Great Plains NP30A 30-foot Operator Manual - Manual (Page 112)

108 NP30A or NP40A Table of Contents Index Great Plains Manufacturing, Inc.
407-502M Table of Contents Index 2014-05-20
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 86
Any two line valves in the system, if both closed with NH
3
present, can trap that ammonia indefinitely. Line valves
include:
tank withdrawal ,
nurse tank hose valve(s) ,
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. If shut-down
procedures were not followed, liquid may be present.
If the state of the equipment is not known with certainty
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 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
a
levels in moments.
This example system line segment is equipped with relief
orifice-equipped ball valves
, but that only bleeds the
downstream side of a valve ball, the outlet valve in this
example. See page 109 for information about ball traps.
The downstream side of this system line segment is
equipped with a bleed valve
. The upstream side of
the inlet valve is equipped with a hydrostatic relief
valve
.
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.
A60 A60
A61 A62
Figure 85
NH
3
Trapped in Line
31616
Valves Open in Use
Valves Closed with
NH
3
Present
A65
A60
A14
A18
A30
A31
a. IDLH: Immediately Dangerous to Life and Health
A58
A60
A61
A62