In addition to losses due to friction in a pipeline system, there are also losses associated with flow through valves and fittings. These losses are called minor losses, but these losses must be considered if the piping system has a lot of such fittings. These losses are treated as equivalent frictional losses.
The minor loss may be treated either as a pressure drop Δp = -KρV2/2 or as a head loss Δh = -KV2/(2g).
The value of the loss coefficient ‘K’ is obtained through experimental data. Generally for valves and fittings, manufacturers provide loss coefficient ‘K’ value.
It may also be calculated from the equivalent length concept: K = fLe/D,
where Le is the equivalent pipe length that has the same frictional loss,
f is the Moody friction factor (Darcy-Weisbach friction factor) and
D is inner diameter of the pipe.
Below table gives loss coefficient (K) and Le/D values for some common fittings.
Loss coefficient ‘K’ values for common fitting types
Type of fitting
Loss coefficient (K)
|3||Diaphram valve, open||2.30||115|
|4||Diaphram valve, half open||4.30||215|
|5||Diaphram valve, 1/2 open||21.00||1050|
|6||Gate valve, open||0.17||9|
|7||Gate valve, half open||4.50||225|
|8||Globe valve, wide open||6.40||320|
|9||Globe valve, half open||9.50||475|
|11||Union and coupling||0.04||2|
For sudden enlargements in the pipeline from smaller diameter (D1) to a larger diameter (D2), the value of loss coefficient ‘K’ can be obtained from the formula:
For sudden contractions in the pipeline from a larger diameter (D2) to a smaller diameter (D1), the value of the loss coefficient ‘K’ can be obtained from the formula:
The above equations should serve as guidelines. Corrections should be made for enlargements and contractions that are gradual. Use values of loss coefficient ‘K’ for fittings whenever provided by the manufacturer.
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