3
V. Description of Operation:
VI. Maintenance:
Any type of liquid may be unloaded through the valve. The
valve will tolerate clean liquids better than others. And
lighter liquids will flow at greater rates than heavier liquids.
Particulates, abrasives and contaminants will not preclude
the valve from opening at the appropriate set pressure.
However, abrasive materials will cause some wear between
the wear sleeve, nozzle, and main seal. While these
components are made with wear resistant surfaces, their life
will be reduced as the abrasives increase in the fluid.
The effective accuracy of set pressure versus opening
pressure is limited by gauge accuracy (1-3%) and regulator
accuracy (1-2%). Weir SPM tests show the accuracy of the
Unloading Valve does not vary more than ± 2.5%.
NOTE:
Under most conditions, the valve will reset completely when
the fluid source is removed and the liquid pressure at the
valve is reduced close to zero. This is generally true if the
initial nitrogen pressure is set above 120 psi.
However, when the initial nitrogen pressure is below 120 psi
(i.e. 3500 psi liquid pressure) it may be necessary to use the
optional override valve to force the relief valve to reset.
Figure 2 illustrates the circuit required to employ the
override valve. The vent line connects to the valve through
the ¼” port, replacing the pipe plug which is normally
installed. The override is a normally closed, spring returned
valve. Once the Unloading Valve has sequenced and is ready
to reset, the override valve must be opened and held open
for approximately 5 seconds to relieve gas pressure in the
bonnet.
Monitor the pressure gauge until it reduces to zero. At this
point the Unloading Valve is reset and the override valve
should be released.
FIG. 2
CAUTION:
It is important for the user to consider the
consequences of a valve that does not fully reseat
after it has relieved. In the event that this might
present a danger to personnel or equipment, SPM
highly recommends that an isolator valve be placed
between the pressure line and the Relief Valve.
separate series of H2S service unloading valve.
CAUTION:
Weir SPM’s 3” standard service Emergency
Unloading Valve is not intended for use in the sour
gas environment. DO NOT USE FOR H2S SERVICE.
Weir SPM does offer a separate series of H2S service
unloading valve. Contact your Weir SPM
representative for advice about sour gas service
applications.
The unloading valve operation is a simple balance between
the nitrogen (or suitable gas) acting on the piston and the
inlet liquid pressure acting on the rod and wear sleeve. The
equation is Pressure times Area equals Force (PxA=F). The
ratio of areas is approximately: 25 : 1
Consequently, a theoretical nitrogen pressure of 100 psi can
counter an inlet line pressure of 2500 psi for the unloading
valve. Variations in actual performance are due to internal
friction and will be discussed in (Section VIII).
With the device installed, as shown in (Fig. 1), any time the
pressure exceeds the set pressure at the Unloading Valve,
the fluid will pass through the valve and out the discharge.
As the rod and wear sleeve rise due to the liquid pressure
the valve goes through a transition zone (Fig. 3). During
this process the effective force from the nitrogen (or gas)
side is reduced to approximately 16% of its initial set
pressure, allowing the valve to open quickly and almost
completely.
a.) Fluid pressure below set pressure-rod and wear sleeve seated.
b.) Fluid pressure slightly above set pressure-rod and wear sleeve
off main seal.
c.) Fluid pressure substantially above set pressure-rod and wear
sleeve off main seal.
d.) Fluid pressure below set pressure-rod and wear sleeve reseated.
FIG. 3
a) b) c) d)
General:
The Weir SPM 3" Emergency Unloading Valve is a well
designed, dependable component that should provide long
term reliable performance for the user. Like any device,
however, it requires routine inspection and servicing to
guarantee that it is fully functional.
The valve is intended to be used as an emergency pressure
relief device only, and should not be subjected to
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