should never be below 200 psi of the system relief valve setting.
When it comes to sequence valve troubleshooting, you’ll need to add pressure gauges to both the primary and secondary ports of the valve. Most valves have a gauge port installed in the unit. You’ll use the readings of the pressure gauge to understand what is occurring in the sequence valve.
If there are contaminants in the hydraulic fluid it can have a negative effect on the sequence valve. It can block internal pilot fluid passage and moving parts can wear down faster.
There are a number of ways that the sequence valve can malfunction:
You may experience premature valve shifting. This can occur when the fluid is not flowing at a high enough pressure. You’ll need to check the sequence valve to confirm if there is a blockage of the drain hole in the main spool. This would allow pressure to build in the spool cavity and therefore shift the spool at the incorrect time. Also check whether the spool is stuck open because it has been damaged or it has contaminants stuck on it.
If you are getting too much pressure in your valve, it’s time to check the sequence valve itself. Check whether the pilot control piston is getting stuck. This could be down to contaminants or even scoring. It could also be that the fluid passage of the pilot has collected contaminants and therefore the pressure is too low to shift the spool.
Any fluctuations in pressure could be down to contaminants or the improper sized drain line.
If you have any experience in the area of sequence valve troubleshooting, feel free to comment below.
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.
An Hydraulic sequencing valve is essentially a normally closed pressure control valve, that is designed to sequence hydraulic operations within hydraulic power packs dependent upon the applied pressure. This valve is used to ensure the beginning of one operation takes place after the first has reached the designed pressure.
When the pressure at inlet port A reaches the set pressure setting, the hydraulic oil is allowed to pass through to the outlet port B at which time it will be able to carry out additional operations in this part of the hydraulic system.
The main spool of the sequence valve is spring loaded to ensure it remains closed at low pressures. The control stage of the valve is fed from port A through a small control orifice. When the pressure at port A reaches the force applied by the adjuster spring the control spool opens allowing the main spool to open. This action allows flow to pass from port A to port B. Pressure in port B will slowly rise due until it matches that of port A. At which time the main spool is fully open and flow may pass freely. It should be noted that the control orifice will pass a pilot flow of typically 1lpm to the tank port to allow operational movements of the control spool.
A typical example of the use of a hydraulic sequence valve would be a bin lifter. Using the sequence valve placed with port A connected to the full bore side of the main lift cylinder. When the main ram reaches the top of the lift stroke the pressure rises opening the sequence valve, the flow then passes through it to the tip cylinder, thus extending it and tipping the bin. Using a second sequence valve in the reverse direction means a single directional control valve can ensure this lift and tip sequence will always be followed.
Meet the relief valve. Contrary to how things might appear, it’s actually the unrecognised superhero of most hydraulic systems. Its heroic contribution is of great importance due to its ability to control the power that can be generated by the system.
Its role is to ensure that the pressure in the system will not go beyond its limit. This will result in less wear and tear of hydraulic components and will go a long way in assuring the safety of workers and operators.
The relief valve is in truth, a very simple component that cannot be matched by any other when it comes to ensuring the efficiency of the hydraulic system.
Prep yourself on more hydraulic system troubleshooting knowledge with our steps on how to handle issues with relief valves:
The first step with understanding what’s going on is to check the pressure with a pressure gauge inserted into the test port. Are you getting a reading that is in line with what the maximum pressure should be?
If so, you’ll next need to check where there is a system pressure problem with the flow. To do this, check the line to see if it’s been blocked. If not, then check whether the relief gauge is working properly as this could also be the underlying issue.
If you don’t find maximum pressure then check whether the machine is under load, it could be that there is too much load for the hydraulics system to cope with. Once you have done that, compare the reading of being under load to the measure of when there is no motion underway. If there is still an issue, you may need to adjust the relief valve.
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In the world of industrial, mobile and aerospace equipment, hydraulic power systems are very popular. They enjoy a high power-to-weight ratio in addition to being able to be stalled, operated intermittently and even reversed. They can also accelerate fast and are quick to respond. Another attractive feature of the fluid power system is that they can be very long lasting in addition to offer reliable operation rates.
