| Valves are, unquestionably, the most important
part of any piping and pumping system because they direct the flow of fluids
and regulate temperatures. Properly used and maintained, they can improve
process efficiency and lower costs. It is wise to apply the basics of proper
valve maintenance in ways that improve their life cycle and operating efficiency.
Here are eight often-overlooked valve maintenance basics that can help
you do just that. |
| 1. Understand Valve Composition. |
| Valves are made of different materials because
fluids such as liquids, gases, and many other materials that flow are often
corrosive, abrasive, or highly contaminated. For consistency, piping materials
must fall into the same categories as the valves, with respect to the fluids
contained within the piping systems. |
| 2. Understand Valve Markings. |
| Most valve bodies include important valve
rating information that indicates where they should be used for safety
and efficiency. In addition to the manufacturer?s name, there is often
a basic service rating, which indicates either steam service or cold service.
The steam rating may be spelled out as 125S, meaning it is safe
to use this valve in a system up to a steam pressure of 125 pounds per
square inch steam. |
| The letters WOG, which stands for cold
water, oil, or gas non-shock, usually indicate cold service ratings. This
means the valve can be used to control most non-corrosive substances at
a pressure normally indicated by the rating. For example, 200 WOG indicates
that the valve is designed to operate at a pressure of 200 pounds per square
inch for cold water, oil, or gas under constant pressure conditions. |
| Arrows are also cast on the body of the valves
to indicate the flow path through the valve. These arrows will help you
correctly install the valve to ensure the proper flow path, which is particularly
important when swing check valves are being installed. |
| 3. Inspect Valves upon Receipt. |
| Valves should be inspected when they are received
at the plant, as this is probably the most critical and most ignored time
in the life of a valve. Unfortunately, it is generally accepted that a
new valve, just out of its packing crate, is at its operational peak. But
this is not always true. The valve must be inspected for in-transit
damage as soon as possible after it is received. Inspection should focus
on the pressure-retaining shell, valve ends, and the valve operating mechanisms,
which include the hand-wheel or actuator and stem. |
| It is also a good practice to dismantle the
valve upon receipt to inspect the internals and discard any anti-movement
restraints that the manufacturer installed for protection during transit.
Swing check valves often have a wooden prop inserted between the disc and
the body and the valves are often installed in that condition. It is also
wise to remove the valve manufacturer?s packing and replace it with in-house
packing that is proven to be reliable in service. |
| 4. Store Valves Properly. |
| Valves should be stored away from weather,
dirt, and transportation driveways. Protective covers should be kept in
place until the time of installation. Slings should never be used around
hand-wheels, stems, or gland adjustment parts. Lift the valves by the main
body. |
| Depending on the valve manufacturer, some
valves are shipped with the disc in the closed position and others with
the disc fully open and back-seated. Whichever way they have been shipped,
the main objective is protecting the seating surfaces. As a general rule,
the disc should be left in the shipping position, if possible, until installation
is completed. |
| 5. Clean Valves Before They are Installed. |
| Before a valve goes into service, all dirt
and foreign matter must be cleaned from inside the piping system. Whenever
possible, the valve and the line should be blown out with clean compressed
air or flushed with clean water to remove dirt and grit that might interfere
with valve operation or shorten valve life. |
| 6. Pack Valve Glands Properly. |
| When valves are properly maintained, they
should remain in service for many months without gland leakage or other
defects. One question that is often asked is how to correctly pack the
gland of a valve. This problem has been the subject of controversy between
the packing manufacturers and the user for many years. It basically boils
down to one thing ? the manufacturers want to sell as much packing as possible,
and as such, their instructions always recommend that the packing rings
be installed with the butts hard against each other. Experience has shown
that valves and pumps that are packed in this manner do not have any great
life expectancy and fail in a short period of time. Packing rings are subjected
to high pressures and high temperatures in the normal course of operation.
Like most other materials, they react to these conditions and expand and
contract accordingly. When the packing rings are operating in those conditions
and cannot breathe, the material will tend to migrate toward the shaft?s
sacrificial sleeve and commence to wear that down as is often observed
by the ridges that are formed on it. The inevitable friction generates
enough heat to cause the lubricant in the packing to lose its lubricity.
This breakdown of packing lubricity creates a braking effect culminating
in extra loading on the driver unit. This, in turn, causes more energy
to be used than is necessary. Hence the higher operational power bills
and greater maintenance costs. |
| From practical experience gained in an operational
environment in steam powered plants over many years, a scarfed 1/16 inch
gap between the packing ring butts will provide sufficient breathing space
to permit the packing to operate comfortably for 10 to 12 times longer
than the other packing method employed. If we consider a gland that requires
16 turns of packing to fill it and we employ the second method of packing,
one inch of packing is saved and the packing will last longer in service.
That represents considerable savings when multiplied by the number of glands
that are in standard power plants. Of course, the operational success of
any gland lies in how it was packed in the first place. Each ring must
be cut on a mandrel before being set squarely in the stuffing box, and
each subsequent ring is installed in the following rotation: |
| Glands with four or more rings: |
-
First ring is installed at the 12 o?clock location
-
Second ring is installed at the 6 o?clock location
-
Third ring is installed at the 3 o?clock location
-
Fourth ring is installed at the 9 o?clock location
-
For subsequent rings, the installation pattern is repeated until the gland
is full.
|
| Glands having only three rings: |
-
First ring is installed at the 12 o?clock location
-
Second ring is installed at the 4 o?clock location
-
Third ring is installed at the 8 o?clock location
|
| 7. Take Caution with Globe Valves. |
| When globe valves that have their bonnets
secured to the body with a large nut are used in a system, special care
must be exercised to ensure that the nut does not slacken back when the
valve opened hard against the back seat. This often happens when a valve
cheater bar or wrench is used to open the valve with disastrous results.
If a valve cheater has to be used to operate a valve, unless there is a
large pressure differential across the seat, that valve should be overhauled
as soon as possible. |
| 8. Connect Valves Correctly. |
| As in the case of piping installation on pumps,
valves must also be connected at both piping connections with zero-cold-spring.
In other words, the pipes should be aligned before installing the valve.
Don?t rely on the valve to pull the piping into alignment. If this is not
observed, valves such as gate valves that operate on internal guide bars,
check valve discs that operate on trunnions affixed to the internal skin
of the valve, and butterfly valve discs will not operate smoothly when
the misalignment stresses cause twisting of the valve body. Particular
attention must be paid to the installation and operation of diaphragm and
motor actuated valves when they have to operate within specified time windows
to ensure there are no ?hang-ups? due to twisting of the body caused by
piping misalignment. It is imperative that valves and their respective
pipe connections are properly aligned at initial installation. |
| Conclusion |
| When in doubt, always check with the valve
manufacturer for proper use and installation guidelines. Valves must be
kept in good operational condition. When they are allowed to fall into
disrepair, chaos reigns supreme. Remember this: Valves are key players
in all fluid systems. Without them, there would be no means of controlling
flows or regulating temperatures. And consider this: How would you
feel with defective valves in your heart? |
