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Comparison of Double 3-Way/ 6-Way Ball Valves to Butterfly Valves in Duplex Strainer

Butterfly valves present a more economical alternative for fluid flow isolation when compared to their more expensive cousins, the ball valves. The lesser number of components in a butterfly valve and their compact construction lends this advantage. However, there are some inherent disadvantages associated with butterfly valves, such as:

1. The butterfly valve disc always remains in the flow path when the valve is completely open leading to flow obstruction. This causes loss of energy. A full-bore ball valve on the other hand, works as an extension of the pipeline with negligible pressure drop.
2. Generally, butterfly valves are used for their price in low pressure systems only. The sealing by rubber seats is effective over a smaller range of pressures compared to the sealing in ball valves using PTFE seats. Though one may argue that butterfly valves using PTFE seats can also be used, they lose part of their price advantage.
3. Structurally, the butterfly valve disc is subjected to a lot of fluid force due to its shape and hence faces limitations. The ball in a ball valve can distribute the forces due to its spherical shape and is suitable for a very wide range of temperatures and pressures.
4. The butterfly valves are suitable for clean, low viscosity fluids whereas the ball valves can be used with a very wide range of fluids.  

There are many manufacturers of Duplex Strainer systems who use four Butterfly valves on their equipment to divert flow to and from pressure vessels on standby. Principally, the reason for choosing Butterfly valves is the lower price and easy availability. Butterfly valves are more widely available in larger sizes when compared to ball valves. However, systems with four Butterfly valves require the operation of all four valves to effect diversion of flow from one filter to another. Also, it is important to first open all the valves before shutting two, to ensure continuous flow. In case all the valves are first closed before opening two, it can lead to catastrophic failure of the system as the downstream process will be starved of the fluid, momentarily, leading to drop in pressure. In critical systems, like turbine oil filtration, such drop in pressure is not tolerated. To prevent such an eventuality, some manufacturers connect all the four valves with linkages for simultaneous operation.

Each Butterfly valve in the system is a possible cause for failure and fluid flow control engineers always try to minimize the failure modes. Hence, a system with lesser number of valves leads to better system reliability. A double 3-way ball valve/ 6-way ball valve is a step in that direction.  

A double 3-way/ 6-way ball valve system is a compact arrangement of only 2 valves used to divert flow from one filter housing to another. The system affords the luxury of operating only one lever to operate both valves simultaneously. In a double 3-way ball valve system, the common lever arrangement is provided to connect the stems of both the valves for simultaneous operation. A 6-way ball valve, which is a more compact arrangement of the two 3-way valves, has an internal coupling to operate both valves simultaneously. The lever or a gear actuator is provided on the top of the valves for easy access. These valves never choke the inlet flow at any point in their operation. There is a very gradual changeover from one filter housing to another. 

There is an option to incorporate a “Pressure Equalization Valve (PE)” within the 3-way valve system. This arrangement is not possible to provide in the 4 Butterfly Valve system. This option is provided to prefill the filter housing before changeover. A pre-filled vessel starts delivery of filtered media immediately upon changeover without an initial drop in pressure. Kavaata® supplies a wide range of 3-way/ 6-way ball valves in sizes from 25FB to 250FB. Various pressure classes are covered. For special applications requiring very high temperatures, metal seated valves are also custom designed.