A request for “one five-port valve for the cylinder” is insufficient for purchasing or acceptance at a Thai factory. The motion may need a 3/2, 5/2 or 5/3 directional valve; an internally piloted model may fail to shift when inlet pressure falls below its model-specific pilot requirement; and a coil labelled DC 24 V may still be incompatible with the PLC output and connector. This guide turns those questions into a specification, an RFQ and FAT/SAT evidence. It concerns pneumatic directional valves on machines. A two-way process valve that simply opens a fluid line is a different product class, illustrated by Emerson ASCO’s two-way series. Any numerical example below is illustrative, never a universal product rating.
Draw the air paths before choosing a part number
Start with the actuator and the work it performs. A single-acting cylinder often calls for a three-port, two-position valve that alternates supply and exhaust at its working port. A double-acting cylinder typically requires alternating supply and exhaust to two chambers, making a five-port valve a common candidate. The machine circuit and hazard assessment may require additional components; the cylinder type alone does not fix a complete design. Festo’s explanation of pneumatic valves distinguishes 3/2, 5/2 and 5/3 by function.
Mark supply, working and exhaust ports, flow paths in every position, spring return, energized state and de-energized state on the pneumatic drawing. P/1, A and B/2 and 4, and R and S/3 and 5 are common conventions, but verify the exact device drawing. The first number in 5/2 counts ports; the second counts switching positions. It says nothing about cylinder bore or flow capacity. The mechanic and PLC engineer must agree whether an ON output extends or retracts the machine axis. Make a state table for normal operation, electrical failure, pressure loss, emergency stop and restoration of both utilities. ISO 4414:2010 addresses general rules and safety requirements for pneumatic systems; an individual machine still needs its own design and validation.
Choose 3/2, 5/2 or 5/3 by the intended state
A 3/2 valve has three ports and two flow positions. For a spring-return single-acting cylinder, define whether the working port exhausts or receives pressure without electricity. Normally closed and normally open describe a valve path; neither term alone proves a safe machine condition. Blow-off air, clamping and a release stroke can require different default states.
A 5/2 valve directs supply to one of two working ports while exhausting the other. A single-solenoid spring-return model differs from a double-solenoid model in its behavior when power disappears. A double-solenoid valve can retain its last commanded spool position instead of returning to the position the process designer expects. Even then, the actuator’s actual motion depends on trapped air, leakage, gravity and external force. Specify whether loss of power should release, retreat or hold the workpiece and test the assembled system. “Double solenoid means safe holding” is not an acceptable design claim.
A 5/3 valve provides a center position. Specify its exact center flow configuration: closed center, exhaust center or pressure center, and check the candidate’s symbol for which ports connect. A closed center can appear to stop a cylinder while internal or external leakage slowly moves it. On a vertical axis with a falling-load hazard, consider a mechanical brake, rod lock or other validated retention according to the hazard assessment. The center position is not by itself a safety-rated holding device. Also define spring centering, transition time and the response to simultaneous coil requests.
| Valve | Typical application | Required specification |
|---|---|---|
| 3/2 | Single-acting cylinder, control air, blow-off | Normal position, supply/exhaust route, power-loss state |
| 5/2 | Double-acting reciprocating axis | Single or double coil, return method, A/B-to-motion mapping |
| 5/3 | Double-acting axis using a center state | Center flow path, centering method, leakage and load retention |
Direct operation versus pilot operation
In a direct-operated valve the electromagnetic force moves the valve mechanism directly. In a pilot-operated valve, a smaller pilot valve shifts the main stage under specified pneumatic conditions. SMC lists direct-operated 3/4/5-port valves separately from pilot-operated 4/5-port valves. Do not infer that every direct valve works at zero pressure or that every pilot valve has higher usable flow. Compare the chosen model’s pressure range, flow data, response and electrical load.
With an internal pilot, inlet air supplies the pilot stage. Measure the lowest pressure at the valve inlet during startup, simultaneous air demand and shutdown. Regulator setting is not the same as dynamic inlet pressure after filters, fittings and piping. An external-pilot version may draw pilot air from a separate supply, but that supply needs its own piping, isolation and failure analysis. SMC’s SZ3000 instructions are an example of model-specific operating and electrical cautions. Never transfer the minimum pressure quoted for one series to another.
Ask what “operation at zero pressure” really means: electrical spool movement with no supply, control of very low pressure air, or a predictable first position when air returns. These are different acceptance tests. In the RFQ state the normal, minimum and maximum pressure at the valve inlet, where each is measured, the pressure ramp and whether an external pilot supply exists. Obtain a written model-specific answer.
