When performing industrial fluid system maintenance, safety is paramount. A single line’s pressure or flow inside represents a hazard to technicians changing a gauge or measurement device. This is why risk managers emphasize the necessity of isolating any fluid system line prior to maintenance.
It has become an industry and safety standard to design and install two block valves in sequence when bleeding out an energized leg in a fluid system. The double block valve sequence is one of the safest ways to achieve an essential state of zero pressure and zero flow.
There are two common approaches engineers typically take when designing two block valves to isolate fluid systems. First, engineers may add a third valve between the two block valves to vent or bleed off any pressure that may be leaking from the first block valve. Another viable option is to have the third valve divert the flow to a bypass loop around the section of the line that is under maintenance. Outlined below are both fluid system configurations, providing safety-focused design options for system isolation and maintenance.
For any fluid system line in a plant or industrial facility that may require maintenance, technicians must have some means of safely isolating that line. Some specific locations that require a configuration for isolation include:
It is industry standard to avoid a situation in which there is only one block valve or no backup. If a single block valve has even a small leak across the seat, pressure can slowly build up in the line under maintenance, creating a safety hazard. Therefore, one of two main configurations for achieving isolation in a fluid line is encouraged:
A double-block-and-bleed configuration, or DBB, is the simplest configuration for isolating a fluid system. It is commonly used transitioning from the process line to an instrumentation line when using a process interface valve, or on a line that leads to an instrument or device, such as a transmitter. The three valves may be configured as a single manifold unit or as three separate components.
A bypass loop is a slightly more complicated configuration that not only isolates the fluid system line under maintenance, but also reroutes the flow so the process can continue to function during maintenance.
For example, the first block valve in bypass configuration may be a three-way valve — diverting flow around the section requiring maintenance. The system filter can now be changed out without requiring downtime. Another reason for a bypass is to avoid hydraulic shock or “water hammer” that results when there is a sudden system flow shutoff.
The first step when preparing for maintenance on any fluid system is depressurization. When doing so, a best practice is to have two block valves in sequence to guard against pressure buildup in the section of the line under maintenance. A good valve should not leak across the seat, but this could happen. For example, if the line is outside, the sun may heat up the line and raise the pressure beyond the valve’s specified rating. Other potential scenarios include if the valve has not been properly maintained or if it’s the wrong valve choice for a positive shutoff. To guard against these and other possible leaks, a second block valve is necessary, along with a vent or bleed valve. Alternatively, in cases where downtime is not an option, a bypass loop that duplicates the components in the line under maintenance is an excellent choice.
The two most common choices for block valves in instrumentation lines are ball valves or needle valves. It is imperative that maintenance technicians check system specifications for the valve that is required for each system.
Beware of inadvertently using the wrong fluid system component in an isolation configuration. Ball valves and some types of needle valves are made for positive shutoff, but regulators are not – even though it’s possible to set up regulators to stop most of the flow. If you need a shutoff around the location of a regulator, it would be safest to install a ball valve upstream.
In addition to block valve selection, it’s good practice to install a pressure indicator downstream from the second block valve in the isolation configuration. This will enable a visual check on pressure at the time of maintenance.
Taking the time to properly isolate your fluid system lines with block valves could result not only in safer maintenance activities, but also increased system uptime and plant profitability. To learn how to safely configure fluid systems in your plant and train your team on best practices, sign up for a Swagelok fluid system essentials training course.