How to isolate systems with block valves
Isolating industrial fluid systems is important; selecting the right valve configuration is critical to safe maintenance
Safe industrial fluid system maintenance requires isolating any line in which work will be performed. Without proper isolation, fluid inside a line — which is often pressurized — could pose significant risks to technicians involved in changing a gauge or measurement device.
One of the most common and effective methods of achieving proper isolation is the use of block valves at proper locations in the system. Two block valves installed in sequence has become the standard for most systems to allow for the bleeding out of an energized segment in a fluid system. This design has proven to be one of the safest, most effective ways to reduce the pressure and flow to zero.
Typically, engineers use one of two approaches in designing a two-block valve system. The first approach is to add a third valve between the other two, which will vent or bleed off any pressure that might squeeze past the first block valve. Or, engineers can use a third valve to redirect any flow to a bypass loop that will eliminate the possibility of fluid flowing through the section of line under maintenance.
Isolation configuration locations
No matter where a fluid system line is in a plant, it will eventually require some maintenance. Consequently, technicians must be able to isolate that line safely.
Among the locations that require a block valve are lines near any device or component that requires regular maintenance like a filter, valve or transmitter. Any system, skid or line that could eventually need to be reconfigured, repaired or replaced — or any section of the main process pipe that may need to be serviced or maintained — could also require block valves.
In addition, any instrumentation line that branches off the process line (e.g., a grab sample station, sampling station or gauges for pressure temperature or flow) could require block valves. Any sampling system for calibration fluids — particularly any sampling streams that may be switched on or off — will also require a block valve configuration.
Isolation configuration options
Having more than one block valve in any given line is critical to prevent leaks, which will cause pressure to increase in the line being maintained and create unsafe conditions. Typically, engineers will solve this challenge with one of two main configurations: a double-block-and-bleed (DBB) setup or a bypass loop.
To isolate a fluid system, a DBB setup is the most common configuration used (see Figure 1). It usually marks a transition from a process line to either an instrumentation line, when using a process interface valve, or to a line that leads to an instrument or device such as a transmitter. Engineers usually design the three-valve system as a single manifold unit or as three separate components.
In contrast, a bypass loop not only isolates the line under maintenance but also redirects the overall flow so the line can continue to function while maintenance is ongoing. In a bypass configuration, for example, a three-way valve might be used to modify the flow around the section of the line under maintenance (see Figure 2). As a result, technicians can change a filter without forcing unnecessary downtime. Bypass valves also eliminate the problem of hydraulic shock (also known as “water hammer”) that occurs when a valve is closed off suddenly.
Depressurize and isolate. Before performing any maintenance, technicians should first depressurize any fluid system. It is generally preferable to have two block valves in sequence when doing so, to keep pressure from building up in the section of the line under maintenance.
While a good valve should not leak across the seat, it occasionally does happen under certain conditions. If the valve has not been properly maintained or if the wrong valve has been chosen for the task, leakage is possible. That is why having a second block valve is critical, in addition to a bleed or vent valve. In cases in which downtime is not possible, a bypass valve may be necessary.
Choosing the right block valve
Needle valves and ball valves are the two most common types of block valves; though, a third option adds additional possibilities to the mix. Maintenance technicians must ensure the system specifications match the application at hand.
Needle valves. Although needle valves are not always used for shutoff purposes, they can be used in situations in which positive shutoff is necessary. In this capacity, needle valves shut down lines gradually, avoiding the hydraulic shock problem. It is critical to choose a needle valve specifically designed as a block valve — one with a rotating or soft-stem tip. Otherwise, metal V-tips can score the metal and cause leakage.
Ball valves. While ball valves can effectively shut off flow quickly and their handles helpfully indicate the direction of flow or shutoff, they also can generate hydraulic shock because they shut the flow off so rapidly. Water hammer can damage pressure indicators, flow meters or other upstream components.
To avoid hydraulic shock, engineers can install a bypass loop into the system. That way, when the ball valve closes, the flow has somewhere to go, which reduces the pressure spike that results from a ball valve shutting off the flow of liquid quickly.
Integral DBBs. One of the latest entries into the block valve market is an integrated DBB configuration, which offers additional reliability. The integrated DBB system contains fewer parts that can leak, takes up less space, weighs less and is easier to install than more traditional block valve components (see Figure 3).
Be careful not to use the wrong component in an isolation configuration accidentally. While ball and needle valves are constructed to deal with positive shutoff, regulators are not (even though they can often stop most of the flow). Do not use the regulator as a block valve — engineers should instead install a ball valve system somewhere upstream.
Engineers also should install a pressure indicator downstream from the second block valve in the isolation configuration because it will allow technicians to check on pressure at a glance and make sure the block valve configuration is working properly.
Properly isolating fluid system lines with block valves will not only improve uptime and plant profitability but also help plants maintain their fluid lines more safely.