by John C. Robertson

Valve (valv), n a device that halts or controls the flow of fluid in a pipeline; a device that makes fluid act differently from the way it would act as if it were traveling along a pipeline without a valve

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.

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.

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 manufacturers 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.

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 manufacturers packing and replace it with in-house packing that is proven to be reliable in service.

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.

Clean valves before installing them
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.

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 shafts 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 oclock location

Second ring is installed at the 6 oclock location

Third ring is installed at the 3 oclock location

Fourth ring is installed at the 9 oclock 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 oclock location

Second ring is installed at the 4 oclock location

Third ring is installed at the 8 oclock location

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.

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. Dont 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?

John Robertson is the maintenance reliability specialist for Strategic Work Systems, a consulting firm with offices in Greenville, S.C., and Mill Spring, N.C.  For more information, call , e-mail or visit www.swspitcrew.com.

MRO Today.  Copyright, 2000.

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