Size the flow through the complete circuit
Undersized flow slows the cylinder and may reduce available force during motion. Oversizing can add cost and make exhaust adjustment harder. Establish cylinder bore, rod diameter, stroke, demanded travel time, load, inlet pressure, tubing bore and length, fittings, speed controller and exhaust silencer. Chamber geometric volume is piston area times stroke; the rod-side volume is smaller. That geometric volume divided by travel time is not directly comparable with catalogue flow quoted at a standard reference condition. Air compressibility, pressure trajectory, tube volume, exhaust backpressure, leakage and acceleration matter.
Use the manufacturer’s flow curves or selection software with consistent test conditions, then measure critical axes on the assembled machine. Two valves with the same thread size can have different internal flow paths. The Festo directional-control range is a starting point; the exact model data sheet is the selection evidence. As an illustrative comparison, a valve beside the cylinder and an identical valve on a remote manifold can produce different movement times because the second arrangement has longer, smaller tubing and more volume to fill. No universal tubing-length limit follows from that example. If a quick exhaust is considered, evaluate impact at the stroke end, noise, exhaust routing and maintainability as well as speed. Record how a clogged silencer changes operation.
Match the DC coil to the PLC output
“DC 24 V” is only the beginning of an electrical interface. Verify whether the PLC transistor output sinks or sources current, the coil’s voltage tolerance and energizing/holding current, per-point and common-terminal output capacity, leakage current, suppression device, polarity, indicator LED and connector pinout. A single-coil valve generally takes one command and a double-coil valve generally takes two; a networked manifold requires its own address map. Record the valve tag, machine action, output address, coil pin and command polarity in one I/O table.
An integrated diode or varistor affects polarity or turn-off behavior. Confirm its combination with PLC output protection and the required stopping time. Include cable length, concurrent coil energization and panel temperature in the electrical review. A lit valve LED does not demonstrate that the main spool shifted. Monitor inlet/pilot pressure and actuator end sensors separately where required. The SMC SZ3000 document illustrates why the actual product wiring diagram and cautions must control the installation.
Interlock opposing commands in PLC logic and time out a failed stroke. Derive the timeout under the worst validated load, pressure, hose and ambient conditions. Avoid endless automatic re-energizing after a fault. A cylinder end sensor proves only its detection condition; it does not by itself prove that the workpiece is seated or clamped with adequate force. Add process-specific checks if that distinction matters.
Specify the manifold as a shared system
A valve manifold reduces discrete wiring and tubing but shares air supply, exhaust, communication, power and sometimes pilot passages among axes. Test whether exhaust backpressure or simultaneous demand affects a neighboring station. Confirm whether the base accepts mixed sizes or internal and external pilot configurations. The Festo ISO 5599-1 catalogue helps check physical interfaces; it does not establish full electrical and pneumatic interchangeability.
Include station count, spare stations, blanking plates, supply and exhaust zoning, electrical connector or bus protocol, address allocation, separate power domains and valve replacement steps in the quotation. Treat loss of bus communication, controller power and valve power as separate faults. Determine whether replacing one valve requires isolation of the whole manifold. At FAT, check addresses and pinout after a model change rather than assuming a compatible-looking replacement is identical.
Define the power-loss state and hazardous-energy control separately
Spring return, last-position retention and spring centering describe valve behavior. Cylinder movement still depends on load, gravity, trapped pressure and leakage. Decide the safe state through a machine hazard assessment and use guards, independently designed isolation/exhaust or mechanical retention where necessary. ISO 4414 is a useful system-level reference, not a declaration that an individual valve is a safety component.
Manual override is useful during commissioning and can move an axis while the PLC output is OFF. Specify push-to-actuate or latching behavior, tool requirement, access location, permitted operating mode and authorized operator. Test its effect on both the driven and return sides while controlling access to the danger area. For lockout and maintenance, isolate electrical and pneumatic sources, vent stored air, restrain gravity or spring loads and verify the specific isolated zone. A gauge at zero in one part of the circuit need not prove all trapped volumes are discharged. SMC’s manifold documentation gives relevant residual-pressure information. A normal directional valve is not automatically a lockout or safety-rated isolation device.
