Fluid Hammer
Fluid hammer:- When the flow of fluid through a system is suddenly halted at one point, through valve closure or a pump trip, the fluid in the remainder of the system cannot be stopped instantaneously as well. As fluid continues to flow into the area of stoppage (upstream of the valve or pump), the fluid compresses, causing a high-pressure situation at that point.
Likewise, on the other side of the restriction, the fluid moves away from the stoppage point, creating a low pressure (vacuum) situation at that location. Fluid at the next elbow or closure along the pipeline is still at the original operating pressure, resulting in an unbalanced pressure force acting on the valve seat or the elbow.
The fluid continues to flow, compressing (or decompressing) fluid further away from the point of flow stoppage, thus causing the leading edge of the pressure pulse to move through the line. As the pulse moves past the first elbow, the pressure is now equalized at each end of the pipe run, leading to a balanced (i.e., zero) pressure load on the first pipe leg. However, the unbalanced pressure, by passing the elbow, has now shifted to the second leg.
The unbalanced pressure load will continue to rise and fall in sequential legs as the pressure pulse travels back to the source (or forward to the sink). The ramp up time of the profile roughly coincides with the elapsed time from full flow to low flow, such as the closing time of the valve or trip time of the pump.
Since the leading edge of the pressure pulse is not expected to change as the pulse travels through the system, the ramp down time is the same. The duration of the load from initiation through the beginning of the down ramp is equal to the time required for the pressure pulse to travel the length of the pipe leg.
What is Fluid Hammer?
Fluid hammer often called hydraulic shock is a sudden surge in pressure that happens when the flow of a liquid inside a pipeline is abruptly stopped, started or redirected. In oil and gas facilities, this can occur when valves close too quickly, pumps are tripped, or flow is suddenly interrupted. The momentum of the moving fluid creates a shockwave that travels through the system.
Causes of Fluid Hammer in Oil & Gas
1. Quick Valve Operation
When valves are opened or closed too quickly, the fluid inside the pipeline is forced to change velocity almost instantly. This sudden deceleration (or acceleration) generates a shockwave that travels through the liquid column. In high-pressure oil and gas systems, such rapid valve action can amplify the surge, leading to vibration, pipe wall stress, and even gasket or joint failures. Ideally, valves should be operated with controlled, gradual closure rates to minimize pressure spikes.
2. Pump Failures or Sudden Starts
Pumps are responsible for maintaining flow and pressure in oil and gas pipelines. If a pump trips unexpectedly, the sudden loss of momentum causes the fluid column to collapse toward the pump, creating a severe pressure drop followed by a rebound surge. On the other hand, if a pump is started too abruptly, the liquid accelerates too fast, producing velocity surges that result in fluid hammer. This is why soft starters and variable frequency drives (VFDs) are often recommended to regulate pump acceleration and deceleration.
3. Check Valve Slamming
Check valves are designed to prevent backflow, but if the flow suddenly reverses, the valve disk or flap can slam shut violently. This sudden closure blocks the backward-moving fluid instantly creating a shockwave that propagates through the system. The higher the flow rate and the quicker the reversal, the more intense the slam impact. This not only contributes to fluid hammer but can also damage the valve internals and connected equipment. Using non-slam check valves or spring-assisted designs helps reduce this risk.
4. Gas/Liquid Pocket Collapse
Sometimes, pockets of air, vapor or entrained gas form within liquid pipelines. These compressible pockets act like cushions while present. However when system conditions change—such as increased pressure—they can suddenly collapse, forcing the surrounding liquid to slam into itself. This implosion generates a severe local shockwave that contributes to fluid hammer. In oil and gas operations where multiphase flow is common, this effect is particularly dangerous. Proper venting, degassing and pipeline slope management are critical to avoid vapor pocket formation.
In summary, fluid hammer in oil and gas systems is most often triggered by rapid changes in fluid velocity or pressure conditions—whether due to valve actions, pump dynamics, valve slamming or collapsing vapor pockets. Each cause has its own mitigation strategy, but the core principle remains the same control fluid movement gradually to prevent destructive pressure surges.
Consequences of Fluid Hammer in Oil & Gas
1. Extreme Pressure Spikes Beyond Design Limits
Fluid hammer generates sudden and intense pressure surges that travel through the pipeline. These spikes can exceed the maximum allowable pressure rating of the piping system. Even if the surge lasts only milliseconds, it can stress gaskets, flanges and welds beyond their design limits. Over time, repeated surges weaken the system and increase the risk of catastrophic failure.
