Do not connect both A ports directly to the dump actuator until the spool states and actuator action are known. A simple parallel tee can connect one solenoid's supply through the actuator line to the other solenoid's exhaust whenever the valves occupy opposite states. For independent dump requests, first prove whether pressure opens the dump valve, closes it, or drives a second actuator chamber; then implement the corresponding pneumatic OR, selector, or reversing function.
What do the symptoms say about the circuit?
The required logic is an OR function: request 1, request 2, or both requests must produce the same dump state. The pneumatic circuit must also define what happens with neither request present. That resting state cannot be inferred from the phrase “energize to dump,” because it may describe the electrical command rather than the actuator's pressure action.
Look at pressure and position together. If one energized solenoid causes continuous exhaust flow while the actuator pressure stays low, the second, de-energized spool is probably providing an exhaust path. If the pressure rises but the dump valve does not move, check actuator action, mechanical travel, and the pressure on any opposing chamber before changing the logic.
| Signal | Source or measurement point | Wrong-value symptom |
|---|---|---|
| Dump request 1 | Controller output or coil terminals | No command at the first solenoid, or an unexpected simultaneous command |
| Dump request 2 | Controller output or coil terminals | No independent operation from the second command |
| Branch outlet pressure | Each solenoid outlet before the common connection | One branch pressurizes while the other vents |
| Common actuator pressure | Line at the dump-valve actuator | Low pressure with continuous airflow indicates a supply-to-exhaust path |
| Opposing chamber pressure | Second actuator port, when fitted | Trapped or maintained pressure prevents the requested travel |
| Valve position | Position indication or observed stem travel | Pressure changes without movement point away from electrical logic |
Why can two parallel 5-port valves blow through?
A directional spool connects working ports differently in its energized and de-energized states. The proposed arrangement—supply to both P ports, both A ports joined to the actuator, exhaust at EA, and both B ports blocked—works only if every relevant spool state prevents the common A line from reaching exhaust while either branch supplies it.
Consider request 1 on and request 2 off. If solenoid 1 connects P to A while solenoid 2 connects A to EA, air flows from the first supply, through the common line, and out the second exhaust. The actuator then receives reduced or unstable pressure, and the circuit consumes air continuously. Reversing the requests produces the same fault through the opposite branch.
Blocking B does not isolate A from the exhaust gallery. It may also deadhead a spool path. Confirm from the SMC SY3120-5LOU-C6 symbol which ports communicate in both coil states, whether the valve permits pressure at a working port, and whether blocked service is allowed. The model code alone is not a substitute for the manufacturer's flow diagram.
Which actuator action must be proved first?
Disconnect control assumptions from actuator behavior. Apply controlled pressure according to the actuator documentation and record which chamber pressure opens the vessel valve, which pressure closes it, and what the valve does when both actuator lines are vented. Isolate the vessel and stored pneumatic energy before changing tubing.
A single-acting, spring-close actuator normally needs pressure in one direction and an exhaust path for return. A pressure-to-shut arrangement instead opens when its closing pressure is removed. A double-acting actuator requires coordinated pressure and exhaust paths on two chambers. These cases need different logic even when the same electrical truth table is requested.
Record four command states: both coils off, coil 1 only, coil 2 only, and both coils on. For each state, specify the required dump-valve position and required pressure at every actuator port. Any state left undefined can become a trapped-pressure, blow-through, or contradictory-command failure.
How should independent dump requests be configured?
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Read both directional symbols. Mark the actual connections among
P,A,B,EA, and every remaining exhaust in each coil state. Confirm permitted flow direction and working-port backpressure in SMC data. - Prove the actuator function. Determine whether the dump command requires pressure, venting, or pressure transfer between two chambers. Include the no-air position in the state table.
- Select the logic architecture. For a single pressure-to-open chamber, use a pneumatic OR function that admits pressure from either solenoid while isolating the inactive branch. A shuttle arrangement supplied by two appropriately configured 3/2 functions can perform this job; the selected assembly must also let the actuator exhaust when both requests are absent.
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Handle pressure-to-shut or double-acting service separately. Use a reversing or selector function that creates complementary actuator pressures without letting one command oppose the other. Candidate pneumatic elements include the Fairchild model
25reversing relay and the Fairchild model90low-selector relay. Their port connections must follow their flow diagrams, actuator action, and required exhaust capacity. - Prefer one authoritative final command where possible. If “independent” refers to two electrical request sources rather than two required pneumatic paths, combine the requests with electrical OR logic and command one correctly sized final directional function. If two physical solenoids are mandatory for redundancy or a protective function, validate the complete architecture against the machine's safety requirements rather than treating a parallel tee as redundancy.
- Build and test at controlled pressure. Fit gauges at both branch outlets and each actuator chamber. Exercise all four command states while checking pressure, exhaust flow, and final valve position.
How is correct operation verified?
Command coil 1 by itself, then coil 2 by itself. Each test must produce the required dump position without sustained flow from the inactive solenoid's exhaust. Command both coils together and verify that the result matches the state table rather than creating opposing actuator pressures.
Remove both commands and observe the defined resting position. Pressure must decay or remain applied exactly as the actuator design requires; a slow or incomplete transition points to restricted exhaust, trapped volume, undersized pneumatic logic, or mechanical friction. Repeat the sequence while monitoring the controller outputs so a wiring or program error is not mistaken for a pneumatic fault.
Complete the test with the real supply-pressure range and expected process load, using ratings taken from the component documentation. A circuit that moves correctly unloaded can stall when vessel differential pressure increases the required actuator force.
What pitfalls recur in dual-solenoid circuits?
- Treating two ordinary directional valves connected by a tee as a pneumatic OR gate.
- Using the electrical phrase “energize to dump” as proof that actuator pressure opens the valve.
- Checking only both-on and both-off states while missing the two opposite-state combinations.
- Blocking an unused working port without checking the spool diagram and allowable service.
- Ignoring pressure trapped in the second chamber of a double-acting actuator.
- Changing control logic before measuring branch pressure, common pressure, exhaust flow, and valve travel.
- Calling duplicated solenoids redundant when one failed or de-energized spool can defeat the other branch.
FAQ
How do I connect two 5-port solenoids to one actuator?
Start with the energized and de-energized flow symbols, then implement the required OR or selector function. Do not directly tee both A ports if either inactive spool can connect the common line to EA.
How do I test whether one solenoid is venting through the other?
Energize only one coil and measure pressure at both branch outlets and the common actuator line. Continuous airflow from the inactive valve's exhaust with low common pressure confirms a supply-to-exhaust path.
How do I control a dump valve that needs pressure to shut?
Define the required closing-chamber pressure for all four coil combinations, then use a reversing or low-selector function that gives the dump command priority without opposing pressure. The Fairchild model 25 and model 90 are candidate functions, but their documented port maps and ratings must match the actuator circuit.
When should I stop troubleshooting and contact official support?
Stop if the SMC SY3120-5LOU-C6 flow symbol, bidirectional-flow allowance, blocked-port service, or actuator fail action cannot be verified from current documentation. Contact official SMC and actuator-manufacturer support with the full model codes, pneumatic schematic, supply pressure, four-state pressure table, and observed valve positions before applying the circuit to the vessel.