How to mitigate reverse flow
Learning Objectives
Review the problems associated with reverse flow.
Learn about mitigating reverse flow problems.
Know the standards required to follow when designing piping systems.
Reverse flow in piping systems is an undesirable occurrence, caused by a loss of pressure. It has adverse implications on processes and flow control devices. These impacts manifest into the form of sporadic pipeline vibrations, media contamination or physical damage.
Installing the correct check valves in the piping system is vital for preventing reverse flow and limiting its impacts. Several factors come into play when selecting valves for industrial applications, commercial buildings, plumbing and heating, ventilation and air conditioning. The crucial considerations are flow capacity needs, the nature of the working media, valve coefficient value and piping orientation.
The performance of a valve generally deteriorates as the number of working cycles increases. Extending valve’s longevity requires regular maintenance, which includes flushing of piping systems to expel debris and valve lubrication.
Water hammer
Water hammer, or sometimes known as hydraulic shock, happens when moving fluid is suddenly stopped or forced to change direction. The sudden impact leads to a build-up of shockwaves that generate a negative pressure downstream. The waves propagate throughout the pipe system and are characterized by banging sounds or vibrations on pipe surfaces.
This is a common phenomenon in piping systems configured for uphill conveyance of fluids. The pumps used in these systems may shut down when an initially open valve closes abruptly, causing an instant loss of fluid pressure. Combining this with low static head pressure increases the intensity of the resultant hydraulic hammer. The impact of water hammer is higher in fast-moving fluids, piping systems with longer branch lines and piping configurations that have fewer elbows or expansion loops.
Singular impacts due to fluid momentum may be inconsequential. As these instances recur, valves become prone to damage and the structural integrity of pipes, welded joints, gaskets and expansion joints decrease. Sharp pressure build-up in pipe bends causes an imbalance in internal and external pipe forces, which causes a positional shift of pipes. As a result, they get detached from their supports and anchors.
Cavitation
Rapid pressure drop due to reverse flow can cause fluid in the downstream side of a valve to condense and evaporate suddenly. As the fluid flows past the valve, there is a creation of a localized low-pressure zone that exerts a force upon the flowing liquid. The externally applied force leads to the formation of cavities in the liquid medium, causing vaporization (formation of fluid bubbles).
As the vaporized fluid travels downstream, there is a steady restoration of system pressure, initiating a forward fluid flow. It results in the condensation of the bubbles characterized by violent implosions.
Piping systems experience different cavitation levels. They are incipient cavitation, an initial stage that manifests as light noises; constant cavitation characterized by incessant rumbling sounds and choked cavitation that limits fluid flow past the valve.
The immediate effect of the implosions is the production of a ringing voice around valves. Repeated bubble implosion causes surface fatigue, which causes the corrosion of pipe surfaces and localized stress on the valve body that causes pitting. Frequent seal or bearing damage, pump impeller corrosion and higher pump power consumption indicate an underlying cavitation problem in a piping system.
Pipes, valves and pumping systems are common in office building mechanical rooms. Courtesy: CFE Media and Technology
Valve leakage and slamming
Frequent reverse flow affects the operation of frequently cycled valves in processes requiring strict emission and leakage control. Conventional check valves are designed to close when downstream pressure exceeds upstream pressure. Events such as pump failures result in an immediate pressure drop, initiating an unwanted reverse flow.
This sudden change affects the closing characteristics of valves, with the flap or swing slamming violently against the valve seat. It results in the formation of shockwaves that persist until frictional losses force the waves to subside.
Shockwaves and nonuniform pressure surges, resulting from frequent flow reversal, exert undue stress on the valve seals and packing material. It weakens the sealing components, increasing the probability of valve leakage and release of volatile compounds to the environment.
Pipe overpressure
The design of piping systems incorporates pipes and valves of different pressure classes, diameters, bends, joints and fittings. During a reverse flow occurrence, there is a pressure wave traveling downstream at the speed of the fluid media. The reversed wave reflects and refracts when it interacts with the different piping components. It initiates the formation of negative and positive sonic waves. The convergence of the waves tends to increase system vibrations.
