In some closed loop systems that serve as a power source of a mechanism, the rapid changes in a fluid’s flow velocity may lead to rapid pressure surges. For example, pressure surges occur frequently in pipelines or the marine loading and unloading system. If these pressure surges are not controlled properly, they are going to exceed the pressure limit of the loop system. Once they do, they not only damage the equipment but become a serious safety and environment issue. In order to avoid such scenarios, the involvement of surge valves in a piping system becomes necessary.
Pressure surges tend to occur when one or multiple of the following situations take place. A pressure surge may be created when an automatic emergency shutdown device (usually an ESD valve) is closed or fails. It may also happen when a manual or a powered valve is opened or closed rapidly. Other situations such as a sudden shut of a non-return valve or the starting and stopping of a pump can lead to the creation of pressure surges. The surge valves are there to control and stabilize the flow velocity when any of the presented situations occurs.
Surge valves are commonly known as surge control valves or anti-surge control valves. As the name suggests, they are used to control pressure surges so that the integrity of a system as well as the safety of personnel and environment can be maintained. A surge control valve relieves and stabilizes pressures by means of directing them out of the piping system; it is called a surge relief valve as a result.
The construction of surge valves differs from model to model but the essential components are basically the same and the working principle is rather simple. There is an inlet and outlet on the valve. Within the surge valves, there is usually a disc, or a similar device, used to retain or release the pressure that enters the valve. The inlet connects to the piping system and the outlet channels the pressures out into the environment. Though the design may be not that complicated, an applicable surge valve should possess certain qualities to best fulfill its purpose.
Quick response refers to the valve response time, which is a rather crucial trait of surge valves regardless of the design. In fact, quick response capability is very important in controlling peak surges. The valve response time varies based on the design of a surge control valve. For example, a gas-loaded relief valve reacts to the surges faster than a pilot-operated valve. When selecting the surge valves for a system, quick response should be a prime consideration.
Valve flow coefficient has to do with the size of an anti-surge control valve. The sizing of a surge control valve is based on the scale of the system the valve is employed. Generally speaking, when a system involves the flow of a larger quantity of fluid, the size of the surge valves should be larger. It is recommended that the value of the required coefficient should not exceed 85% of that of the selected valve size. Again, the coefficient differs based on the design of a valve.
Bias pressure is the normal pressure that acts in an environment. In a piping system, the bias pressure refers to the force that keeps the surge control valve closed. When the line pressure exceeds the force, the valve opens. The bias pressure varies based on the type of the valve and the operating mechanism it applies. The selection of a valve should be based on the pressure a system normally creates.
The valve control curve defines the relationship between both the opening and closing characteristics of a valve and the flow rate under constant pressure conditions. The opening and closing characteristics refer to the strokes of a valve. It is the percent opening of the valve. The three basic valve characteristic curves include linear, equal percentage, and fast. For most types of valve, the characteristic is inherent in design and cannot be changed. Therefore, the valve type should be selected based on the application.
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