Valve use and maintenance basic knowledge: installation of valves must pay attention to the matters and details

Valve use and maintenance basic knowledge: installation of valves must pay attention to the matters and details

Some of the basic parameters have national and departmental standards.
The nominal diameter of the inlet and outlet passage of the valve is called the nominal diameter of the valve. It is represented by Dg (national standard Dn for trial), in millimeters (mm). The nominal diameter of the valve is specified in the national standard GB1074-70. Nominal diameter series of valves are shown in Table 1-1.
Under normal circumstances, the nominal diameter of the valve is consistent with the actual diameter. There is a phenomenon that the nominal diameter is not consistent with the actual diameter of the forged valve used in high pressure chemical industry and petroleum.
The nominal pressure of the valve is called the nominal pressure of the valve. It is expressed by Pg (the national standard is expressed by PN, the pressure unit is bar), and the unit is kg force /cm2 (kgf/cm2). If Pg16 is marked on the valve, the nominal pressure of the valve is 16 kg force/cm 2. The nominal pressure of the valve is specified in the national standard GB1048-70. Nominal pressure series of valves are shown in Table 1-2. The actual pressure capacity of the valve is often much larger than the nominal pressure of the valve, which is designed to consider the safety factor. In the strength of the valve pressure test, according to the provisions of the allowable more than nominal pressure, in the working state of the valve, is strictly divided over pressure work, generally choose less than the nominal pressure value.
Three, the relationship between the working pressure of the valve and the working temperature of the valve in the working state of the pressure is called the working pressure of the valve, which is related to the material and the working temperature of the medium of the valve. Represented by P, the figure in the lower right corner of the P word is the medium ** high temperature divided by 10 integer. For example, P42 indicates the working pressure of the valve medium at the highest temperature of 425℃. Valve working temperature and corresponding large working pressure change table for short. See Table 1-3, 4, 5.
Application example: a 40kg force/cm 2 carbon steel valve in the medium working temperature of 425℃ on the pipeline, its maximum working pressure is how much is the first six out of table 1-3 carbon steel column, find out the working temperature of 425℃ a grid to look down, and then check the nominal pressure column of 40 kg force/cm 2 a grid to look down, The number at the intersection of the two bars is the maximum working pressure P4222 kg/cm 2 of this carbon steel valve
The suitable medium of the valve is a factor to be considered in the design and selection of the valve. How to master the “valve medium” please read the valve sample and anti-corrosion manual as well as the use and maintenance of the valve
Knowledge: installation valve must pay attention to the matters and details of the valve installation position, must be convenient for operation: even if the installation is temporarily difficult, but also for the long-term work of the operator. It is better that the valve handwheel is aligned with the chest (generally 1.2 meters away from the operating floor), so that it is easier to open and close the valve. Ground valve handwheel should be upward, do not tilt, so as to avoid awkward operation. The wall machine depends on the valve of the equipment, but also leave room for the operator to stand. Before installation, the valve should be inspected to check the specification and type and identify whether there is any damage, especially for the valve stem.
Valve installation quality, directly affect the use, so must pay careful attention to.
(1) Direction and position
Many valves have directional, such as globe valves, throttle valves, reducing valves, check valves, etc., if inverted inverted, it will affect the use effect and life (such as throttle valves), or do not work at all (such as reducing valves), and even cause danger (such as check valves). General valve, direction mark on the valve body; If not, it should be correctly identified according to the working principle of the valve. The valve chamber of the globe valve is asymmetrical, the fluid should let it from bottom to top through the valve port, so that the fluid resistance is small (determined by the shape), open the force saving (due to the medium pressure up), closed after the medium does not pressure packing, easy maintenance, this is why the globe valve can not be inverted. Other valves also have their own characteristics.
