During polyurethane foam spraying and polyurea coating applications, excessive pressure imbalance (Pressure Imbalance) is one of the most common equipment failures. When the machine detects that the pressure difference between the A and B sides exceeds the preset value, it will immediately trigger a pressure imbalance alarm. In severe cases, the machine will automatically stop spraying to prevent material ratio imbalance and avoid affecting construction quality.

Excessive pressure imbalance not only reduces construction efficiency but also causes abnormal foam density, poor foaming performance, incomplete polyurea curing, and other problems. Therefore, when a pressure imbalance alarm occurs, it is essential to quickly identify the cause and eliminate the fault in time.

From the working principle of the equipment, the essence of excessive pressure imbalance can only be caused by two situations:

• The low-pressure side has insufficient material supply;
• The high-pressure side has increased flow resistance caused by blockage in the material flow path.

Therefore, when troubleshooting a pressure imbalance fault, the correct maintenance procedure should be followed step by step, using the simplest methods to quickly locate and solve the problem.

Step 1: First eliminate the possibility of a booster pump failure

Regardless of the brand of polyurethane spray machine, whether GRACO, PMC, or other brands, when a pressure imbalance alarm occurs, it is recommended to check whether the booster pump is operating normally first.

This is because the booster pump is responsible for continuously delivering material to the main machine. Once internal leakage occurs inside the booster pump, that side will not be able to maintain stable pressure, resulting in a pressure imbalance alarm.

How to determine whether the booster pump has leakage?

The specific inspection method is as follows:

1. Start the equipment and allow the pressure on both A and B sides to reach normal working pressure.
2. Stop spraying, close the spray gun, and place the equipment in a standby pressure-holding condition.
3. Observe the pressure changes on the A and B side pressure gauges.

Judgment results

Situation 1: Pressure remains stable

If the pressure on both A and B sides remains stable for a long period of time without obvious pressure drop, it indicates that the booster pump sealing performance is normal, and the booster pump failure can basically be ruled out.

Situation 2: Pressure on one side continues to drop

If the pressure on one side continues to decrease while the other side remains stable, it indicates that internal leakage exists on the pressure-drop side.

The following components should be inspected:

• Booster pump packing
• Booster pump piston seals
• Check valve
• Ball valve sealing
• Booster pump repair kit

Wear or sealing failure of these components can cause material backflow, preventing the system from maintaining stable pressure.

Only after confirming that the booster pump is working properly should other components be checked. Otherwise, the subsequent troubleshooting process may lead to incorrect diagnosis.

Step 2: Check whether the high-pressure side has a blockage

If the booster pump is working normally but the equipment still shows a pressure imbalance alarm, in most cases the high-pressure side has a blockage.

A blockage increases material flow resistance, causing the pressure on that side to continuously rise and creating an imbalance between the A and B sides.

First check the spray gun

The spray gun is the most common location for blockage and is also the most frequent cause of pressure imbalance alarms.

Focus on checking:

• Mixing chamber
• Nozzle
• Side seal
• Internal spray gun flow passages

Even a small amount of cured material or contamination inside the spray gun can restrict material flow, causing a significant increase in pressure on that side.

It is recommended to replace the mixing chamber or nozzle with a new one for testing first, as this is the fastest verification method.

Then check the heated hose and material lines

If the spray gun is confirmed to be normal, continue checking the material delivery system.

Focus on checking:

• Whether the heated hose is blocked
• Whether JIC fittings are blocked
• Whether the material lines are bent or damaged
• Whether there is crystallized material or contamination inside the lines
• Whether the front gun fittings are blocked

Any blockage in these areas can cause pressure increase and trigger a pressure imbalance alarm.

Step 3: Check whether the material supply system is insufficient

If no blockage is found on the high-pressure side, then the low-pressure side material supply system should be checked.

Insufficient material supply prevents that side from building normal pressure and is another major cause of pressure imbalance alarms.

The following components should be checked:

Feed pump

Check whether the feed pump is operating normally and confirm that material supply is continuous and stable.

If the feed pump is worn, the seals are damaged, or the suction capacity is reduced, insufficient material supply may occur.

Material inlet filter

A blocked filter directly affects material flow.

Remove the filter screen and check for:

• Cured material
• Contaminants
• Metal particles
• Material crystallization

If blockage is found, clean or replace the filter immediately.

Material suction tube

Check whether the suction tube has:

• Air leakage
• Loose connections
• Cracks or damage
• Air entering the system

Air leakage in the suction tube can also cause insufficient material supply and result in pressure drop.

Material temperature

When the material temperature is too low, the viscosity will increase significantly, increasing material flow resistance.

Therefore, confirm that:

• The main machine heating temperature is normal;
• The heated hose temperature is normal;
• The material drum temperature meets the spraying requirements.

How to prevent spray machine pressure imbalance alarms?

To reduce pressure imbalance failures, regular equipment maintenance is recommended:

• Thoroughly clean the spray gun after each working day.
• Regularly clean or replace the material inlet filters.
• Regularly inspect booster pump seals and check valves.
• Check whether the suction tube is leaking or aging.
• Maintain the recommended material and equipment temperatures.
• Regularly inspect heated hoses and all connection fittings.
• Use clean raw materials that meet requirements to prevent contamination from entering the system.

Good maintenance practices can effectively reduce the occurrence of pressure imbalance alarms, extend equipment service life, and improve spraying efficiency.

Summary

A pressure imbalance on a spray machine is not an independent failure but a symptom of abnormal equipment operation.

From the working principle, there are only two fundamental causes:

• Insufficient material supply on the low-pressure side;
• Blockage or increased flow resistance on the high-pressure side.

During actual maintenance, always follow the troubleshooting sequence:

First check the booster pump, then check for blockage on the high-pressure side, and finally check the low-pressure side material supply system.

Do not disassemble the equipment blindly. By following a scientific fault diagnosis process, problems can be located quickly, repair time can be reduced, and the operating stability of polyurethane spray foam machines and polyurea spraying machines can be improved, ensuring consistent construction quality.