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There are two main types of safety valves: Spring-loaded safety valves and pilot-operated safety valves. Also relevant are so-called change-over valves, which ensure continuous system availability.
Change-over valves are switching valves used for the redundant installation of safety valves. Change-over valves switch the flow of media between two different safety valves. The forced control incorporated into the design ensures that one inflow and/or outflow is always open.
Change-over valves are used to connect two safety valves with a pipe connection to a pressure system, in order to increase operational availability. One safety valve is in operation and one safety valve is on standby. The standby safety valve can be disassembled and serviced, for example during running operation. The pressure system continues to be protected against impermissible pressure. This way, shutdown periods of the plant can be planned independent of the maintenance cycles of the safety valves.
Change-over valves exist in several designs. These are presented and compared below.
In change-over valves with a rotor design, a rotating actuator ensures that one flow path is blocked and the other unblocked. A change-over valve such as this is selected based on the same nominal size as the inlet and/or outlet of the safety valve. But the switching process itself is rather complicated.
With this type of valve and an optimized flow path and low pressure drop, it is possible to do without additional reducers. Only one pressure-drop coefficient is known for each nominal size, however. Effects caused by options or other nominal pressures are not specified. For that reason, there are some uncertainties involved in calculating the pressure drop for these change-over valves.
Change-over valves with shuttle design have 2 x 90° deflections. This results in a comparatively high pressure drop. Because of this, they can only be designed in the same nominal sizes as the inlets of the safety valve when the safety valve only has to have a low capacity or the inlet lines in question are relatively short.
For high-capacity safety valves or long piping in the inlet, the shuttle change-over valve must be at least one nominal size larger than the nominal inlet size of the safety valve. This is the only way to achieve the maximum inlet pressure drop of 3%, thereby ensuring the stable functioning of the safety valve.
As with the rotor design, a pressure-drop coefficient is often specified for each nominal size. The effects of reducers or other nominal pressures are not factored in here either. As a result, these valves are subject more uncertainties as rotor changeover valves when calculating the pressure drop.
The special design of change-over valves with a pendulum design makes it possible for the active safety valve to be switched reliably during maintenance. The shut-off disc travels along a circular path back and forth between the two outlets. The optimized flow path ensures a minimal pressure drop. Clearly defined flow resistance coefficients in each configuration make it easier to calculate the inlet pressure drop precisely.
The 3% criterion is therefore easy to implement. Designed for maintenance-free longevity and reliable, 24/7 uptime, the new change-over valves have undergone lifecycle tests with 1,000 changeovers with hot, cold and particulate media and at high operating pressure
Rotor Design
Inlet Pressure Drop
Optimised flow level
Reliable Uptime (prevent simultaneous closure of both sides)
Complicated change-over in three steps
Reliable Uptime (ensure seal tightness)
Sealing problems
Coupling on the inlet and outlet sides in lockable combinations
Lockable combination with different nominal sizes of change-over valves on the inlet and outlet of safety valves
Shuttle Design
Inlet Pressure Drop
Comparatively higher pressure drop due to 2 x 90° deflections (risk of chatter or larger valves required)
Reliable Uptime (prevent simultaneous closure of both sides)
Easy to operate using a hand wheel
Reliable Uptime (ensure seal tightness)
Robust and long service life
Coupling on the inlet and outlet sides in lockable combinations
Lockable combination only possible with change-over valves with the same nominal size on the inlet and outlet of safety valves (high weight, reducers)
Pendulum Design
Inlet Pressure Drop
Optimised flow path, pressure drop slightly higher than with the rotor design
Reliable Uptime (prevent simultaneous closure of both sides)
Easy to operate using a hand wheel
Reliable Uptime (ensure seal tightness)
Robust and long service life
Coupling on the inlet and outlet sides in lockable combinations
Lockable combination with different nominal sizes of change-over valves on the inlet and outlet of safety valves
Change-over valves are safety valves that switch the medium flow between two different safety valves, thus ensuring higher plant availability by ensuring that one inlet or outlet is always open.
What is a CoV Valve?The abbreviation „CoV“ stands for “Change-Over-Valve” and refers to the safety valve type of the same name.
