What is the effect of back seating in a two-way service valve?

Back seating in a two-way service valve is designed to maintain system flow while closing off the service port. This is particularly useful when maintenance or testing of the system is required without having to completely shut down the entire system.

By back seating the valve, the valve stem is adjusted so that the sealing surface at the back of the valve handle is engaged, thus closing the service port, but allowing the fluid to continue to flow through the valve body. This feature is critical in operational efficiency, as it means that technicians can access the service port for repairs or inspections without disrupting the flow of the system, which could be crucial depending on the application and the fluids involved.

The other responses address aspects that are not true for back seating in a two-way service valve. While it is true that back seating restricts access to the service port, it does not completely shut off the system flow, which differentiates it from options that suggest flow is entirely restricted or blocked. Thus, the ability to maintain flow during maintenance while closing off the port is the defining characteristic of back seating.