Boiler house
Blowdown vessels
2.9.9
Sizing and selection
The selection of a blowdown vessel depends on the flowrate and the proportion of flash steam released.
The following factors affect the selection:
-
The boiler pressure.
-
The number of boilers.
-
The duration of blowdown.
-
The blowdown line size.
-
The length of blowdown line between the boiler and the blowdown vessel.
-
The blowdown regime.
For example the sizing below assumes the blowdown line has an 'equivalent straight length' of at least 7 m.
When estimating 'equivalent straight length' the lengths shown in Table 1 should be added to the length of straight pipe to allow for
valves and fittings.
From Table 1 it will be seen that in most practical applications it would be unusual for the equivalent straight length to be less than 7 m.
If the length is less than 7 m multiply the actual boiler pressure by 1.15 before using the sizing table, Table 2.
The blowdown regime:
-
Typical normal bottom blowdown of, perhaps, 5 seconds duration.
-
Weekly low level alarm testing where practice is in accordance with BG01 and INDG436 - steam down from normal level to first low
water level and blow down from first to second low water level.
-
Blowdown discharges from sequencing valves on external chambers, gauge glasses etc.
-
Automatic TDS control discharges.
The selection table (Table 2) only applies if the blowdown duration is no longer than 20 seconds in total, starting with a cold
vessel (water at a temperature of 15 °C to 20 °C).
If this time is exceeded, carryover of water with the flash steam may occur through the vent. The water may also be too hot to discharge
safely and legally into a public drainage system.
Always incorporate a cooling water system for multi-boiler applications or in instances where the frequency or duration of blowdown is
considerably in excess of the requirements stated in PM60, BG01 and INDG436.
If there is any doubt about correct vessel selection, please contact Spirax Sarco outlining the specific conditions and
blowdown regime.
Table 1 Equivalent straight lengths
Table 2 Blowdown vessel selection
Blowdown line size
Pipe fitting or valve
Long radius bend
Manifold inlet
Globe valve
Check valve
Blowdown valve
25 mm 32 mm 40 mm
(1")
(11⁄4")
(11⁄2")
50 mm
(2")
Blowdown line size
Boiler pressure
25 mm 32 mm 40 mm 50 mm
(1")
(11⁄4")
(11⁄2")
(2")
Blowdown vessel BDV60/_
3
3
3
4
3
3
4
5
3
4
4
6
3
4
5
6
4
4
5
8
4
5
6
8
5
6
8
10
5
6
8
10
6
8
8
-
Equivalent length in metres
0.5
1.1
9.6
3.6
0.3
0.7
0.8
1.5
1.7
12.2
13.9
4.3
5.0
0.4
0.4
0.9
2.2
17.8
6.3
0.5
bar g
5.5
7.6
8.3
psi g
80
110
120
10.3
150
12.1
175
17.2
250
20.7
300
24.1
350
27.6
400
Note: For intermediate pressures go to the next higher pressure.
Page 4 of 7
TI-P405-33 EMM Issue 8
BDV60 Blowdown Vessels
