Piston Valve: Types, Mechanism, Purpose, Design & Advantages
What is Piston Valve?
Piston Valve: Types, Mechanism, Purpose, Design & Advantages :- A piston valve is a flow regulating device which is used to control the flow of fluid inside the pipeline. It is generally applied in the case when either fully open or fully close operation of a fluid is required, such as water pipeline. The partial opening can also be done but this will create a huge amount of turbulence which may ends up damaging the valve sealings.
Generally, the piston valves are operated manually but some times it is also actuated electrically or hydraulically, depending upon the requirement.
The piston valves are generally not designed for the partial opening conditions; it must be used in fully opened or fully closed portion. In the fully opened condition, only bottom surface of the valve comes in contact with the flowing fluid and rest of the valve body is protected by the upper sealing of the valves. This function not only increases the valve life but also make it safe from the erosion from the flowing fluid, consequently, the valve remains leak proof after a long term use.
Design of Piston Valve
The piston valve generally consists of two sealings to avoid the leakage that is, upper and lower sealing rings that are held in place by lantern bushings. As, the valve opens the piston side wall comes in contact with the upper sailings and preventing leakage throughout the valve stem and bonnet. As, the valve closes, the bottom surface of the piston comes in contact with the lower sealing rings and thus create a leak proof packing of the fluid inside the pipeline. The pistons are designed so that they can have only linear motion; up and down motion, and do not rotate with the rotation of the hand wheel.
Although the piston of the valve goes fully upward in open condition and let the fluid move linearly, but due to sharp edges of the piston and irregularity in pipe geometry at the valve zone, turbulence occurs which causes loss of fluid energy in the pipeline.
Manufacturing of Piston Valves
The sealing of the valve are manufactured by different types of flexible materials, but in general reinforced graphite or sometimes rubber or PTFE sealing rings are also used in the system. The sealing rings are known as the heart of the piston valve. To avoid the corrosion of piston, different grades of stainless steels are used to manufacture the piston as per application and working environment.
Types of Piston Valves
1. Single and Double Actuation Piston Valves
There are ‘N’ numbers of piston valves available in the market, which can be categorized on the basis of actuation mechanism, piston arrangements, piston design and working requirements. On the basis of actuating mechanism, the valve can be categorized as manual, electrical and hydraulic type. But, in general, manual operation is preferred the most.
There can be two different types of piston arrangement done in the piston valve; single actuating piston and double actuating piston. In single actuating piston the only one piston is operated to perform the closure or opening of the fluid flow. In this case, the piston moves upward to open the flow and downward to close the flow.
In double actuating piston two pistons are present, opposite to each other, such that the bottom flat face of piston is facing each other. To open the flow, both the piston moves backward and the gap is created for the fluid flow whereas to close the flow, each piston moves towards each other and after establishing the complete contact the fluid flow is completely closed. The single actuating piston valve is generally operated manually whereas the double operating piston valve is operated automatically.
2. Regulating and Normal Piston Valves
As per the piston design the piston valve is categorised into two categories- regulating type and normal type. The normal type of piston valve consists of a piston which is having flat bottom face whereas the regulating type piston which consists of a piston with inclined bottom face. This inclination gives throttling effect to the flowing fluid and helps in regulating the fluid with minimal losses.
Sometimes to incorporate the throttling effect and smooth operation of the valve, the angle seat piston valve is also used in which the piston enters into the flowing fluid at certain angle. This valve is generally operated automatically; by electrical or hydraulic actuation techniques. The operating condition divides the piston valve into two categories; balanced type and unbalanced type. The balanced type piston valve is designed for the high pressure application whereas the unbalanced type piston valves are designed for low pressure application.
Purpose of Piston Valve
Piston valves are primarily used for saturated and superheated steam, hot water and normal water services in power plants, refineries, pulp and paper mills, municipality etc. Piston valves can be also used for handling gases such as oxygen, nitrogen, and hydrogen. Moreover, they are used as well with heat transfer oils and various chemicals. Apart from the industrial application, the piton valves are also used in the musical instruments such as trumpet etc.
Advantages of Piston Valves
The piston valve is very convenient to use, operate and maintain. Due to large sealing area and valve size, it provides the reliable shut offs to the flowing fluid. The actuation process is so easy as there is no need of skilled personnel to operate the valve. The actuation can be done through rotating hand wheel for mechanical actuation and by automatically for electrical or hydraulic actuation. Apart from it the maintenance of the valve is quite easy. The sealings of the valve can be replaced simply while the body remains in the pipeline. By just loosing the bonnet bolts, the stem and piston assembly can be withdrawn and the upper and lower sealings are replaced. If piston is damaged then it can be replaced in the same way.
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