Thai-factory environment and spares
Ambient heat and humidity, condensate, dusty air, washdown and chemical exposure can affect a valve even when nominal pressure is within range. Record where condensation forms, dryer/filter placement, drainage maintenance, dust and oil mist, required seal material and the candidate’s specified air quality and enclosure rating. Check local availability of coils, connectors, seals, manifold bases and silencers using supplier quotations and stock answers, not a guessed universal lead time. Deliver pneumatic drawings, electrical drawings, I/O lists and replacement instructions in languages the local maintenance team can use.
RFQ/RFP: request comparable evidence
State the machine operation; single- or double-acting cylinder; valve ports and positions; normal and center paths; single/double coil; direct/internal/external pilot; inlet pressure range and measurement point; flow demand and operating assumptions; target cycle; tube and fitting conditions; air quality; DC electrical interface; connector and surge suppression; override; manifold layout; failure state; spares and service method. Mark irrelevant fields “not applicable” rather than leaving them ambiguous.
Request the full order code, current data sheet, pressure and flow selection rationale, pneumatic and electrical drawings, mounting dimensions, delivery quote and spare-parts list. Compare the same scope of base, cables, fittings, silencers, installation and test work across suppliers. If a safety function or certification is required, obtain evidence naming the exact model and applicable function; an unsupported “equivalent” claim is inadequate.
| RFQ line | Example input | Evidence to request |
|---|---|---|
| Circuit | Double-acting, 5/2, single coil, retraction requested on power loss | Port diagram and power-loss test plan |
| Pressure | Normal/minimum/maximum at valve inlet | Model range and pilot method |
| Flow | Load, stroke, travel time, tubing | Calculation and model flow curves |
| Electrical | Coil rating, PLC output, pinout | Wiring diagram, I/O and concurrent-load calculation |
| Service | Isolation, venting, replacement and restart | Instructions and local spares response |
The circuit row is an illustrative specification, not a recommendation for every machine. Retraction itself may create a hazard on another machine.
FAT: prove the valve works in the machine
Use representative workpieces, the intended tubing, actual PLC outputs and power supply. Test coil commands, extension/retraction, response, minimum and maximum cycle, continuous operation, simultaneous axes and the lowest specified inlet pressure. Do not force a product outside its rated range simply to create a test. Log PLC output, coil input, valve inlet/pilot pressure, actuator sensor and workpiece state on a common timeline. Simulate power loss, communication loss, air-pressure reduction, controlled manual override and the order in which power and air return, using an approved safe procedure and restricted access.
Observe a 5/3 closed-center axis for leakage-related drift rather than declaring retention after a momentary pause. Record pressure, load, sample count, command/response timing, deviations, corrective action and retest. Tie the record to the valve order code, drawing revision and PLC version so later maintenance can identify changes.
SAT: repeat the evidence under site conditions
At the Thai plant, use the real compressor header, simultaneous consumers, installed hoses, cabinet power, communications and operator movements. Check dynamic inlet pressure during morning start and production peak, kinked tubing, silencer placement and condensate. Accept the process outcome—workpiece seating, grip/release, product quality and cycle time—alongside valve motion. An end switch may be ON while a clamp has insufficient force. Carry FAT items that could not be reproduced into a SAT list with owner, test date and decision rule. Ask local maintenance personnel to demonstrate a valve replacement and safe restart.
Close the selection meeting with documented evidence, not merely a part number
When purchasing asks for an equivalent, matching the port thread and coil voltage is insufficient. Pilot method, manual override, return mechanism, current draw, terminal layout and minimum pressure can differ. Put the existing and proposed full order codes side by side. Compare circuit symbol, every port assignment, normal state, electrical connection, mounting dimensions, minimum and maximum pressure, flow characteristics, ambient temperature and replacement parts line by line. An unverified item should remain an open test with an owner, rather than being marked “equivalent.” Store the comparison where night-shift maintenance can use it during a replacement.
Flow values also require a common basis. One supplier may quote effective area, another Cv, and another reference-condition flow. The larger printed number does not necessarily indicate the better valve. Check the test standard, inlet and outlet pressure, air temperature and flow direction; ask suppliers to compare the same operating point where necessary. A silencer or speed controller on the exhaust may not be included in a valve-only catalogue figure. Tie the purchasing comparison to the complete proposed circuit and its measured performance.
A spare coil alone will not restore production if the failed item is a connector, gasket, manifold base or exhaust silencer. Decide the spare set from fault history, the number of identical installed devices, production loss on the critical process and actual Thai stock responses. Record order codes, storage conditions and stock movement dates because stored electrical and elastomer parts can deteriorate. When an alternative is approved, revise the pneumatic drawing, electrical drawing, PLC I/O and FAT/SAT baseline. This prevents a later purchase based on an obsolete drawing.