2. Noise and Vibrations Echoing Through the Piping
When a pressure wave slams through a liquid column, it produces loud banging noises (often called water hammer knocks) and severe vibrations along the pipeline. These vibrations propagate through the structure, creating resonance effects in pipe supports, hangers and even nearby equipment. Prolonged exposure can loosen fittings, accelerate wear in joints and lead to misalignment of sensitive rotating machinery.
3. Pipe Wall Fatigue or Rupture if Not Controlled
Repeated exposure to sudden surges stresses the pipe wall material. This phenomenon, known as fatigue stress, causes microscopic cracks that slowly propagate with every surge event. If not addressed, these cracks can grow into full ruptures, releasing high-pressure oil, gas, or multiphase fluids into the environment. In extreme cases, this may lead to leaks, fire hazards, or explosions—posing safety and environmental risks.
4. Equipment Failure in Pumps, Compressors and Separators
Fluid hammer does not only affect pipelines—it also transmits damaging loads to connected equipment. Pumps may experience cavitation, seal damage, or bearing misalignment. Compressors and separators may suffer from sudden pressure shocks that disrupt internal components, reduce separation efficiency, or even cause mechanical breakdowns. Repairing or replacing such critical equipment can be both time-consuming and expensive.
5. Reduced System Reliability and Costly Downtime
Ultimately, uncontrolled fluid hammer reduces the overall reliability and efficiency of oil and gas systems. Frequent shutdowns for inspection, repair or emergency maintenance lead to significant production losses. In addition, unplanned downtime often requires mobilizing extra manpower and spare parts, increasing operational costs. In competitive oil and gas markets, this downtime directly impacts profitability.
In short: Fluid hammer is not just a noise problem its a serious integrity threat that can lead to structural damage, equipment failures and unplanned shutdowns. Preventing it ensures safer operations, longer equipment life and maximum system uptime.
Mitigation Measures for Fluid Hammer in Oil & Gas
1. Install Slow-Acting or Controlled Valves
One of the primary triggers of fluid hammer is sudden valve operation. By using slow-acting valves or installing actuated valves with controlled closing and opening speeds, the momentum of the moving fluid is gradually reduced. This prevents sudden pressure surges and minimizes shock waves in the line. In critical applications, engineers often use electrically or hydraulically actuated valves that can be programmed for smooth operation.
2. Use Surge Relief Devices (Accumulators, Air Chambers or Surge Tanks)
Surge relief systems are designed to absorb excess pressure before it damages the pipeline. Devices like accumulators, expansion chambers or surge tanks act as cushions that take in sudden fluid energy and release it slowly back into the system. For example, an air chamber provides compressible volume to reduce wave impact, while a hydraulic accumulator stores fluid under pressure to dampen spikes. This keeps surge pressures well within safe limits.
3. Ensure Gradual Pump Start/Stop Procedures
Pumps that are started or stopped abruptly can cause sudden changes in flow velocity, resulting in hammering effects. To prevent this, pumps should be equipped with soft starters, variable frequency drives (VFDs) or controlled shutdown systems. These ensure that the acceleration and deceleration of fluids happen smoothly, reducing the chance of harmful surges. In critical facilities, backup systems are also added to handle unexpected pump trips.
4. Design Proper Pipeline Supports to Absorb Stress
Even if fluid hammer occurs, well-designed pipeline supports and restraints can help contain the resulting forces. Properly spaced guides, anchors and shock-absorbing supports prevent excessive vibration and keep the piping from shifting. In offshore platforms and refineries, engineers often use spring hangers or dampers that allow controlled movement while resisting damaging loads caused by surge waves.
5. Maintain Correct Venting and Slope to Avoid Vapor Collapse
Trapped air or vapor pockets in pipelines are another major cause of hammer. When these pockets collapse, they generate strong shock waves. To mitigate this, systems must be designed with correct slopes, air vents and drain points. This ensures that gases and vapors are safely vented out while preventing the formation of voids. Proper venting not only reduces hammering but also improves flow stability and efficiency.
In summary: Fluid hammer can never be eliminated entirely, but with the right engineering practices such as controlled valve operations, surge protection devices, pump control strategies, robust pipeline supports and proper venting—it can be managed effectively. These measures protect critical equipment, extend pipeline life, and ensure safe and reliable oil & gas operations.
In oil and gas operations, fluid hammer is not just noise its a dangerous pressure surge that can damage pipelines, separators and rotating equipment. Preventing it is essential for plant safety and reliability.