What is Hammer Effect in Pressure Gauges?

Understanding Water Hammer and Its Effects on Gauges

When fluids flow through pipes such as water, oil, or other liquids sudden changes to that flow can cause serious pressure shocks. These shocks are known as water hammer (also “hydraulic shock” or “fluid hammer”). In industrial systems, these sudden pressure surges can damage sensitive instruments like pressure gauges and transmitters.

In this article, we will explain:

1. What “hammer effect” means in gauges

2. What causes water hammer

3. How water hammer damages pressure sensing devices

4. Ways to protect gauges and transmitters

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What Is the “Hammer Effect” in Gauges?

In the context of pressure gauges and transmitters, the “hammer effect” refers to the sudden spike or shock in pressure that impacts the measuring element.

• Gauges and transmitters are built to measure pressures up to some rated value (their “full scale”).
• The internal sensor such as a Bourdon tube or diaphragm has some flexibility to deform and return to its original shape under pressure.
• However, a sudden pressure surge from water hammer can exceed that flexibility, causing permanent deformation, loss of accuracy, or even rupture.

Thus, the hammer effect is the transient overload that “slams” into the sensor beyond normal operating conditions.

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What Causes Water Hammer?

The water hammer effect arises when a fluid in motion is forced to stop or change direction very quickly. Because liquids are nearly incompressible, they resist sudden changes. Some common causes are:

• A valve is closed too quickly
• A pump stops suddenly
• A sudden change in flow direction
• Elevation changes or abrupt pipeline changes
• Poorly designed piping systems

When the fluid is abruptly stopped, the kinetic energy of the moving fluid becomes a pressure wave that travels through the pipe. This pressure wave can reflect back and forth until the system dampens it. 

A simple household example: shutting off a faucet quickly may create a “thump” in the plumbing. That is water hammer in action. 

In pipelines, this effect is magnified and can generate extremely high pressures much higher than normal operating pressure. 

Magnitude of the Pressure Spike

Although the spike lasts only for milliseconds, it can reach many times the system’s steady pressure. Some sources suggest spikes as high as 8× the normal pressure under certain conditions. 

The actual pressure surge depends on factors such as:

• Fluid velocity
• Pipe cross-section
• Compressibility of fluid and elasticity of pipe walls
• Speed of valve closure

How Does Water Hammer Affect Pressure Gauges and Transmitters?

Damage Mechanisms

When a pressure spike reaches a gauge or transmitter, the sensor element is exposed to forces beyond its design. Possible damage includes:

• Permanent deformation of the diaphragm or Bourdon tube so that it no longer returns to zero
• Loss of repeatability and accuracy
• Rupture of the sensing element or internal connections
• Fatigue failure over repeated cycles

Many gauges are rated to handle 1.3× their full scale for short durations; some diaphragm gauges can go up to 5× in extreme cases. Transmitters may withstand 1.5× to 2× for short times. 

If the pressure spike exceeds these tolerances, the instrument can be permanently damaged. So do not consider this hammer effect lightly.

Why Gauges Fail Even When System Pressure Is Lower

Often, users are surprised: “How can my gauge, rated for 1000 kPa, fail when my system pressure is only 600 kPa?” The culprit is the transient spike from water hammer. 

Even though the average system pressure is within limits, those microsecond surges can overwhelm the sensor if not mitigated.

Let us take an example:

When the speed of a liquid suddenly changes like when a pump is started or stopped quickly, or when the liquid is forced to change direction sharply it creates a surge. This surge is commonly called water hammer.

In most process plants, water hammer happens when a valve closes too fast. If a valve (manual or automatic) shuts in about 1.5 seconds or less, the liquid flow stops abruptly. This sudden stoppage creates a pressure wave that can be up to five times higher than the normal working pressure.

These waves travel at the speed of sound in liquids, which can be more than 4000 feet per second—fast enough to damage piping, gauges, and transmitters.

How to Calculate the Pressure Increase

There are online calculators for water hammer, but they often give very different results. A simple formula you can use is:

P = (0.070VL/t) + P1

Where:

• P = total pressure after surge (psi)
• P1 = normal inlet/system pressure (psi)
• V = flow velocity (ft/s)
• t = valve closing time (seconds)
• L = length of upstream pipe (ft)

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Protecting Gauges and Transmitters Against Water Hammer Effect

Because water hammer is often unavoidable in fluid systems, protection of sensitive devices is critical. Here are common techniques:

1. Snubbers / Dampers

A pressure snubber (or damper) is a fitting inserted between the process line and the instrument. It slows or limits the rate of pressure change reaching the sensor. 

They typically consist of a porous material, a narrow orifice, or sintered metal disc. 

Variants include:

• Threaded snubbing screw
• Capillary tubes added to snubbers
• Needle valves
• Adjustable flow restrictors
• Piston-type snubbers

These devices don’t block steady pressure but attenuate sudden spikes. 

2. Overpressure Protectors

In severe cases with sustained high pressures, specialized overpressure protectors are needed. These are devices designed to bypass or relieve excess pressure so that the instrument is not overloaded. 

3. Slower Valve Closure / Control Systems

One root cause is abrupt valve action. Solutions include:

• Use valves that close slowly (multiturn valves rather than quarter-turn) 
• Use control valves instead of reactive check valves
• Automate pump start/stop sequences for gradual change 

4. Reducing Fluid Velocity and Proper Piping Design

Lower velocities mean less kinetic energy to convert into pressure spikes. Designing pipelines to avoid abrupt changes and ensuring smooth flow paths helps. 

5. Surge Tanks, Air Cushions, Expansion Chambers

These devices absorb the energy of a pressure pulse. For example:

• Surge or accumulator tanks
• Air chambers
• Expansion vessels

They serve as “shock absorbers” in the fluid system. 

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Example Scenario & Tips

Scenario: You have a pipeline delivering fluid at a moderate pressure. A valve downstream is closed quickly. The sudden stoppage causes a pressure wave upstream that hits a pressure transmitter. The transmitter, rated for 1 MPa, sees a short spike of 4 MPa, permanently deforming its diaphragm.

Mitigation steps:

1. Insert a snubber or restrictor between the pipeline and the instrument.

2. Use a valve that closes slowly.

3. Ensure the pump shutoff is gradual (soft start/stop).

4. Lower the fluid velocity or change pipe sizing.

5. Add surge tank or air cushion near risk points.

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What we learn today?

• Hammer effect in gauges refers to a sudden pressure shock acting on the sensor.
• Water hammer is a hydraulic phenomenon arising from abrupt changes in fluid flow, creating pressure waves.
• These pressure spikes may far exceed normal system pressure and can damage or destroy gauges and transmitters.
• Common protective techniques include snubbers, overpressure protectors, slow valve control, surge tanks, and good piping design.
• Being proactive in safeguarding instruments is much cheaper than replacing damaged sensors.