Sizing Examples

Problem #1

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An existing 3" main is supplying one department of a plant with 3,200 lbs/hr of steam at 125 psig.

Could the steam use in this department be increased to 7,000 lbs/hr without a new supply line?

Steam Pipe Capacity at 125 psig - Schedule 40 pipe

Pipe Size

(IN)

Pressure drop per 100 ft of pipe length

1/2

3/4

1/2
3/4
1
1 1/4
1 1/2
2
2 1/2
3
3 1/2
4
5
6
8
10
12

23
54
109
241
375
760
1,250
2,280
3,430
4,840
8,950
14,710
30,650
56,000
89,900

29
66
134
296
460
930
1,535
2,815
4,200
5,930
11,000
18,070
37,550
68,500
110,200

33
77
155
341
532
1,075
1,775
3,245
4,850
6,850
12,700
20,800
43,400
79,100
127,100

47
109
220
483
751
1,520
2,550
4,590
6,850
9,700
17,950
29,500
61,400
112,000
179,600

75
172
347
764
1,185
2,410
3,960
7,260
10,880
15,350
28,400
46,500
97,100
177,000
284,100

Above

Velocities Less Than

Violet

Yellow

Blue

Red

Green

6,000 fpm

8,000 fpm

10,000 fpm

12,000 fpm

15,000 fpm

If the 5 psi pressure drop and the corresponding decrease in steam temperature are also acceptable, then the existing 3" pipe can be used. If the pressure or temperature drop is not acceptable, then a second 3" pipe is necessary. Alternatively, a single 4" pipe would provide approximately 7,000 (6,850) lbs/hr steam at 125 psig with only a 1 psi pressure drop. The flow velocity in the 4" pipe would remain under 6,000 fpm.

1. Find the
correct pipe
size.

Problem #2

The boiler supplies steam at 15 psig through the steam main with a 2 psig pressure drop. You need to feed a 300-ton steam absorption machine, which requires 12 psig steam. The absorption machine condenses steam at the rate of 6,000 lbs/hr. The required branch line is 50' long and has three elbows and a gate valve. Find the size line required to provide a maximum of 1 psi pressure drop and maintain the velocity below 10,000 fpm.

Each elbow has an equivalent length of 11'. The valve has an equivalent length of 2.2'. The total equivalent length for the valve and fittings is 35.2'. Add the 35.2' to the 50' length of branch line for a total effective length of approximately 85'. Find the maximum allowable pressure drop per 100' by dividing the 1 psi maximum drop allowed for the branch by the total effective length of the branch: 1/.85 = 1.18 psi per 100'.

Equivalent Length of Pipe to be
Added for Fittings - Schedule 40 Pipe

Pipe
Size
(in)

Equivalent Length in Feet to be Added Run

Standard
Elbow

Side
Outlet
Tee

Gate
Valve*

Globe
Valve

Angle
Valve

1/2
3/4
1
1 1/4
1 1/2
2
2 1/2
3
3 1/2
4
5
6
8
10
12

1.3
1.8
2.2
3.0
3.5
4.3
5.0
6.5
8.0
9.0
11.0
13.0
17.0
21.0
27.0

3
4
5
6
7
8
11
13
15
18
22
27
35
45
53

0.3
0.4
0.5
0.6
0.8
1.0
1.1
1.4
1.6
1.9
2.2
2.8
3.7
4.6
5.5

14
18
23
29
34
46
54
66
80
92
112
136
180
230
270

7
10
12
15
18
22
27
34
40
45
56
67
92
112
132

*Valve in full open position

Steam Pipe Capacity at 15 psig - Schedule 40 pipe

On this basis, the next pipe size would have to be chosen. The equivalent length of the fittings is now 41.8'. Total effective length is approximately 92 feet. Allowable pressure drop per 100' is 1.08 psi. This rate of condensation (7,150 lbs/hr) in a 6" inch pipe has only 1/2 psi per 100' pressure drop and has a velocity less than 10,000 fpm.

5. Read up the column and find the pressure drop per 100' of pipe is 5 psi

1a. Locate the correct chart for the correct pressures.

3. We are being
provided with
3200 lbs/hr
steam.

2. We currently
have a 1psi per
100' pressure
drop using 125
psig.

4. Read across the chart until you reach 7,000 lb/hr (7,260 lb/hr). That portion of the chart is yellow, so the velocity of the steam is between 6,000 and 8,000 fpm. This velocity is acceptable.

Read across the 5" row until you find 6,000 (6160) lbs/hr.

Read up the row to find a 1 psi per 100 ft. pressure drop and note that this is within the 1.18 psi maximum. Notice, however, that the velocity for the required flow rate is in the range of 10,000 - 12,000 fpm, which exceeds the specified velocity.