Alex Schick, product marketing manager, and Joe DeMonte, global midstream oil and gas and LNG industry manager, both with Emerson Automation Solutions, wrote an article for the April 2019 issue of Flow Control magazine titled Specifying Ball Valves in Pipeline Applications. They point out that pipeline ball valves (as shown in Figure 1) are routinely misapplied in the field.
For example, isolation valves are often used in control applications, leading to poor flow control and possible valve damage. This is because their seal and packing designs are typically unsuited for throttling applications.
Their article covers the proper application of ball valves for isolation, control, general service and severe service applications.
Figure 1. Proper selection of pipeline ball valves, such as these Fisher Model V260A severe service valves, can provide appropriate control and reduce maintenance.
Inside Isolation Valves
On pipelines, isolation valves are often used to isolate process fluid from moving downstream when a nearby control valve is closed. The authors say most control valves should have an isolation valve installed upstream and downstream.
Isolation valves use a nonmechanically retained seal to leverage seal injection ports for tight shutoff. These valves have O-ring style packing (see Figure 2), ideal for low-cycling applications, and the packing includes a fire-safe gasket. These valves can either be manually operated, or automatically operated by an actuator.
Figure 2: Isolation valves have several features designed for bubble-tight shutoff and fire safety.
When selecting an isolation valve, look for:
General Service Control Valves
General service applications do not require noise attenuation, cavitation protection or tight control; therefore, general service valves do not have splined shafts or attenuators, but they are still equipped for dynamic applications.
A general service, trunnion-mounted and full-bore pipeline ball valve (as shown in Figure 3) can be used in both control and isolation applications. It has the mechanically retained seals necessary for throttling without the risk of seal failure.
Figure 3. A general service control valve, such as this Fisher Model V270, is designed for line pressures of up to 1480 psi.
When selecting a general service control valve, look for:
A general service control valve can be used in almost any application that does not require noise reduction and extreme precision. However, a double-D or pinned ball shaft connection does not provide the highest performance because general service applications generally do not have stringent throttling requirements.
Severe service applications are those in which excessive noise or cavitation is expected.
Severe Service Valves
Typical severe service applications require high-performance actuation. This could be needed for fast stroking speeds or higher cycles. To provide better control of valve position, a severe service, trunnion-mounted and full-bore pipeline ball valve (as shown in Figure 4) has splined ball-to-shaft and shaft-to-actuator connections
Figure 4. A severe service, anti-surge valve, such as this Fisher Model V260A, has a rugged actuator and other features for fast stroke, long life and high performance in demanding applications.
Cavitation can be a problem in severe service applications. When pipeline fluid flows through a fixed restriction like a control valve, the pressure drops. Depending on the severity of the pressure drop and the fluid properties associated with the process, this can cause cavitation.
Pipeline ball valves can be supplied with various types of anti-cavitation and noise-protection trims to stage the pressure drops and prevent cavitation. This allows the valve to not take the entire pressure drop at once.
When selecting a severe service ball valve, look for:
Pipeline Applications
Ball valves can be used in the following pipeline applications:
As pipeline gas travels from higher-pressure transmission pipelines to lower-pressure distribution pipelines, the gas passes through a gate station with worker and/or monitor valves to reduce pressure. An over-pressure protection, or super monitor valve, often performs as a general service valve. A pump station pressure-control valve minimizes the pressure drop when fully open.
Compressor stations may be placed every 50 to 100 miles on a pipeline to help boost pressure. Typically, an anti-surge control valve is used to prevent the compressor from experiencing surge, which can cause costly damage to the equipment.
Conclusion
Proper ball valve selection will result in more uptime and better performance, and it will prevent “sucking out” of seals and damage to the drive train. The correct valve will provide appropriate control and reduce required valve maintenance.