Pipe Friction Loss Theory
Pipe friction loss is a large design factor influencing performance of a snowmaking system. When water or air flows through a pipe, there is resistance between the fluid and the inside walls of the pipe. This resistance to flow is termed pipe friction. At low flow speeds, the friction is quite low. As velocity of flow increases, friction increases dramatically. Friction loss evidences itself by a corresponding loss of pressure.
A residential plumbing system can be used to illustrate pipe friction loss. Assume that a pressure gauge at the water meter reads 50 psi with no water flowing through the meter. This is static pressure (the pressure available in the street water main), and may be thought of as the potential water energy available.
A meter at the point of usage, a spigot on the second floor, may read just 15 psi, 25 psi having been lost to friction and 10 to lifting the water “uphill” to the second floor. (For every 2.31 feet of lift we loose 1 psi.) We have therefore lost 35 psi of potential energy just pushing the water through our piping distribution system.
Losses of this magnitude are, of course, not acceptable to snowmaking systems but the theory remains the same. The snow engineer, knowing the amount of water he expects to be flowing through a pipe, and knowing the vertical lift the pump must overcome, can select pipe diameters which will allow acceptable friction losses within the system parameters. Air piping is sized in the same manner except that vertical lift is not considered, as the weight of air is negligible.
Friction Loss in New System Design
The engineer of a snowmaking system must constantly balance system performance parameters against cost considerations.
For this reason, system piping sizes are always a compromise, being sized somewhat less than ideal. The engineer has to estimate the amounts of air and water to flow through a given section of piping at extremes of the system operating range, while assuming a “normal” distribution of snowguns throughout the system. Any future system expansions have to be considered in the original design.
Friction Losses in Existing Systems
Existing systems are often found to have improper initial design, or friction losses that have occurred due to design, adding extra air or water capacity or extending the length of existing pipelines.
These systems must be carefully analyzed to determine maximum snowgun concentrations and distribution during operation. A rule of thumb is that every time an amount of fluid flowing is doubled, the friction loss goes up four times.
Pipe Wall Thicknesses
The most common mistake in selecting pipe wall thickness is to use the pipe mill test pressure as a guide. The pipe mill test pressure may range from 500 to 2,800 psi and is applied for about 5 seconds to test weld quality and metal uniformity.
Actual pipeline operating pressures result in internal prolonged stresses that have to be calculated. As a rule, the bigger the pipe diameter, the thicker the walls to withstand the same amount of internal pressure.
Wall thicknesses may be determined by the following formula:
t = 8/7 (pD / (2S + 0.8p) ) + C)
t is nominal pipe thickness, inches (see note 4)
p is maximum internal service pressure, psig (see note 1)
D is outside pipe diameter, inches
S is allowable stress psi, in accordance with ASA B 31.1 Section 1 Table 2A (water) or Section 2, Table 4 (air). See note 2.
C is allowance for threading, grooving or corrosion inches (see note 3.
Note 1: In determining the value of P, include such factors as positive head pump inlet, pump shut-off pressure, and elevation effects if pipe location is below pump elevation. Allowance must also be made for pipe installation at higher elevations and system water hammer.
Note 2: Values of S for ASTM A-53 Grade B steel pipe are as ollows:
S is 15,000 psi for seamless pipe
S is 12,750 psi for electric resistance welded (ERW) pipe
S is 6,750 psi for butt-welded pipe
Note 3: The value of C shall be the depth of thread if pipe is threaded, the depth of Victaulic grooves if the grooves are machined, and in no case less than .065 inch for water service or 0.05 inch for air service.
Note 4: For machined Victaulic groove construction, the pipe wall thickness shall not be less than that recommended by the Victaulic Company for cut groove use.
Table I shows some results of calculations using the above formula. The table of results given here is not meant to eliminate the need for professional engineering help for your system. It is meant as an illustration of typical pressures. Table represents typical values as calculated for welded pipeline.
| Outside Pipe Diameter | Maximum Expected Service Pressure | End Use | Electric Resistance Welded Pipe — Calculated Wall | Electric Resistance Welded Pipe — Normally Used Wall | Butt-Welded Pipe — Calculated Wall | Butt-Welded Pipe — Normally Used Wall |
|---|---|---|---|---|---|---|
| 4-1/2″ | 125 PSI | Compressed Air | .082″ | .125″-.156″ | .125″ | .125″-.156″ |
| 4-1/2″ | 250 PSI | Water | 107″ | .125″-.156″ | .168″ | .188″ |
| 4-1/2″ | 500 PSI | Water | .173″ | .188″ | .259″ | None normally Available |
| 6-5/8″ | 125 PSI | Compressed Air | .094″ | .125″-.156″ | .127″ | .188″ |
| 6-5/8″ | 250 PSI | Water | .148″ | .156″ | .212″ | .219″ |
| 6-5/8″ | 500 PSI | Water | .220″ | .250″ | .346″ | None normally Available |
| 8-5/8″ | 125 PSI | Compressed Air | .096″ | .188″ | .147″ | .188″ |
| 8-5/8″ | 250 PSI | Water | .170″ | .188″ | .254″ | .277″ |
| 8-5/8″ | 500 PSI | Water | .264″ | .277″ | .428″ | None normally Available |
| 10-3/4″ | 125 PSI | Compressed Air | .117″ | .188″-.219″ | .170″ | .188″ |
| 10-3/4″ | 250 PSI | Water | .194″ | .219″ | .298″ | .307″ |
| 10-3/4″ | 500 PSI | Water | .311″ | .312″ | .516″ | None normally Available |
If a ski area is experiencing failures of pipelines under pressure, it is not usually “Bad Pipe”. It is bad engineering. The wall thicknesses were selected too thin.
