Self-acting SAV and SR provide double safety
Self-acting surge relief valves for peak loads (SR) and surge anticipation valves (SAV) are used for mechanical pressure surge control.
Pressure surges occur when the flow of a fluid is suddenly interrupted, e.g. by fluctuations in the power supply, pump failure or the sudden closing of a shut-off valve downstream of a pump station.
SR and SAV protect the plant
The surge relief valve (SR) is typically used across pumps, discharge headers and at critical points along the pipeline in the bypass when dynamic pressure peaks need to be relieved. The valve opens immediately to atmosphere when a set response pressure is reached and exceeded, i.e. directly when the high pressure wave hits it, in order to discharge the medium from the system. The valve closes slowly in a controlled manner when the pressure falls below the set pressure again.
Since the SR does not have a pre-opening function like the SAV, it often cannot react quickly enough to the returning overpressure wave. SR should therefore only be used for gradual pressure rises.
The sudden closing of a shut-off valve downstream of a pump station is an exemplary application for the SR. Due to the sudden retardation of the flow at the shut-off valve, up to a standstill, the liquid is strongly compressed and the pressure surge occurs (see also Joukowsky surge). This high-pressure wave is reflected at the shutoff valve and flows upstream back to the pump, propagating throughout the piping system at the speed of sound. Behind the closed shut-off valve, the fluid column continues to move through the pipeline due to its own momentum, creating a strong negative pressure. At this point, bleeding and venting valves with built-in anti-pressure surge function can intercept and dampen the pressure surges in the downstream piping system.
The surge anticipation valve (SAV) is designed for installation in a bypass line. It opens with the aid of high-pressure and low-pressure pilot valves when the pressure falls below or goes beyond the set threshold values, and does so immediately with the onset of the vacuum wave. Thus, the SAV already senses the initial stage of the high-pressure wave and is already open when the high-pressure wave finally reaches the valve. By pre-opening in this way, the SAV prevents the pressure rise before it occurs. After the high-pressure wave decays, the valve closes slowly to prevent pressure peaks. If the pipeline pressure is between the two set pressures, the valve remains closed.
A sudden pump failure is an exemplary application for the SAV. Here, the negative pressure occurs in the immediate vicinity of the pump. The fluid column continues to move through the pipeline away from the pump due to its own dynamics and creates a strong negative pressure. The negative pressure slows down the fluid column. The pressure differences in the pipeline cause the direction of movement of the fluid column to reverse and the fluid column moves back to the pump. Should it encounter closed check valves, a pressure surge is created which propagates through the entire piping system at the speed of sound.
An SAV should be preferred instead of an SR when high pressure peaks occur so quickly that the SR does not have sufficient time to respond and open.
Self-acting for more safety
Self-acting control valves are also referred to as self-operated regulators (German abbreviation: RoH). They are medium-controlled and do not require an external energy source or actuators to operate properly.
Many manufacturers do not design valves of this type to be self-acting, but rather nitrogen-pressurized versions. The nitrogen gas is used to pressurize the valve actuator and keep it in the closed position during normal operation. This has some disadvantages, as nitrogen must be available at all times, which makes the valve much more expensive to operate. The system also requires more space due to the arrangement of the nitrogen cylinders.
The use of self-acting valves avoids these disadvantages. Even in remote areas with inadequate infrastructure or in pipelines laid over long distances, self-acting valves perform their service reliably without any external control. Accidental maloperation is thus ruled out. Even power failures or computer errors in the plant are no problem - self-acting valves reliably ensure safe operation in exceptional situations.
Pressure surges? Important for your plant design:
- Determine the proper flow rate so that an SR or SAV can be properly sized. An oversized SAV will release too much pressure to the atmosphere when the surge occurs and will not be able to recover the static piping pressure, causing the system to run dry. The main valve remains open until there is no more differential pressure across the valve to close it. Re-pressurizing costs a lot of energy. So the valve manufacturer will design the smallest possible SAV for the task in the plant. Oversizing could lead to damage.
- Have a transient analysis prepared by a specialized service provider. The type and extent of the dynamic pressure changes must be known in order to plan and operate the system with precision, e.g. start-up, switching pumps over or off, or changes to valve settings.
Do not leave it to the valve supplier to provide a complete pressure surge analysis. When designing the valve, the maximum pressure surge on the longest pipeline section is what counts for him; he does not take into account any pipelines to be fed in or changes in the pipeline diameter.