Hydraulic systems are able to work as they contain incompressible liquid. In many situations, it’s much preferred to use hydraulics to move machinery. For one, fluid systems do not produce the same amount of wear as a dryer method would. It also does not require so many moving parts as a different type of system would.
The pressure of fluid in the hydraulic system is controlled by the valve. They also handle the flow rate and which way the flow is going. The funny thing about hydraulic valves is that they can change name depending on how they are being used and according to which system that are part of. Used in combination with cylinders and hydraulic pumps to control the flow of liquid, hydraulic valves are powerful.
The classification of hydraulic valves is determined by how much pressure they can handle. It is also related to the flow and how many directional control valves there are in them. They may also be classified on their looks and extra features such as needle valves, spools and poppets.
Apart from their ability at moving very heavy objects, one of the reasons that hydraulic systems are so popular is because they can operate at very low noise levels. In the manufacturing industry, a low noise level is sought after, in particular at less than 70dB. The hydraulic system and pump is able to accomplish this.
The hydraulic control valve is a clever piece of kit. Browse hydraulic valves here.
Made of a number of precision machined components, hydraulic equipment and systems require care and maintenance to get the best of them and in order to give them what they need for long lasting and trouble-free operation.
We’ve covered troubleshooting before, and if you want to know more in further detail then check out our blog posts on:
Troubleshoot Hydraulics: Basic Knowledge
Troubleshooting Hydraulic Relief Valves
This checklist is for those who have not had extensive experience with troubleshooting hydraulics and it might suit your interns or newbies in the workshop.
Let’s get started.
First off, it’s essential to keep all of the hydraulic system clean. This includes the hydraulic fluid. Oil and oil filters need to be changed at regular intervals. You could say that dirt and grime is your and your equipment’s worst enemy and it’s your role to keep it at bay and prevent it from messing up your machinery and its peripherals.
Here is a checklist of what is at the root of most trouble:
1. The fluid or oil being used is not of the correct viscosity.
2. There is not enough fluid or oil in the system
3. There is a leak
4. There is dirt, moisture or there is another foreign body in the system
5. There is air in the system
6. Structural failure.
7. Adjustments have been made but they are wrong
Here are some shortcuts to answers that you can refer to when getting started with hydraulic troubleshooting:
Your Pump is Operating Incorrectly and not delivering fluid or oil. It could be down to any of these reasons:
· The fluid is too low in the reservoir. You may need to check its level and refill if necessary.
· If there is a hole in your intake pipe allowing air to pass through, you may hear a noise or experience erratic results. This will need to be repaired. Alternatively there could be a blockage in your filter. In which case, you will need to clean it.
· The oil is too thick and the viscosity is too heavy. Check the specs suggested by the manufacturer.
· The pump shaft is rotating in the wrong direction. Reverse it otherwise you will cause irreparable damage as there won’t be enough lubricant.
· Dirt in the pump – clean it.
Your System is Not Developing Pressure
The most likely causes for this type of situation are:
· The pump is not delivering fluid (see info above – with remedies listed)
· The relief valve is malfunctioning either through leakage, incorrect settings or because the valve spring is broken. You may need to check the settings according to the manufacturer, check the valve seat to look for either dirt or scoring or even replace the spring and then adjust it as suitable.
· The valves may be allowing the oil to be recirculated through the system. Check the directional valve to ascertain what the situation is with it.
· There is leakage internally in the valves or the cylinders. Check these components and their condition.
Pump is making noise
· The intake line or the filter is not allowing fluid to pass. Clean these and assure there are no kinks or anything to stop them being fully open.
· There are air leaks either in the intake pipe at the joints, at the pump shaft packing or through the inlet pipe opening. You can check the joints for leaks by pouring on oil. Also check the shaft by pouring oil onto it and check that the inlet pipes are below the oil level in the reservoir to ensure that suction is strong enough.
· If you are seeing air bubbles, you may need to use an oil with a foam depressant.
· Check the reservoir air vent to see if it’s plugged, if so clean it.
· You may find that the pump is running too fast, in which case you will need to refer to the manufacturer’s specifications.