Valve installation position, must be convenient for operation: even if the installation is temporarily difficult, but also for the long-term work of the operator. It is better that the valve handwheel is aligned with the chest (generally 1.2 meters away from the operating floor), so that it is easier to open and close the valve. Ground valve handwheel should be upward, do not tilt, so as to avoid awkward operation. The wall machine depends on the valve of the equipment, but also leave room for the operator to stand. To avoid lifting operation, especially acid and alkali, toxic media, otherwise it is not safe. Do not flip the gate (i.e. hand wheel down), otherwise the medium will remain in the bonnet space for a long time, easy to corrod the stem, and for some process requirements contraindicated. It is inconvenient to change the packing at the same time. Open stem gate valve, do not install in the ground, otherwise due to damp corrosion exposed stem. Lift check valve, installation to ensure that the disc vertical, so that flexible lifting. Swing check valves, when installed to ensure that the pin level, so that flexible swing. The reducing valve should be installed upright on the horizontal pipe, and do not tilt in any direction.
(2) Construction operations
Installation and construction must be careful not to strike valves made of brittle materials.
Before installation, the valve should be inspected to check the specification and type and identify whether there is any damage, especially for the valve stem. Also turn a few times to see if it is skewed, because in the process of transportation, it is easy to bump the valve stem. Also the debris in the valve. When lifting the valve, the rope should not be tied to the handwheel or stem to avoid damage to these components, should be tied to the flange.
For valves connected to the pipeline, be sure to clean. Compressed air can be used to blow away iron oxide chips, sand, welding slag and other debris. These sundries, not only easy to scratch the sealing surface of the valve, including large particles of sundries (such as welding slag), but also can block the small valve, so that it fails. When installing the screw valve, the sealing packing (thread and aluminum oil or polytetrafluoroethylene raw material belt) should be wrapped on the pipe thread, do not get into the valve, so as not to valve memory product, affect the flow of media.
When installing flanged valves, pay attention to tighten the bolts symmetrically and evenly. Valve flange and pipe flange must be parallel, reasonable clearance, so as to avoid excessive pressure, or even cracking valve. Especially for brittle materials and valves with low strength. Valves to be welded with pipes should be spot-welded first, then fully open the closing parts, and then welded dead.
(3) Protective measures
Some valves must also have external protection, which is insulation and cooling. Hot steam lines are sometimes added to the insulation. What kind of valve should be insulated or kept cold, according to production requirements.
In principle, where the medium in the valve reduces the temperature too much, it will affect the production efficiency or freeze the valve, you need to keep heat, or even mix heat; Where exposed valves, adverse to production or cause frost and other adverse phenomena, it is necessary to keep cold. Insulation materials are asbestos, slag wool, glass wool, perlite, diatomite, vermiculite, etc.; Cooling materials include cork, perlite, foam, plastic and so on.
(4) Bypass and instrument
Some valves, in addition to the necessary protection facilities, but also have bypass and instrumentation. A bypass was installed. Easy to repair the trap. Other valves, also installed by bypass. Whether to install bypass, depends on the valve condition, importance and production requirements.
(5) packing replacement
Inventory valve, some packing is not good, some with the use of media does not match, this needs to change the packing. Valve manufacturers can not consider the use of thousands of units of different media, stuffing box is always filled with ordinary root, but when used, must let the filler in the medium to adapt.
When replacing the filler, press in round by round. Each ring seam to 45 degrees is appropriate, ring and ring open 180 degrees. The packing height should consider the room for the gland to continue to press. At present, the lower part of the gland should press the packing chamber to an appropriate depth, which can generally be 10-20% of the total depth of the packing chamber. For high demand valves, the joint Angle is 30 degrees. The seam between the rings is staggered by 120 degrees. In addition to the above fillers, can also according to the specific situation, the use of rubber O-ring (natural rubber resistance below 60 degrees Celsius weak alkali, butadiene rubber resistance below 80 degrees Celsius oil crystal, Fluorine rubber resistance below 150 degrees Celsius a variety of corrosive media) three-piece stacked polytetrafluoroethylene ring (resistance below 200 degrees Celsius strong corrosive media) nylon bowl ring (resistance below 120 degrees Celsius ammonia, alkali) and other forming filler. A layer of polytetrafluoroethylene (PTFE) raw material tape is wrapped outside the common asbestos disc, which can improve the sealing effect and reduce the electrochemical corrosion of the valve stem.
When pressing the seasoning, it is necessary to rotate the valve stem at the same time to keep it uniform around and prevent it from being too dead. It is necessary to tighten the gland evenly and not tilt.