Manage changes and transfer acceptance baselines to operations
Increasing the takt target, lengthening a hose, adding an axis to the same manifold or replacing a PLC output card changes the operating conditions even when the valve order code remains the same. A change record should show the purpose; before-and-after circuit and I/O; minimum inlet pressure; number of simultaneous axes; power capacity; exhaust backpressure; power-loss response; and required repeat tests. “Only one axis added” can still affect existing axes through shared supply and exhaust. Define which existing acceptance results must be repeated.
Give operations and maintenance the measured normal pressure and cycle-time baselines, including the FAT/SAT conditions under which they were measured. Include how to read fault history, restrictions on manual override, locations for isolation and residual-pressure venting, replacement parts, restart checks and contacts. A number posted without its load, product, pressure and test configuration can be misused after a product change. If motion slows, the work instruction should call for checking supply shortage, a blocked silencer and mechanical interference before simply increasing regulator pressure.
Agree who maintains the final pneumatic and electrical drawings and who approves the motion test after valve replacement: machine supplier, electrical contractor, site maintenance or production owner. If site commissioning changes a setting that passed FAT, retest the affected operation and associated failure response. That turns the delivery-day result into a reproducible standard for long-term maintenance and acceptance.
During manifold commissioning, match each PLC output address to its physical valve station one at a time. Starting with every axis together can mix up wiring faults, swapped A/B tubes and wrong station placement. First run each station at a controlled safe speed and record the command, coil indication, working-port pressure, cylinder direction and end sensor on the same sheet. Then run two or more axes together, measure supply pressure and exhaust backpressure, and compare their cycle times with the single-axis tests. Shared passages make this an assembly-level acceptance test; a catalogue value for an isolated valve cannot replace it.
Unused stations need an acceptance decision too. Specify whether a blanking plate closes the unused position, whether supply and exhaust are reserved for future expansion, and whether only an electrical address is reserved. If the quotation merely says “room for expansion” without verifying passage capacity or output capacity, adding one axis later can slow existing axes. Preserve the FAT station layout, supply and exhaust zones, number of simultaneously energized coils and measured pressures as the baseline. After connecting the installed site tubing, repeat the same sequence at SAT and record test-condition differences separately from result differences.
Periodic checks after acceptance should go beyond “the axis moved.” Record valve inlet pressure at the most air-intensive production step and compare it with the FAT/SAT baseline under comparable conditions. If the exhaust sound changes, consider a clogged silencer, altered tubing and incomplete valve switching; measure pressure and motion time before deciding. The maintenance handover should capture regulator and speed-controller settings, before-and-after replacement motion checks, and confirmation that manual override has been released. Tailor these checks to the selected model and machine hazard assessment.
Troubleshooting order and common questions
When a cylinder does not move, first make the machine safe. Then follow the chain: PLC command, output terminal, coil input, valve inlet pressure, pilot pressure, working-port pressure, exhaust restriction and mechanical load. An ON LED does not narrow that chain to the valve alone. For a slow axis inspect dynamic supply drop, filter/silencer clogging, concurrent demand, speed-controller setting, tubing changes and workpiece friction. For drifting at stop inspect center function, leakage, gravity, retention and sensor tolerance. Record measurements before replacing parts; simply lengthening a PLC timeout can hide a failure.
Does 5/2 work for every double-acting cylinder? It is a circuit candidate, but flow, pressure, electrical and failure-state requirements still decide the model. Does a 5/3 closed center safely hold a vertical load? No guarantee follows from center closure; test leakage and provide validated retention where the hazard assessment requires it. Does a lit coil LED prove a defective valve? No; measure electrical and pneumatic conditions. Can ISO 5599-1 models always be swapped? Interface dimensions do not establish coil pinout, pilot routing or performance equivalence.
Conclusion
Define the required flow paths and power-loss behavior first. Then select 3/2, 5/2 or 5/3 and any center function; check direct or pilot actuation, minimum inlet pressure, complete-circuit flow, coil/PLC interface, manifold behavior and hazardous-energy control. Require a documented rationale in the RFQ, verify assembly behavior at FAT and repeat it with real site utilities at SAT.
For support with pneumatic circuits, PLC I/O and machine acceptance in Thailand, contact TOMAS TECH. For the actuator decision, see our air-cylinder selection guide and electric-cylinder selection guide.
