Many valves are directional, such as gate valves, throttle valves, pressure reducing valves, and check valves. If installed upside down, their performance and lifespan will be affected (e.g., throttle valves), or they may not function at all (e.g., pressure reducing valves), or even cause danger (e.g., check valves). Generally, valves have directional markings on the valve body; if not, the correct orientation should be determined based on the valve's operating principle. Gate valves have asymmetrical valve chambers; fluid should flow from bottom to top through the valve port. This minimizes fluid resistance (due to the shape), makes opening easier (because the medium pressure rises), and ensures that the medium does not press against the packing when closed, facilitating maintenance. This is why gate valves must not be installed backwards. Other valves also have their own characteristics.
The installation location of valves must be convenient for operation: even if installation is temporarily difficult, the long-term working conditions of operators must be considered. The valve handwheel should be level with chest height (generally 1.2 meters from the operating floor), making opening and closing the valve easier. For floor-mounted valves, the handwheel should face upwards, not tilted, to avoid awkward operation. Valves near walls or equipment should also allow sufficient standing space for operators. Avoid operating with your back to the ceiling, especially with acidic, alkaline, or toxic media, as this is very unsafe. Gate valves should not be installed upside down (i.e., handwheel facing downwards), otherwise the medium will remain in the valve cover space for a long time, easily corroding the valve stem, and is prohibited by some process requirements. It also makes changing the packing extremely inconvenient. Rising stem gate valves should not be installed underground, otherwise the exposed valve stem will corrode due to moisture. For lift check valves, ensure the valve disc is vertical during installation for flexible lifting and lowering. For swing check valves, ensure the pin is horizontal during installation for flexible swinging. Pressure reducing valves should be installed upright on horizontal pipelines, without tilting in any direction.
(II) Construction Operations
Installation and construction must be carried out with care, and valves made of brittle materials should never be struck.
Before installation, the valve should be inspected to verify its specifications and model, and to check for any damage, especially to the valve stem. It should also be rotated a few times to check for any tilting, as the valve stem is most easily bent during transportation. Any debris inside the valve should also be removed.
When lifting valves, do not tie the rope to the handwheel or valve stem to avoid damaging these parts; instead, tie it to the flange.
The pipelines connected to the valves must be thoroughly cleaned. Compressed air can be used to blow away iron oxide filings, mud, sand, welding slag, and other debris. These impurities can easily scratch the valve's sealing surface, and large particles (such as welding slag) can clog small valves, causing them to malfunction. When installing threaded valves, the sealing packing (hemp fiber with aluminum oil or PTFE tape) should be wrapped around the pipe threads, avoiding getting it inside the valve to prevent accumulation and obstruction of media flow.
When installing flanged valves, ensure the bolts are tightened symmetrically and evenly. The valve flange and pipe flange must be parallel with a reasonable clearance to prevent excessive pressure on the valve, which could lead to cracking. This is especially important for valves made of brittle materials or those with low strength. For valves that must be welded to pipes, tack weld first, then fully open the shut-off mechanism, and finally weld it shut.
(III) Protective Measures
Some valves require external protection, which involves heat insulation and cold insulation. Sometimes, heated steam pipelines are also installed inside the insulation layer. Which valves should be insulated or cold-insulated depends on production requirements.
In principle, if the temperature of the medium inside the valve drops too much, it will affect production efficiency or damage the valve by freezing, then insulation or even heating is required; if the valve is exposed, which is detrimental to production or causes frost or other adverse phenomena, then cold insulation is required. Insulation materials include asbestos, slag wool, glass wool, perlite, diatomaceous earth, vermiculite, etc.; cold insulation materials include cork, perlite, foam, plastics, etc.
(iv) Bypass and Instrumentation
Some valves, in addition to necessary protective facilities, also require bypasses and instruments. Installing a bypass facilitates the maintenance of the steam trap. Other valves may also have bypasses installed. Whether or not to install a bypass depends on the valve's condition, its importance, and production requirements.
(V) Packing Replacement
Some of the valve packings in the inventory are no longer working properly, and some are not compatible with the medium being used, so the packings need to be replaced.
Valve manufacturers cannot consider the myriad different media used by users, so they always fill the stuffing box with ordinary packing. However, when using it, the packing must be compatible with the medium.
When replacing packing, it should be pressed in one ring at a time. The joint angle between each ring should ideally be 45 degrees, and the rings should be spaced 180 degrees apart. The packing height should allow for further tightening of the gland, while also ensuring the lower part of the gland presses into the packing chamber to an appropriate depth, generally 10-20% of the total depth of the packing chamber. For valves with high requirements, the joint angle should be 30 degrees. The joints between rings should be staggered by 120 degrees. In addition to the packing mentioned above, depending on the specific situation, shaped packings such as rubber O-rings (natural rubber is resistant to weak alkalis below 60 degrees Celsius, nitrile rubber is resistant to oil crystals below 80 degrees Celsius, and fluororubber is resistant to various corrosive media below 150 degrees Celsius), three-piece stacked PTFE rings (resistant to strong corrosive media below 200 degrees Celsius), and nylon cup rings (resistant to ammonia and alkalis below 120 degrees Celsius) can be used. Wrapping a layer of PTFE raw material tape over ordinary asbestos packing can improve the sealing effect and reduce electrochemical corrosion of the valve stem.
