Page 77 - CITS - Welder - Trade Theory

WELDER - CITS

Hydraulic backpressure valve and flash back arrestor

Objectives : At the end of this lesson you shall be able to
• state the necessity of the hydraulic back pressure valve
• explain the Working principle of a hydraulic back pressure valve.
• explain the Working principle of a flash back arrestor.

Necessity of Hydraulic Safety Valve: In the low pressure system the oxygen pressure is always greater than the
generated acetylene gas pressure.
During welding, due to back fire or nozzle hole blockage, the high pressure oxygen may enter into the acetylene
passage and enter the acetylene generator, which will lead to an explosion.
To prevent the entry of high pressure oxygen or back fire to the generator, a hydraulic safety valve must be fitted
in the acetylene pipe line between the blow pipe and the generator or purifier.

The pipe (P) has a baffle plate fixed a tit slowerend.
In the vent of backfire or flash back (backpressure) from the blow pipe side, the water level (A )is pushed down
and water is forced up into the vent pipe until the hole (H) is exposed.
Constructional Features Of Hydraulic Back Pressure valve : It is a cylindrical shaped device having 250mm
depth and 50 to 100mm dia, as per the generator’s capacity.(Fig 1)
Working Principle
The cylindrical device is filled with water through the VENTPIPE(v) upto the level of water level (D).(Fig1)Gas
enters from the generator through the in let pipe valve and comes down the center pipe(P),bubbles through the
outlet pipe(R) and valve(S). The burnt gases in case of aback fire, or the back pressure gases, pass up the vent
pipe in to the atmosphere and are prevented from getting in to the generator.(Fig 2)
Flash back arrestor
Introduction : It is a safety device and fitted with carbide to water acetylene generator. It is made out of mild steel
cylindrical body.
Parts : Flash back arrestor has the following parts
1 Inlet
2 Drain plug
3 Non-return value

4 Water level

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CITS : C G & M - Welder - Lesson 14-26 CITS : C G & M - Welder - Lesson 14-26