· The oil is at the wrong viscosity. Again, check the manufacturer’s specifications for details.
· Check whether the filter is of the correct size, as this could also be a problem. Refer to manufacturer’s specs for details.
· Check for work or broken components and parts, and replace as necessary.
If you’re experiencing an external oil leak around the pump, you may need to look for:
· Worn shaft packing which needs replacing
· Head packing damaged, again replace it.
· Loose or broken parts, which may need to be tightened or replaced.
Excessive wear can be caused by and remedied as follows:
· Abrasive material or dirt in oil being circulated. Clean and/or replace the filter and change the oil.
· If the viscosity of the oil is too low, check what is recommended by the manufacturers.
· Pressure could be too high for maximum rating of the pump. In which case you may need to check the settings of the relief valve or regulator valve.
· The drive is not aligned correctly. Check this and correct as appropriately.
· Air is in the system. This will need to be removed.
Broken pump parts can come about from:
· Pressures are above the maximum pump rating check the relief or regulator valve settings.
· Seizure from lack of oil in the system. Check the level of the reservoir, the oil filter and the suction line.
· Dirt or material in the pump – clean it and check the filter.
· The head is screwed on too tight – check the specifications as listed by the manufacturer.
Follow our blog for more handy hydraulic system troubleshooting checklists.
There are four main types of hydraulic valves that Hydraproducts offer, all of which are compliant with CETOP standards. Hydraulic valves that are recognised by CETOP are interchangeable within most types of hydraulic equipment, making it easy to find a supplier that can provide valves suitable for use in your hydraulic equipment.
Valves in hydraulic equipment control and regulate the flow of hydraulic fluid through a system, but as there are different requirements for fluid flow in certain applications it is important that the right valve is used for each purpose. Flow control valves can be configured to regulate the flow of fluid in two directions and are adjustable to allow for fine tuning of the flow rate. They control the movement of fluid in one direction, but in reverse the flow is free and cannot be regulated in the same was as it is when flowing forwards. A pilot operated check valve is used to control the flow of hydraulic fluid to a cylinder, stopping it when needed to prevent unwanted movement of the cylinder. A pressure relief valve is often used in conjunction with a pilot operated check valve or a solenoid valve to release the pressure contained by a pilot operated check valve, or to limit pressure in a control line leading to a solenoid valve. Our solenoid valves have a four-landed control spool and are used to change the direction of fluid flow. They are all rated to ISO4401-03-02-0-94 and DIN 240 340-A6 and can handle a flow rate of up to 60 litres per minute and pressures up to 320 bar. With a distribution network covering the UK, Germany, Australia, New Zealand, Finland, Lithuania and Bulgaria our hydraulic valves can be delivered to a wide range of locations very quickly.
Solenoid valves are often chosen over flow control valves for their ability to regulate the directional flow, instead of only being able to regulate in one direction. A flow control valve is suitable for applications where movement of a cylinder in one direction needs to be carefully regulated, but when it returns to the starting position no control is needed over the flow and speed of movement. They can be mounted in either direction, so it is up to the user which direction requires the control. For applications where control is needed over the flow rate in both directions (for example, in technologies that open and close gates or doors at a set speed) a solenoid valve is better as it allows for that fine control of movement and speed in both directions.
In most cases a hydraulic valve will be replaced with one of the same type, and buying from Hydraproducts ensures you will get a valve that is ideal as a replacement even if your equipment is made by a different manufacturer. Sometimes a hydraulic valve needs to be replaced with one of a different type when machine specifications or usage changes, an example being a piece of machinery that used to use a flow control valve, but where there are operational advantages to having better control over the hydraulic flow in two directions.
Choosing the right type should be easy when replacing a valve, as you can use the same type as before, but when designing a new piece of hydraulic machinery, the decision is a little harder. Understanding the basic function the machinery is required to perform is a must, but going a little further into the other functionality that may be required, or thinking about performance improvements that could be made with a different type of valve can inform a better design and result in hydraulic machinery that is more user friendly or that can fulfil more than the basic functions specified at the design stage.
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