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Low Flow Capacity: This could be caused by an undersized
internal valve, undersized or long downstream
piping, plugged screens, some other restriction in the
down-stream system, or by the bypass valve sticking in
the open position. The bypass valve could also be set
too low and be opening prematurely.
Principle of Operation
Refer to the schematic drawing, Figure 4 In View 1, the
valve is held closed by both tank pressure and the valve's
closing spring. There is no leakage past the resilient
seats in the poppet to the valve outlet in this position.
The valve is opened by moving the operating lever to
approximately mid-point in its travel range (Figure 4 View
2). This allows the roller cam to place the "Excelerator"
equalization portion of the valve stem in the pilot opening,
permitting a larger amount of product to bleed
downstream than if the operating lever was moved to the
full open position.
When tank and downstream pressures have equalized in
the bleed position, (after a few seconds), the excess flow
spring pushes open the main poppet (Figure 4 View 3) and
the operating lever can be moved to the full open position.
If tank pressure is greater than the valve's outlet pressure,
the main poppet will remain in the closed position. If valve
outlet piping is closed off by other valves, however,
product bleeding through the pilot will increase until it
nearly equals tank pressure and the main poppet opens.
Note
The main poppet will not open if valve outlet piping is
not closed off so that the outlet pressure can approach
tank pressure.
Once the main poppet opens, a flow greater than the
valve's excess flow spring rating or a sufficient surge in
flow forces the main poppet closed against the excess flow
spring (View 4). The pilot valve allows a small amount of
product to bleed, but much less than (View 2) where the
rapid equalization portion of the stem is placed in the pilot
opening. When the operating lever is moved to the closed
position, the valve closes completely and seals tightly
(View 1).
Maintenance
!CAUTION!
Do not use these internal valves if they leak, fail to work
properly or have been damaged or have missing parts.
Prompt repairs should be made by a properly trained
serviceman. Continued use without repair can create a
hazardous situation which could result in serious or fatal
personal injury or property damage.
FORM #1132 ME990-3F IOM
ME990-3F
A simple preventative maintenance program for the valve
and its controls will eliminate a lot of potential problems.
MEC recommends these steps be conducted once a
month. Also refer to the Department of Transportation
(DOT) CFR 49 Sections 180.416 and 180 Appendix A and B
which specific monthly maintenance and inspection tests
for cargo tank service internal valves and their actuation
controls.
1.
Inspect the operating lever to see that it moves freely
and smoothly. Also examine the stub shaft bonnet
nuts for leakage using a soap solution. If there is
leakage, the bonnet packing will have to be replaced.
A sticking lever indicates mechanism wear or trapped
dirt. This could mean the need for new shaft seals,
shaft bushings or stem bushings.
2.
Check for tight closure of the seat disks. Any detected
leakage, which is normally caused by disk wear or
dirt, scale or debris embedded in the disk, requires
that the internal valve be removed from service and
repaired. Repair most often requires the replacement
of valve disks. To check for leakage:
a.
Close the internal valve and exhaust
downstream pressure. Close the first valve
downstream from the internal valve, and note
any pressure buildup, using a pressure gauge,
between the closed valve and the internal
valve. If piping is cold, allow it to warm to
ambient temperature.
b.
Refer to CFR 49 Section 180 Appendix B for
Meter Creep Test Methods.
3.
All operating controls should be inspected, cleaned
and oiled. The controls should be
checked to see that they fully open but not over
travel the internal valve operating lever and work
freely to close the valve.
4.
Standard construction internal valves must be
removed if the container is to be steam cleaned. Heat
can damage the valve's seats and seals.
5.
Standard construction internal valves are not
designed for water service. Immediately after a
container is hydrostatically tested, remove all water
and allow the container to thoroughly dry out.
Parts Ordering
Use only genuine MEC replacement parts. Components
that are not supplied by MEC should not, under any
circumstances, be used in any MEC valve, because they
might adversely affect the performance of the valve, and
could give rise to personal injury and property damage.
When corresponding about this equipment, always
reference the equipment model or series number found
on the nameplate.
Important
Rev C 2/3/21
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