When it comes to linear valve installations, gravity is our friend. The most common and preferred installation for these valves is to have the stem in the vertical position. This orientation, with an assist from the laws of gravity, aligns the stem to the disc and the disc to the body guides, which keeps everything in proper orientation throughout the open and closed cycles.
For this reason, it would be terrific if every linear valve could be installed in the stem vertical position; however, the realities of plant construction often dictate horizontal stem installation applications. So what is the big deal about valves laying on their sides, backs or bellies? Aren’t they supposed to operate in all positions anyway? The answer is yes, no, maybe and it depends.
American Petroleum Institute (API) Standard 600, Steel Gate Valves, Flanged & Butt-welding Ends, Bolted Bonnets states that valves built to this standard should be capable of operating in all orientations. But sometimes that “operation” comes with a painful price in terms of operational valve damage.
The latest edition of API 600, published in January 2015, included additional requirements designed to help valves operate in the horizontal position. The language in paragraph 5.6.3 states:
The body and gate shall have guide surfaces to minimize wear of the gate seats during operation of the valve, to accurately position the gate throughout the travel distance to its seat, and to ensure the alignment of the gate and stem in all orientations without gate binding or galling. For sizes DN 650 (NPS 26) and above, as a minimum, wedge guides and body guides shall be hardfaced and machined with appropriate tolerances and clearances to allow for proper valve operation in any orientation, including the effects of wear or galling.
THE COCK AND LOCK SYNDROME
The hardfacing will help. But it is not a 100% cure for the “cock and lock” syndrome in which a valve disc rocks within the guides during the opening cycle until the disc bites into the body guide area, locking the disc in place and keeping the valve from operating. In severe cases this cocking action can disorient the stem foot-to-disc connection and create so much stress that the stem can break off. This is especially an issue for hardened (and brittle) martensitic 410ss stems.
Because of differences in manufacturing techniques, the cock and lock syndrome as well as other issues related to stem and disc misalignment were rare to non-existent 40 years ago. Back then, plants building gate valves would generally cast the valve body guides well oversize and employ a machine tool called a “slotter” to plane the guides to precision dimensions as well as create a very smooth surface finish. Today, virtually all mass-produced commodity gate valves are made with “as cast” body guides. The cast guides are susceptible to a host of dimension-killing casting problems, which can render the already non-precision-machined body guides useless for keeping the disc snug and secure, and in perfect alignment during opening and closing cycles.
Pressure seal valves used in nuclear service are usually manufactured with precision-machined guides to ensure reliability during operation. Large outer diameter waterworks valves sometimes include special design features to enhance their horizontal operability. The predominant design uses rollers on the disc and body guide-ways that ensure free movement of the disc while on its side. Scrapers are also inserted ahead of the wheel path to ensure no debris gets under the wheels.
Although the cock and lock syndrome occurs when the stem is horizontal and the flow is also horizontal (the valve is laying on one side), stem horizontal and flow vertical applications can have problems as well. The same rough guides or misaligned cast guides can cause the disc to engage the body seats at an unfavorable angle, causing potential damage to either the seats or disc. Other problems such as mismatched inside diameters of body and bonnet castings can keep discs from moving at all when in the horizontal position.
Another major issue with horizontally aligned stems occurs because of improper machining or misalignment of components during the manufacturing process. While the great majority of the individual parts may meet design tolerances for each component, occasionally the individual tolerance offsets, when added together, can cause major misalignment issues. This issue really comes to the forefront when large valves have actuators installed. In this situation, there is no subtlety to the movement of the actuator-powered stem, and it will drag or push the stem through, over or around anything in its path. Even a slight misalignment with a backseat bushing easily creates a new linear gouge. Likewise, if a misalignment with the packing gland occurs, scratching can also happen. When stems are severely scratched, the valve has to be disassembled and the stem repaired or replaced. The valve also has to be re-tested. All these steps mean money out of someone’s pocket.
The best way to ensure that the valve stem is properly aligned and not prone to gouging or scratching is to perform a manual, pre-actuation, open and close cycle test while closely examining the stem for defects as it moves up and down. This pre-test should be performed with the valve in the stem vertical position for the best possible alignment. If no visible damage is seen, then the actuator can be installed. If scratches occur, it is time to cease the actuation installation and determine what is out of alignment.
Today, many large valves are shipped from offshore manufacturing plants in crates lying in the horizontal position. This extended period of time on the ocean, with an eccentric load on the compression packing, can cause the stem to literally fall out of alignment by crushing one side of the packing. Where this problem really comes into play is in the realm of low-
emissions valves. If the stem has deformed the packing because of an off-center, vibration-enhanced, four-week cruise in a box, there is very little chance that packing will provide much fugitive emissions containment.
End-users or owners ideally should communicate through the supply chain when a valve will be installed in the horizontal position. This can alert the manufacturer to take extra steps to confirm the horizontal operability of the valve or even have an outside facility test the valve in the horizontal position before delivery. For those actuating a large horizontal installation valve, the stem vertical manual pre-test should be considered. Additionally, the actuation process should be performed in the stem vertical position if at all possible.
For parties repairing gate valves 14 inches and larger, a close examination of disc and body guides is in order. If the valve owner indicates that the valve may be installed in the stem horizontal position, the guides should be hardfaced to prevent the dreaded cock and lock.
In an ideal valve world, the gate valve body guides would all be finish-machined and the tolerances would be tighter. The relational dimensions between yoke bushing, stem, stuffing box, body and bonnet also would be tighter, and actuators would all be mounted in the stem vertical position. However, in the real world, we have to be vigilant and not assume anything.
One fact is for certain: The cost to remove a large stuck gate valve from a plant pipeline can be very high because expensive riggers and a crane may be needed to remove the valve for repair. For this reason, taking a few extra steps and precautions before installation or during the repair process to ensure proper gate valve operation in all orientations may be money well spent.