Check valves can be unreliable and unavailable, partly because they contain moving parts and rarely receive the maintenance they require. We need to find a solution that allows a reverse flow rate less than 0.5% of the normal flow rate should the inlet and outlet pressures be reversed. Ideally we would like a simple, mechanical system free from moving parts. What do you suggest?
Use a fluidic check valve
You can use a fluidic check valve (diodes), shown in the figure. They are not as well-known as they should be.
Trevor A. Kletz, safety consultant
Fluidic Check Valve
Install in vertical pipe
Install the check valve in a vertical pipe run with manually operated valves above and below it so that removal, inspection and replacement can be conducted if needed. The check valve should have a seat, ball and basket section. These components should be made of materials that do not deteriorate in the presence of the process stream.A check valve operates on two principles: gravity and pressure differences. The design has only one moving part — the floating ball. It does not require springs or other mechanisms to function.The design of the ball is such that under normal operating conditions it “floats” in the process stream flow. This means that the ball is suspended above the seat, but not raised to a position where it contacts the cage. In low-flow conditions, the flow will push past the ball between the seat and ball. Proper material selection will minimize potential wear for low-flow conditions. In high-flow conditions, the flow will drive the ball against the basket, possibly deforming the ball or cage. George M. Hudak, manufacturing consultant
GN Manufacturing Consultants Inc., Summerfield, N.C.
Change position relative to ground
Raise the inlet and/or lower the outlet height of the check valve relative to the ground such that any pressure change provides insufficient head to cause reverse flow.
Ed Leibner, principal process engineer
Tyco Healthcare/Mallinckrodt, St. Louis
Consider four options
The following solutions may be applied:
1. Install a differential pressure cell around an on/off valve in the line. Use logic to close the control valve when the differential pressure cell reads about 1 in. to 2 in. H2O.
2. A magnetic flow meter can sense the direction of flow. If you have one in the line in addition to an on/off valve, you can close the valve when the magmeter senses reverse flow.
3. You can use a barometric leg to compensate for the highest pressure expected from the downstream vessel.
4. Use a failsafe dip tube with no mechanical moving parts if the line flows to a vessel that does not contain a slurry.
Dilip K. Das, P.E., principal engineer
Bayer CropScience, Kansas City, Mo.
Double up on valves
1. Put two check valves in series. Ideally they would be different styles, such as a swing check and a center-guided disc.
2. If you have a programmable logic controller (PLC) or other control system that can sense pressure reversal, use it to close any appropriate control valve(s).
3. Again, if you have a PLC, you can use an actuated ball valve to stop reverse flow. It should be a spring-return, normally closed valve, so that loss of power or instrument air would close the valve.
Rick Torchia, production project team
Aera Energy LLC, Bakersfield, Calif.
A pinch may help
How about a pinch valve?
Gregory Peak, director, technical service
Spears Manufacturing Co., Sylmar, Calif.
Opt for standpipes
We use standpipes. If they’;re made of C-276 materials, they are cheaper and more reliable than check valves.
Piston rings on piston pumps or check valves on some diaphragm pumps and on acid-egg type pumps may serve your purposes without additional moving parts. This equipment also gives clues about its operability when pump performance is monitored.
Float valves can be more or less reliable than check valves, depending on their service. Under some circumstances it is easier to determine whether a float floats than to determine whether a swing or ball check seats properly.
Andrew K. Schwartz Jr., chairman
Keeshan & Bost Chemical Co., Manvel, Texas
Switch to a U-trap
Perhaps you could use a U-type trap with a downstream reservoir installed at the point where the U-bend starts to go vertical. This reservoir would come off the pipe horizontally and hold enough higher-density fluid such that when the flow reverses, the reservoir empties enough to create a differential head great enough to stop the flow. For example, if the flowing fluid is air, the fluid acting as the check fluid could be water.
Jim Darby, process design engineer
Lanxess Inc., Sarnia, Ontario
Ask a plumber
You should consider using a reduced-pressure-zone-type backflow preventer (available at a plumbing supply house).
Don Bowlby, P.E., consulting engineer
West Chester, Pa.
In lieu of a check
Depending on the pressure differential and the line size, many applications use a bi-directional floating type ball valve in lieu of a check valve. The bi-directional ball valve also can serve as a shut-off valve if needed.
Don MacDonald, product specialist
MacDonald Specialties Ltd., Oakville, Ontario
Install on/off valve
Install a normally closed ball or globe valve that would actuate fully open or closed (shut-off) as a function of pressure differential. The valve could be automated or a pressure sensing, linear-to-rotary actuated valve with a “cracking” pressure below the upstream pressure.
Ron Johnson, process engineer
Alpharma Pharmaceuticals, Baltimore