6” MCFM - 12000
01.2021
Page 13 of 61
PREVENTION OF REVERSE FLOW
Fine screen collapse is caused by hydraulic pressure acting externally upon the screen. The hydraulic back pressure
results from a differential pressure across the screen, where the external pressure is greater than the internal pressure.
The process conditions leading to this screen failure have been identified through failure studies of actual installed
filters.
The major process conditions are:
Process media backflow through the screen.
Air entrapment in the fine screen compartment of the filter house.
The four-layer screen is very sensitive to back pressure; small differential back pressure acting upon a blocked screen
shall cause collapse.
Preventing reverse flow
Reverse flow can be avoided in these ways:
1.
Install a check (non-return) valve downstream of the filter. The check valve shall be of the quick acting
type, and the disc's return should be spring loaded. The valve manufacturer's installation
recommendations must be conformed, to prevent any back flow through the valve.
2.
Avoid additional pressure sources between the filter and the process. When this cannot be avoided,
isolate the filter before introducing these pressure sources.
3.
Close the filter manual outlet valve, and then the inlet valve before stopping the process pump.
Start the pump before opening the filter inlet and outlet valves.
4.
Avoid situations of automatic process pump starts and stops where back flow may result.
Air entrapment
Air entrapment must be minimized in the fine screen chamber. This is not only to protect the screen from collapse, but
to allow the entire fine screen to be submerged in the media and participate in the filtration. This is more important in
low pressure applications.
When air entrapment is present, any sudden release of hydraulic pressure shall allow the air pocket to expand and
push media in a backflow direction through the fine screen.
Preventing air entrapment
Air entrapment damage can be avoided in these and other ways:
1.
Vent entrapped air from the fine screen chamber.
2.
Do not isolate the filter from the pressure source until the flush valves are closed.
3.
Do not open the flush valves after the filter has been isolated from the pressure source and until the filter has been
depressurized.
4.
To depressurize, and when opening the filter for maintenance, release the internal pressure slowly. This can be
achieved by a ¼" valve.
5.
When draining the filter, ensure that the filter has been depressurized first.
Technical solutions to remove trapped air
The MCFM filter housings are provided with a connection port for an optional air release valve.
