In today’s world, abrasive cost has increased dramatically, labour costs have increased,
and the cost for disposal of spent abrasive is much higher than just a few years ago.
This has created a great incentive for surface preparation companies to invest in
technology with the highest level of abrasive utilization as possible. The same companies
are also demanding higher production rates than what the legacy equipment is capable of
achieving. (Higher Efficiency)
A major step in improving abrasive utilization is in the migration from non-pressure hold
(NPH) equipment to pressure hold (PH) equipment.
NPH control systems are the most common in the world, they were designed when
abrasive blasting was considered a necessary nuisance with cheap abrasive and cheap
labour, very few people put much thought into the process.
NPH equipment start of blast:
Starting with abrasive in the vessel and the vessel is at atmospheric pressure. The
operator engages a Deadman Control which sends a signal to the vessel. The controls
simultaneously pressurize the blast hose and the vessel. Due to the blast hose being much
smaller in volume than the vessel, it takes longer for the vessel to pressurize than the
blast hose. At this point no abrasive can enter the blast hose, it is not until the vessel
equals pressure with the blast hose that abrasive can enter and blasting begins. This is a
loss of productivity.
NPH equipment stopping blast:
Once the operator releases the Deadman Control, the controls stop the flow of air to
the blast hose and vessel at the same time. The vessel must depressurize in order to stop
the blasting process. The blast hose will depressurize faster than the vessel. Because of
this, the vessel will continue pushing abrasive into the blast hose until the vessel is fully
depressurized.
The loss of abrasive utilization occurs at the next start up; all the dormant abrasive in
the blast hose must be purged out through the nozzle before blasting can occur again.
PH equipment start of blast:
The vessel has abrasive and is already pressurized. The operator engages the Deadman
Control sending a signal to two valves on the vessel, the air valve pressurizes the blast
hose, the abrasive metering valve opens and allows abrasive to flow into the air stream.
Due to the vessel already being pressurized, blasting occurs immediately.
PH equipment stopping blast:
The operator releases the Deadman Control, the air valve stops the flow of air to the
blast hose and the abrasive metering valve closes. This stops the flow of abrasive but the vessel remains pressurized, therefore upon the next start up, there is minimal dormant
abrasive in the blast hose that must be purged out before blasting can occur again.
Summary:
NPH equipment cannot achieve the same abrasive efficiency as PH systems. Companies
that migrate to PH systems are capable of achieving between 15% and 20% higher abrasive
savings. There are many factors which affect abrasive utilization, abrasive metering, air
supply, air cleanliness, how many times the operator starts and stops but the operational
differences between NPH and PH cannot be argued.
Bill Nelson
Vice President
Schmidt
