Installation of the shut-off and control valve
Shut-off and control valves belong to the group of drive switchgears that are mounted on pipelines. The flow rate is changed by means of a plunger /rod, which translationally moves as a result of a control signal emanating from an external actuator.
Factors that are important to consider during installation
The main problem is the accuracy of the valve installation. Since the axes of the valve and the pipeline must completely coincide, their dimensions and flatness are crucial.
Most standards contain a list of tolerances that the manufacturer must comply with, including:
- The thread accuracy or the circumference diameter of the mounting bolts (respectively for coupling and flange valves);
- Additionally – tolerances for the displacement of bolt hole sizes;
- Tolerance for overall (especially axial) dimensions of the shut-off and control valve;
- Non-perpendicularity of the surfaces relative to which the drive operates.
At the same time, the external dimensions are usually considered free.
The tolerances are set depending on the method of manufacturing the valve body (casting or hot stamping). Foundry slopes are usually larger than stamping ones. If the slope exceeds the tolerance, the coupling is installed unsteadily, and the flange bolt may be bent when tightening. If the coupling and the flange are manufactured by other manufacturers, then the flanges may differ in different requirements for flatness and alignment.
Taking into account all of the above, before installing the shut-off and control valve, the surfaces of the joints and connections with the drive parts are machined. Ultimately, the diameter of the fit spot or the thread gaps in the coupling should be such that a reliable sealing of the device is ensured, the axes of all drive parts are parallel to each other.
Influence of dimensions and weight
For the manufacture of valves of the type in question, gray cast iron grades from SCH 15-36 and above are often used. However, cast iron is brittle, so the slightest misalignment leads to radial forces on the actuator, which cause dangerous bending stresses in the valve body. Therefore, the installation of such connections is significantly complicated. If possible, the shut-off and control valve housings are made of more ductile metals – stainless steel, bronze or red brass.
Particular attention is paid to the roughness of the mating surfaces. Flanged type valves should have shallow grooves at the end, which facilitate the sealing of the gasket and prevent its ejection when the control drive is abruptly switched on. To dampen the valve body designed for high pressures, epoxy resin coating, reinforcement with water- and oil-resistant synthetic fabrics are used, and high-strength and elastic gaskets are also used.