Sizing blow-off valve

[Joe1001]

Has anyone sized a blow-off valve based on the time it will take to empty a section of pipeline? The flow rate varies according to the water height which changes with time. The slope of the pipe(s) aslo has an effect of the total drainage time. Usually there are several pipe slopes up-stream and down-stream of the valve. This seems like quite an involved calculation.

 

[cvg]

use a pump to drain the line

 

[bimr]

Blowoff valves. A valve outlet installed at the low point in a force main, and arranged to drain or flush the pipeline, is termed a blowoff. Normally, blowoffs will be required only on long depressed sections of force main, or where an accumulation of solids is likely to occur. Blowoff connections will be installed in manholes or valve structures, and will be protected against freezing. A means of discharging to a suitable location materials flushed from the system will be provided. The pipe size of the outlet connection should coincide with the size of the force main.

As the specification above notes, the size of the blowoff valve is typically the same size as the force main. There is no need to calculate the size.

 

[cvg]

bimr - I have to disagree as our practice is quite different for sizing blowoffs than yours. To preface, my experience is in waterlines, not sewer force mains. We typically size blowoffs between 4 - 8 inches diameter. Fire hydrants are sometimes used as blowoffs also. I have not been involved with any waterlines smaller than 8" that required blowoffs, so the outlet is never the same size as the mainline. We design for draining, not generally for flushing. Flushing a line is difficult at best through a blowoff unless the line is quite small. To achieve adequate flushing velocity (2 fps or faster) through a large line requires huge quantities of water and rarely is there ability to dispose of the large quantities of water easily. To fully drain a (underground) waterline through a blowoff will require a pump in all cases.

 

[Joe1001]

Usually we would size the blow-off and air-intake requirements simultaneous as sizing the blow-off too big might cause negative pressures in the pipeline if the air valves are too small. To calculate the time to drain the pipe is not so easy (i think). On big pipelines, say 2m diameter, this could be significant as the pipe might take days to drain depending on length etc.