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5.5 ComboValve® (blast vessel pressurization/blowdown)
The ComboValve (#4) is a dual-purpose valve that controls both the blast vessel pressurization
and the blast operations. At one end the valve pinches the 3/4" blowdown hose (#9) to seal it
and allow air to pressurize the blast vessel. At the other end, the ComboValve opens and allows
air to flow to the blast vessel (#1) and through the blast air piping to the blast nozzle. See the
ComboValve drawing in Section 9.6.
The ComboValve opens and blasting starts when the deadman lever (#12) is pressed down. The
blast vessel will pressurize.
The ComboValve closes and blasting stops when the deadman lever (#12) is released.
The blast vessel will depressurize (blowdown).
When the ComboValve closes the pinch ram on the blowdown hose (#9) is released and the air
inside the blast vessel (#1) will exhaust through the blowdown hose. The MV3 Valve abrasive
blaster (#1) remains depressurized when the ComboValve (#4) is closed. The abrasive blaster
must be depressurized before filling with abrasive or before performing any maintenance.
The MV3 Valve abrasive blaster is a pressurized vessel. Propelled objects will cause serious
injury or death. Read and follow all pre-operation and operating procedures prior to
pressurizing the abrasive blaster. See Sections 6.0 and 7.0.
Airborne particles and loud noise hazards from blowdown exhaust air can cause serious injury
and loss of hearing. Wear approved eye and ear protection. Stay clear of blowdown air path.
DO NOT place hands or other body parts in the blowdown air path. Make sure no personnel are
in the blowdown air path.
Note: All Schmidt MV3/Combo blasters (except for 1.5cf) are equipped with the VBS II
Volumetric Blowdown Suppression System which reduces noise level and floor damage caused
by the blowdown exhaust flow. Refer to Section 5.16.
5.6 Choke Valve
The choke valve (#13) is used to clear any trash that may get into the blast vessel and block the
MV3 Valve (#14) orifice. Whenever trash (paint chips, cigarette butts, etc.) blocks the MV3
Valve orifice the procedure is to fully open the MV3 Valve by backing out the knob, then press
down the deadman lever (#12) to begin blasting. While blasting, have an assistant close the
choke valve completely for about one second. This creates differential pressure at the MV3
Valve (high pressure above; low pressure below). The higher pressure from the blast vessel
should be enough to force the trash through the MV3 Valve orifice. Keep the choke valve fully
open at all other times while blasting to minimize excess MV3 Valve wear.
Do not aim blast nozzle towards yourself or any person. System malfunction can cause
accidental start up and result in injury to personnel.
A secondary purpose of the choke valve is as a manual shut off valve for the blast air supply.
When the choke valve (#13) is closed it will shut off the blast air supply to the blast outlet.
See Section 11.3.2 for further safety information regarding the choke procedure.
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