Difference between single seated & double seated control valves

Single Seated


Single seated valves are one form of globe valve that are very common and quite simple in design. These valves have few internal parts. They are also smaller than double seated valves and provide good shut off capability.
Maintenance is simplified due to easy access with top entry to the valve components. Because of their widespread usage, they are available in a variety of trim configurations, and therefore a greater range of flow characteristics are available. They also produce less vibration due to the reduced plug mass.

Advantages

– Simple design.
– Simplified maintenance.
– Smaller and lighter.
– Good shutoff.

Disadvantages

– More complex designs required for balancing

 

Double Seated

 


Another globe valve body design is double seated. In this approach, there are two plugs and two seats that operate within the valve body. In a single seated valve, the forces of the flow stream can push against the plug, requiring greater actuator force to operate the valve movement. Double seated valves use opposing forces from the two plugs to minimise the actuator force required for control movement. Balancing is the term used when the net force on the
stem is minimised in this way. These valves are not truly balanced. The result of the hydrostatic forces on the plugs may not be zero due to the geometry and dynamics. They are therefore termed semibalanced. It is important to know the combined loading due to the amount of balancing and dynamic forces when sizing the actuator. Shutoff is poor with the double seated valve and is one of the downfalls with this type of construction. Even though manufacturing tolerances may be tight, due to different forces on the plugs it is not possible for both plugs to make contact at the same time. Maintenance is increased with the added internal parts required. Also these valves tend to be quite heavy and large.
These valves are an older design that have fewer advantages compared with the inherent disadvantages. Although they can be found in older systems, they are seldom used in newer applications.

Advantages

– Reduced actuator force due to balancing.
– Action easily changed (Direct/Reverse).
– High flow capacity.

Disadvantages

– Poor shutoff.
– Heavy and bulky.
– More parts to service.
– Only semi-balanced.

Also read

Control Valve Noise and Cavitation

What are motor operated valves?

How a Globe valve works?

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