AirSprayTech Academy Certificate Program

Portable Plural-Component Coating Systems for Contractors

Article 12 of 24

Filters, Valves, Gauges, Sensors, and Protective Controls

The proportioning pumps may provide the power, but filters, valves, gauges, sensors, alarms, and relief devices determine whether that power remains controlled, observable, and safe.

Small Components Can Control the Entire System

A partially blocked filter, leaking check valve, inaccurate pressure gauge, damaged sensor, or defeated alarm can create unstable operation, off-ratio material, excessive pressure, poor mixing, equipment damage, or a serious injection hazard.

These devices must be selected, installed, inspected, and maintained as parts of one engineered fluid system. They are not interchangeable accessories.

Learning Objectives

After completing this article, the contractor should be able to:

• Explain the functions of fluid filters, valves, gauges, and sensors.
• Select filtration appropriate for the material and downstream components.
• Recognize the difference between isolation, check, dosing, restriction, sampling, and relief valves.
• Use pressure and temperature readings as process information without treating them as proof of ratio.
• Recognize protective controls that must never be bypassed.
• Create a practical inspection and maintenance program for these components.

Four Functions of Protective Fluid Components

Fluid Filters and Strainers

Filters protect pumps, check valves, dosing valves, flow meters, heaters, mix manifolds, spray guns, and tips from foreign material. Filtration also creates resistance. The correct filter removes damaging contamination without preventing required particles from passing or starving the equipment.

Filtration may be located at:

• The pickup tube or feed-pump inlet
• The proportioning-pump inlet
• The proportioner outlet
• The mix-manifold inlet
• The spray gun or another manufacturer-approved location

Finer Filtration Is Not Always Better

A screen that is too fine may remove functional fillers, load rapidly, produce excessive pressure loss, or starve one component pump. Highly filled, fiber-reinforced, aggregate-containing, or abrasive materials may require coarse filtration or no filter at certain locations.

Follow the coating and equipment manufacturers' filtration recommendations. Never select a filter solely by spray-tip size without evaluating every material constituent and protected component.

Understanding Mesh and Opening Size

Mesh count generally describes the number of openings per linear inch, but actual opening size also depends on wire diameter and screen construction. Two screens with the same nominal mesh count may not have identical open area or flow capacity.

Use the exact filter element specified for the housing and material. Record the manufacturer, part number, opening or mesh designation, material of construction, pressure rating, and installation location.

Signs of a Restricted Filter

• Increasing pressure upstream of the filter
• Decreasing pressure or flow downstream
• Pump cavitation or irregular cycling
• A growing pressure difference between A and B
• Ratio, flow, or pressure alarms
• Poor spray-pattern development or reduced production
• A component that heats abnormally from restriction or repeated circulation

Relieve pressure before opening any filter housing. High-pressure coating can penetrate skin, and a housing may remain pressurized even after the main pump stops.

Isolation and Shutoff Valves

Isolation valves stop or redirect flow so equipment can be operated, tested, flushed, serviced, or removed. A closed valve can also trap pressure or create a dead-ended pump condition.

Every operator should know:

• Which component the valve controls
• The normal operating position
• Whether closing it traps pressure
• Whether it is part of a circulation, sampling, flushing, or relief path
• The correct sequence for opening and closing it

Permanently label valves. Do not rely on memory, handle color, or an unmarked diagram kept away from the equipment.

Check Valves

Check valves permit flow in one direction and resist reverse flow. They are used in pumps, dosing assemblies, mix manifolds, flush circuits, and other locations.

A leaking check valve can allow backflow, pressure decay, cross-contamination, inaccurate dosing, pump slippage, or one component entering another component's passage.

Check valves must be installed in the correct direction and maintained with the specified seat, ball, needle, spring, seals, and torque. Do not substitute parts merely because they fit physically.

Dosing and Injection Valves

Electronically controlled systems may use fast-acting valves to meter or inject one component into the flow of another. Ratio performance depends on consistent valve opening, closing, response time, sealing, and control signals.

Dosing-valve problems can result from:

• Contamination or cured coating on the needle or seat
• Incorrect packing adjustment
• Insufficient actuation-air pressure
• Worn seals, needle, seat, or actuator
• Material pressure outside the approved range
• Electrical, pneumatic, or controller faults

Restriction and Back-Pressure Valves

Restriction or back-pressure valves can help establish controlled circulation, pressure relationships, or stable operation. They should be adjusted only according to the equipment procedure.

Do not use a restriction valve merely to make two gauge needles appear equal. Excessive restriction can generate heat, increase pump loading, accelerate wear, or overpressurize an upstream component.

Relief Valves Are Protective Devices

Positive-displacement pumps can continue building pressure against a closed or blocked discharge. Approved relief devices or automatic shutdown controls are required where specified to prevent pressure from exceeding safe system limits.

Relief devices must discharge to an approved return point or safe location. They must not be plugged, isolated, defeated, wired out, or adjusted beyond the manufacturer's setting.

A pressure gauge warns only if someone sees and correctly interprets it. A gauge is not a replacement for a required relief device.

Pressure Gauges

Pressure gauges help the operator observe inlet, outlet, component, circulation, mix-manifold, air-supply, or hydraulic conditions. Gauge location determines what the reading represents.

