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Troubleshooting Pressure Release Pots

 

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What is differential pressure?

The best way to troubleshoot a pressure release pot is to understand differential pressure, which comes in two forms; Positive and Negative. Each form of pressure delivers problematic “symptoms” which helps in diagnosing WHERE the actual problem may lie.

Before explaining differential pressure however, it’s first helpful to review how a properly working blast pot with balanced pressure works.

 

Balanced pressure

A properly working blast pot maintains the same pressure in both the vessel and the push line. Abrasive drops via gravity (like an hour glass) and is picked up by the pressure stream going through the push line and is then carried toward the nozzle.

 

Positive differential pressure

When the vessel’s pressure is GREATER than the push line pressure it’s called positive differential pressure. This imbalance causes the vessel to force the abrasive into the lower metering valve (as opposed to the hour glass analogy) which in turn prematurely wears out the valve.

Blasters experiencing a consistent stream of too much abrasive should suspect the vessel is force-feeding the line – and thus the system suffers from positive differential pressure.

The cause of this is typically a leak in the push line hose or valve, which is bleeding pressure away from the push line, and thus creating an imbalance where the vessel pressure is greater.

 

Negative differential pressure

When the vessel’s pressure is LOWER than the push line pressure it’s called negative differential pressure. The result of this imbalance is that the push line pressure, being greater, actually creates an updraft and lifts the abrasive as it drops, causing turbulence and chaos in the valve while simultaneously delaying the delivery of the abrasive.

Blasters experiencing a sporadic supply of abrasive – it’s there, then it’s not – should suspect negative differential pressure is preventing the abrasive from entering the air stream. When the weight of the obstructed abrasive becomes great enough however, it breaks through the updraft and once again fills the blast stream. This is the reason for the sporadic delivery of abrasive. Updraft stops the abrasive… and then it breaks through. It stops the abrasive again… and then it breaks through.

The cause of this is typically a leak in the blowdown hose, pop-up valve, or handway, which bleeds pressure away from the vessel.

 

Review

Positive differential pressure

Results in: Down pressure

Indicators: Way too much abrasive coming through blast stream

Premature valve wear

Probable Causes: Leaks or valve problems in Push Line

 

Negative differential pressure

Results in: Upward pressure / Up draft

Indicators: Sporadic delivery of abrasive through nozzle

Probable Causes: Leak in blowdown hose, pop-up valve, or handway

Intentional choking

 

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