As the waves traverse through the pipeline, internal pipe pressure rises sharply to exceed the minimum working pressure of the system. The excess pressure crashes against the piping, causing damage to valves, pipes, bends and fittings.
Contamination
Piping systems such as public water supply pipelines or industrial chemical lines, have strict requirements for contamination control. Their systems are designed to offer minimum resistance to forward flow, with sufficiently sealed valves. These systems are not immune to reverse flows so check valves are critical.
When frequent reverse flows happen, the risk of contamination increases as fatigue affects valve sealing properties. Contamination of water systems poses health risks to the general public. The presence of foreign chemical content in unregulated volumes could result in violent reactions, which escalate safety risks within production facilities.
Two ways to mitigate reverse flow problems
Eliminating reverse flow troubles begins with the correct design of the entire piping system. Having the appropriate system configuration lays a foundation for sizing pipes, fittings and valves. Below are two of the ways for tackling reverse flow-related issues.
1. Selecting the correct check valve
Swing check valves and tilting valves rely on flow reversal and gravity to revert to a closed position in the case of a pressure drop in the system. Although check valves should close instantly, it is common to experience a lag that lasts a few seconds before full flap closure. Conventional swing check valves use flappers that travel a shorter distance and have low inertia to facilitate quick closing. Double-check valves, silent check valves and spring check valves have better shut-off characteristics, which are necessary for eliminating backflows.
A double-check valve contains two independent, spring-loaded check valves assembled in a series configuration. When one check valve fails, the other guarantees protection against reverse flow. When one valve closes, it limits the pressure difference across the other check valve to form a tight seal that inhibits reverse flow. These valves are suitable for domestic water services such as lawn sprinklers. It is not advisable to use them in highly hazardous applications.
Swing check valves amplify the impacts of water hammers in pressurized piping systems. On the other hand, spring check valves eliminate these problems. These valves use a spring that exerts a force on the flapper, returning it to an initially closed position without relying on pipeline pressure differences. They provide positive sealing as long as fluid pressure is below the valve’s cracking pressure. They operate over a wide temperature range and provide timely valve closure irrespective of the vertical orientations. Some application areas of these valves are industrial gas and chemical service, public water supply systems, boilers and fuel pipelines.
Silent check valves or non-slam check valves combat pipe overpressure and vibration, caused by shockwaves created by water hammers. Unlike the other check valves that use a flapper, the silent check valves use a spring-loaded piston with short strokes. The spring restricts the axial movement of the piston toward the direction of flow. Combining spring action with the short axial strokes enables the valve to rapidly open and close, permitting smooth fluid flow. These valves are essential for clean water supply systems, food processing systems and high-pressure chemical applications.
2. Power-assisted valves for critical operations
Applications like water circulation for condenser cooling in coal power plants are prone to reverse flow. Such critical applications rely on PAVs that are programmed to respond to pump trips, temperature variations and circulating water volume variations. The valves are powered using electric motors, hydraulic operators or pneumatic operators.
The PAV is used in combination with conventional check valves, creating a redundant system that permits controlled closure of valves. The check valves close first when a pressure drop occurs in the piping system, while the PAV acts as a backup solution, protecting the entire system from flow reversal shockwaves. PAVs provide flow isolation to facilitate repairs of the pumping units.
In some cases, system pressure may remain below the full vacuum pressure for a significant amount of time, despite using a combination of check valves and PAVs. This condition creates perfect conditions for cavitation to occur. For the prevention of cavitation, a vacuum breaker or air inlet is required. It automatically allows the injection of air into the system whenever a vacuum is detected. Vacuum breakers are limited to systems that are insensitive to air.
Reverse flow standards
The design of piping systems for commercial buildings, production facilities, HVAC systems, civil pipelines and residential buildings follow specific piping standards and codes. The standards define the minimum requirements for pipe sizes, valve type and orientation, pressure and velocity limits. Establishing efficient piping systems, devoid of reverse flows, demands adherence to these standards.
The reversed fluid flow impacts the performance and structural integrity of a piping system. It equally poses health and safety risks to occupants of a facility. Such occurrences inhibit compliance with statutory regulations on emissions, contamination control and environmental protection.
– ValveMan is a CFE Media and Technology content partner.
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