Avoid trapped anhydrous, Avoid line traps, Clearing a line trap – Great Plains NH3 Safety User Manual
Page 41: En when completely disconnected. see “avoid
Great Plains Manufacturing, Inc.
Ammonia Maintenance Safety
37
2013-07-15
407-551M
Avoid Trapped Anhydrous
Before servicing anhydrous equipment, or storing it near
untrained individuals, be absolutely certain that there is
no liquid NH
3
trapped in the system, and no NH
3
vapor
present (or only trace amounts).
Follow bleeding and discharge instructions carefully.
Beware of places in the systems where NH
3
can be
trapped.
Avoid Line Traps
Refer to Figure 30
Any two line valves in the system, if both closed with NH
3
present, can trap that ammonia indefinitely. Line valves
include:
tank withdrawal
(page 16),
nurse tank hose valve
(page 13),
check valves at a disconnected breakaway,
emergency shut-off valve,
control valve at rate zero
On/Off solenoid valve, and
aftermarket section valves.
In a properly designed system, segments not open to the
atmosphere are protected by hydrostatic relief valves,
but the operation of such valves only releases a small
portion of the trapped ammonia.
Normal shut-down instructions, after closing the tank
withdrawal valve, are intended to bleed most of the
system NH
3
into the soil. However, there can still be
substantial vapor in the system after that. And if shut-
down procedures were not followed (or the state of the
equipment is unknown at time of servicing), you need to
assume that a closed valve represents a risk of trapped
NH
3
.
The illustration at right depicts a nurse tank
hose
(page 12) in use, and shut-off while still full of
liquid NH
3
. If this hose is removed to an enclosed space,
then opened, it represents an extreme hazard. Vapor
concentrations could rise to well above IDLH levels in
moments.
This example system line segment is equipped with relief
orifice-equipped ball valves
(page 13), but that only
bleeds the downstream side of a valve ball, the outlet
valve in this example. See page 38 for information about
ball traps.
The downstream side of this system line segment is
equipped with a bleed valve
(page 13). The
upstream side of the inlet valve is equipped with a
hydrostatic relief valve
(page 14).
If this line segment warms, pressure can rise high
enough to cause periodic releases at the relief valve.
This protects the segment from rupture, but does not
clear the line.
Clearing a Line Trap
To clear a line segment with possible trapped NH
3
:
▲ Move the equipment outdoors.
▲ If calm, wait for wind.
▲ Stand up-wind of the bleed valve.
▲ Point the bleed valve orifice down-wind and away from all
personnel.
▲ If it’s a hose bleed valve, make sure you can set the hose
down with the valve still pointing in a safe direction.
▲ Open the valve very slowly. Once any discharge is
observed, (set any hose down) depart up-wind.
▲ Remain up-wind until the discharge completes.
Figure 29
NH
3
Trapped in Line
31616
Valves Open in Use
Valves Closed with
NH
3
Present
A61
A62
A61
A63
A66
A61
A30
A59
A61
A62
A63