Select gauges with:

• A working-pressure rating suitable for the maximum system pressure
• A scale that places normal operation in a readable portion of the range
• Wetted parts compatible with the component and flushing materials
• Adequate resistance to pulsation, vibration, heat, and jobsite damage
• The accuracy and calibration status required by the quality plan

Pressure Does Not Prove Ratio

Two components can have similar pressure readings while being delivered at the wrong ratio. They can also operate at different pressures while being delivered at the correct ratio.

Use gauges to identify restrictions, starvation, leakage, abnormal loading, or pressure imbalance. Use the approved ratio-monitoring and ratio-verification procedure to determine actual proportioning performance.

Recognizing an Unreliable Gauge

• The pointer does not return to zero after pressure is relieved.
• The pointer sticks, jumps, or vibrates excessively.
• The lens is broken, clouded, or filled with coating.
• The case, connection, or pressure-isolation device leaks.
• Two gauges exposed to the same verified pressure disagree beyond their permitted accuracy.
• The gauge has been overpressurized, dropped, frozen, overheated, or physically damaged.
• Its calibration or inspection status is unknown when documented accuracy is required.

Pressure Sensors and Transducers

Electronic pressure sensors convert fluid pressure into a signal used by the controller. The system may compare A and B pressures, identify abnormal conditions, display trends, issue alarms, or stop operation.

A sensor can produce a believable but incorrect value. Diagnosis may require comparison with an approved reference, inspection of wiring and connectors, confirmation of calibration, and verification that the sensor passage is not plugged or isolated from the actual fluid pressure.

Temperature Sensors

Temperature sensors may monitor a heater, fluid passage, hopper, heated hose, or mixed-material location. The displayed value applies to the sensor's location, not automatically to the entire fluid system.

Investigate a questionable temperature reading when:

• The reading changes unrealistically fast or does not change at all.
• The heater operates continuously but reported temperature remains low.
• Material behavior does not agree with the displayed temperature.
• An independent measurement differs significantly.
• The sensor has poor contact, damaged wiring, a loose connection, or an incorrect controller setting.

Stroke Sensors and Flow Meters

Stroke sensors monitor pump movement. Flow meters measure or infer the material passing through a fluid circuit. Controllers may use this information to regulate ratio, record consumption, detect a stalled pump, or identify an empty-container condition.

A stroke sensor confirms movement; it does not by itself prove that a pump chamber filled completely or delivered its full displacement. A flow meter must operate within its approved material, viscosity, flow, pressure, and temperature range.

Calibration, minimum-flow limits, cleanliness, wiring, installation orientation, and mechanical condition all affect measurement reliability.

Low-Level and Dry-Running Protection

Low-level controls, pump-cycle monitoring, inlet-pressure sensors, and dry-running protection can warn of an empty container or inadequate material supply. These controls help prevent loss of prime, pump damage, air entry, and off-ratio application.

They do not replace an assigned operator checking container levels. Sensors can fail, floats can stick, material can bridge, and a pickup tube can draw air while material remains elsewhere in the container.

Alarms, Interlocks, and Automatic Shutdowns

Modern proportioners may monitor ratio, pressure, temperature, flow, pump movement, component supply, pot life, heater operation, electrical condition, emergency-stop status, or communication between controllers.

Resetting an Alarm Is Not Correcting the Problem

An alarm identifies a condition—not necessarily its root cause. Repeatedly resetting an alarm can permit off-ratio material, excessive pressure, overheating, component starvation, or equipment damage.

Record the alarm code and time, stop application, identify the potentially affected coating area, diagnose the cause, complete required testing, and document authorization before spraying resumes.

Emergency Stop Versus Normal Shutdown

An emergency stop is intended to stop designated equipment functions quickly. It may not automatically relieve trapped fluid pressure, flush mixed material, close every valve, remove every energy source, or make the equipment safe for service.

After an emergency stop, follow the manufacturer's emergency, pressure-relief, flushing, electrical-isolation, and restart procedures. Never assume the system is depressurized because the pumps have stopped.

Daily Inspection Checklist

Maintenance and Replacement Controls

• Relieve pressure and isolate all energy before opening or removing components.
• Use the manufacturer's exact repair kit, seal material, spring, seat, screen, or sensor.
• Keep A-side and B-side tools and parts separated when cross-contamination is possible.
• Clean parts only with approved, compatible materials.
• Inspect removed material for metal, cured particles, fibers, seal fragments, or other evidence of failure.
• Restore guards, labels, grounding, wiring, relief paths, and controller settings.
• Complete pressure, leakage, functional, calibration, and ratio tests required after service.
• Document the work, parts used, technician, test results, and return-to-service approval.

Stop-Work Conditions

• A required relief device, guard, sensor, alarm, or interlock is missing or bypassed.
• A filter housing, valve, gauge, sensor, fitting, or manifold leaks under pressure.
• A gauge remains above zero after the approved pressure-relief procedure.
• A valve cannot be positively identified or its safe operating position is unknown.
• Pressure or temperature readings are implausible or cannot be verified.
• Repeated alarms occur without an identified and corrected cause.
• A component's pressure rating or material compatibility cannot be confirmed.
• The system cannot maintain stable pressure, flow, temperature, or ratio at production output.