2000 Hook-up Book

Sizing Steam Lines On Velocity

Fig. 1 lists steam capacities of pipes under various pressure and velocity conditions. EXAMPLE: Given a steam heat ing system with a 100 psig inlet pressure ahead of the pressure reducing valve and a capacity of 1,000 pounds of steam per hour at 25 psig, find the smallest sizes of upstream and downstream pip ing for reasonable quiet steam velocities. Upstream Piping Sizing Enter the velocity chart at A for 1,000 pounds per hour. Go over to point B where the 100 psig diagonal line intersects. Follow up vertically to C where an intersection with a diagonal line falls inside the 4,000-6,000 foot-per-minute velocity band. Actual velocity at D is about 4,800 feet per minute for 1-1/2 inch upstream piping. Downstream Piping Sizing Enter the velocity chart at A for 1,000 pounds per hour. Go over to point E where the 25 psig diag onal line intersects. Follow up vertically to F where an intersec tion with a diagonal line falls inside the 4,000-6,000 foot-per minute velocity band. Actual velocity at G is 5,500 feet per minute for 2-1/2 inch downstream piping. Pressure Drop in Steam Lines Always check that pressure drop is within allowable limits before selecting pipe size in long steam mains and whenever it is critical. Fig. 2 and Fig. 3 provide drops in Sch. 40 and Sch. 80 pipe. Use of the charts is illustrated in the two examples. EXAMPLE 1 What will be the smallest sched ule 40 pipe that can be used if drop per 100 feet shall not exceed 3 psi when flow rate is 10,000 pounds per hour, and steam pressure is 60 psig? Solution: 1. Find factor for steam pres sure in main, in this case 60 psig. Factor from chart = 1.5.

Figure 1: Steam Velocity Chart

SYSTEM DESIGN

Pipe Size (Schedule 40 pipe)

Reasonable Steam Velocities in Pipes

20000

1"

1/2"

1-1/4"

1-1/2"

12000 10000

3/4"

Process Steam 8000 to 12000 ft/min

2"

2-1/2"

8000 6000 5000 4000 3000

F

3"

D G

Heating Systems 4000 to 6000 ft/min

4"

C

5"

6"

8"

2000

10"

12"

14"

16"

Velocity ft/min

1000

50000 40000 30000

Multiply chart velocity by factor below to get velocity in schedule 80 pipe Pipe Size Factor 1/2" 3/4" & 1" 1-1/4" & 1-1/2" 2" to 16" 1.30 1.23 1.17 1.12

20000

10000 8000 6000 5000 4000 3000

250

200

150

125

100

75

50

25

10

5

2000

0

A

E

B

Capacity lb/h

1000 800 600

250

200

300 400 500

150

125

100

75

50

25

10

200

5

0

Steam Velocity Chart

100

Steam Pressure psig (Saturated Steam)

2. Divide allowable pressure drop by factor 3–. . 1.5 = 2 psi. 3. Enter pressure drop chart at 2 psi and proceed horizontal ly to flow rate of 10,000 pounds per hour. Select pipe size on or to the right of this point. In this case a 4" main. EXAMPLE 2 What will be the pressure drop per 100 feet in an 8" schedule 40 steam main when flow is 20,000 pounds per hour, and steam pres sure is 15 psig?

Solution: Enter schedule 40 chart at 20,000 pounds per hour, proceed verti cally upward to 8" pipe curve, then horizontally to pressure drop scale, read 0.23 psi per 100 feet. This would be the drop if the steam pressure were 100 psig. Since pressure is 15 psig, a cor rection factor must be used. Correction factor for 15 psig = 3.6 0.23 x 3.6 = 0.828 psi drop per 100 feet for 15 psig

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