Linear Motion Valves are divided to be three notes; there are single port valve, double port valve and three-way valve which should be considered for design.
Single Port Valve Body
The most common control valve body style is single ported as shown in Figure 1, which has widely used in process control applications, particularly in sizes NPS (Nominal Pipe Size) 1 through NPS 4. Normal flow direction is most often flow-up through the seat ring.
Single ported valves are available in various forms, such as globe, angle, bar stock, forged and split constructions.
Since high pressure fluid is normally loading the entire area of the port, the unbalance force created must be considered in selecting actuators for single ported control valve bodies. Single ported valves are known to work well in small sizes but it can often be used in 4 inch to 8 inch sizes with high thrust actuators.
Many modern single ported valve bodies use cage style construction to retain the seat ring, provide guiding to the valve plug, and means for establishing a particular flow characteristic. Cage style trim offers advantages in ease of maintenance and flexibility in changing the cages to alter valve flow characteristics.
Cage style single seated valve bodies can also be easily modified by change of trim parts to provide reduced capacity flow, noise attenuation, or reduction or elimination of cavitations.
Port guided single port valve bodies :
- Usually limited to 150 psi (10 bar) maximum pressure drop.
- Susceptible to velocity-induced vibration.
- Typically provided with screwed in seat rings which might be difficult to remove after use.
These valves are generally specified for applications with stringent shutoff requirements. Metal to metal seating surfaces or “soft seating” with nitrile or other elastomeric materials forming the seal, can handle most service requirements
Double Port Valve Body
Double port valve is a special trim design used to fill the same purpose as pressure balanced trim: to reduce the effect of the process forces on the plug, thereby lowering the thrust requirement and allowing the use of smaller actuators.
Flow is directed by the inlet port to the body gallery and the trim, which features two seats and a single plug that features two plug heads. The control valve body in the picture is assembled for push down to open valve plug action.
Double ported designs are typically used in refineries on highly viscous fluids or where there is a concern about dirt, contaminants, or process deposits on the trim.
Port Guided Double Ported Valve Bodies :
- Dynamic force on plug tends to be balanced as flow tends to open one port and close the other.
- Bodies normally have higher capacity than single ported valves of the same line size.
- Many double-ported bodies reverse, so the valve plug can be installed as either push-down-to-open or push-down-to-close.
- Port guided valve plugs are often used for on-off or low pressure throttling service. Top-and-bottom guided valve plugs furnish stable operation for severe service conditions
Three Way Valve Body
Three-way valves have two designs; double seated valve and single seated valve. Each of design has specific application.
Double seated usually is used for diverging (flowsplitting), where as single seated is used for mixing (flow-mixing) service. These valve bodies are best designs use cage-style trim for positive valve plug guiding and ease of maintenance.
Three-way valves are often found in refineries and chemical plants is around heat exchanger to control heat transfered.
fig 1 - single seat design for mixing service in heat exchanger
fig 2 - double seat design for diverging service in heat exchanger
Below Figure explains the actuator pushes a disc or pair of valve plugs between two seats increasing or decreasing the flow through ports A and B in a corresponding manner.
fig 3 - three way valve
Each valve is shown in figure 1 and 2 its preferred placement, with the flow through each port inclining to open that port. There would be a harmful of the valve slamming onto its seat, with releasing line vibration, especially in the larger sizes, as a valve moves through its seat, the reduction in flow rate caused by this movement lowers the downstream pressure, so that increasing the pressure drop across the valve, if the valve were installed in such a placement that the flow would inclined to close the port.