Sequence valves manage the sequence of operations between two different circuit branches, allowing one branch to activate another branch automatically. Sequence valves are used in automobile engines — fuel must pass from the tank into the cylinders before the piston can rotate the crankshaft. A sequence valve installed in the gas tank transfer lines controls this sequence, helping protect the engine.
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Sequence valves are actually similar to pressure relief valves because they both operate based on pressure changes. The difference is that once the set pressure level is reached, sequence valves have an actuator that diverts the medium to another part of the system to execute the next command in the programmed sequence. Many different types of sequence valves exist; most are managed with pressure, but some are controlled mechanically.
There are two different ways to classify sequence valves. The first class is according to its mechanism and structure, divided into two types: direct action and pilot. The second class is according to the pressure control method: internal control or external control. The sequence valve is equipped with a one-way sanitary valve. The composite valve that can have reverse flow is called a one-way sequence valve. In general, this kind of valve is often used.
The basic function of a sequence valve is to control the sequential operation of multiple actuators. Sequence valves have an actuator that diverts the medium to another part of the system to execute the next command in the programmed sequence. This function is critical to the performance of many different industrial systems, including automobiles, aerospace, fluid processing, and more.
There are a few different names for sequence valves, depending on their application: back pressure valve, unloading valve, and balance valve, respectively. The performance of the sequence valve is basically the same as that of the overflow valve. However, due to different functions, there are special requirements for sequence valves.
1) In order for the actuators to accurately achieve sequential action, it’s required that the precision of pressure regulating be high and the deviation be small.
2) In order for the sequential action to be accurate, there must be only a small amount of internal leakage when the valve is closed.
3) For a one-way sequence valve, it’s required that the reverse pressure loss and the forward pressure loss be small.
1) Controlling the sequential action of multiple components.
2) Use in the pressure-keeping circuit.
3) To be used as a balance valve to prevent the free fall of cylinder pistons caused by self-weight.
4) Externally-controlled sequence valves can be used as unloading valves to unload the pump.
5) Internally-controlled sequence valves can be used as back pressure valves.
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With their versatile mounting structures, sequence valves can be used for different circuit functions to replace overflow valves and balance valves.
If the opening and closing characteristics of the sequence valve are too poor, when the flow is large, the pressure is too high and the efficiency of the circuit is reduced. The opening and closing characteristics include hysteresis, the opening pressure is smaller than the closing pressure. This should be noted when the load flow changes.
For valves with too low an opening pressure, when the pressure is lower than the set pressure, leakage occurs, leading to the malfunction of the actuator. When the flow through the valve is far smaller than the rated flow, vibration or other instabilities can occur, and you must take maintenance measures on the circuit.
Figure 14-6. Three types of counterbalance valves
An internal pilot-operated counterbalance valve shifts to allow excess fluid to flow to the outlet when pressure at the inlet increases to the pressure set by the pressure adjustment. Pressure at the inlet never drops below set pressure when there is flow at the outlet. Flow from the inlet to the outlet is just enough so that backpressure on the actuator never drops below set pressure. This means the actuator moves only as fast as it is supplied and stops when Inlet flow ceases.
Pressure adjustment on the Internal-piloted counterbalance valve is usually made by first screwing the pressure adjustment all the way in. To assure that the valve is capable of high enough pressure, start the pump and raise the load a small amount. Then center the directional valve — which connects the cylinder rod-end port to tank — to see if it holds. If the load holds, next raise the load in increments — checking for load stop every few inches. With the load suspended, start reducing set pressure on the counterbalance valve slowly until the load creeps forward. When the load starts drifting down slowly, increase pressure until movement stops, then turn the pressure adjustment another quarter to half turn higher. This method of adjusting usually wastes less energy while it always stops and holds the load.
The main disadvantage of an internal pilot-operated counterbalance valve is that backpressure is constant and it holds back even when the actuator needs maximum force. Another disadvantage is that to maintain optimum performance, an Internal-piloted counterbalance valve must be readjusted every time the load changes. The valve’s main advantage is that it produces smooth cylinder action while advancing to the work.
An external pilot-operated counterbalance valve shifts to allow excess fluid flow to the outlet when pressure at the opposite cylinder port reaches the pressure set by the pressure adjustment. Pressure at the inlet never drops below load-induced pressure plus pressure set on the pressure adjustment when there is flow at the outlet. Flow from inlet to outlet is just enough that the actuator moves only as fast as it is supplied and stops when flow to the actuator ceases.
Pressure adjustment on the external pilot-operated counterbalance valve can be made on a test stand by setting the pressure adjustment at 100 to 200 psi. If pressure must be set on the machine, set the pressure adjustment higher than 200 psi and lift the load a small distance to make sure it stops and holds. If it holds, continue to raise the load high enough to have some time for the next step. Now, power the load down and observe pump pressure. Pump pressure while lowering the load should not exceed 200 psi. Continue this action until pump pressure is between 100 and 200 psi while the load is lowering. This method of adjusting usually wastes less energy while always stopping and holding the load.
The main disadvantage to an external pilot-operated counterbalance valve is that it may cause lunging or even stop cylinder action while advancing to the work. The main advantage is that backpressure is only present when the actuator is advancing to the work. At work contact, pressure at the actuator inlet increases and forces the counterbalance valve wide open, thus eliminating all backpressure. Another advantage is that an external pilot-operated counterbalance valve does not need to be readjusted when the load changes.
Internal and external pilot-operated counterbalance valves shift when pressure at the internal pilot area reaches the pressure set on the pressure adjustment and allows excess flow to go to the outlet. Pressure at the Inlet never drops below set pressure when there is flow at the outlet. Flow from the inlet to the outlet is just enough that backpressure on the actuator never drops below set pressure. This means the actuator moves only as fast as it is supplied and stops when Inlet flow ceases.
Pressure adjustment on an internal and external pilot-operated counterbalance valve is usually made by first screwing the pressure adjustment all the way in. To assure that the valve is capable of high enough pressure, start the pump and raise the load a small amount. Then center the directional valve that has the cylinder rod-end port connected to tank — to see if it holds. If the load holds, then raise the load in increments — checking for load stop every few inches. With the load suspended, start reducing set pressure slowly until the load creeps forward. When the load starts drifting down slowly, increase pressure until movement stops, then turn the pressure adjustment another quarter to half turn higher. This method of adjusting usually wastes less energy while always stopping and holding the load.
An internal and external pilot-operated counterbalance valve lowers loads smoothly and opens fully when pressure at the actuator inlet increases upon contact with the work. The valve does need to be readjusted when loads change, but this is a small price to pay for good control.
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